the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Spatial variability of the modern radiocarbon reservoir effect in the high-altitude lake Laguna del Peinado (Southern Puna Plateau, Argentina)
Paula Andrea Vignoni
Francisco Elizalde Córdoba
Rik Tjallingii
Carla Santamans
Liliana Concepción Lupo
Achim Brauer
Abstract. The high-altitude lakes of the Altiplano-Puna Plateau in the Central Andes often have large radiocarbon reservoir effects. This combined with the general scarcity of terrestrial organic matter makes obtaining a reliable and accurate chronological model based on radiocarbon ages a challenge. As a result, age-depth models based on radiocarbon dating are often constructed by correcting for the modern reservoir effect, however, commonly without consideration of spatial variations of reservoir ages within the lake and across the basin. In order to get a better constrain on the spatial variability of the radiocarbon reservoir effects, we analyse 14C ages of modern terrestrial and aquatic plants from the El Peinado basin in the Southern Puna Plateau, which hosts the Laguna del Peinado lake fed by hydrothermal springs. The oldest 14C ages of modern samples (> 18,000 and > 26,000 BP) were found in hot springs discharging into the lake likely resulting from the input of 14C-depleted carbon from old groundwater and 14C-free magmatic CO2. In the littoral and central part of Laguna del Peinado, modern samples 14C ages were several thousand years lower (> 13,000 and > 12,000 BP) compared to the inflowing waters as a result of CO2 exchange with the atmosphere. Altogether, our findings reveal a spatial variability of up to 14,000 14C years of the modern reservoir effect between the hot springs and the lake in the El Peinado basin. This study has implications for high-precision dating and accurate 14C-based chronologies in paleoclimate studies in the Altiplano-Puna Plateau and similar settings. Our results highlight the need to consider spatial and likely also temporal variations in the reservoir effects when constructing age-depth models.
Paula Andrea Vignoni et al.
Status: open (until 25 Apr 2023)
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RC1: 'Comment on gchron-2023-3', Anonymous Referee #1, 27 Mar 2023
reply
In their study, Vignoni and colleagues present an interesting case study into the “Spatial variability of the modern radiocarbon reservoir effect in the high-altitude lake Laguna del Peinado (Southern Puna Plateau, Argentina)”. Whilst the phenomenon described is not in itself new, the dataset presented provides a useful example (that I shall no doubt cite myself in future!) of both spatial and temporal variability in the radiocarbon reservoir effect of a high altitude freshwater body, with fairly extreme effects (>>10,000 14C yrs) evident.
The manuscript is well written, and I recommend acceptance, pending very minor revisions, which I list below. N.B., not all suggestions/questions necessarily require revision of the text. I thank the authors (and editor!) for the opportunity to read this interesting submission, and wish them well for their future research.
L16-17: You obviously go on to talk about TEMPORAL variation in reservoir effects too, but I felt that this should also be mentioned right at the start here, along with your noting of “spatial variations”.
L24-25: You’re absolutely right about the implications of this study affecting both precision and accuracy of 14C-derived chronologies, and they’re obviously interwoven, but I wonder if these should be inverted to reflect the greater importance of accuracy over precision? (I.e., accuracy is fundamental – there’s no point having inaccurate chronology is there?! – and, after that, increased precision then makes the data increasingly useful, no?)
L31: You list “endorheic basins that host numerous saline lakes, playa-lakes and salars”; is there scope for these basins to episodically dry out completely, with consequent impacts (hiatuses!) upon age modelling/palaeoenvironmental reconstruction?
L48-49: “Sometimes, even assumptions on temporal variations of the reservoir effect are included in the construction of age-depth models”; please could you include one or two references to support this statement.
L119-124: In order to interpret any radiocarbon data, it is essential to specify what chemical pre-treatment procedures have been applied. (I take on trust that this has been performed robustly, but this needs to be fully clarified, and is probably the most significant of my comments.)
L123-124 (and also for Table 1): Why only calibrate the post-bomb 14C measurements, but not the pre-bomb?
L125-127: Again, what chemical pre-treatment procedures were applied to these samples prior to d13C analysis?
L129: Surely this is “precision” rather than “accuracy”?
L143: Your samples were collected in 2019… and so the latter age (2018-2019 cal CE) makes sense. But how do you explain the former age (1994-1996 cal CE)? A freshwater reservoir effect wouldn’t ENHANCE the 14C (112.39 pMC c.f. 101.61 pMC). Precisely what was the material sampled (for both of these samples)? Is the former sample more woody material (with an associated inbuilt “storage age”)? Please give more information around these samples, and suggest what has led to this.
L166-167: I would say that this wording is misleading; Yes, terrestrial plants are “expected to provide modern radiocarbon ages without any reservoir effect involved” (generally speaking! Although there could be rare examples where the expectation may differ…) BUT aquatic plants obviously take on their carbon from the water, and so they wouldn’t be “expected to” provide modern radiocarbon ages, surely? Isn’t that a fundamental premise of the present paper? I just find the wording of this sentence unnecessarily misleading, taken in isolation.
L168-169: This is really interesting. I am not a biologist – is the aged C being taken in by the grass from the air (localised atmospheric depletion from C release from the hydrothermal spring), or is the aged C being taken in through the roots (in the water taken up by the plant)?
L169: Clarify again that here you are referring to aquatic species(?).
L182-184: Give an approximate representation of the values given for the cited study.
L191-194: “The dissolution of carbonate-rich sediments or rocks in the catchment area is usually considered a main source of 14C-dead carbon influx into a lake (Macdonald et al., 1991; Ascough et al., 2010). However, the dissolution of catchment carbonates can only be a minor source of 14C-dead carbon into Laguna del Peinado because the lithology of the basin is dominated by volcanic rocks”. Does this contradict what was written earlier on (“Abundant carbonate precipitation takes place in the El Peinado basin…”, L81), or do I misunderstand? (Even if the latter, perhaps clarification is still needed?)
L213 and 216: Can you clarify what you mean by the terms “old” and “ancient” groundwater? (Is it the “100-10,000 years or longer” noted below, L219?)
L279: Is it possible to measure 14C on (the DIC/dissolved gasses of) the water itself? And would/could this, in combination with other isotope measures (including d13C and d3H, mentioned earlier) help to understand the “dominant process” question?
L287: I would actually say that “corrections of 14C chronologies based on a single reservoir age for an entire lake…” would result in INACCURATE results, rather than just “large uncertainties” (which, as I noted earlier would be a bigger problem). You would only end up with “large uncertainties” if these uncertainties were ACTUALLY accounted for and, the point that I think you’re making (which I totally agree with!) is that often these “large uncertainties” are NOT properly accounted for (…producing small uncertainties, but inaccurate chronologies).
Finally, a more general question relating to your Discussion: If the C assimilated by the species in the hydrothermal pool were solely sourced from magmatic C (rather than “old groundwater”), this would yield "infinitely old" 14C ages… And so, in that scenario, even the older 14C sample would still include some proportion of "modern" C input? (Is that reasonable to assume?) Why not perform a quick endmember "mixing model" to estimate the proportion of C (for each sample) that is from a modern (2019 CE atmospheric) source and what proportion from geologically old (14C dead) C? (N.B. this is a simple “back of an envelope” calculation, rather than requiring “proper” modelling!) I suggest that this will give a "better" impression of the differing contributions (of old vs modern C), which can be skewed by the exponential nature of the 14C decay curve, which can then carry through to all of your samples through the lake. (I.e., for each sample, what proportion of C is sourced from "modern" vs geologically "dead" sources?)
(Non-comprehensive) typo/wording suggestions:
L14: Insert comma after “This”.
L17: Change “constrain” to “constraint”.
L24: Here, do you mean the “centre of the lake” specifically?
L114: Missing word: “littoral [zone]”?
L115: Spell out “macrofossil”… Perhaps even “plant macrofossil”.
L127: “Mile” should read “mille”.
L143: “cal CE” is a suffix, and so should come after the date (e.g., “1994-1996 cal CE”).
L246: Even though I agree that your explanation is the overwhelmingly most likely one, is “proving” still too strong a word to use?
L278: I would say that “>26,000 14C years” is more than “up to several thousand years”?!
Citation: https://doi.org/10.5194/gchron-2023-3-RC1
Paula Andrea Vignoni et al.
Paula Andrea Vignoni et al.
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