the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2021
Paleomagnetic secular variation for a 21,000-year sediment sequence from Cascade Lake, north-central Brooks Range, Arctic Alaska
Abstract. Two > 5-m-long sediment cores from Cascade Lake (68.38° N, 154.60° W), Arctic Alaska, were analyzed to quantify their paleomagnetic properties over the past 21,000 years. Alternating-field demagnetization of the natural remanent magnetization, anhysteretic remanent magnetization, isothermal remanent magnetization, and hysteresis experiments reveal a strong, well-defined characteristic remanent magnetization carried by a low coercivity magnetic component that increases up core. Maximum angular deviation values average < 2°, and average inclination values are within 4° of the geocentric axial dipole prediction. Radiometric ages based on 210Pb and 14C were used to correlate the major inclination features of the resulting paleomagnetic secular variation (PSV) record with those of other regional PSV records, including two geomagnetic field models and the longer series from Burial Lake, located 200 km to the west. Following around 6 ka (cal BP), the ages of PSV fluctuations in Cascade Lake begin to diverge from those of the regional records, reaching a maximum offset of about 2000 years at around 4 ka. Several correlated cryptotephra ages from this section (reported in a companion paper by Davies et al., this volume) support the regional PSV-based chronology and indicate that some of the 14C ages at Cascade Lake are variably too old.
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RC1: 'Comment on gchron-2021-19', Anonymous Referee #1, 19 Jul 2021
Review report for;
gchron-2021-19 “Paleomagnetic secular variation for a 21,000-year sediment sequence from Cascade Lake, north-cenral Brooks Range, Arctic Alaska” by Steen et al.
Summary
This article presents a new data set of paleomagnetic secular variation from lake sediment cores from Cascade Lake, an interesting place of Arctic region. The authors have obtained paleomagnetic data by progressive alternating-field demagnetizations of u-channel samples, and discussed magnetic carriers and grain sizes with non-thermal rock-magnetic experiments. The secular variation (SV) features of inclination from Cascade Lake relatively well correlate with those from Burial Lake, 200 km to the west, and partly with those from the global SV models (CALS10b.1b, pm9k.1b). The two data sets (Cascade Lake and Burial Lake) may contribute to construct a regional type SV curve by stacking, which is useful for local magnetochronology. However, the Cascade Lake paleomagnetic record has no reliable age model with the measured radiocarbon dates, possibly affected by old carbon. Therefore, the authors transferred the age model with 14C dates less affected by old carbon of Burial Lake to the Cascade Lake record (PSV-1 age model) by wiggle matching of SV features between Cascade Lake and Burial Lake. The correlation of SV features between different sites/basins is more or less flexible, so that the correlation needs supports of reliable age constraints. In this paper, four tephra ages published in the companion paper by Davies et al. may play an important role for it.
One of the main conclusions of this paper is that the PSV-1 age model shows evidence for the old carbon effect on the Cascade Lake 14C dates above a composite depth of 160 cm. However, the evidence is weak because the correlation is flexible, and the possibility of downward-shifted recording of paleomagnetic field by authigenic iron-sulfide ferrimagnets is remained. The authors developed a discussion assuming that the main magnetic carrier is magnetite. Hence, I cannot recommend accepting this paper in the present form for publication in Geochronology. But the SV data of Cascade Lake may be useful as supporting evidence for the companion paper by Davies et al., although additional magnetic experiments, e.g. progressive thermal demagnetizations of NRMs, thermomagnetic analyses (Js-T), and/or FORC experiments, are necessary to estimate magnetic carriers/particle sizes.
Individual comments
(1) P. 4, lines 125-126
The authors calibrated the 14C dates of this study using the IntCal 13 calibration curve (Reimer et al., 2013). Is the curve consistently used in the calibrations for other SV records in and around Alaska, and the global SV models (Figs. 9 and 11)? We have the IntCal20 calibration curve, now (Reimer et al., 2020).
(2) P. 6 to 7, “4.2 S-ratios and kARM/klf”
The authors interpret as the increasing up-core trends of the S-ratio and kARM/klf reflect the progressive addition of a separate fine-grained ferromagnetic component to the magnetic assemblage dominated by high-coercivity particles in the lower part. This interpretation would be correct. However, we should note that a similar up-core increasing trend lies in the organic material content (OM), while up-core decreasing trends are present in the magnetic susceptibility (klf) and IRM (Fig. 2). These trends must be discussed, together with the trends in the proxies of soft-component (S-ratio) and magnetic grain size (kARM/klf), which can be associated with the gradually increased anoxic environments that cause dissolution of fine magnetites, and formation of super-fine authigenic ferrimagnets, e.g., greigites.
(3) P.7, “4.3 Hysteresis, magnetic grain size and mineralogy”
The sediments contain a high coercivity mineral of hematite (or goethite), in addition to a low coercivity ferrimagnet (magnetite?). Further, the possibility of containing iron-sulfide ferrimagnets is still remained. Thus, the authors need to reconsider the interpretation of the domain states with the Day diagram, which is originally for titanomagnetite (Fig. 3B). FORC diagram may be suitable for a mixture of magnetic minerals to estimate domain state (Roberts et al., 2017, JGR, 123, 2618–2644).
(4) P.8, “4.4 Characteristic remanent magnetization”
The ChRM determined with a small MAD value is a strong point of this study. However, the orthogonal projections of demagnetization data seem to show that the magnetization vector does not decay toward the origin. Doesn’t this result indicate the presence of a higher coercivity component except hematite/goethite? A PDRM component by detrital hematite/goethite particles should have a component with a similar direction with that of detrital magnetite particles.
(5) P.10, “4.5 Normalized remanence (relative paleointensity)”
The main magnetic carriers of Cascade Lake sediments comprise a high-coercivity mineral (probably hematite) and a low-coercivity mineral (possibly magnetite/greigite). I consider the NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, because they are less affected by high-coercivity components. Unfortunately, no curves of normalizers ARM20-70mT and IRM20-70mT are shown in Fig. 7, so that we cannot evaluate the correlation between the RPI and normalizers (“R” is not helpful). The kLF, ARM45mT, and IRM45mT in Fig. 7 include both mineral components, and in addition the contributions of high-coercivity particles increased in the ARM45mT and IRM45mT , compared with those before AFD.
(6) P. 13, “5.1 Magnetic assemblage”
The authors indicate that the fine grained and low-coercivity ferromagnetic component is carried by (titano)magnetite. As mentioned in (2), greigite is also a candidate. Authigenic greigite particles are fine, with coercivity ranges similar to magnetite. Therefore, it is difficult to separate the components of magnetite and greigite by AF demagnetization. To reject the possibility of the presence of greigite, the authors should show evidence with thermal experiments, e.g. progressive thermal demagnetizations of NRMs, thermos-magnetic analysis (Js-T), and so on.
(7) P. 13, line 330
I do not agree the reason of the linear depth-age relation over the past 17 ka for the Burial Lake 14C age model’s being more realistic than the Cascade Lake 14C age model.
(8) P. 15, Figure 9
The correlation of inclination features between Cascade Lake and Burial Lake seems to be generally good (Fig. 9). But I do not agree with the correlation with the global SV models. Many tie-points between I3 and I8 are flexible. For example, the oldest part of the pfm9k.1b Inc-GAD may not reach the inclination feature I7 of the Burial Lake inc-GAD, and the I6-I8 of the pfm9k.1b may correlate in phase to around I4 of the Burial Lake. The correlations in Fig. 9 are not robust, and thus the authors must be careful with using the tie-point ages for age models and others.
(9) P. 16, “5.3 Age model discrepancy”
For the age model discrepancy, the authors discussed two possible causes; the old carbon of aquatic organic materials and the lock-in depth of PDRM. If the authors do not show evidence for the absence of authigenic greigite, readers would concern the effect of it. Large downward shifts of paleomanetic signals in the record carried by greigites are likely (e.g., Robert et al. (2005) GRL).
The authors mention that four tephra ages published in the companion paper by Davies et al. provide strong evidence for the discrepancy. If they clearly show the discrepancy without help of PSV data, the authors can construct a new age model with selected measured 14C dates and the tephra ages for the Cascade Lake SV curve. In this case, they should not mention the old carbon effect in the conclusion. In place, they would have a merit of making a type SV curve in Alaska by stacking the Cascade Lake and Burial Lake SV data, both of which have independent age models.
(10) P. 17, lines 414-415
The authors mention that high-amplitude inclination shifts at this time are contemporaneous with low relative paleointensity estimates (Fig. 11). However, the relative paleointensity curve plotted in Fig. 11 is after 15.3 ka, which does not show paleointensity values around 17 ka. Readers may want to see the global VADM values plotted until about 20 ka.
(11) P. 17, lines 434-435
“Magnetic grain-size estimation (Fig. 3 and 4) suggests fine PSD magnetites”
As mentioned in (3), we cannot estimate grain sizes (domain states) with a Day plot of magnetic mineral assemblages of low-coercivity ferrimagnets and hematites, which are suggested by the relatively small S-ratio values ranging from 0.5 to 0.88 throughout the sequence.
Citation: https://doi.org/10.5194/gchron-2021-19-RC1 -
AC1: 'Reply on RC1', Darrell Kaufman, 26 Aug 2021
We thank the reviewer for their helpful and constructive comments. Our responses and proposed revisions are in bold font below excerpts of the reviewer’s verbatim comments.
Summary
… the Cascade Lake paleomagnetic record has no reliable age model with the measured radiocarbon dates, possibly affected by old carbon.
While we do present evidence that the radiocarbon dates have been influenced by old carbon, we also present evidence for a reliable age model based on independent evidence using tephra ages (from Davies et al. companion paper) and constraints from other regional paleomagnetic secular variation (PSV) records.
However, the evidence is weak because the correlation [among PSV records] is flexible…
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
… and the possibility of downward-shifted recording of paleomagnetic field by authigenic iron-sulfide ferrimagnets is remained.
Although it is a remote possibility, there is no evidence for authigenic Fe-S (greigite) as a dominant magnetic mineral within this assemblage, and we doubt that it is present. Because authigenic greigite is a diagenetic product of sulphate reduction (Roberts et al., 2011; Roberts, 2015), it is most often found in strongly reducing marine deposits, or in lake sediments that transitioned from marine settings (e.g., Loch Lomond, Snowball and Thompson, 1990), or vice versa (e.g., Black Sea, Nowacyzk et al., 2012). When present in lake sediments, it is typically characterized by highly variable intensities often at centimeter scale (Stockhausen and Zolitschka, 1999; Frank, 2007), and by poor quality natural remanent magnetization. Evidence for Greigite is often associated with a gyro remanent magnetization (Stephenson and Snowball, 2001; Snowball, 1997; Roberts et al., 2011) and extremely high SIRM/k or IRM/k ratios (Snowball et al., 1991; Nowaczyk et al., 2012, 2020). Instead, the consistency of the Holocene assemblage at Cascade Lake, including its AF demagnetization behavior and remanence records, is reminiscent of data from Scandinavian lakes where high-quality magnetization is associated with a magnetite-dominated magnetic assemblage, likely produced by microbes (e.g., Snowball et al., 2013; Haltia-Hovi et al., 2011). Although we cannot prove that the assemblage is a result of biogenic magnetite, the data are consistent with this interpretation, and more importantly, they afford a high-quality magnetization record by a fine-grained ferrimagnetic component, with no evidence for greigite.
… additional magnetic experiments, e.g. progressive thermal demagnetizations of NRMs, thermomagnetic analyses (Js-T), and/or FORC experiments, are necessary to estimate magnetic carriers/particle sizes.
We agree that extensive additional analyses would be needed to prove the magnetic mineralogy of the sediments, but such an undertaking would be beyond the scope of this paper. More importantly, our use of AF demagnetization is commonly used across many (hundreds?) of high-quality paleomagnetic studies of lake sediments, especially for Holocene sediments, which typically preserve well-defined NRM. Furthermore, the thermal demagnetization suggested by the reviewer can be problematic when working with wet lake sediments because: i) additional subsampling disturbs the soft sediment and can alter the paleomagnetic signal; ii) desiccation during heating can alter the directional record; iii) combusting organic-rich lake sediment in a restricted environment (magnetically shielded) can be hazardous; iv) thermal sample alteration can generate new magnetic minerals; and v) after heating, the sediment can no longer be used for paleointensity or environmental magnetic studies.
Individual comments
(1) P. 4, lines 125-126. The authors calibrated the 14C dates of this study using the IntCal 13 calibration curve (Reimer et al., 2013). Is the curve consistently used in the calibrations for other SV records in and around Alaska, and the global SV models (Figs. 9 and 11)? We have the IntCal20 calibration curve, now (Reimer et al., 2020).
The other PSV records to which we are correlating were calibrated using earlier versions of IntCal, so we are hesitant use a different version. More importantly, the differences are relatively small within the timeframe of the Cascade Lake record.
(2) P. 6 to 7, “4.2 S-ratios and kARM/klf” The authors interpret as the increasing up-core trends of the S-ratio and kARM/klf reflect the progressive addition of a separate fine-grained ferromagnetic component to the magnetic assemblage dominated by high-coercivity particles in the lower part. This interpretation would be correct. However, we should note that a similar up-core increasing trend lies in the organic material content (OM), while up-core decreasing trends are present in the magnetic susceptibility (klf) and IRM (Fig. 2). These trends must be discussed, together with the trends in the proxies of soft-component (S-ratio) and magnetic grain size (kARM/klf), which can be associated with the gradually increased anoxic environments that cause dissolution of fine magnetites, and formation of super-fine authigenic ferrimagnets, e.g., greigites.
Both susceptibility (k) and IRM decrease subtly up core, reflecting an increase in an extremely fine-grained ferrimagnetic assemblage that is not present in the older part of the sequence. This component increases up core, as indicated by a decrease in k and IRM, relative to ARM that is biased toward fine magnetite (Banerjee et al., 1981). We interpret this fine-grained ferrimagnetic component as biogenic magnetite, which is consistent with the expected evolution of the lake and its catchment toward increasing productivity following the last ice age. Biogenic magnetite also explains the well- defined, high-quality magnetization record, and the coercivity of the magnetization. Alternatively, the fine-grained magnetite might be dust. Dust seems to be present in lake sediment in the region (Burial Lake, Dorfman et al., 2015); however, the dust is most abundant in the Pleistocene sediment. The up-core increase inferred biogenic magnetite is consistent with the corresponding increase in organic matter associated with warmer climates of the Holocene, as found at other high-latitude lakes (e.g., Snowball et al., 2013; Haltia-Hovi et al., 2011). As explained above, there is no evidence for the presence of greigite: poor quality magnetization, gyro remanent magnetization typically between 50 and 80 mT (Ron et al., 2007; Nowacyzk et al., 2020), high IRM/k, coercivity lower than hematite as indicated by the S-ratio.
(3) P.7, “4.3 Hysteresis, magnetic grain size and mineralogy” The sediments contain a high coercivity mineral of hematite (or goethite), in addition to a low coercivity ferrimagnet (magnetite?). Further, the possibility of containing iron-sulfide ferrimagnets is still remained. Thus, the authors need to reconsider the interpretation of the domain states with the Day diagram, which is originally for titanomagnetite (Fig. 3B). FORC diagram may be suitable for a mixture of magnetic minerals to estimate domain state (Roberts et al., 2017, JGR, 123, 2618–2644).
Production of FORC diagram would further document a complex magnetic mineral assemblage (again, however, there is no evidence for Fe-S greigite), but would do little to address the primary question much further than the evidence already provided. Additionally, a full accounting of the magnetic mineralogy is not the goal of this study and would entail an exceptional amount of work with equipment not readily available.
(4) P.8, “4.4 Characteristic remanent magnetization” The ChRM determined with a small MAD value is a strong point of this study. However, the orthogonal projections of demagnetization data seem to show that the magnetization vector does not decay toward the origin. Doesn’t this result indicate the presence of a higher coercivity component except hematite/goethite? A PDRM component by detrital hematite/ goethite particles should have a component with a similar direction with that of detrital magnetite particles.
Small deviations in directions at high field strengths are commonly observed and generally considered to reflect increased noise relative to signal in that part of the demagnetization diagram, with many potential causes including general measurement noise. While it is possible that detrital hematite or goethite might contribute to the signal following the demagnetization of magnetite, and while we appreciate the suggestion, it is difficult to know where to take this because neither mineral is considered to carry a quality magnetic signal in Holocene lake sediments.
(5) P.10, “4.5 Normalized remanence (relative paleointensity)” The main magnetic carriers of Cascade Lake sediments comprise a high-coercivity mineral (probably hematite) and a low-coercivity mineral (possibly magnetite/greigite). I consider the NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, because they are less affected by high-coercivity components. Unfortunately, no curves of normalizers ARM20-70mTand IRM20-70mT are shown in Fig. 7, so that we cannot evaluate the correlation between the RPI and normalizers (“R” is not helpful). The kLF, ARM45mT, and IRM45mT in Fig. 7 include both mineral components, and in addition the contributions of high-coercivity particles increased in the ARM45mT and IRM45mT , compared with those before AFD.
We agree that NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, but not “because they are less affected by high-coercivity components”, but rather because k is not a remanence carrier and influenced by other things. However, the main point is that the similarity between all three normalizers lends confidence to the record of geomagnetic intensity. Plotting ARM, IRM as a stack or from a single demagnetization step makes little practical difference, but this can be changed and addressed.
(6) P. 13, “5.1 Magnetic assemblage”. The authors indicate that the fine grained and low-coercivity ferromagnetic component is carried by (titano)magnetite. As mentioned in (2), greigite is also a candidate. Authigenic greigite particles are fine, with coercivity ranges similar to magnetite. Therefore, it is difficult to separate the components of magnetite and greigite by AF demagnetization. To reject the possibility of the presence of greigite, the authors should show evidence with thermal experiments, e.g. progressive thermal demagnetizations of NRMs, thermos-magnetic analysis (Js-T), and so on.
We would agree with this if there was evidence for greigite (gyro remanence, high SIRM/k, magnetic dissolution, detection hydrogen sulfides during coring or splitting, oxidative changes in magnetic properties or coloring, sulfidic lake, down core increase in greigite indicators, etc.). Again, a full accounting of the magnetic mineralogy is not the goal of this study and would entail an exceptional amount of work with equipment not readily available.
(7) P. 13, line 330. I do not agree the reason of the linear depth-age relation over the past 17 ka for the Burial Lake 14C age model’s being more realistic than the Cascade Lake 14C age model.
We will omit the phrase, “… and because the sedimentation rate at Burial Lake is rather linear over the past ~ 17 kyr.”
(8) P. 15, Figure 9. The correlation of inclination features between Cascade Lake and Burial Lake seems to be generally good (Fig. 9). But I do not agree with the correlation with the global SV models. Many tie-points between I3 and I8 are flexible. For example, the oldest part of the pfm9k.1b Inc-GAD may not reach the inclination feature I7 of the Burial Lake inc-GAD, and the I6-I8 of the pfm9k.1b may correlate in phase to around I4 of the Burial Lake. The correlations in Fig. 9 are not robust, and thus the authors must be careful with using the tie-point ages for age models and others.
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
(9) P. 16, “5.3 Age model discrepancy”. For the age model discrepancy, the authors discussed two possible causes; the old carbon of aquatic organic materials and the lock-in depth of PDRM. If the authors do not show evidence for the absence of authigenic greigite, readers would concern the effect of it. Large downward shifts of paleomanetic signals in the record carried by greigites are likely (e.g., Robert et al. (2005) GRL).
See our responses above regarding the lack of evidence for greigite.
The authors mention that four tephra ages published in the companion paper by Davies et al. provide strong evidence for the discrepancy. If they clearly show the discrepancy without help of PSV data, the authors can construct a new age model with selected measured 14C dates and the tephra ages for the Cascade Lake SV curve. In this case, they should not mention the old carbon effect in the conclusion. In place, they would have a merit of making a type SV curve in Alaska by stacking the Cascade Lake and Burial Lake SV data, both of which have independent age models.
We understand this alternative approach and appreciate the suggestion. However, the primary goal of the study is generating a reliable age model for Cascade Lake, and not improving the regional PSV curve.
(10) P. 17, lines 414-415. The authors mention that high-amplitude inclination shifts at this time are contemporaneous with low relative paleointensity estimates (Fig. 11). However, the relative paleointensity curve plotted in Fig. 11 is after 15.3 ka, which does not show paleointensity values around 17 ka. Readers may want to see the global VADM values plotted until about 20 ka.
We will attempt to extend the field intensity curve in Figure 11, as suggested.
(11) P. 17, lines 434-435. “Magnetic grain-size estimation (Fig. 3 and 4) suggests fine PSD magnetites”. As mentioned in (3), we cannot estimate grain sizes (domain states) with a Day plot of magnetic mineral assemblages of low-coercivity ferrimagnets and hematites, which are suggested by the relatively small S-ratio values ranging from 0.5 to 0.88 throughout the sequence.
We agree, at least as a strict grain-size indicator. However, hysteresis data provide information on the magnetic assemblage and our Day plots suggest that fine-grained ferrimagnetic minerals are a substantial component.
References cited in authors’ replies
Banerjee, S.K., King, J. Marvin, J., 1981. A rapid method for magnetic granulometry with applications to environmental studies. Geophysical Research Letters, 8, 333-336.
Dorfman, J.M., J.S. Stoner, M.S. Finkenbinder, M.B. Abbott, C. Xuan, & G. St-Onge 2015: A 37,000-year Environmental Magnetic Record of Aeolian Dust Deposition from Burial Lake, Arctic Alaska. Quaternary Science Reviews, 128, 81-97.
Frank, U. 2007, Palaeomagnetic investigations on lake sediments from NE China: a new record of geomagnetic secular variations for the last 37 ka. Geophys. J. Int. (2007) 169, 29–40
Haltia-Hovi, E., Nowaczyk N., Saarinena T., 2011. Environmental influence on relative palaeointensity estimates from Holocene varved lake sediments in Finland. Physics of the Earth and Planetary Interiors 185, 20–28
Li, C.G., Zheng, Y., Wang, M., Sun, Z., Jin, C., and Hou, J., 2021. Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau. Quaternary Geochronology, 62, 101146.
Nowaczyk, N.R., Arz, H.W., Frank, U., Kind, J.& Plessen, B., 2012. Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments, Earth Planet. Sci. Lett., 351–352, 54–69.
Nowaczyk N.R., Liu, J. Arz, H.W., 2020 Records of the Laschamps geomagnetic polarity excursion from Black Sea sediments: magnetite versus greigite, discrete sample versus U-channel data. Geophys. J. Int. (2021) 224, 1079–1095
Roberts, A.P., 2015. Magnetic mineral diagenesis. Earth-Science Reviews, 151 1–47.
Roberts, A.P., Chang, L., Rowan, C.J., Horng, C.S., Florindo, F., 2011. Magnetic properties of sedimentary greigite (Fe3S4): an update. Rev. Geophys. 49, RG1002.
Ron, H., Nowaczyk, N.R., Frank, U., Schwab, M.J., Naumann, R., Striewski, B. & Agnon, A., 2007. Greigite detected as dominating remanence carrier in Late Pleistocene sediments, Lisan formation, from Lake Kinneret (Sea of Galilee), Israel, Geophys. J. Int., 170, 117–131.
Snowball, I.F., 1997. Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden, Geophys. J. Int., 129, 624–636.
Snowball, I.F., Thompson, R., 1990. A mineral magnetic study of Holocene sediment yields and deposition patterns in the Llyn Geirionydd catchment, north Wales. The Holocene 2, 238–248.
Snowball, I.F., 1991. Magnetic hysteresis properties of greigite (Fe3S4) and a new occurrence in Holocene sediments from Swedish Lappland. Phys. Earth Planet. Inter. 68, 32–40
Snowball, I., Mellström, A., Ahlstrand, E., Haltia, E., Nilsson, A., Ning, W., Muscheler, R., Brauer, A., 2013. An estimate of post-depositional remanent magnetization lock-in depth in organic rich varved lake sediments. Glob. Planet. Change 110, 264–277.
Stephson and Snowball, A large gyromagnetic effect in griegite. (2001) Geophys. J. Int. 145, 570–575.
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Citation: https://doi.org/10.5194/gchron-2021-19-AC1
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AC1: 'Reply on RC1', Darrell Kaufman, 26 Aug 2021
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RC2: 'Comment on gchron-2021-19', Anonymous Referee #2, 21 Jul 2021
The manuscript by Steen et al. is part of a “twin-submission” that collectively build an age-depth model for a sediment sequence in Cascade Lake. Steen et al. focus on a paleomagnetic secular variation (PSV) record that extends back to 21 ka, which is a strength of the paper. The companion paper by Davies et al. (Gchron-2021-18) focuses on late Holocene cryptotephra in Cascade Lake and the construction of an age model based on radiometric methods (210Pb and 14C) and correlated ages of the cryptotephra. Davies et al.’s dating results are used to support Steen et al.’s conclusion that there are significant differences compared to (i) the radiometric-based age model and (ii) one that is based on relative paleomagnetic secular variation (PSV) correlations to the predictions of time-varying geomagnetic field models and the trends in an independently dated PSV record from Burial Lake (only 200 km away). The two manuscripts come to the same conclusion that several radiocarbon ages are too old, which is well known for lake sediment samples that contain more or less terrestrial derived organic matter that is older than the age of deposition. A general problem is that the two manuscripts frequently cross-reference each other and it is quite difficult to understand how either one can stand alone. Some raw data are put into both manuscripts (e.g. the 14C dates and tables) which gives future writers an unnecessary choice about which source to cite. Davies et al. state that details of the radiometric age-model construction are in Steen et al., but there are more details in the former than the latter. The editor could consider asking the authors to combine the papers into one so that problems that arise from twin-submissions are negated.
There is also reference to a Masters thesis by Steen (2016) that is not easily accessible and Davies et al. refer to a submitted manuscript by Jensen et al., which might be crucial because it concerns the age of a cryptotephra.
In addition to a number of specific comments listed further on, I have a couple of general doubts about the manuscript by Steen et al. First, there are rather limited data, which are not replicated. Palaeomagnetic studies of lacustrine and marine sites should rely on three of more cores from each site so that there is replication of results, which can be stacked, and also the possibility to reject outliers. In this study two cores (CASC-4A & CASC-2D) were taken and measured for paleomagnetic properties. The authors state that CASC-4A has a more complete record of unit L3 and that CASC-2D data from unit L3 are rejected. Then the authors take higher resolution data from unit L2 in CASC-2D and splice them onto CASC-4A by matching the inclination records. No details about this match are provided. This exercise assumes the data from CASC-4A are good and forces the CASC-2D data from L2 to fit. It is better to use an independent parameter or proxy (magnetic susceptibility, for example) to match the records and then stack the paleomagnetic data and consider uncertainties.
The second major reservation concerns the tie-points (control points) in Figure 9. These tie-points give the impression that there are high-frequency matches between the composite Cascade Lake inclination record, the field-model predictions and the Burial Lake record. But, the long-term trends seem very different, particularly for the Holocene. Specifically, there are tie-points within the interval 4-8 ka. Models predict high inclination but the data (the composite record) show low inclination. The authors state in the introduction that major directional (PSV) features predicted from models can be used for age control, but they subsequently ignore the differences in the long-term major directional features. Why? An associated problem concerns the tie-points between the model predictions and the Burial Lake record that are older than 5 ka. These are very large differences in age (2-3 ka) rather than the Burial Lake ages being “somewhat older”. If the Burial Lake 14C-based chronology is valid (as the authors argue) one has to deduce that the geomagnetic model predictions are poor, and thus should not be used to provide correlation tie-points).
In general, the manuscript contains a lot of detail that could be removed through combining the two papers into one. There are aspects about the temporal and spatial development of the geomagnetic field that would be better suited for submission (and review) by a specialised geophysics journal.
Specific comments
The start of the introduction could be more general and focus on paleomagnetic secular variation and its advantages and limitations as a relative dating method. The authors use the term wiggle-matching, which is often used by the radiocarbon community to objectively (statistically) match established changes D14C, but which here is really visual (and quite subjective) matching of trends in the PSV data and model predictions.
The geomagnetic field models are not perfectly constrained, anywhere.
Section 2.2 “Geomagnetic setting” contains details about the origin of the geomagnetic field and its manifestation on Earth’s surface that are unnecessary in this study’s context. The discussion and conclusions do not refer back to these details, so I suggest that they are omitted.
Section 3.1 The individual sections of the cores were not relatively aligned to each other or absolutely aligned to an azimuth, which seems like an experimental error if the purpose of the study was PSV. I appreciate that t is difficult to obtain whole cores that are oriented to an azimuth, but it is relatively easy to keep sections oriented when the core is cut into sections. Why was this orientation not done?
Section 3.2 What is the approximate half-width of the signal that the 35 mm Bartington loop measures? That distance is equally important as the measuring increment (1 cm).
The authors mention that a couple of segments were not measured. The reason should be stated in the methods section. I think that the segments were measured, but the results were bad due to saturation of the SQUIDs by highly magnetic layers.
Section 3.3 These methods (and also the results) are duplicated in the twin paper by Davies et al.
Section 4.1 The authors state that the sediments recovered are undeformed by the coring procedure. This statement contrasts with an earlier statement made in section 3.2 about the samples being taken from as close as possible from the centre of the core, unless “appeared less disturbed by minor coring deformation”. Please be consistent, and how do you know that the sediments are undeformed and/or deformed?
Please avoid the use of terms like “are significantly lower” where there is no known significance.
The authors mention a “hint at the authigenic creation of greigite” but there is no proof. It would be better to state that the cause of the highly magnetic layers is unknown so that there is no speculation.
The average inclinations are close to the GAD model prediction for the site latitude, which the authors use to argue that the PSV record is good, but earlier on the authors state that the geomagnetic field might be different at high latitudes due to the tangent cylinder. The logic seems a bit circular. All palaeomagnetic data (that are ideally oriented to an azimuth) test the GAD model.
The data shown in Figure 5 were obtained based on analyses of the raw palaeomagnetic data, with examples shown in Figure 6, so it might be better to place current Figure 6 before current Figure 5.
Figure 2 is very cramped. The reader is unable to obtain any useful information from the different coloured lines that show all the ARM and IRM demagnetization data. I recommend simplifying the figure.
Figure 3 shows that the hysteresis loops are not closed at 1T, which means that a slope correction does not only correct for paramagnetic (and diamagnetic) contributions. The correction will include an unknown part of the unsaturated anti-ferromagnetic component, which seems relatively high in this case.
Section 4.4 The maximum angular deviation (MAD) is really a measure of how well the ChRM can be defined, rather than a measure of magnetic stability. It is influenced by the stability of the equipment used to demagnetize and measure remanence and the signal-to-noise ratio. Low MADs are not a guarantee that the data reflect the ancient geomagnetic field direction.
Section 4.5 The relevance of the attempt to reconstruct a relative paleointensity (RPI) record is perhaps out of context with the aims of the journal (geochronology) and particularly the twin submission by Davies et al.
Section 4.7 As previously mentioned, the radiometric age model is presented in more detail by Davies et al. To avoid duplicating raw data I suggest either combining to the two manuscripts, or allowing Davies et al. to present the age model in detail, which Steen et al. test using the paleomagnetic data in a subsequent manuscript. The data in Table 1 show the 210Pb activity for the upper 3.6 cm, which is not useful for the PSV data set (no PSV data are from the short core)
Section 5.2 I have made a general comment about this section in my opening paragraph. There are several problems with correlations, mainly associated with the (dis)similarity of the different curves. I do not understand why the authors consider that the Burial Lake radiometric age-model is more reliable than the Cascade lake because the sedimentation rate is rather linear. What is the reason for this argument? The Burial Lake radiometric age-model is definitely better because the 14C dated material did not contain terrestrial organic matter.
There is a reference to a Masters thesis by Steen (2016) and an alternative PSV age model, which has been rejected by this study. I leave it up to the editor(s) to decide if this reference is suitable.
Section 5.3 This section contains quite a lot of speculation about the reason for possibly too old 14C dates. The authors need to consider that a paleomagnetic lock-in depth (delay) might also apply to the Burial Lake record, but the comparisons with the predictions of field models suggest that the offset would be quite large, possible unreasonable, in terms of depth (time).
Section 5.4 Much of this section is not relevant to the journal (Geochronology) because it concerns the development of the geomagnetic field (using paleomagnetism) and would be better suited to a submission and review by a specialised geophysical journal. The comparisons with regional records (and a global VADM) in Figure 11 seem unnecessary in the context of the aims of the twin submissions. Figure 11A has no subjective tie-points (unlike Figure 9) and I do not see much similarity between the different records. If these records were plotted against each other (using age as the control) I doubt that one would find a significant correlation. Have you tried to statistically check the similarity in this way and how adjustment of the age-models might improve a correlation coefficient?
Citation: https://doi.org/10.5194/gchron-2021-19-RC2 -
AC2: 'Reply on RC2', Darrell Kaufman, 26 Aug 2021
We appreciate the thorough and detailed review by Anonymous Referee #2. Our responses and proposed changes to the text are outlined below. Author comments are in bold below corresponding referee comments.
A general problem is that the two manuscripts frequently cross-reference each other and it is quite difficult to understand how either one can stand alone. Some raw data are put into both manuscripts (e.g. the 14C dates and tables) which gives future writers an unnecessary choice about which source to cite. Davies et al. state that details of the radiometric age-model construction are in Steen et al., but there are more details in the former than the latter.
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 210 Pb data will be shifted into a supplement for the Davies et al. companion paper.
The editor could consider asking the authors to combine the papers into one so that problems that arise from twin-submissions are negated.
Prior to submitting the papers, and now following this review, the authors of both papers have considered how the papers could be combined into one. We have made a concerted effort but cannot find a path that enables us to present all the relevant data. We submitted these two manuscripts to Geochronology because we aim to convey the important details about the individual dating methods to this specialized audience. We believe that attempting to do this with a single paper would be unwieldy for readers: both manuscripts date sediments from this high-latitude lake using two completely unrelated methods and reporting this in a meaningful fashion requires two separate papers that can properly present the methods and results for these independent approaches.
There is also reference to a Masters thesis by Steen (2016) that is not easily accessible and Davies et al. refer to a submitted manuscript by Jensen et al., which might be crucial because it concerns the age of a cryptotephra.
Steen’s (2016) thesis is available as open access through ProQuest: https://www.proquest.com/docview/1808501293. We will add the full URL link to the reference cited list. The submitted Jensen et al. manuscript is an invited review and, following its scheduled timeline and assuming its eventual acceptance, it should be publicly available by the time this manuscript may be published. However, this information is not crucial, as explained in Davies’ response to reviewers of the companion paper.
In addition to a number of specific comments listed further on, I have a couple of general doubts about the manuscript by Steen et al. First, there are rather limited data, which are not replicated. Palaeomagnetic studies of lacustrine and marine sites should rely on three of more cores from each site so that there is replication of results, which can be stacked, and also the possibility to reject outliers. In this study two cores (CASC-4A & CASC-2D) were taken and measured for paleomagnetic properties. The authors state that CASC-4A has a more complete record of unit L3 and that CASC-2D data from unit L3 are rejected. Then the authors take higher resolution data from unit L2 in CASC-2D and splice them onto CASC-4A by matching the inclination records. No details about this match are provided. This exercise assumes the data from CASC-4A are good and forces the CASC-2D data from L2 to fit. It is better to use an independent parameter or proxy (magnetic susceptibility, for example) to match the records and then stack the paleomagnetic data and consider uncertainties.
The procedure used to create the composite sequence, as explained by the reviewer, is described on lines 261-271. We will add a reference to Figure 2.7 in Steen (2016), which illustrates the splicing. We understand the advantage of replication and stacking to generate a composite record. We also believe that splicing the lower part of one core, taken from where sedimentation rates are higher, onto the base of the other core is a valid approach. This strategy follows, for example, the recent development of a premier global marine oxygen-isotope record (Westerhold et al., 2020). Rather than stacking records, which is known to smooth variability, the record was spliced together from sites where resolution was highest for each interval. In addition, while further replication is always better, we believe that the available data are sufficient for the purpose of this study: helping to constrain the chronology of the sedimentary sequence. We also note the relative paucity, and therefore the value, of this type of record, which reflects the logistical challenges in this remote region.
The second major reservation concerns the tie-points (control points) in Figure 9. These tie-points give the impression that there are high-frequency matches between the composite Cascade Lake inclination record, the field-model predictions and the Burial Lake record. But, the long-term trends seem very different, particularly for the Holocene. Specifically, there are tie-points within the interval 4-8 ka. Models predict high inclination but the data (the composite record) show low inclination. The authors state in the introduction that major directional (PSV) features predicted from models can be used for age control, but they subsequently ignore the differences in the long-term major directional features. Why? An associated problem concerns the tie-points between the model predictions and the Burial Lake record that are older than 5 ka. These are very large differences in age (2-3 ka) rather than the Burial Lake ages being “somewhat older”. If the Burial Lake 14C-based chronology is valid (as the authors argue) one has to deduce that the geomagnetic model predictions are poor, and thus should not be used to provide correlation tie-points).
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
Additionally, we will assign different levels of certainty to difference subsections of the final age model, depending on the number of chronological methods (PSV, 14C, and cryptotephra) used to construct each section. This will explicitly show where the available data are in good agreement versus where they are more limited. While we accept that there are uncertainties to some of the PSV interpretations, especially in the lower part of the sequence where they are not validated by other methods, we also see value in presenting these data.
In general, the manuscript contains a lot of detail that could be removed through combining the two papers into one. There are aspects about the temporal and spatial development of the geomagnetic field that would be better suited for submission (and review) by a specialised geophysics journal.
Prior to submitting the papers, and now following this review, the authors of both papers have considered how the papers could be combined into one. We have made a concerted effort but cannot find a path that enables us to present all the relevant data. Both manuscripts present first attempts to date sediments from this high-latitude lake using two completely unrelated methods and reporting this in a meaningful fashion requires two separate papers that can focus on properly presenting the methods and results for these independent approaches. We acknowledge that the information included in Section 2.2 on the geomagnetic setting of the field area is not necessarily relevant to the discussion and main conclusions of this study, which focus on improving the chronology of the sedimentary sequence using paleomagnetic data. We therefore intend to omit this section to avoid diluting the main points of this study with extraneous information.
Specific comments
The start of the introduction could be more general and focus on paleomagnetic secular variation and its advantages and limitations as a relative dating method. The authors use the term wiggle-matching, which is often used by the radiocarbon community to objectively (statistically) match established changes D14C, but which here is really visual (and quite subjective) matching of trends in the PSV data and model predictions.
We will omit the term “wiggle matching” to avoid confusion with the procedure used in 14C dating.
The geomagnetic field models are not perfectly constrained, anywhere. Section 2.2 “Geomagnetic setting” contains details about the origin of the geomagnetic field and its manifestation on Earth’s surface that are unnecessary in this study’s context. The discussion and conclusions do not refer back to these details, so I suggest that they are omitted.
We are comfortable omitting this section to avoid diluting the main points of this study with extraneous information.
Section 3.1 The individual sections of the cores were not relatively aligned to each other or absolutely aligned to an azimuth, which seems like an experimental error if the purpose of the study was PSV. I appreciate that t is difficult to obtain whole cores that are oriented to an azimuth, but it is relatively easy to keep sections oriented when the core is cut into sections. Why was this orientation not done?
The core sections were not aligned relative to each other or absolutely aligned to an azimuth because the cores were not initially collected with the goal of obtaining paleomagnetic data.
Section 3.2 What is the approximate half-width of the signal that the 35 mm Bartington loop measures? That distance is equally important as the measuring increment (1 cm).
The spatial resolution of the Bartington MS2C is 20 mm. We will add this detail.
The authors mention that a couple of segments were not measured. The reason should be stated in the methods section. I think that the segments were measured, but the results were bad due to saturation of the SQUIDs by highly magnetic layers.
U-channels were not collected from Lithologic Unit 1 (L1) because it is a stony diamicton and would therefore not provide a reliable record of PSV. The interval from 387 – 251 cm blf in core CASC-2D was not measured for SIRM due to saturation of the SQUID electronics, as suggested by the Referee’s comment. Please see lines 160 – 166 of the Preprint for additional information on this topic.
Section 3.3 These methods (and also the results) are duplicated in the twin paper by Davies et al.
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 201Pb data will be shifted into a supplement for the Davies et al. paper.
Section 4.1 The authors state that the sediments recovered are undeformed by the coring procedure. This statement contrasts with an earlier statement made in section 3.2 about the samples being taken from as close as possible from the centre of the core, unless “appeared less disturbed by minor coring deformation”. Please be consistent, and how do you know that the sediments are undeformed and/or deformed?
We are grateful to the reviewer for pointing out this contradiction. Indeed, the statement in Section 3.2 is not correct. It actually refers to the cores from Shainin Lake, which was included in Steen’s (2016) thesis. In fact, the cores collected from Cascade Lake do not display visual evidence of deformation. The statement regarding samples being “taken as close as possible to the center axis of the core, unless either side of a core section appeared less disturbed by minor coring deformation” will be removed. We will also include clarification that sediment layers are undeformed in the Cascade Lake cores.
Please avoid the use of terms like “are significantly lower” where there is no known significance.
Terms like “are significantly lower” will be removed, as suggested.
The authors mention a “hint at the authigenic creation of greigite” but there is no proof. It would be better to state that the cause of the highly magnetic layers is unknown so that there is no speculation.
We agree and will add a sentence to acknowledge that this hypothesis cannot be substantiated without further analysis.
The average inclinations are close to the GAD model prediction for the site latitude, which the authors use to argue that the PSV record is good, but earlier on the authors state that the geomagnetic field might be different at high latitudes due to the tangent cylinder. The logic seems a bit circular. All palaeomagnetic data (that are ideally oriented to an azimuth) test the GAD model.
We intend to omit this section to avoid diluting the main points of this study with extraneous information on geomagnetic field dynamics and the tangent cylinder.
The data shown in Figure 5 were obtained based on analyses of the raw palaeomagnetic data, with examples shown in Figure 6, so it might be better to place current Figure 6 before current Figure 5.
We appreciate the suggestion, but it is our opinion that the order of Figures 5 and 6 is good in the current configuration, and that there would be little benefit to the reader if the order were switched.
Figure 2 is very cramped. The reader is unable to obtain any useful information from the different coloured lines that show all the ARM and IRM demagnetization data. I recommend simplifying the figure.
We understand the suggestion that Figure 2 could be simplified, however data for demagnetization of the IRM and ARM highlight some unique magnetic properties of the sedimentary sequence. For example, it shows the anomalous saturation of the SQUID electronics during IRM demagnetization of CASC-2D between ~ 250 and 300 cm blf, while these issues were not encountered in other core sections.
Figure 3 shows that the hysteresis loops are not closed at 1T, which means that a slope correction does not only correct for paramagnetic (and diamagnetic) contributions. The correction will include an unknown part of the unsaturated anti-ferromagnetic component, which seems relatively high in this case.
We will add a sentence to acknowledge that hysteresis values used in the Day diagram are estimates because of the issue raised by the reviewer.
Section 4.4 The maximum angular deviation (MAD) is really a measure of how well the ChRM can be defined, rather than a measure of magnetic stability. It is influenced by the stability of the equipment used to demagnetize and measure remanence and the signal-to-noise ratio. Low MADs are not a guarantee that the data reflect the ancient geomagnetic field direction.
We understand that low MAD values do not guarantee a high-quality paleomagnetic record that preserves ancient geomagnetic field direction, however, low MAD values certainly have been accepted as a prerequisite for such records (Stoner and St-Onge, 2007). In our experience using this equipment, such low MAD values are rare, pointing to the extremely well-resolved magnetization of Cascade Lake sediment. This inference is supported by inclination values that vary around GAD predictions and show variations consistent with known PSV.
Section 4.5 The relevance of the attempt to reconstruct a relative paleointensity (RPI) record is perhaps out of context with the aims of the journal (geochronology) and particularly the twin submission by Davies et al.
While the relative paleointensity (RPI) record is perhaps not directly related to the main goal of this paper, it is common for similar paleomagnetic data studies to include estimates of RPI when possible. It is also reasonable to assume that these data could be useful for future regional studies of RPI or the construction of geomagnetic field models.
Section 4.7 As previously mentioned, the radiometric age model is presented in more detail by Davies et al. To avoid duplicating raw data I suggest either combining to the two manuscripts, or allowing Davies et al. to present the age model in detail, which Steen et al. test using the paleomagnetic data in a subsequent manuscript. The data in Table 1 show the 210Pb activity for the upper 3.6 cm, which is not useful for the PSV data set (no PSV data are from the short core)
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 201Pb data will be shifted into a supplement for the Davies et al. companion paper. The 210Pb data are important as an independent constraint on sedimentation rate.
Section 5.2 I have made a general comment about this section in my opening paragraph. There are several problems with correlations, mainly associated with the (dis)similarity of the different curves. I do not understand why the authors consider that the Burial Lake radiometric age-model is more reliable than the Cascade lake because the sedimentation rate is rather linear. What is the reason for this argument? The Burial Lake radiometric age-model is definitely better because the 14C dated material did not contain terrestrial organic matter.
We agree and state that the terrestrial organic matter used for radiocarbon dating at Burial Lake is the main reason why the Burial Lake radiometric age model is preferred. We will omit the statement about the Burial Lake age model being “linear” as justification for its reliability.
There is a reference to a Masters thesis by Steen (2016) and an alternative PSV age model, which has been rejected by this study. I leave it up to the editor(s) to decide if this reference is suitable.
Section 5.3 This section contains quite a lot of speculation about the reason for possibly too old 14C dates. The authors need to consider that a paleomagnetic lock-in depth (delay) might also apply to the Burial Lake record, but the comparisons with the predictions of field models suggest that the offset would be quite large, possible unreasonable, in terms of depth (time).
Dorfman (2015) estimated that Burial Lake PSV features were ~ 200 years older than similar features in western North American records (Hagstrum and Champion, 2002), and that this difference could be attributed to post-depositional lock-in, concluding that Burial Lake could place an effective “maximum age” on geomagnetic features in this region.
Section 5.4 Much of this section is not relevant to the journal (Geochronology) because it concerns the development of the geomagnetic field (using paleomagnetism) and would be better suited to a submission and review by a specialised geophysical journal. The comparisons with regional records (and a global VADM) in Figure 11 seem unnecessary in the context of the aims of the twin submissions. Figure 11A has no subjective tie-points (unlike Figure 9) and I do not see much similarity between the different records. If these records were plotted against each other (using age as the control) I doubt that one would find a significant correlation. Have you tried to statistically check the similarity in this way and how adjustment of the age-models might improve a correlation coefficient?
We believe that there is value in comparing our Cascade Lake data with other regional records (Fig. 11). These comparisons are part of our comprehensive and balanced presentation of evidence of a geomagnetic signal. Based on this and other review comments, we intend to improve our tie-point correlation procedures.
References cited in Authors’ replies
Hagstrum, J.T., Champion, D.E., 2002. A Holocene paleosecular variation record from 14C‐dated volcanic rocks in western North America. Journal of Geophysical Research: Solid Earth (1978–2012) 107, EPM–8.
Jensen, B. J. L., Davies, L. J., Nolan, C., Pyne-O’Donnell, S. D. F., Monteath, A. J., Ponomareva, V. V., Portnyagin, M. V., Cook, E., Plunkett, G., Booth, R. K., Hughes, P. D. M., Bursik, M., Luo, Y., Cwynar, L. C. and Pearson, D. G.: in revision. A latest Pleistocene and Holocene composite tephrostratigraphic framework for 765 paleoenvironmental records for northeastern North America, Quat. Sci. Rev., n.d.
Li, C.G., Zheng, Y., Wang, M., Sun, Z., Jin, C., and Hou, J., 2021. Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau. Quaternary Geochronology, 62, 101146.
Steen, D.P., 2016. Late Quaternary paleomagnetism and environmental magnetism at Cascade and Shainin Lakes, north-central Brooks Range, Alaska, MS Thesis, Northern Arizona University. https://www.proquest.com/docview/1808501293
Stoner, J.S., and St-Onge, G., 2007, Chapter Three: Magnetic Stratigraphy in Paleoceanography: Reversals, Excursions, Paleointensity, and Secular Variation, Developments in Marine Geology, 1, 99-138, doi:10.1016/S1572-5480(07)01008- 1.
Westerhold, T., Marwan, N., Drury, A. J., Liebrand, D., Agnini, C., Anagnostou, E., ... & Zachos, J.C., 2020. An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science, 369(6509), 1383-1387.
Citation: https://doi.org/10.5194/gchron-2021-19-AC2
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AC2: 'Reply on RC2', Darrell Kaufman, 26 Aug 2021
Interactive discussion
Status: closed
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RC1: 'Comment on gchron-2021-19', Anonymous Referee #1, 19 Jul 2021
Review report for;
gchron-2021-19 “Paleomagnetic secular variation for a 21,000-year sediment sequence from Cascade Lake, north-cenral Brooks Range, Arctic Alaska” by Steen et al.
Summary
This article presents a new data set of paleomagnetic secular variation from lake sediment cores from Cascade Lake, an interesting place of Arctic region. The authors have obtained paleomagnetic data by progressive alternating-field demagnetizations of u-channel samples, and discussed magnetic carriers and grain sizes with non-thermal rock-magnetic experiments. The secular variation (SV) features of inclination from Cascade Lake relatively well correlate with those from Burial Lake, 200 km to the west, and partly with those from the global SV models (CALS10b.1b, pm9k.1b). The two data sets (Cascade Lake and Burial Lake) may contribute to construct a regional type SV curve by stacking, which is useful for local magnetochronology. However, the Cascade Lake paleomagnetic record has no reliable age model with the measured radiocarbon dates, possibly affected by old carbon. Therefore, the authors transferred the age model with 14C dates less affected by old carbon of Burial Lake to the Cascade Lake record (PSV-1 age model) by wiggle matching of SV features between Cascade Lake and Burial Lake. The correlation of SV features between different sites/basins is more or less flexible, so that the correlation needs supports of reliable age constraints. In this paper, four tephra ages published in the companion paper by Davies et al. may play an important role for it.
One of the main conclusions of this paper is that the PSV-1 age model shows evidence for the old carbon effect on the Cascade Lake 14C dates above a composite depth of 160 cm. However, the evidence is weak because the correlation is flexible, and the possibility of downward-shifted recording of paleomagnetic field by authigenic iron-sulfide ferrimagnets is remained. The authors developed a discussion assuming that the main magnetic carrier is magnetite. Hence, I cannot recommend accepting this paper in the present form for publication in Geochronology. But the SV data of Cascade Lake may be useful as supporting evidence for the companion paper by Davies et al., although additional magnetic experiments, e.g. progressive thermal demagnetizations of NRMs, thermomagnetic analyses (Js-T), and/or FORC experiments, are necessary to estimate magnetic carriers/particle sizes.
Individual comments
(1) P. 4, lines 125-126
The authors calibrated the 14C dates of this study using the IntCal 13 calibration curve (Reimer et al., 2013). Is the curve consistently used in the calibrations for other SV records in and around Alaska, and the global SV models (Figs. 9 and 11)? We have the IntCal20 calibration curve, now (Reimer et al., 2020).
(2) P. 6 to 7, “4.2 S-ratios and kARM/klf”
The authors interpret as the increasing up-core trends of the S-ratio and kARM/klf reflect the progressive addition of a separate fine-grained ferromagnetic component to the magnetic assemblage dominated by high-coercivity particles in the lower part. This interpretation would be correct. However, we should note that a similar up-core increasing trend lies in the organic material content (OM), while up-core decreasing trends are present in the magnetic susceptibility (klf) and IRM (Fig. 2). These trends must be discussed, together with the trends in the proxies of soft-component (S-ratio) and magnetic grain size (kARM/klf), which can be associated with the gradually increased anoxic environments that cause dissolution of fine magnetites, and formation of super-fine authigenic ferrimagnets, e.g., greigites.
(3) P.7, “4.3 Hysteresis, magnetic grain size and mineralogy”
The sediments contain a high coercivity mineral of hematite (or goethite), in addition to a low coercivity ferrimagnet (magnetite?). Further, the possibility of containing iron-sulfide ferrimagnets is still remained. Thus, the authors need to reconsider the interpretation of the domain states with the Day diagram, which is originally for titanomagnetite (Fig. 3B). FORC diagram may be suitable for a mixture of magnetic minerals to estimate domain state (Roberts et al., 2017, JGR, 123, 2618–2644).
(4) P.8, “4.4 Characteristic remanent magnetization”
The ChRM determined with a small MAD value is a strong point of this study. However, the orthogonal projections of demagnetization data seem to show that the magnetization vector does not decay toward the origin. Doesn’t this result indicate the presence of a higher coercivity component except hematite/goethite? A PDRM component by detrital hematite/goethite particles should have a component with a similar direction with that of detrital magnetite particles.
(5) P.10, “4.5 Normalized remanence (relative paleointensity)”
The main magnetic carriers of Cascade Lake sediments comprise a high-coercivity mineral (probably hematite) and a low-coercivity mineral (possibly magnetite/greigite). I consider the NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, because they are less affected by high-coercivity components. Unfortunately, no curves of normalizers ARM20-70mT and IRM20-70mT are shown in Fig. 7, so that we cannot evaluate the correlation between the RPI and normalizers (“R” is not helpful). The kLF, ARM45mT, and IRM45mT in Fig. 7 include both mineral components, and in addition the contributions of high-coercivity particles increased in the ARM45mT and IRM45mT , compared with those before AFD.
(6) P. 13, “5.1 Magnetic assemblage”
The authors indicate that the fine grained and low-coercivity ferromagnetic component is carried by (titano)magnetite. As mentioned in (2), greigite is also a candidate. Authigenic greigite particles are fine, with coercivity ranges similar to magnetite. Therefore, it is difficult to separate the components of magnetite and greigite by AF demagnetization. To reject the possibility of the presence of greigite, the authors should show evidence with thermal experiments, e.g. progressive thermal demagnetizations of NRMs, thermos-magnetic analysis (Js-T), and so on.
(7) P. 13, line 330
I do not agree the reason of the linear depth-age relation over the past 17 ka for the Burial Lake 14C age model’s being more realistic than the Cascade Lake 14C age model.
(8) P. 15, Figure 9
The correlation of inclination features between Cascade Lake and Burial Lake seems to be generally good (Fig. 9). But I do not agree with the correlation with the global SV models. Many tie-points between I3 and I8 are flexible. For example, the oldest part of the pfm9k.1b Inc-GAD may not reach the inclination feature I7 of the Burial Lake inc-GAD, and the I6-I8 of the pfm9k.1b may correlate in phase to around I4 of the Burial Lake. The correlations in Fig. 9 are not robust, and thus the authors must be careful with using the tie-point ages for age models and others.
(9) P. 16, “5.3 Age model discrepancy”
For the age model discrepancy, the authors discussed two possible causes; the old carbon of aquatic organic materials and the lock-in depth of PDRM. If the authors do not show evidence for the absence of authigenic greigite, readers would concern the effect of it. Large downward shifts of paleomanetic signals in the record carried by greigites are likely (e.g., Robert et al. (2005) GRL).
The authors mention that four tephra ages published in the companion paper by Davies et al. provide strong evidence for the discrepancy. If they clearly show the discrepancy without help of PSV data, the authors can construct a new age model with selected measured 14C dates and the tephra ages for the Cascade Lake SV curve. In this case, they should not mention the old carbon effect in the conclusion. In place, they would have a merit of making a type SV curve in Alaska by stacking the Cascade Lake and Burial Lake SV data, both of which have independent age models.
(10) P. 17, lines 414-415
The authors mention that high-amplitude inclination shifts at this time are contemporaneous with low relative paleointensity estimates (Fig. 11). However, the relative paleointensity curve plotted in Fig. 11 is after 15.3 ka, which does not show paleointensity values around 17 ka. Readers may want to see the global VADM values plotted until about 20 ka.
(11) P. 17, lines 434-435
“Magnetic grain-size estimation (Fig. 3 and 4) suggests fine PSD magnetites”
As mentioned in (3), we cannot estimate grain sizes (domain states) with a Day plot of magnetic mineral assemblages of low-coercivity ferrimagnets and hematites, which are suggested by the relatively small S-ratio values ranging from 0.5 to 0.88 throughout the sequence.
Citation: https://doi.org/10.5194/gchron-2021-19-RC1 -
AC1: 'Reply on RC1', Darrell Kaufman, 26 Aug 2021
We thank the reviewer for their helpful and constructive comments. Our responses and proposed revisions are in bold font below excerpts of the reviewer’s verbatim comments.
Summary
… the Cascade Lake paleomagnetic record has no reliable age model with the measured radiocarbon dates, possibly affected by old carbon.
While we do present evidence that the radiocarbon dates have been influenced by old carbon, we also present evidence for a reliable age model based on independent evidence using tephra ages (from Davies et al. companion paper) and constraints from other regional paleomagnetic secular variation (PSV) records.
However, the evidence is weak because the correlation [among PSV records] is flexible…
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
… and the possibility of downward-shifted recording of paleomagnetic field by authigenic iron-sulfide ferrimagnets is remained.
Although it is a remote possibility, there is no evidence for authigenic Fe-S (greigite) as a dominant magnetic mineral within this assemblage, and we doubt that it is present. Because authigenic greigite is a diagenetic product of sulphate reduction (Roberts et al., 2011; Roberts, 2015), it is most often found in strongly reducing marine deposits, or in lake sediments that transitioned from marine settings (e.g., Loch Lomond, Snowball and Thompson, 1990), or vice versa (e.g., Black Sea, Nowacyzk et al., 2012). When present in lake sediments, it is typically characterized by highly variable intensities often at centimeter scale (Stockhausen and Zolitschka, 1999; Frank, 2007), and by poor quality natural remanent magnetization. Evidence for Greigite is often associated with a gyro remanent magnetization (Stephenson and Snowball, 2001; Snowball, 1997; Roberts et al., 2011) and extremely high SIRM/k or IRM/k ratios (Snowball et al., 1991; Nowaczyk et al., 2012, 2020). Instead, the consistency of the Holocene assemblage at Cascade Lake, including its AF demagnetization behavior and remanence records, is reminiscent of data from Scandinavian lakes where high-quality magnetization is associated with a magnetite-dominated magnetic assemblage, likely produced by microbes (e.g., Snowball et al., 2013; Haltia-Hovi et al., 2011). Although we cannot prove that the assemblage is a result of biogenic magnetite, the data are consistent with this interpretation, and more importantly, they afford a high-quality magnetization record by a fine-grained ferrimagnetic component, with no evidence for greigite.
… additional magnetic experiments, e.g. progressive thermal demagnetizations of NRMs, thermomagnetic analyses (Js-T), and/or FORC experiments, are necessary to estimate magnetic carriers/particle sizes.
We agree that extensive additional analyses would be needed to prove the magnetic mineralogy of the sediments, but such an undertaking would be beyond the scope of this paper. More importantly, our use of AF demagnetization is commonly used across many (hundreds?) of high-quality paleomagnetic studies of lake sediments, especially for Holocene sediments, which typically preserve well-defined NRM. Furthermore, the thermal demagnetization suggested by the reviewer can be problematic when working with wet lake sediments because: i) additional subsampling disturbs the soft sediment and can alter the paleomagnetic signal; ii) desiccation during heating can alter the directional record; iii) combusting organic-rich lake sediment in a restricted environment (magnetically shielded) can be hazardous; iv) thermal sample alteration can generate new magnetic minerals; and v) after heating, the sediment can no longer be used for paleointensity or environmental magnetic studies.
Individual comments
(1) P. 4, lines 125-126. The authors calibrated the 14C dates of this study using the IntCal 13 calibration curve (Reimer et al., 2013). Is the curve consistently used in the calibrations for other SV records in and around Alaska, and the global SV models (Figs. 9 and 11)? We have the IntCal20 calibration curve, now (Reimer et al., 2020).
The other PSV records to which we are correlating were calibrated using earlier versions of IntCal, so we are hesitant use a different version. More importantly, the differences are relatively small within the timeframe of the Cascade Lake record.
(2) P. 6 to 7, “4.2 S-ratios and kARM/klf” The authors interpret as the increasing up-core trends of the S-ratio and kARM/klf reflect the progressive addition of a separate fine-grained ferromagnetic component to the magnetic assemblage dominated by high-coercivity particles in the lower part. This interpretation would be correct. However, we should note that a similar up-core increasing trend lies in the organic material content (OM), while up-core decreasing trends are present in the magnetic susceptibility (klf) and IRM (Fig. 2). These trends must be discussed, together with the trends in the proxies of soft-component (S-ratio) and magnetic grain size (kARM/klf), which can be associated with the gradually increased anoxic environments that cause dissolution of fine magnetites, and formation of super-fine authigenic ferrimagnets, e.g., greigites.
Both susceptibility (k) and IRM decrease subtly up core, reflecting an increase in an extremely fine-grained ferrimagnetic assemblage that is not present in the older part of the sequence. This component increases up core, as indicated by a decrease in k and IRM, relative to ARM that is biased toward fine magnetite (Banerjee et al., 1981). We interpret this fine-grained ferrimagnetic component as biogenic magnetite, which is consistent with the expected evolution of the lake and its catchment toward increasing productivity following the last ice age. Biogenic magnetite also explains the well- defined, high-quality magnetization record, and the coercivity of the magnetization. Alternatively, the fine-grained magnetite might be dust. Dust seems to be present in lake sediment in the region (Burial Lake, Dorfman et al., 2015); however, the dust is most abundant in the Pleistocene sediment. The up-core increase inferred biogenic magnetite is consistent with the corresponding increase in organic matter associated with warmer climates of the Holocene, as found at other high-latitude lakes (e.g., Snowball et al., 2013; Haltia-Hovi et al., 2011). As explained above, there is no evidence for the presence of greigite: poor quality magnetization, gyro remanent magnetization typically between 50 and 80 mT (Ron et al., 2007; Nowacyzk et al., 2020), high IRM/k, coercivity lower than hematite as indicated by the S-ratio.
(3) P.7, “4.3 Hysteresis, magnetic grain size and mineralogy” The sediments contain a high coercivity mineral of hematite (or goethite), in addition to a low coercivity ferrimagnet (magnetite?). Further, the possibility of containing iron-sulfide ferrimagnets is still remained. Thus, the authors need to reconsider the interpretation of the domain states with the Day diagram, which is originally for titanomagnetite (Fig. 3B). FORC diagram may be suitable for a mixture of magnetic minerals to estimate domain state (Roberts et al., 2017, JGR, 123, 2618–2644).
Production of FORC diagram would further document a complex magnetic mineral assemblage (again, however, there is no evidence for Fe-S greigite), but would do little to address the primary question much further than the evidence already provided. Additionally, a full accounting of the magnetic mineralogy is not the goal of this study and would entail an exceptional amount of work with equipment not readily available.
(4) P.8, “4.4 Characteristic remanent magnetization” The ChRM determined with a small MAD value is a strong point of this study. However, the orthogonal projections of demagnetization data seem to show that the magnetization vector does not decay toward the origin. Doesn’t this result indicate the presence of a higher coercivity component except hematite/goethite? A PDRM component by detrital hematite/ goethite particles should have a component with a similar direction with that of detrital magnetite particles.
Small deviations in directions at high field strengths are commonly observed and generally considered to reflect increased noise relative to signal in that part of the demagnetization diagram, with many potential causes including general measurement noise. While it is possible that detrital hematite or goethite might contribute to the signal following the demagnetization of magnetite, and while we appreciate the suggestion, it is difficult to know where to take this because neither mineral is considered to carry a quality magnetic signal in Holocene lake sediments.
(5) P.10, “4.5 Normalized remanence (relative paleointensity)” The main magnetic carriers of Cascade Lake sediments comprise a high-coercivity mineral (probably hematite) and a low-coercivity mineral (possibly magnetite/greigite). I consider the NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, because they are less affected by high-coercivity components. Unfortunately, no curves of normalizers ARM20-70mTand IRM20-70mT are shown in Fig. 7, so that we cannot evaluate the correlation between the RPI and normalizers (“R” is not helpful). The kLF, ARM45mT, and IRM45mT in Fig. 7 include both mineral components, and in addition the contributions of high-coercivity particles increased in the ARM45mT and IRM45mT , compared with those before AFD.
We agree that NRM20-70mT/ARM20-70mT and NRM20-70mT/IRM20-70mT are better RPI proxies, but not “because they are less affected by high-coercivity components”, but rather because k is not a remanence carrier and influenced by other things. However, the main point is that the similarity between all three normalizers lends confidence to the record of geomagnetic intensity. Plotting ARM, IRM as a stack or from a single demagnetization step makes little practical difference, but this can be changed and addressed.
(6) P. 13, “5.1 Magnetic assemblage”. The authors indicate that the fine grained and low-coercivity ferromagnetic component is carried by (titano)magnetite. As mentioned in (2), greigite is also a candidate. Authigenic greigite particles are fine, with coercivity ranges similar to magnetite. Therefore, it is difficult to separate the components of magnetite and greigite by AF demagnetization. To reject the possibility of the presence of greigite, the authors should show evidence with thermal experiments, e.g. progressive thermal demagnetizations of NRMs, thermos-magnetic analysis (Js-T), and so on.
We would agree with this if there was evidence for greigite (gyro remanence, high SIRM/k, magnetic dissolution, detection hydrogen sulfides during coring or splitting, oxidative changes in magnetic properties or coloring, sulfidic lake, down core increase in greigite indicators, etc.). Again, a full accounting of the magnetic mineralogy is not the goal of this study and would entail an exceptional amount of work with equipment not readily available.
(7) P. 13, line 330. I do not agree the reason of the linear depth-age relation over the past 17 ka for the Burial Lake 14C age model’s being more realistic than the Cascade Lake 14C age model.
We will omit the phrase, “… and because the sedimentation rate at Burial Lake is rather linear over the past ~ 17 kyr.”
(8) P. 15, Figure 9. The correlation of inclination features between Cascade Lake and Burial Lake seems to be generally good (Fig. 9). But I do not agree with the correlation with the global SV models. Many tie-points between I3 and I8 are flexible. For example, the oldest part of the pfm9k.1b Inc-GAD may not reach the inclination feature I7 of the Burial Lake inc-GAD, and the I6-I8 of the pfm9k.1b may correlate in phase to around I4 of the Burial Lake. The correlations in Fig. 9 are not robust, and thus the authors must be careful with using the tie-point ages for age models and others.
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
(9) P. 16, “5.3 Age model discrepancy”. For the age model discrepancy, the authors discussed two possible causes; the old carbon of aquatic organic materials and the lock-in depth of PDRM. If the authors do not show evidence for the absence of authigenic greigite, readers would concern the effect of it. Large downward shifts of paleomanetic signals in the record carried by greigites are likely (e.g., Robert et al. (2005) GRL).
See our responses above regarding the lack of evidence for greigite.
The authors mention that four tephra ages published in the companion paper by Davies et al. provide strong evidence for the discrepancy. If they clearly show the discrepancy without help of PSV data, the authors can construct a new age model with selected measured 14C dates and the tephra ages for the Cascade Lake SV curve. In this case, they should not mention the old carbon effect in the conclusion. In place, they would have a merit of making a type SV curve in Alaska by stacking the Cascade Lake and Burial Lake SV data, both of which have independent age models.
We understand this alternative approach and appreciate the suggestion. However, the primary goal of the study is generating a reliable age model for Cascade Lake, and not improving the regional PSV curve.
(10) P. 17, lines 414-415. The authors mention that high-amplitude inclination shifts at this time are contemporaneous with low relative paleointensity estimates (Fig. 11). However, the relative paleointensity curve plotted in Fig. 11 is after 15.3 ka, which does not show paleointensity values around 17 ka. Readers may want to see the global VADM values plotted until about 20 ka.
We will attempt to extend the field intensity curve in Figure 11, as suggested.
(11) P. 17, lines 434-435. “Magnetic grain-size estimation (Fig. 3 and 4) suggests fine PSD magnetites”. As mentioned in (3), we cannot estimate grain sizes (domain states) with a Day plot of magnetic mineral assemblages of low-coercivity ferrimagnets and hematites, which are suggested by the relatively small S-ratio values ranging from 0.5 to 0.88 throughout the sequence.
We agree, at least as a strict grain-size indicator. However, hysteresis data provide information on the magnetic assemblage and our Day plots suggest that fine-grained ferrimagnetic minerals are a substantial component.
References cited in authors’ replies
Banerjee, S.K., King, J. Marvin, J., 1981. A rapid method for magnetic granulometry with applications to environmental studies. Geophysical Research Letters, 8, 333-336.
Dorfman, J.M., J.S. Stoner, M.S. Finkenbinder, M.B. Abbott, C. Xuan, & G. St-Onge 2015: A 37,000-year Environmental Magnetic Record of Aeolian Dust Deposition from Burial Lake, Arctic Alaska. Quaternary Science Reviews, 128, 81-97.
Frank, U. 2007, Palaeomagnetic investigations on lake sediments from NE China: a new record of geomagnetic secular variations for the last 37 ka. Geophys. J. Int. (2007) 169, 29–40
Haltia-Hovi, E., Nowaczyk N., Saarinena T., 2011. Environmental influence on relative palaeointensity estimates from Holocene varved lake sediments in Finland. Physics of the Earth and Planetary Interiors 185, 20–28
Li, C.G., Zheng, Y., Wang, M., Sun, Z., Jin, C., and Hou, J., 2021. Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau. Quaternary Geochronology, 62, 101146.
Nowaczyk, N.R., Arz, H.W., Frank, U., Kind, J.& Plessen, B., 2012. Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments, Earth Planet. Sci. Lett., 351–352, 54–69.
Nowaczyk N.R., Liu, J. Arz, H.W., 2020 Records of the Laschamps geomagnetic polarity excursion from Black Sea sediments: magnetite versus greigite, discrete sample versus U-channel data. Geophys. J. Int. (2021) 224, 1079–1095
Roberts, A.P., 2015. Magnetic mineral diagenesis. Earth-Science Reviews, 151 1–47.
Roberts, A.P., Chang, L., Rowan, C.J., Horng, C.S., Florindo, F., 2011. Magnetic properties of sedimentary greigite (Fe3S4): an update. Rev. Geophys. 49, RG1002.
Ron, H., Nowaczyk, N.R., Frank, U., Schwab, M.J., Naumann, R., Striewski, B. & Agnon, A., 2007. Greigite detected as dominating remanence carrier in Late Pleistocene sediments, Lisan formation, from Lake Kinneret (Sea of Galilee), Israel, Geophys. J. Int., 170, 117–131.
Snowball, I.F., 1997. Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden, Geophys. J. Int., 129, 624–636.
Snowball, I.F., Thompson, R., 1990. A mineral magnetic study of Holocene sediment yields and deposition patterns in the Llyn Geirionydd catchment, north Wales. The Holocene 2, 238–248.
Snowball, I.F., 1991. Magnetic hysteresis properties of greigite (Fe3S4) and a new occurrence in Holocene sediments from Swedish Lappland. Phys. Earth Planet. Inter. 68, 32–40
Snowball, I., Mellström, A., Ahlstrand, E., Haltia, E., Nilsson, A., Ning, W., Muscheler, R., Brauer, A., 2013. An estimate of post-depositional remanent magnetization lock-in depth in organic rich varved lake sediments. Glob. Planet. Change 110, 264–277.
Stephson and Snowball, A large gyromagnetic effect in griegite. (2001) Geophys. J. Int. 145, 570–575.
Stockhausen, H., Zolitschka, B., 1999. Environmental changes since 13,000 cal. BP reflected in magnetic and sedimentological properties of sediments from Lake Holzmaar (Germany). Quat. Sci. Rev. 18, 913–925.
Citation: https://doi.org/10.5194/gchron-2021-19-AC1
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AC1: 'Reply on RC1', Darrell Kaufman, 26 Aug 2021
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RC2: 'Comment on gchron-2021-19', Anonymous Referee #2, 21 Jul 2021
The manuscript by Steen et al. is part of a “twin-submission” that collectively build an age-depth model for a sediment sequence in Cascade Lake. Steen et al. focus on a paleomagnetic secular variation (PSV) record that extends back to 21 ka, which is a strength of the paper. The companion paper by Davies et al. (Gchron-2021-18) focuses on late Holocene cryptotephra in Cascade Lake and the construction of an age model based on radiometric methods (210Pb and 14C) and correlated ages of the cryptotephra. Davies et al.’s dating results are used to support Steen et al.’s conclusion that there are significant differences compared to (i) the radiometric-based age model and (ii) one that is based on relative paleomagnetic secular variation (PSV) correlations to the predictions of time-varying geomagnetic field models and the trends in an independently dated PSV record from Burial Lake (only 200 km away). The two manuscripts come to the same conclusion that several radiocarbon ages are too old, which is well known for lake sediment samples that contain more or less terrestrial derived organic matter that is older than the age of deposition. A general problem is that the two manuscripts frequently cross-reference each other and it is quite difficult to understand how either one can stand alone. Some raw data are put into both manuscripts (e.g. the 14C dates and tables) which gives future writers an unnecessary choice about which source to cite. Davies et al. state that details of the radiometric age-model construction are in Steen et al., but there are more details in the former than the latter. The editor could consider asking the authors to combine the papers into one so that problems that arise from twin-submissions are negated.
There is also reference to a Masters thesis by Steen (2016) that is not easily accessible and Davies et al. refer to a submitted manuscript by Jensen et al., which might be crucial because it concerns the age of a cryptotephra.
In addition to a number of specific comments listed further on, I have a couple of general doubts about the manuscript by Steen et al. First, there are rather limited data, which are not replicated. Palaeomagnetic studies of lacustrine and marine sites should rely on three of more cores from each site so that there is replication of results, which can be stacked, and also the possibility to reject outliers. In this study two cores (CASC-4A & CASC-2D) were taken and measured for paleomagnetic properties. The authors state that CASC-4A has a more complete record of unit L3 and that CASC-2D data from unit L3 are rejected. Then the authors take higher resolution data from unit L2 in CASC-2D and splice them onto CASC-4A by matching the inclination records. No details about this match are provided. This exercise assumes the data from CASC-4A are good and forces the CASC-2D data from L2 to fit. It is better to use an independent parameter or proxy (magnetic susceptibility, for example) to match the records and then stack the paleomagnetic data and consider uncertainties.
The second major reservation concerns the tie-points (control points) in Figure 9. These tie-points give the impression that there are high-frequency matches between the composite Cascade Lake inclination record, the field-model predictions and the Burial Lake record. But, the long-term trends seem very different, particularly for the Holocene. Specifically, there are tie-points within the interval 4-8 ka. Models predict high inclination but the data (the composite record) show low inclination. The authors state in the introduction that major directional (PSV) features predicted from models can be used for age control, but they subsequently ignore the differences in the long-term major directional features. Why? An associated problem concerns the tie-points between the model predictions and the Burial Lake record that are older than 5 ka. These are very large differences in age (2-3 ka) rather than the Burial Lake ages being “somewhat older”. If the Burial Lake 14C-based chronology is valid (as the authors argue) one has to deduce that the geomagnetic model predictions are poor, and thus should not be used to provide correlation tie-points).
In general, the manuscript contains a lot of detail that could be removed through combining the two papers into one. There are aspects about the temporal and spatial development of the geomagnetic field that would be better suited for submission (and review) by a specialised geophysics journal.
Specific comments
The start of the introduction could be more general and focus on paleomagnetic secular variation and its advantages and limitations as a relative dating method. The authors use the term wiggle-matching, which is often used by the radiocarbon community to objectively (statistically) match established changes D14C, but which here is really visual (and quite subjective) matching of trends in the PSV data and model predictions.
The geomagnetic field models are not perfectly constrained, anywhere.
Section 2.2 “Geomagnetic setting” contains details about the origin of the geomagnetic field and its manifestation on Earth’s surface that are unnecessary in this study’s context. The discussion and conclusions do not refer back to these details, so I suggest that they are omitted.
Section 3.1 The individual sections of the cores were not relatively aligned to each other or absolutely aligned to an azimuth, which seems like an experimental error if the purpose of the study was PSV. I appreciate that t is difficult to obtain whole cores that are oriented to an azimuth, but it is relatively easy to keep sections oriented when the core is cut into sections. Why was this orientation not done?
Section 3.2 What is the approximate half-width of the signal that the 35 mm Bartington loop measures? That distance is equally important as the measuring increment (1 cm).
The authors mention that a couple of segments were not measured. The reason should be stated in the methods section. I think that the segments were measured, but the results were bad due to saturation of the SQUIDs by highly magnetic layers.
Section 3.3 These methods (and also the results) are duplicated in the twin paper by Davies et al.
Section 4.1 The authors state that the sediments recovered are undeformed by the coring procedure. This statement contrasts with an earlier statement made in section 3.2 about the samples being taken from as close as possible from the centre of the core, unless “appeared less disturbed by minor coring deformation”. Please be consistent, and how do you know that the sediments are undeformed and/or deformed?
Please avoid the use of terms like “are significantly lower” where there is no known significance.
The authors mention a “hint at the authigenic creation of greigite” but there is no proof. It would be better to state that the cause of the highly magnetic layers is unknown so that there is no speculation.
The average inclinations are close to the GAD model prediction for the site latitude, which the authors use to argue that the PSV record is good, but earlier on the authors state that the geomagnetic field might be different at high latitudes due to the tangent cylinder. The logic seems a bit circular. All palaeomagnetic data (that are ideally oriented to an azimuth) test the GAD model.
The data shown in Figure 5 were obtained based on analyses of the raw palaeomagnetic data, with examples shown in Figure 6, so it might be better to place current Figure 6 before current Figure 5.
Figure 2 is very cramped. The reader is unable to obtain any useful information from the different coloured lines that show all the ARM and IRM demagnetization data. I recommend simplifying the figure.
Figure 3 shows that the hysteresis loops are not closed at 1T, which means that a slope correction does not only correct for paramagnetic (and diamagnetic) contributions. The correction will include an unknown part of the unsaturated anti-ferromagnetic component, which seems relatively high in this case.
Section 4.4 The maximum angular deviation (MAD) is really a measure of how well the ChRM can be defined, rather than a measure of magnetic stability. It is influenced by the stability of the equipment used to demagnetize and measure remanence and the signal-to-noise ratio. Low MADs are not a guarantee that the data reflect the ancient geomagnetic field direction.
Section 4.5 The relevance of the attempt to reconstruct a relative paleointensity (RPI) record is perhaps out of context with the aims of the journal (geochronology) and particularly the twin submission by Davies et al.
Section 4.7 As previously mentioned, the radiometric age model is presented in more detail by Davies et al. To avoid duplicating raw data I suggest either combining to the two manuscripts, or allowing Davies et al. to present the age model in detail, which Steen et al. test using the paleomagnetic data in a subsequent manuscript. The data in Table 1 show the 210Pb activity for the upper 3.6 cm, which is not useful for the PSV data set (no PSV data are from the short core)
Section 5.2 I have made a general comment about this section in my opening paragraph. There are several problems with correlations, mainly associated with the (dis)similarity of the different curves. I do not understand why the authors consider that the Burial Lake radiometric age-model is more reliable than the Cascade lake because the sedimentation rate is rather linear. What is the reason for this argument? The Burial Lake radiometric age-model is definitely better because the 14C dated material did not contain terrestrial organic matter.
There is a reference to a Masters thesis by Steen (2016) and an alternative PSV age model, which has been rejected by this study. I leave it up to the editor(s) to decide if this reference is suitable.
Section 5.3 This section contains quite a lot of speculation about the reason for possibly too old 14C dates. The authors need to consider that a paleomagnetic lock-in depth (delay) might also apply to the Burial Lake record, but the comparisons with the predictions of field models suggest that the offset would be quite large, possible unreasonable, in terms of depth (time).
Section 5.4 Much of this section is not relevant to the journal (Geochronology) because it concerns the development of the geomagnetic field (using paleomagnetism) and would be better suited to a submission and review by a specialised geophysical journal. The comparisons with regional records (and a global VADM) in Figure 11 seem unnecessary in the context of the aims of the twin submissions. Figure 11A has no subjective tie-points (unlike Figure 9) and I do not see much similarity between the different records. If these records were plotted against each other (using age as the control) I doubt that one would find a significant correlation. Have you tried to statistically check the similarity in this way and how adjustment of the age-models might improve a correlation coefficient?
Citation: https://doi.org/10.5194/gchron-2021-19-RC2 -
AC2: 'Reply on RC2', Darrell Kaufman, 26 Aug 2021
We appreciate the thorough and detailed review by Anonymous Referee #2. Our responses and proposed changes to the text are outlined below. Author comments are in bold below corresponding referee comments.
A general problem is that the two manuscripts frequently cross-reference each other and it is quite difficult to understand how either one can stand alone. Some raw data are put into both manuscripts (e.g. the 14C dates and tables) which gives future writers an unnecessary choice about which source to cite. Davies et al. state that details of the radiometric age-model construction are in Steen et al., but there are more details in the former than the latter.
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 210 Pb data will be shifted into a supplement for the Davies et al. companion paper.
The editor could consider asking the authors to combine the papers into one so that problems that arise from twin-submissions are negated.
Prior to submitting the papers, and now following this review, the authors of both papers have considered how the papers could be combined into one. We have made a concerted effort but cannot find a path that enables us to present all the relevant data. We submitted these two manuscripts to Geochronology because we aim to convey the important details about the individual dating methods to this specialized audience. We believe that attempting to do this with a single paper would be unwieldy for readers: both manuscripts date sediments from this high-latitude lake using two completely unrelated methods and reporting this in a meaningful fashion requires two separate papers that can properly present the methods and results for these independent approaches.
There is also reference to a Masters thesis by Steen (2016) that is not easily accessible and Davies et al. refer to a submitted manuscript by Jensen et al., which might be crucial because it concerns the age of a cryptotephra.
Steen’s (2016) thesis is available as open access through ProQuest: https://www.proquest.com/docview/1808501293. We will add the full URL link to the reference cited list. The submitted Jensen et al. manuscript is an invited review and, following its scheduled timeline and assuming its eventual acceptance, it should be publicly available by the time this manuscript may be published. However, this information is not crucial, as explained in Davies’ response to reviewers of the companion paper.
In addition to a number of specific comments listed further on, I have a couple of general doubts about the manuscript by Steen et al. First, there are rather limited data, which are not replicated. Palaeomagnetic studies of lacustrine and marine sites should rely on three of more cores from each site so that there is replication of results, which can be stacked, and also the possibility to reject outliers. In this study two cores (CASC-4A & CASC-2D) were taken and measured for paleomagnetic properties. The authors state that CASC-4A has a more complete record of unit L3 and that CASC-2D data from unit L3 are rejected. Then the authors take higher resolution data from unit L2 in CASC-2D and splice them onto CASC-4A by matching the inclination records. No details about this match are provided. This exercise assumes the data from CASC-4A are good and forces the CASC-2D data from L2 to fit. It is better to use an independent parameter or proxy (magnetic susceptibility, for example) to match the records and then stack the paleomagnetic data and consider uncertainties.
The procedure used to create the composite sequence, as explained by the reviewer, is described on lines 261-271. We will add a reference to Figure 2.7 in Steen (2016), which illustrates the splicing. We understand the advantage of replication and stacking to generate a composite record. We also believe that splicing the lower part of one core, taken from where sedimentation rates are higher, onto the base of the other core is a valid approach. This strategy follows, for example, the recent development of a premier global marine oxygen-isotope record (Westerhold et al., 2020). Rather than stacking records, which is known to smooth variability, the record was spliced together from sites where resolution was highest for each interval. In addition, while further replication is always better, we believe that the available data are sufficient for the purpose of this study: helping to constrain the chronology of the sedimentary sequence. We also note the relative paucity, and therefore the value, of this type of record, which reflects the logistical challenges in this remote region.
The second major reservation concerns the tie-points (control points) in Figure 9. These tie-points give the impression that there are high-frequency matches between the composite Cascade Lake inclination record, the field-model predictions and the Burial Lake record. But, the long-term trends seem very different, particularly for the Holocene. Specifically, there are tie-points within the interval 4-8 ka. Models predict high inclination but the data (the composite record) show low inclination. The authors state in the introduction that major directional (PSV) features predicted from models can be used for age control, but they subsequently ignore the differences in the long-term major directional features. Why? An associated problem concerns the tie-points between the model predictions and the Burial Lake record that are older than 5 ka. These are very large differences in age (2-3 ka) rather than the Burial Lake ages being “somewhat older”. If the Burial Lake 14C-based chronology is valid (as the authors argue) one has to deduce that the geomagnetic model predictions are poor, and thus should not be used to provide correlation tie-points).
We will improve the objectivity and quantification of correlations between PSV records by using: (1) an established tie-point identification algorithm, such as QAnalySeries, to detect points of correlation between records, and (2) Pearson correlation coefficients to evaluate the strength of alternative tie-point correlations. This procedure has been used in similar studies, including recently (Li et al., 2021).
Additionally, we will assign different levels of certainty to difference subsections of the final age model, depending on the number of chronological methods (PSV, 14C, and cryptotephra) used to construct each section. This will explicitly show where the available data are in good agreement versus where they are more limited. While we accept that there are uncertainties to some of the PSV interpretations, especially in the lower part of the sequence where they are not validated by other methods, we also see value in presenting these data.
In general, the manuscript contains a lot of detail that could be removed through combining the two papers into one. There are aspects about the temporal and spatial development of the geomagnetic field that would be better suited for submission (and review) by a specialised geophysics journal.
Prior to submitting the papers, and now following this review, the authors of both papers have considered how the papers could be combined into one. We have made a concerted effort but cannot find a path that enables us to present all the relevant data. Both manuscripts present first attempts to date sediments from this high-latitude lake using two completely unrelated methods and reporting this in a meaningful fashion requires two separate papers that can focus on properly presenting the methods and results for these independent approaches. We acknowledge that the information included in Section 2.2 on the geomagnetic setting of the field area is not necessarily relevant to the discussion and main conclusions of this study, which focus on improving the chronology of the sedimentary sequence using paleomagnetic data. We therefore intend to omit this section to avoid diluting the main points of this study with extraneous information.
Specific comments
The start of the introduction could be more general and focus on paleomagnetic secular variation and its advantages and limitations as a relative dating method. The authors use the term wiggle-matching, which is often used by the radiocarbon community to objectively (statistically) match established changes D14C, but which here is really visual (and quite subjective) matching of trends in the PSV data and model predictions.
We will omit the term “wiggle matching” to avoid confusion with the procedure used in 14C dating.
The geomagnetic field models are not perfectly constrained, anywhere. Section 2.2 “Geomagnetic setting” contains details about the origin of the geomagnetic field and its manifestation on Earth’s surface that are unnecessary in this study’s context. The discussion and conclusions do not refer back to these details, so I suggest that they are omitted.
We are comfortable omitting this section to avoid diluting the main points of this study with extraneous information.
Section 3.1 The individual sections of the cores were not relatively aligned to each other or absolutely aligned to an azimuth, which seems like an experimental error if the purpose of the study was PSV. I appreciate that t is difficult to obtain whole cores that are oriented to an azimuth, but it is relatively easy to keep sections oriented when the core is cut into sections. Why was this orientation not done?
The core sections were not aligned relative to each other or absolutely aligned to an azimuth because the cores were not initially collected with the goal of obtaining paleomagnetic data.
Section 3.2 What is the approximate half-width of the signal that the 35 mm Bartington loop measures? That distance is equally important as the measuring increment (1 cm).
The spatial resolution of the Bartington MS2C is 20 mm. We will add this detail.
The authors mention that a couple of segments were not measured. The reason should be stated in the methods section. I think that the segments were measured, but the results were bad due to saturation of the SQUIDs by highly magnetic layers.
U-channels were not collected from Lithologic Unit 1 (L1) because it is a stony diamicton and would therefore not provide a reliable record of PSV. The interval from 387 – 251 cm blf in core CASC-2D was not measured for SIRM due to saturation of the SQUID electronics, as suggested by the Referee’s comment. Please see lines 160 – 166 of the Preprint for additional information on this topic.
Section 3.3 These methods (and also the results) are duplicated in the twin paper by Davies et al.
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 201Pb data will be shifted into a supplement for the Davies et al. paper.
Section 4.1 The authors state that the sediments recovered are undeformed by the coring procedure. This statement contrasts with an earlier statement made in section 3.2 about the samples being taken from as close as possible from the centre of the core, unless “appeared less disturbed by minor coring deformation”. Please be consistent, and how do you know that the sediments are undeformed and/or deformed?
We are grateful to the reviewer for pointing out this contradiction. Indeed, the statement in Section 3.2 is not correct. It actually refers to the cores from Shainin Lake, which was included in Steen’s (2016) thesis. In fact, the cores collected from Cascade Lake do not display visual evidence of deformation. The statement regarding samples being “taken as close as possible to the center axis of the core, unless either side of a core section appeared less disturbed by minor coring deformation” will be removed. We will also include clarification that sediment layers are undeformed in the Cascade Lake cores.
Please avoid the use of terms like “are significantly lower” where there is no known significance.
Terms like “are significantly lower” will be removed, as suggested.
The authors mention a “hint at the authigenic creation of greigite” but there is no proof. It would be better to state that the cause of the highly magnetic layers is unknown so that there is no speculation.
We agree and will add a sentence to acknowledge that this hypothesis cannot be substantiated without further analysis.
The average inclinations are close to the GAD model prediction for the site latitude, which the authors use to argue that the PSV record is good, but earlier on the authors state that the geomagnetic field might be different at high latitudes due to the tangent cylinder. The logic seems a bit circular. All palaeomagnetic data (that are ideally oriented to an azimuth) test the GAD model.
We intend to omit this section to avoid diluting the main points of this study with extraneous information on geomagnetic field dynamics and the tangent cylinder.
The data shown in Figure 5 were obtained based on analyses of the raw palaeomagnetic data, with examples shown in Figure 6, so it might be better to place current Figure 6 before current Figure 5.
We appreciate the suggestion, but it is our opinion that the order of Figures 5 and 6 is good in the current configuration, and that there would be little benefit to the reader if the order were switched.
Figure 2 is very cramped. The reader is unable to obtain any useful information from the different coloured lines that show all the ARM and IRM demagnetization data. I recommend simplifying the figure.
We understand the suggestion that Figure 2 could be simplified, however data for demagnetization of the IRM and ARM highlight some unique magnetic properties of the sedimentary sequence. For example, it shows the anomalous saturation of the SQUID electronics during IRM demagnetization of CASC-2D between ~ 250 and 300 cm blf, while these issues were not encountered in other core sections.
Figure 3 shows that the hysteresis loops are not closed at 1T, which means that a slope correction does not only correct for paramagnetic (and diamagnetic) contributions. The correction will include an unknown part of the unsaturated anti-ferromagnetic component, which seems relatively high in this case.
We will add a sentence to acknowledge that hysteresis values used in the Day diagram are estimates because of the issue raised by the reviewer.
Section 4.4 The maximum angular deviation (MAD) is really a measure of how well the ChRM can be defined, rather than a measure of magnetic stability. It is influenced by the stability of the equipment used to demagnetize and measure remanence and the signal-to-noise ratio. Low MADs are not a guarantee that the data reflect the ancient geomagnetic field direction.
We understand that low MAD values do not guarantee a high-quality paleomagnetic record that preserves ancient geomagnetic field direction, however, low MAD values certainly have been accepted as a prerequisite for such records (Stoner and St-Onge, 2007). In our experience using this equipment, such low MAD values are rare, pointing to the extremely well-resolved magnetization of Cascade Lake sediment. This inference is supported by inclination values that vary around GAD predictions and show variations consistent with known PSV.
Section 4.5 The relevance of the attempt to reconstruct a relative paleointensity (RPI) record is perhaps out of context with the aims of the journal (geochronology) and particularly the twin submission by Davies et al.
While the relative paleointensity (RPI) record is perhaps not directly related to the main goal of this paper, it is common for similar paleomagnetic data studies to include estimates of RPI when possible. It is also reasonable to assume that these data could be useful for future regional studies of RPI or the construction of geomagnetic field models.
Section 4.7 As previously mentioned, the radiometric age model is presented in more detail by Davies et al. To avoid duplicating raw data I suggest either combining to the two manuscripts, or allowing Davies et al. to present the age model in detail, which Steen et al. test using the paleomagnetic data in a subsequent manuscript. The data in Table 1 show the 210Pb activity for the upper 3.6 cm, which is not useful for the PSV data set (no PSV data are from the short core)
We see the duplication of the datasets as minimal, and the inclusion of radiometric dates as important for both papers. The removal of these data from either paper would hinder its ability to stand alone. To avoid overt duplication, however, the 14C and 201Pb data will be shifted into a supplement for the Davies et al. companion paper. The 210Pb data are important as an independent constraint on sedimentation rate.
Section 5.2 I have made a general comment about this section in my opening paragraph. There are several problems with correlations, mainly associated with the (dis)similarity of the different curves. I do not understand why the authors consider that the Burial Lake radiometric age-model is more reliable than the Cascade lake because the sedimentation rate is rather linear. What is the reason for this argument? The Burial Lake radiometric age-model is definitely better because the 14C dated material did not contain terrestrial organic matter.
We agree and state that the terrestrial organic matter used for radiocarbon dating at Burial Lake is the main reason why the Burial Lake radiometric age model is preferred. We will omit the statement about the Burial Lake age model being “linear” as justification for its reliability.
There is a reference to a Masters thesis by Steen (2016) and an alternative PSV age model, which has been rejected by this study. I leave it up to the editor(s) to decide if this reference is suitable.
Section 5.3 This section contains quite a lot of speculation about the reason for possibly too old 14C dates. The authors need to consider that a paleomagnetic lock-in depth (delay) might also apply to the Burial Lake record, but the comparisons with the predictions of field models suggest that the offset would be quite large, possible unreasonable, in terms of depth (time).
Dorfman (2015) estimated that Burial Lake PSV features were ~ 200 years older than similar features in western North American records (Hagstrum and Champion, 2002), and that this difference could be attributed to post-depositional lock-in, concluding that Burial Lake could place an effective “maximum age” on geomagnetic features in this region.
Section 5.4 Much of this section is not relevant to the journal (Geochronology) because it concerns the development of the geomagnetic field (using paleomagnetism) and would be better suited to a submission and review by a specialised geophysical journal. The comparisons with regional records (and a global VADM) in Figure 11 seem unnecessary in the context of the aims of the twin submissions. Figure 11A has no subjective tie-points (unlike Figure 9) and I do not see much similarity between the different records. If these records were plotted against each other (using age as the control) I doubt that one would find a significant correlation. Have you tried to statistically check the similarity in this way and how adjustment of the age-models might improve a correlation coefficient?
We believe that there is value in comparing our Cascade Lake data with other regional records (Fig. 11). These comparisons are part of our comprehensive and balanced presentation of evidence of a geomagnetic signal. Based on this and other review comments, we intend to improve our tie-point correlation procedures.
References cited in Authors’ replies
Hagstrum, J.T., Champion, D.E., 2002. A Holocene paleosecular variation record from 14C‐dated volcanic rocks in western North America. Journal of Geophysical Research: Solid Earth (1978–2012) 107, EPM–8.
Jensen, B. J. L., Davies, L. J., Nolan, C., Pyne-O’Donnell, S. D. F., Monteath, A. J., Ponomareva, V. V., Portnyagin, M. V., Cook, E., Plunkett, G., Booth, R. K., Hughes, P. D. M., Bursik, M., Luo, Y., Cwynar, L. C. and Pearson, D. G.: in revision. A latest Pleistocene and Holocene composite tephrostratigraphic framework for 765 paleoenvironmental records for northeastern North America, Quat. Sci. Rev., n.d.
Li, C.G., Zheng, Y., Wang, M., Sun, Z., Jin, C., and Hou, J., 2021. Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau. Quaternary Geochronology, 62, 101146.
Steen, D.P., 2016. Late Quaternary paleomagnetism and environmental magnetism at Cascade and Shainin Lakes, north-central Brooks Range, Alaska, MS Thesis, Northern Arizona University. https://www.proquest.com/docview/1808501293
Stoner, J.S., and St-Onge, G., 2007, Chapter Three: Magnetic Stratigraphy in Paleoceanography: Reversals, Excursions, Paleointensity, and Secular Variation, Developments in Marine Geology, 1, 99-138, doi:10.1016/S1572-5480(07)01008- 1.
Westerhold, T., Marwan, N., Drury, A. J., Liebrand, D., Agnini, C., Anagnostou, E., ... & Zachos, J.C., 2020. An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science, 369(6509), 1383-1387.
Citation: https://doi.org/10.5194/gchron-2021-19-AC2
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AC2: 'Reply on RC2', Darrell Kaufman, 26 Aug 2021
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