The paper presents new geochronological data in the form of amino acid racemisation of N. pachyderma and C. wuellerstorfi from a range of cores in the Arctic Ocean. New age estimates are derived for these cores and the discrepancies between different dating techniques in this area are addressed in detail. This work also demonstrates the utility of C. wuellerstorfi for AAR, a species which has not been previously investigated in detail using modern separation methods. I therefore recommend this paper for publication with minor revisions, detailed below.

Specific comments:

Line 159: It should be stated that during hydrolysis, Asn and Gln irreversibly hydrolyse to Asp and Gln, so the ‘Asp’ and ‘Glu’ reported in this study also includes any Asn and Gln present in the biomineral (see Hill 1965 https://doi.org/10.1016/S0065-3233(08)60388-5).

Table 3: Table 3 gives a count of subsamples destroyed during analysis, but this is not mentioned in the text.

Figures 4 and 5: The authors mention the reduced confidence in the power function for C. wuellerstorfi from the Alpha Ridge due to the small number of C. wuellerstorfi samples analysed at this site; as only 4-5 samples of N. pachyderma were analysed from the Alpha Ridge and of C. wuellerstorfi from the Iceland Sea, this uncertainty should also be discussed for these sites/species.

Lines 293-300: Equations for a simple power model applied to the data are given here. These should be given when the models are first introduced (in Section 3c). I would also suggest briefly summarising the precedent of using simple power functions to model racemisation (e.g. Clarke and Murray-Wallace 2006 https://doi.org/10.1016/j.quageo.2006.12.002), justifying why the authors used this model and how the exponent for each model was derived.

Line 391-409: The authors suggest that differences in microbial environments during diagenesis may account for some of the discrepancy between racemisation and other dating methods. As this should only affect open-system inter-crystalline material, it would be worth addressing recent work on isolating the intra-crystalline fraction in foraminifera (see Penkman et al. 2008 https://doi.org/10.1016/j.quageo.2007.07.001 for IcPD overview and Wheeler et al. 2021 https://doi.org/10.1016/j.quageo.2020.101131 for IcPD of N. pachyderma), as the intra-crystalline approach may minimise or eliminate environment-specific effects on racemisation rates.

Technical corrections:

All figures: increase the line thickness of error bars – they are very difficult to see, especially for green/yellow data. As figure 8 presents data from only one species (thus placing Asp on the lefthand plot and Glu on the righthand plot), I would recommend switching the layout of the other plots that the amino acid is faceted horizontally and the species vertically, so that all figures are consistent in this respect.

Line 19: ‘large geographical area, from the Greenland’.

Lines 23, 241, 244, 403: ‘foraminifer species/taxa/tests’ or ‘species/taxa/tests of foraminifera’ - the singular 'foraminifer' should be used in the adjectival form.

Line 41-42: ‘the protein amino acid isoleucine over time in samples of the planktic foraminifer Neogloboquadrina pachyderma and the benthic species Cibicidoides wuellerstorfi’.

Line 69: Consider removing the dashes around ‘undetermined’ to improve the readability of this sentence.

Line 110-111: Consider listing the Nordic Sea cores here explicitly, e.g. ‘Cores from the Nordic Seas (ODP151/907A and PS17/1906-2) primarily relied on oxygen isotope stratigraphy’.

Line 122, and in general: Consider putting references at the end of a sentence/clause to improve readability.

Line 149, and in general: L should be capitalised in mL and µL.

Line 155-159: Consider breaking the sentence starting ‘The peak-area ratio…’ into two sentences at ‘extent of racemisation, but this study’.

Table 3: Full stop at end of table caption.

Line 200: Consider giving the name of the stratigraphically reversed sample removed from the species comparison.

Figures 4, 5 and 9: Increase line thickness on open circles/diamonds – these are challenging to see, especially for the pale data points.

Figure 6: Define black/green/red lines in each figure. Also consider changing line style of the blue line (e.g. to dashed or dotted) to make it clear that it is an age model derived from other data, especially as the colour of this line is to denote data from this paper in other figures.

Line 264: In abstract, a standard deviation is given for the difference between racemisation rates of the two species; this should be quoted here rather than ‘approximately 16 %’.

Line 280: Consider changing ‘since we established that AAR is faster’ to ‘as racemisation proceeds more quickly’.

Figures 7 and 8: Consider adjusting palette – blue and purple are very similar, and yellow and green challenging to separate under red-green colourblind conditions. I would also recommend not reusing blue for the LMROG12-PC03 data and the Kaufman 2013 model, as this implies a connection between them.

Line 315: ‘what are interpreted as’

Line 325: Make sure that ’marine isotope stage’ is defined in its first instance and that the acronym is used thereafter.

Table 4: Left-align text on row 2, as justified text is difficult to read in narrow columns.

Line 384: Consider changing ‘seems untenable’ to ‘is unlikely’ or ‘is highly unlikely’ – more scientific language.

This is a very interesting and in many respects, throught-provoking manuscript because it tries to come to terms with a complex marine geological problem, while at the same time, resolve some of the typically challenging issues in Quaternary aminiostratigraphy.  The latter involves the potentially different diagenetic temperatures and a genus-effect on racemization with their ultimate influence on the measured extent of amino acid racemization in fossils, and ultimatelty an assessment of age of the marine successions.   The manuscript discusses the potential difficulties of inferring the age of the successions in question and clearly outlines many geological and environmental attributes that confound a conclusive age interpretation.   While concluding that the higher amino acid D/L values for the foraminifer C. wuellerstorfi is not due to a genus effect, a whole set of new questions arise to reconcile the basis for the extent of racemization observed in these individuals compared with other Arctic Ocean deep sea cores. In this sense, the existing manuscript is to some extent open-ended in its conclusions. Perhaps the conclusions can be more decisive?

Some additional more specific comments include:

Line 21 and other instances - oxygen isotope stratigraphy and magnetostratigraphy are, strictly speaking, not 'dating methods' in themselves, although with appropriate calibration using geochronological methods have an obvious role in unravelling Earth history.

Please be consistent in the spelling of racemization (either s or z but please be consistent throughout the text).

Line 24 and other instances 'n' italic font

Line 40 epimerization

Line 51 and elsewhere - do you mean calcium carbonate?

Line 62 'upper Quaternary' is not a stratigraphically recognised term - please be more specific

Line 82 sample mass

Table 1 please indicate unit of measurement for temperature

Line 119 correlated with

Line 149 mL and for microlitre later in the same paragraph

Line 186 high serine content - please quantify and explain in what sense.

Line 214 fossil age (sample is something that you have collected)

Figure 4 caption - uncertainties rather than 'error bars' - they are not really an error, meaning something that is incorrect

Line 249 compared with

Line 323 as above

Line 334 validity of this assumption?

Lines 341 to 345 Is this a manifestation of the kinetics of racemization and overall form of the extent of AAR with time?

Line 364 but is this valid?

Figure 9 benthic oxygen isotope curve - perhaps have times arrow reading to the right-hand side of the page?

Line 391 calcium carbonate.

Colin Murray-Wallace

Review of “Amino acid racemization in Neogloboquadrina pachyderma

and Cibicidoides wuellerstorfi from the Arctic Ocean and its

implications for age models» by West et al. for Geochronology

This paper present new amino acid data from a series of sediment cores from the Arctic Ocean and the Nordic Seas. The new data include analyses of the planktonic species N. pachyderma and the benthic species C. wuellerstorfi and it is focused on the racemization reaction of Aspartic and Glutamic acid. The results are discussed in the context of the challenges in establishing robust chronologies for Arctic Ocean deep sea sediments. The paper is well written and has a sound scientific approach and should definitely be published. However, after reading the paper I think the authors should be a little bit stronger on their conclusion regarding the implications of their findings. To me it seems clear that the results suggest that either the established chronology is to young or the bottom waters have been warmer.

In the revision of the paper I would like the authors to consider expand with the following:

1.    It is fine that the aa data generally support the correlation of cores based on density. However, merging all data  on the depth scale of the ACEX core may introduce more scatter and uncertainty than needed.

2.    Present all Arctic Ocean cores with the basis for correlation also on depth scales.

3.    Expand the section with some hard data on the basis for the ACEX established chronology and expand the discussion of the aa results relative to the other methods used. 

4.    A review/figure/profile depicting the present main water masses in the Arctic Ocean may be useful. Perhaps some references to modelling work concerning possible temperature conditions in the Arctic ocean during the glacial stages may be useful-

Miinor suggestions on text

30    … bottom water temperatures may have been similar.

33    ….models. Also a better understanding of temperature histories at the investigated sites and other possible environmental factors that may influence rasemisation rates in the central Arctic ocean, is needed.

64    Bottom water of Atlantic and Pacific sites are presently generally a few degrees warmer than the Nordic Seas and Arctic ocean.

100    …..influence of Atlantic surface water, and…

110     …cores have been developed….

141    ..with 21(?) samples…

172     Where the reversal confirmed for both species at the same level?

214    Considering what we know about past and present hydrography the samples from the Nordic Seas most likely have been exposed to the same water masses through time. Also the datapoints you have, seems not allow for establishing different pathways at the two sites. Suggest you make one polynomial fit but keep the coloring of points.

249    This is surprising taken into account that the temperatures potentially have been lower than the “global” ocean.

Fig. 2

Suggest that the density data is plotted on individual core depths with correlation to ACEX indicated with lines.

Fig. 5

It would have been nice to see the data for each core plotted on core depth in a separate figure..