Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Thomas, Nicola C.; Ford, Heather L.; Greaves, Mervyn; Hodell, David A.

doi:10.5194/egusphere-2025-4566

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https://doi.org/10.5194/egusphere-2025-4566

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01 Oct 2025

| 01 Oct 2025

Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Abstract. We report basinal and global compilations of deep-water temperature and δ¹⁸O_seawater for the past 1.5 million years using tandem oxygen isotopic and Mg/Ca measurements of benthic foraminifera. Across the Middle Pleistocene Transition (MPT), interbasinal gradients suggest North Atlantic deep-water became colder and Pacific deep-water saltier during glacial periods after ~900 thousand years ago. Salinity in source areas increased in the marginal seas around Antarctica by decreased meltwater discharge from ice sheets and increased sea ice extent, which led to increased density stratification of the abyssal ocean. The deep ocean became a more effective carbon trap and lowered glacial atmospheric carbon dioxide, leading to expansion of continental ice sheets and longer glacial cycles. Results support a physical role for abyssal ocean stratification in explaining the MPT. Collectively, our deep ocean stacks lend support to hypotheses proposing that the MPT resulted from a progressive drawdown in glacial atmospheric pCO₂, a conclusion that awaits verification from the Beyond EPICA–Oldest Ice core from Antarctica.

Received: 16 Sep 2025 – Discussion started: 01 Oct 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.

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19 Aug 2026

Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Clim. Past, 22, 1559–1584, https://doi.org/10.5194/cp-22-1559-2026,https://doi.org/10.5194/cp-22-1559-2026, 2026

Short summary

Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2025-4566', Peter U. Clark, 22 Oct 2025

It would be helpful to add uncertainties to all time series shown in Figure 12.

Citation: https://doi.org/10.5194/egusphere-2025-4566-CC1
RC1:
'Review', Anonymous Referee #1, 27 Oct 2025

This paper measures and analyses Mg/Ca-based deep ocean temperature (DOT) and d18O_benthic and the previously obtained DOT to determine d18O_seawater of the Iberian Margin site U1385 across the last 1.5 Ma covering the Mid-Pleistocene Transition (MPT) and sets results into context of others by (a) making basin- and global-wide averages using 4 other sites, and (b) discussing especially difference to the recent finding of Clark et al. (2025) on mean ocean temperature (MOT) and the deconvolution of d18O_benthic into temperature and seawater components. It concludes that while there is hardly any change in DOT across the MPT there is a drop in d18O_seawater around 900 ka, rather the opposite of what is proposed in Clark et al. (2025).
I cannot judge the methods, so my comments will solely be on the interpretation.
I think this is a solid piece of work on a new data set across the MPT. The discrepancy with the Clark et al interpretation somehow illustrates the state of knowledge that we are facing right now. So, more details here on what is compared and why things differ would help the reader to set things better into context. See my comments below for improvements:
Major issues:
- The used equations for calibration (Eq. 2-4) come with uncertainties, which are completely ignored here. For example, in Weldeab, Arce and Kasten (2016) the full equation is given as Mg/Ca [mmol/mol] = (0.36 ± 0.02) ∗ BWT [°C] + (2.22 ± 0.19). Furthermore, both the Mg/Ca-T calibrations are rather scatted in the temperature range used here (-1 to +6°C). Therefore I suggest to add discussions how these ignored uncertainties impact the results, at best recalculate propagated errors using uncertainties on all the parameters that go into the calibration curves. The 3 equations 2-4 should also be reworked to properly state units (so far it is only mentioned, that T comes as °C).
- Several times Raymo et al. (2006) is discussed as an probably cause for the deduced salinity changes. However, the global SST compilation in Clark et al (2024) had no power in precession which is the dominant frequency in the Raymo et al (2006) hypothesis. This is discussed in part 3 of the Clark et al papers, just published in Science (doi: 10.1126/science.adv8389). I therefore think this hypothesis can be drop, or at least add this discussion point made in Clark et al to this idea.
- In the comparison to Clark et al (Figure 12) it is important to plot both, the own and the Clark data with uncertainties. Only then can we see if there is overlap or complete disagreement in certain periods. I actually see at least two main differences in T or both reconstructions. First, the drop in T across the MPT in Clark, missing here (discussed by the authors). Second, the much larger G/IG amplitudes in T after the MPT in the Clak data (not discussed). Thus, both T data sets never show similar things. For the part after the MPT Clark et al. used a ratio of GMSST:MOT=1.1, thus no information from deep ocean T are used which could explain the offset between both T. Furthermore, the d18O_sw from Clark contain the removal of a long-term increase of 0.083 permil/My in d18O_benthic, which is not included in d18O_sw of this study. Although this effect is small (1.5 Myr + 0.083 = 0.12 permil) it needs to be mentioned to understand the comparison.
- How can we learn even more from the comparison to the Clark et al. paper? Your stacked d18O_benthic compares very good to the Prob-stack d18O_benthic for the last 400 kyr, but differs somehow before (Fig. 9a). Maybe this illustrates, that for the last 400 kyr your stack is representative of the global signal, but not so much before. Maybe calculate the difference between both d18O_benthic time series to use as a proxy of global representativeness. If you plot this difference then together with the T and d18O_sw comparison to Clark (Figure 12), the reader would see, how much your stack are (or are not) global signals. Maybe this difference can also be exploided as a correction for d18O_sw and/or T, but I am not sure it will work or if it is a good idea. Furthermore, Fig 12 needs different y-axis labels. Your stack of d18O_sw is NOT a global stack (while d18O_sw of Clark is), it is a stack of globally distributed cores from waters below 2km. Actually, maybe at best you should not compare to the MOT from Clark but to the time series of T changes below 2km (Fig 16d in Clark et al., 2025, data should be available from the authors), which uses GMSST and a change in heat storage efficiency (=Delta MOT / Delta GMSST). This has large uncertainties 1.5-0.9 Myr, but basically allows(more or less) a long-term flat line across the MPT, similarly as here (no trend in temperatures).
- So all together, (a) considering if a long-term trend in d18O_benthic is included in plotted data or not, (b) which kind a T curves are compared, (c) highlighting that your approach is a least partially not representing global signals, (d) having some more emphasis on (missed) uncertainties, both your data and the Clark data seem to be not so much in disagreement as so far discussed.

Minor Details:
- Not all SI figures are cited in the main text. Either cite (and order the SI figures according to citation) them or delete them.
- Maybe I missed it, but why does Fig. 2 only show the time 700-1500 ka. Is this the only new part and 0-700 ka was already published elsewhere? Details can probably be extend in section 2.
- Line 435 or so: I believe Pacific bottom water temperature being near-freezing throughout the MPT and thereafter has first been shown in Siddall et al. (2010: doi:10.1016/j.quascirev.2009.05.011).
- Line 668: Raymo et al (2006) was published in Science, not Nature.

Citation: https://doi.org/10.5194/egusphere-2025-4566-RC1
- AC2: 'Reply on RC1', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC2
RC2:
'Short addition to my review', Anonymous Referee #1, 28 Oct 2025

In a paper published yesterday in this journal

Larsson, V. and Jung, S.: Persistent contamination in benthic-foraminifera-based Mg ∕ Ca thermometry using standard cleaning methods, Clim. Past, 21, 1871–1894, https://doi.org/10.5194/cp-21-1871-2025, 2025.

a different calibration for Uvigerina peregrina than the one used here (their Fig. 12) and some contamination issues are discussed. Please add some thoughts, if and how their findings are relevant for your study here.

Citation: https://doi.org/10.5194/egusphere-2025-4566-RC2
- AC3: 'Reply on RC2', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC3
RC3:
'Comment on egusphere-2025-4566', Anonymous Referee #2, 25 Nov 2025
Thomas et al. present combined benthic foraminiferal Mg/Ca and oxygen isotope data from IODP Site U1385 at the Iberian Margin to reconstruct δ¹⁸O_sw (seawater oxygen isotopes) covering the past 1.5 Myrs. The record is compiled with similar datasets from the Atlantic and Pacific sectors of the Southern Ocean as well as from the North Pacific, allowing the authors to produce a deep-ocean δ¹⁸O_sw and temperature stack that reflects global changes. The study uses a (to my knowledge) novel approach by applying a volume-weighted averaging scheme to calculate the global δ¹⁸O_swstack, that accounts for the relative sizes of the major ocean basins. This method is contrasted with previous approaches that did not include ocean volume-based corrections, providing new insights into the interpretation of global signals. The main findings of the study indicate increased salinity in the deep Pacific and reduced deep-water temperatures in the Atlantic during glacial periods after 900 ka, compared to the interval between 1.5 and 0.9 Ma. The authors interpret these results as evidence for enhanced glacial water mass stratification in the Pacific after 0.9 Ma, leading to a greater sequestration of atmospheric carbon dioxide in the deep ocean, as well as increased salinity in surface waters within regions of southern component water formation around the Antarctic margins.
The study draws on an extensive dataset, and the methodology appears sound and appropriate to the study’s objectives. The resulting dataset has the potential to make a valuable contribution to our understanding of global ocean temperature, salinity and ice volume changes across the Mid-Pleistocene Transition (MPT). However, the strength of the methodological and analytical work is not matched by the clarity and depth of the data presentation and interpretation. In its current form, the manuscript does not fully exploit the scientific potential of the dataset, and several key aspects of the analysis and discussion require revision and elaboration. Below I outline my major and minor points of criticism in the hope that the authors find them useful in improving and revising their manuscript.
In the following, I will first outline several broad, major concerns that affect the overall interpretation and structure of the manuscript. I will then provide detailed, section-by-section comments and suggestions aimed at improving the scientific robustness and overall comprehensiveness of the manuscript.
The manuscript warrants more transparency in the many nuances of the interpretation and should clearly outline the limitation of the data interpretation. For instance, the authors base their conclusion of an enhanced density stratification throughout the entire deep Pacific after 900 ka (compared to 1.5–0.9 Ma) primarily on data from only one Site in the South Pacific and one in the North Pacific. This interpretation is presented as a definitive conclusion, rather than as a working hypothesis supported by limited regional evidence. Potential variations in salinity throughout the entire water column—and their corresponding effects on vertical stratification—are not discussed, and possible regional hydrographic differences are insufficiently addressed in my opinion.

Throughout the manuscript, several sections lack a clear explanation and important arguments are presented in a way that they appear somewhat ambiguous. This occurs in multiple places and affects the overall clarity of the manuscript (e.g. comments on lines 42, 127, 172-177, 191, section 4.1). I recommend using more precise wording in many sections to avoid confusion.

In the abstract and discussion, the authors attribute the observed increase in deep Pacific salinity after 900 ka to changes in the source regions of Southern Component Waters (SCW), i.e., Antarctic margin regions (e.g., the Weddell and Ross Seas). However, this interpretation (i.e. a key finding in the abstract) is not substantiated by any evidence presented in the study. In my view, the manuscript provides insufficent discussion of existing literature and data that would support such large-scale changes in SCW source regions during this interval. Revision is needed here. Moreover, as PDW is a mixture of several water masses, the possibility of temporal changes in PDW end-member composition—and their influence on reconstructed salinity—should be addressed, to support the suggested changes in SCW across the MPT. The authors could consider potential variations in inter-basin connectivity within the Southern Ocean, e.g. using the Antarctic Circumpolar Current strength reconstruction of Lamy et al. (2024), which could help assess whether large-scale circulation changes—rather than solely local shifts in AABW formation—may contribute to the observed Atlantic–Pacific differences across the MPT.

Throughout the manuscript, statistical analyses are insufficiently applied when comparing different datasets and time intervals. Statements referring to “significant” differences are frequently made without presenting the results of any formal statistical tests. To substantiate these claims, appropriate statistical analyses (e.g., t-tests, ANOVA) should be conducted and reported. Including quantitative measures of variability and statistical significance would greatly strengthen the robustness of the interpretations and allow readers to assess whether observed differences between regions or time periods are statistically significant.

In my opinion, the introduction lacks a clear structure and does not fully establish the central research question. Integrating relevant key studies (e.g. Quin et al., 2022)—many of which are discussed in the discussion (e.g. Hines et al., 2024; Pena and Goldstein 2014, Farmer et al., 2019; Lear et al., 2016) —more prominently at the start would help frame the topic, clarify existing knowledge gaps, and better motivate the study within its scientific context. Moreover, a concise overview of the main hypotheses proposed to explain the MPT—including e.g. the regolith hypothesis (e.g. Clark and Pollard, 1998, Clark et al., 2006), the expansion of North American and/or East Antarctic ice sheets (e.g. An et al., 2024; Bintanja and van de Wal, 2008; Raymo et al., 2006), or feedback mechanisms involving changes of the marine carbon cycle (e.g. Chalk et al., 2017; Willeit et al., 2019)—would provide important context and more clearly situate the study within the broader landscape of MPT research.

Overall, the figures would benefit from improved clarity and more explicit information to help the reader follow the comparisons and interpretations of the many timeseries presented. It would be helpful to include the location of each record directly in the figure, for instance by labelling the regions (e.g., Pacific, Southern Ocean, Atlantic…) in the figures. For the Iberian Margin record, it should be clearly indicated which data were generated in this study and which were compiled from previous publications (different symbols or colours). Finally, it would be beneficial to show key time intervals discussed in the manuscript at higher temporal resolution, rather than referring exclusively to the long composite records. This would allow for a clearer evaluation of the main features and transitions emphasised in the text. If feasible, reducing the overall number of figures may improve the readability of the manuscript, as the large number of time series and figures can be somewhat overwhelming for the reader.

Salinity on the Practical Salinity Scale (PSS-78) is a dimensionless quantity, and the use of the unit “PSU” (Practical Salinity Unit) is therefore incorrect. The authors should avoid using “PSU” throughout the manuscript. Instead, it should be stated upon first mention that all salinity values are reported on the Practical Salinity Scale (PSS-78) and are thus unitless. It would be preferable to adopt the TEOS-10 (Thermodynamic Equation of Seawater, since 2009) scale and thus report Absolute Salinity in g/kg. However, given that global datasets such as GLODAP and WOA report salinity on the PSS-78 scale, calculated from conductivity, temperature and pressure, clarification in the text that values are dimensionless and follow PSS-78 are sufficient.

Detailed comments:
Line 13, 14: The study would benefit from more cautious wording, as it presents no direct evidence for changes in the carbon storage capacity.
Line 23: One of the main conclusions of the study concerns a salinity change across the entire deep Pacific after 900 ka based on two Sites. However, the text states that δ¹⁸O_sw only reflects variations in global ice volume and local hydrographic effects. This phrasing is somewhat ambiguous and should be clarified.
Line 32: This statement is too vague. Since this appears to be one of the main motivations of the present study, the divergent interpretations should be clearly introduced in the Introduction.
Line 40-43: Please specify what specific concerns are being referred to here and clarify whether “previous findings” refers to Yu & Broecker (2010) or Sosdian & Rosenthal (2009).
Table 1: Please use a consistent coordinate format. The authors should also cite the sources or datasets from which present-day temperature and salinity data were obtained.
Fig. 1: Please indicate in panel (a) where the transects shown in panels (b) and (c) were taken. Given potential differences between the eastern and western Atlantic, this spatial context is important. It would also be useful to include a single transect (covering the Pacific, Southern Ocean, and Atlantic) directly in the main manuscript rather than in the Supplement, as temperature gradients between these basins are a key aspect of the study.
Fig. 3: Please use a consistent capitalisation for “Marine Isotope Stage” throughout the manuscript (either always “Marine isotope stage” as in line 81 or “Marine Isotope Stage,” as in line 103). In addition, clarify the basis of the 2σ error bars mentioned in lines 82-83; are these derived from replicate measurements or represent analytical uncertainty or both?
Line 89 and 92: The phrase “and have similar seawater physical properties” is somewhat vague. Please specify which properties are referred to (e.g., temperature, salinity, density) or include a reference to Table 1 for clarification.
Line 107-109: Do all studies use the same approach for their age model?
Line 122-123: To my knowledge the MD01-2444 record spans ~50–35 ka, which does not include the LGM.
Line 123-124: Please Specify whether this offset is based on overlapping intervals between the cores or on modern temperature differences (which are not listed in Table 1).
Line 127: I cannot fully understand this sentence. If the text refers to a specific taxonomic level, please use the term “species” to avoid confusion.
Line 128: Please clarify how comparable the results are between the two genera when both were measured.
Line 143-144: Please specify which error the authors are referring to. No corresponding uncertainty is shown in Fig. 2a, so it is unclear what uncertainty estimate is referred to or how it was derived.
Line 146: It seems likely that the intended method was inductively coupled plasma–optical emission spectrometry (ICP OES), rather than "spectrophotometry".
Line 166: I think the intention was to refer to the excluded samples when using the term “the remaining samples.” However, lines 166–169 read as if this refers to the samples actually used in the dataset. Please replace “remaining” with “excluded” to avoid confusion.
Line 168: Please remove the “<” before 0.03 mol/mol⁻¹ or instead use “< 0.1 mol/mol⁻¹,” as Barker et al. (2003) only rejected samples with Fe/Mg < 0.1 mol/mol⁻¹, whereas the 0.03 mol/mol⁻¹ value cited here represents the typical range of their samples rather than an exclusion criterion.
Line 171: units are missing.
Line 172-177: The reasoning in this paragraph is difficult to follow. The use of “rather” and “more likely” is confusing. "Ferromanganese oxide overgrowths" and "authigenic Mn–Fe-oxide coatings" refer to the same type of contamination.
Line 174: What is meant by elevated? Please quantify.
Line 175: Please rephrase "this contamination" as not all Mn in foraminiferal calcite necessarily indicate contamination. It could also reflect (partly) natural incorporation of Mn during biomineralisation.
Line 191: This phrasing is ambiguous. Do the authors mean a 0.32 mmol/mol correction for Mg/Mn_coating within formula 1, or a constant correction for all foraminiferal Mg/Ca data?
Equation 2: How do the core-top Mg/Ca-derived temperatures compare with modern and/or pre-industrial deep-water temperatures at the study site? Please also justify the use of the Elderfield et al. (2012) calibration (originally introduced in Elderfield et al., 2010). The Elderfield et al., 2010 calibrations lower end is ~1°C. Roberts et al. (2016) specifically adjusted the Elderfield calibration for temperatures below 0 °C.
Line 225: Is this uncertainty based on replicate measurements of the same standard material, or of multiple aliquots of the same sample?
Figure 4: Is there a reason why some interglacials and glacials are missing their labels in panel (a)? For example, MIS 13, 14, 17, and 18 are not indicated. Please clarify or add the missing MIS numbers for consistency.
Line 283: Please ensure consistent use of time units throughout the manuscript. Earlier, “Ma/ka” was used for specific ages or time periods in the past, whereas “Myr/kyr” was used for durations. Here, “1.5 Myrs” is inconsistent with that convention. The same issue appears in line 285. Please standardise the terminology.
Line 284 - 285: This statementis difficult to follow. Is the observed cooling occurring during interglacials, glacials, or consistently across both climate states? Are uncertainties associated with this trend quantified? How are these numbers calculated?
Line 313-316:The comparison is difficult to follow, as it mixes different quantities: interbasin glacial–interglacial temperature differences for 1.5–0.9 Ma with glacial-only temperatures before and after 0.9 Ma. This makes the intended conclusion unclear. Please clarify the rationale for comparing these metrics and include uncertainties.
Line 319-321: If the authors already write “We interpret the greater increase …,” this should strictly not appear in the Results section.
Figure 9: The purpose of the dashed lines in Figure 9 is unclear. In panel 9b, it appears that the upper line might represent the mean interglacial δ¹⁸Oₛ_w across all interglacials and the lower line the maximum values—but this is not explained. Were these lines calculated from the data, or were they arbitrarily chosen? Similarly, in panel 9c, it is not evident how the dashed line is intended to represent “constant deep-ocean temperatures (line 343).” It appears to be placed at ~0 °C rather than reflecting a calculated mean. If this line is meant to represent an average, please show the associated variability (e.g., 2 SD).
Line 351-353: Please add numbers for the Temperature decrease
Line 357-360: There may also be an important interbasin difference between the western and eastern North Atlantic, which challenges the assumption that Iberian Margin records can be treated as representative of the entire North Atlantic basin (e.g., Chalk et al., 2019). This aspect should be discussed, especially in relation to the statement in lines 287–290 that the carbonate ion effect cannot fully explain the temperature differences between Sites 607 and U1385. If the carbonate ion effect is not the sole driver of interbasin- and species Mg/Ca temperature offsets, the authors should explicitly consider and discuss alternative explanations, including east–west basin-scale hydrographic differences, which are currently not addressed anywhere in the manuscript.
Section 4.1: I would appreciate it if this entire section were given more structure. It is difficult to follow the connections between the different hypotheses and literature references and to identify the main message of the section. Throughout the paragraph, several mechanisms are mentioned (e.g., standing volume effect, NADW cooling, SCW salinity increase, ice sheet/shelf melting, and a stratification inversion in the North Atlantic), but they are not sufficiently integrated into a coherent narrative or overarching context in my opinion. For example, it is not clear to me whether the authors are arguing that the North Atlantic was bathed in NCW, in SCW, or favouring some other scenario.
Line 400: Please remove strong, as it is relative.
Figure 11c: Please ensure that your own data points are clearly visible. They are currently obscured by the Grimmer et al. (2025) dataset, which prevents a proper comparison between the two records.
Line 425: The period mentioned (2.75–2.15 Ma, according to the time frame defined in line 38) is not covered by the dataset. Please rephrase.
Line 427-429: Please clarify which MDOT dataset you are referring to and indicate the corresponding figure.
Line 431: See comment on line 400
Line 431-433: I would strongly recommend being more cautious with the argument that the records from Sites 607 and U1313 are complicated by multi-species measurements, since the record presented in this study (U1385) is also a derived from three different species.
Figure 12: Please indicate uncertainties
Line 453-456: Are there any similar approaches applied to more recent, data-rich intervals? A comparison would help assess uncertainties.
Conclusion: The conclusion includes a substantial amount of important and relevant literature. I would appreciate it if these key studies were introduced in more detail in the Introduction, allowing the conclusion to focus more clearly on the study’s own findings.
Line 465-467: Most of the studies cited in this context examine glacial–interglacial changes—often limited to the last glacial cycle—and do not address variability across the MPT. It is therefore unclear how these references support the conclusions drawn here or how they relate to the long-term patterns discussed in the study.
References cited in this review that do not appear in the manuscript:
Roberts et al.: Evolution of South Atlantic density and chemical stratification across the last deglaciation, Proc. Natl. Acad. Sci. U.S.A., 113 (3) 514-519, https://doi.org/10.1073/pnas.1511252113 (2016).
B. Chalk et al.: Dynamic storage of glacial CO₂ in the Atlantic Ocean revealed by boron [CO₂^3-] and pH records, Earth Planet. Sc. Lett., 510, 1–11, https://doi.org/10.1016/j.epsl.2018.12.022, (2019).
Lamy et al.: Five million years of Antarctic Circumpolar Current strength variability. Nature, 627, 789–796, https://doi.org/10.1038/s41586-024-07143-3, (2024).
An et al.: Mid-Pleistocene climate transition triggered by Antarctic Ice Sheet growth. Science, 385, 560-565, https://doi.org/10.1126/science.abn4861, (2024).
U. Clark & D. Pollard: Origin of the middle Pleistocene transition by ice sheet erosion of regolith. Paleoceanography, 13, 1–9. https://doi.org/10.1029/97PA02660, (1998).
Willeit et al.: Mid-Pleistocene transition in glacial cycles explained by declining CO₂ and regolith removal. Sci. Adv., 5, 7337, https://doi.org/10.1126/sciadv.aav7337, (2019).
B. Qin et al.: Sustained Deep Pacific Carbon Storage After the Mid‐Pleistocene Transition Linked to Enhanced Southern Ocean Stratification, Geophys. Res. Lett., 49, (4), 1944-8007, https://doi.org/10.1029/2021GL097121, (2022).
Citation: https://doi.org/10.5194/egusphere-2025-4566-RC3
- AC1: 'Reply on RC3', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (12 Feb 2026) by Lorraine Lisiecki

AR by Nicola Thomas on behalf of the Authors (27 Apr 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 May 2026) by Lorraine Lisiecki

RR by Anonymous Referee #1 (27 May 2026)

Suggestions for revision or reasons for rejection

I am in principle happy with the revision apart from the few points below which need further attention:

1. I made the point that the hypothesis of Raymo et al. (2006) might not be relevant anymore, since it relies on changes in precession and I argued that precession is not in the recent SST data compilation of Clark et al. (2024) as discussed in Clark et al. 2025 (Science). In the response letter the authors argue that „the absence of detection of significant precession in global mean SST does not preclude precession-paced influences on regional surface salinity or ice–ocean interactions“. Note, that the is also no power in precession in the regional SST stacks in Clark et al. (2024) (northern / southern extratropical or tropical SST). While I can follow this argument I disagree that this is taken to change nothing at all to the discussion of the relevance of the hypothesis of Raymo et al. If this is kept in the paper it should be accompanied with some lines of discussion that the Clark et al data do not support it due to the missing power in precession, but that the authors have have reasons to believe that this shows not the whole story. This is even more relevant since the Raymo et al. hypothesis has been directly addressed in the Clark et al. (2025) paper in Science.

2. MOT from Allan Hills and MOT from Clark et al. (2025): In the response letter and in the draft (line 571) the authors cite Wolff (2026), which is a News & Views on the Shackleton et al. (2026) Allan Hills MOT data. There, E. Wolff writes: „In my view, the results of Shackleton and colleagues undermines the basis of that study [-> MOT from Clark et al. 2025], because they show that the scaling of sea surface to deep ocean temperature is invalid.“ Since this is a personal opinion in a comment (without further evidences) I do not think this should be taken as argument in a peer-reviewed paper, and I disagree with that opinion. Furthermore, in Shackleton et al. (2025) the two MOT data sets are never plotted with the uncertainties in both data sets attached. If you do that and if you furthermore consider the suggested cold bias of up to 0.5°C in the Allan Hills data and that they are probably weighted towards interglacials you would find that both MOT data sets agree within their uncertainties with each other. I therefore ask that the corresponding statements around line 571 are revised accordingly and that the citation to Wolff 2026 is dropped. Related to that one might also say that within uncertainties MDOT of Clark et al. (2025) and this paper (Fig. 10d) are not so different and agree within the uncertainties with each (even if the mean values diverge before the MPT). One might argue that there is missing G/IG variability in the Clark DMOT record, but this is due to the way it was reconstructed (see Clark et al. (2025) in ClimPast for details), because of which the uncertainties are really important and tell that both approaches are not completely disagreeing with each other.

3. d18O_sw: The caption to Fig 11 contains that Clark et al. (2025) corrected d18O by 0.083 permil/Myr due to assumed diagenesis. While this might be unproven (as said there), it was brought up by Raymo et al. (2018) in order to understand Pliocene sea level reconstructions. It is up to the authors if they want to add these details here. But I strongly suggest that the discussion on these details is not only found in the figure captions, but also in the main text, where it is missing so far.

4. References to Figures. It is not clear to me, why in line 425 Fig S11c is cited for MDOT, while this is also plotted in Fig 8c (but with different weightening). Maybe use both. I think this might also be the case for other figures, where the version on the SI and the version in the main text show similar things, but differ only by small details (weighted by ocean volume or unweighted).

5. All figures are really low resolution and details are hardly detectable. This need improvements.

6. line 543: At the end of the line „MPT“ is missing.

Hide

RR by Anonymous Referee #3 (22 Jun 2026)

Suggestions for revision or reasons for rejection

Thomas and colleagues provide a new Mg/Ca-based deep water temperature record and d18Osw reconstruction from IODP Site U1385 in addition to global compilations of deep ocean temperature and d18Osw. Although I did not review the previous version, it is apparent from the response to reviewers and authors’ tracked changes that the authors have made significant modifications in response to the previous reviewers’ concerns. To that end, I intend to provide a lighter touch review that mostly focuses on readability. Overall I found this manuscript well suited for publication in Climate of the Past following a series of minor changes, below.

Abstract L8-22. Two minor issues here:
1. The abstract is missing an opening 1-2 sentences of essential framing for this study. Why is it important to have basin-scale and globally distributed compilations of deep-water temperature and d18Osw? The authors provide this in L24-61; they just need include the executive summary of the problem here.
2. Surprisingly, in the abstract the authors do not mention the major data product of their work: A new Mg/Ca-derived bottom water temperature reconstruction from Site U1385. That should be added as a “Here we” sentence coming prior to the current 1st abstract sentence.

L24. Is 0.64 million a typo, perhaps from being one key off to the left? Usually the MPT is noted as 1.25 to 0.75 Ma (e.g., Herbert, 2023).

L32. Also cite Chalk et al. 2017 and Farmer et al. 2019, here, as both studies delved into the NH vs SH controls on the MPT.

L75-78. Not necessary, but this would be a useful distinction to visualize in a supplementary figure.

L79-90, and again on L119. A question on the veracity of the stacking approach: Is it to be expected that U1385, at a 2578m and in the northeastern basin, would record the same watermass characteristics as DSDP 607, at 3427m and in the northwestern basin? On L119 the authors state that “All study sites are deeper than 2500m and are therefore assumed to be representative of the deep ocean”. That is a major oversimplification. And it is noteworthy to this reviewer that the temperature estimates in Table 1 are about 0.7°C offset for the two sites, which is significant for the deep ocean and suggests that the two sites may reside in different “limbs” of modern NADW. I should note that the results section supports the stacking approach, but it would help to have a bit more modern support for the method.
To address this, the authors should compare the modern density of seawater at these two locations to see if they fall within the same watermass (see, e.g., Liu & Tanhua, Ocean Sciences, 2021).
Liu, M. and Tanhua, T., 2021. Water masses in the Atlantic Ocean: characteristics and distributions. Ocean Science, 17(2), pp.463-486.

L93. Simplify: “Although there are few deconvolved benthic…”

L148. meters

L153-163. If this wasn’t a revision and I wasn’t providing a review for the first time, I’d recommend that the authors use a consistent age model approach (ProbStack) for all records. But, alas, I ‘m not sure that’s an entirely fair new comment at the revision stage, so I’ll defer to the authors if such consistency is necessary for their stacking approach.

L265-290. Solving for d18Osw. A question here concerns the alignment of data generated from different foraminifera species with different depth habitats. G. affinis can be found quite deep (up to 15 cm), whereas C. wuellerstorfi is thought to be epifaunal (e.g., Corliss, 1985). In the case where the authors are calculating d18Osw from d18O in C. wuellerstorfi and BWT derived from Mg/Ca in G. affinis, how sure are they that the two proxy measurements are in fact contemporaneous in time? One might expect that the C. wuellerstorfi d18O data lead the G. affinis data.

To address this, the authors could compare d18O from C. wuellerstorfi and G. affinis in the same depths; if they are comparable with known species offsets, then this depth habitat offset is probably negligible. They could also note the high sedimentation rate of the site (10 cm/kyr, Hodell et al. 2015) which would make any depth habitat difference minor in time. And the 3kyr interpolation (L305) probably makes this a moot point. But worth adding a sentence or two in the methods to note/refute this habitat depth issue.
Corliss, B.H., 1985. Microhabitats of benthic foraminifera within deep-sea sediments. Nature, 314(6010), pp.435-438.

L378. For ease of comparison, please express the equivalent amounts of cooling at U1385 over the two time intervals.

L383-384. Agreed.

L385-386. Please delete this. The comparison to SST is not a point for results, but rather a point for the discussion further down when these data are presented in Fig. 9 and L468-470.

Figures 3-7 & throughout. It would really help the reader if all records from one region maintained a consistent color family throughout these figures. For instance, Site 1123 shifts from light blue (Fig. 3, 5) to dark blue (Fig. 4, 6) to pink/red/purple in the stacks in Fig. 7. Why not keep Atlantic red-scale and Pacific blue-scale, or vice versa?

L416-417. How are you evaluating that Atlantic d18Osw increased across the MPT? I cannot see any notable change.

L424-425. It would be worth reminding the reader if this is the stack in question here is volumetrically weighted.

L474. Missing period at end of sentence

L474-478. Suggest reversing the order of these sentences, with the eNd interpretive caveat following the eNd data sentence, but before the d13C gradient sentence.

L479-480. Worth stating the assumption that the salinity of NCW was maintained.

L522-540. The authors’ might be able to make their lives much simpler here if they reframe this discussion in terms of how the processes they discuss impact the global efficiency of the biological pump (e.g., Sigman et al., 2010). The relevant processes needed to sequester atmospheric CO2 in the deep ocean require that a greater fraction of the nutrients in the ocean interior are derived from the remineralization of sinking organic matter (“regenerated”) instead of advected via deep water formation (“preformed”). In ocean models, any increase in the regenerated/preformed nutrient content of the deep ocean has been shown to sequester atmospheric CO2 (Marinov et al. 2006; 2008). This could happen via (1) a decrease in the rate of upwelling (“stratification”) and/or (2) an increase in the consumption of major nutrients.
Marinov, I., Gnanadesikan, A., Toggweiler, J.R. and Sarmiento, J.L., 2006. The southern ocean biogeochemical divide. Nature, 441(7096), pp.964-967.
Marinov, I., Gnanadesikan, A., Sarmiento, J.L., Toggweiler, J.R., Follows, M. and Mignone, B.K., 2008. Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO2. Global Biogeochemical Cycles, 22(3).

L542. it is

L545-546. I agree, but nonetheless recommend deleting. We should not assume that an unpublished record is going to solve a current problem. What if the gas record is compromised by diffusion or respiration, or if the samples are lost?

L561. Change “older than” to “before”, and “after which” to “but after 1200 ka the amplitude…”

L579. Typo: Site 607

L584-585. I’m starting to wonder if the authors were paid to advertise the Beyond EPICA project! See point above. It would be sufficient to say “higher resolution MOT data are needed to capture the full amplitude of glacial-interglacial variations”.

L608-609. What does it mean for Site 1208 to have “anomalously high temperatures during MIS 22”? What is the point for comparison?

L611. Specify who “They” refers to, as the previous sentence switched orientation from the authors’ results, to Clark et al.’s results, back to the authors’ results.

L639-641. Wouldn’t it be more direct to look for evidence of dissolution in the tests used, e.g. Poirier et al. (2021)? B/Ca would tell you the carbonate ion saturation of the overlying water column, which if anything could be buffered by dissolution.
Poirier, R.K., Gaetano, M.Q., Acevedo, K., Schaller, M.F., Raymo, M.E. and Kozdon, R., 2021. Quantifying diagenesis, contributing factors, and resulting isotopic bias in benthic foraminifera using the foraminiferal preservation index: Implications for geochemical proxy records. Paleoceanography and Paleoclimatology, 36(5), p.e2020PA004110.

L642. Delete space between sigma and end-parenthesis.

L655. Reference typo: Shackleton and Opdyke, 1973.

L655. The MPT is thought to involve a fundamental change…

L658. Suggest, or show?

L669-671. Now here is fine to include Beyond EPICA.

Hide

ED: Publish subject to minor revisions (review by editor) (03 Jul 2026) by Lorraine Lisiecki

AR by Nicola Thomas on behalf of the Authors (20 Jul 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 Jul 2026) by Lorraine Lisiecki

AR by Nicola Thomas on behalf of the Authors (04 Aug 2026) Manuscript

Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2025-4566', Peter U. Clark, 22 Oct 2025

It would be helpful to add uncertainties to all time series shown in Figure 12.

Citation: https://doi.org/10.5194/egusphere-2025-4566-CC1
RC1:
'Review', Anonymous Referee #1, 27 Oct 2025

This paper measures and analyses Mg/Ca-based deep ocean temperature (DOT) and d18O_benthic and the previously obtained DOT to determine d18O_seawater of the Iberian Margin site U1385 across the last 1.5 Ma covering the Mid-Pleistocene Transition (MPT) and sets results into context of others by (a) making basin- and global-wide averages using 4 other sites, and (b) discussing especially difference to the recent finding of Clark et al. (2025) on mean ocean temperature (MOT) and the deconvolution of d18O_benthic into temperature and seawater components. It concludes that while there is hardly any change in DOT across the MPT there is a drop in d18O_seawater around 900 ka, rather the opposite of what is proposed in Clark et al. (2025).
I cannot judge the methods, so my comments will solely be on the interpretation.
I think this is a solid piece of work on a new data set across the MPT. The discrepancy with the Clark et al interpretation somehow illustrates the state of knowledge that we are facing right now. So, more details here on what is compared and why things differ would help the reader to set things better into context. See my comments below for improvements:
Major issues:
- The used equations for calibration (Eq. 2-4) come with uncertainties, which are completely ignored here. For example, in Weldeab, Arce and Kasten (2016) the full equation is given as Mg/Ca [mmol/mol] = (0.36 ± 0.02) ∗ BWT [°C] + (2.22 ± 0.19). Furthermore, both the Mg/Ca-T calibrations are rather scatted in the temperature range used here (-1 to +6°C). Therefore I suggest to add discussions how these ignored uncertainties impact the results, at best recalculate propagated errors using uncertainties on all the parameters that go into the calibration curves. The 3 equations 2-4 should also be reworked to properly state units (so far it is only mentioned, that T comes as °C).
- Several times Raymo et al. (2006) is discussed as an probably cause for the deduced salinity changes. However, the global SST compilation in Clark et al (2024) had no power in precession which is the dominant frequency in the Raymo et al (2006) hypothesis. This is discussed in part 3 of the Clark et al papers, just published in Science (doi: 10.1126/science.adv8389). I therefore think this hypothesis can be drop, or at least add this discussion point made in Clark et al to this idea.
- In the comparison to Clark et al (Figure 12) it is important to plot both, the own and the Clark data with uncertainties. Only then can we see if there is overlap or complete disagreement in certain periods. I actually see at least two main differences in T or both reconstructions. First, the drop in T across the MPT in Clark, missing here (discussed by the authors). Second, the much larger G/IG amplitudes in T after the MPT in the Clak data (not discussed). Thus, both T data sets never show similar things. For the part after the MPT Clark et al. used a ratio of GMSST:MOT=1.1, thus no information from deep ocean T are used which could explain the offset between both T. Furthermore, the d18O_sw from Clark contain the removal of a long-term increase of 0.083 permil/My in d18O_benthic, which is not included in d18O_sw of this study. Although this effect is small (1.5 Myr + 0.083 = 0.12 permil) it needs to be mentioned to understand the comparison.
- How can we learn even more from the comparison to the Clark et al. paper? Your stacked d18O_benthic compares very good to the Prob-stack d18O_benthic for the last 400 kyr, but differs somehow before (Fig. 9a). Maybe this illustrates, that for the last 400 kyr your stack is representative of the global signal, but not so much before. Maybe calculate the difference between both d18O_benthic time series to use as a proxy of global representativeness. If you plot this difference then together with the T and d18O_sw comparison to Clark (Figure 12), the reader would see, how much your stack are (or are not) global signals. Maybe this difference can also be exploided as a correction for d18O_sw and/or T, but I am not sure it will work or if it is a good idea. Furthermore, Fig 12 needs different y-axis labels. Your stack of d18O_sw is NOT a global stack (while d18O_sw of Clark is), it is a stack of globally distributed cores from waters below 2km. Actually, maybe at best you should not compare to the MOT from Clark but to the time series of T changes below 2km (Fig 16d in Clark et al., 2025, data should be available from the authors), which uses GMSST and a change in heat storage efficiency (=Delta MOT / Delta GMSST). This has large uncertainties 1.5-0.9 Myr, but basically allows(more or less) a long-term flat line across the MPT, similarly as here (no trend in temperatures).
- So all together, (a) considering if a long-term trend in d18O_benthic is included in plotted data or not, (b) which kind a T curves are compared, (c) highlighting that your approach is a least partially not representing global signals, (d) having some more emphasis on (missed) uncertainties, both your data and the Clark data seem to be not so much in disagreement as so far discussed.

Minor Details:
- Not all SI figures are cited in the main text. Either cite (and order the SI figures according to citation) them or delete them.
- Maybe I missed it, but why does Fig. 2 only show the time 700-1500 ka. Is this the only new part and 0-700 ka was already published elsewhere? Details can probably be extend in section 2.
- Line 435 or so: I believe Pacific bottom water temperature being near-freezing throughout the MPT and thereafter has first been shown in Siddall et al. (2010: doi:10.1016/j.quascirev.2009.05.011).
- Line 668: Raymo et al (2006) was published in Science, not Nature.

Citation: https://doi.org/10.5194/egusphere-2025-4566-RC1
- AC2: 'Reply on RC1', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC2
RC2:
'Short addition to my review', Anonymous Referee #1, 28 Oct 2025

In a paper published yesterday in this journal

Larsson, V. and Jung, S.: Persistent contamination in benthic-foraminifera-based Mg ∕ Ca thermometry using standard cleaning methods, Clim. Past, 21, 1871–1894, https://doi.org/10.5194/cp-21-1871-2025, 2025.

a different calibration for Uvigerina peregrina than the one used here (their Fig. 12) and some contamination issues are discussed. Please add some thoughts, if and how their findings are relevant for your study here.

Citation: https://doi.org/10.5194/egusphere-2025-4566-RC2
- AC3: 'Reply on RC2', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC3
RC3:
'Comment on egusphere-2025-4566', Anonymous Referee #2, 25 Nov 2025
Thomas et al. present combined benthic foraminiferal Mg/Ca and oxygen isotope data from IODP Site U1385 at the Iberian Margin to reconstruct δ¹⁸O_sw (seawater oxygen isotopes) covering the past 1.5 Myrs. The record is compiled with similar datasets from the Atlantic and Pacific sectors of the Southern Ocean as well as from the North Pacific, allowing the authors to produce a deep-ocean δ¹⁸O_sw and temperature stack that reflects global changes. The study uses a (to my knowledge) novel approach by applying a volume-weighted averaging scheme to calculate the global δ¹⁸O_swstack, that accounts for the relative sizes of the major ocean basins. This method is contrasted with previous approaches that did not include ocean volume-based corrections, providing new insights into the interpretation of global signals. The main findings of the study indicate increased salinity in the deep Pacific and reduced deep-water temperatures in the Atlantic during glacial periods after 900 ka, compared to the interval between 1.5 and 0.9 Ma. The authors interpret these results as evidence for enhanced glacial water mass stratification in the Pacific after 0.9 Ma, leading to a greater sequestration of atmospheric carbon dioxide in the deep ocean, as well as increased salinity in surface waters within regions of southern component water formation around the Antarctic margins.
The study draws on an extensive dataset, and the methodology appears sound and appropriate to the study’s objectives. The resulting dataset has the potential to make a valuable contribution to our understanding of global ocean temperature, salinity and ice volume changes across the Mid-Pleistocene Transition (MPT). However, the strength of the methodological and analytical work is not matched by the clarity and depth of the data presentation and interpretation. In its current form, the manuscript does not fully exploit the scientific potential of the dataset, and several key aspects of the analysis and discussion require revision and elaboration. Below I outline my major and minor points of criticism in the hope that the authors find them useful in improving and revising their manuscript.
In the following, I will first outline several broad, major concerns that affect the overall interpretation and structure of the manuscript. I will then provide detailed, section-by-section comments and suggestions aimed at improving the scientific robustness and overall comprehensiveness of the manuscript.
The manuscript warrants more transparency in the many nuances of the interpretation and should clearly outline the limitation of the data interpretation. For instance, the authors base their conclusion of an enhanced density stratification throughout the entire deep Pacific after 900 ka (compared to 1.5–0.9 Ma) primarily on data from only one Site in the South Pacific and one in the North Pacific. This interpretation is presented as a definitive conclusion, rather than as a working hypothesis supported by limited regional evidence. Potential variations in salinity throughout the entire water column—and their corresponding effects on vertical stratification—are not discussed, and possible regional hydrographic differences are insufficiently addressed in my opinion.

Throughout the manuscript, several sections lack a clear explanation and important arguments are presented in a way that they appear somewhat ambiguous. This occurs in multiple places and affects the overall clarity of the manuscript (e.g. comments on lines 42, 127, 172-177, 191, section 4.1). I recommend using more precise wording in many sections to avoid confusion.

In the abstract and discussion, the authors attribute the observed increase in deep Pacific salinity after 900 ka to changes in the source regions of Southern Component Waters (SCW), i.e., Antarctic margin regions (e.g., the Weddell and Ross Seas). However, this interpretation (i.e. a key finding in the abstract) is not substantiated by any evidence presented in the study. In my view, the manuscript provides insufficent discussion of existing literature and data that would support such large-scale changes in SCW source regions during this interval. Revision is needed here. Moreover, as PDW is a mixture of several water masses, the possibility of temporal changes in PDW end-member composition—and their influence on reconstructed salinity—should be addressed, to support the suggested changes in SCW across the MPT. The authors could consider potential variations in inter-basin connectivity within the Southern Ocean, e.g. using the Antarctic Circumpolar Current strength reconstruction of Lamy et al. (2024), which could help assess whether large-scale circulation changes—rather than solely local shifts in AABW formation—may contribute to the observed Atlantic–Pacific differences across the MPT.

Throughout the manuscript, statistical analyses are insufficiently applied when comparing different datasets and time intervals. Statements referring to “significant” differences are frequently made without presenting the results of any formal statistical tests. To substantiate these claims, appropriate statistical analyses (e.g., t-tests, ANOVA) should be conducted and reported. Including quantitative measures of variability and statistical significance would greatly strengthen the robustness of the interpretations and allow readers to assess whether observed differences between regions or time periods are statistically significant.

In my opinion, the introduction lacks a clear structure and does not fully establish the central research question. Integrating relevant key studies (e.g. Quin et al., 2022)—many of which are discussed in the discussion (e.g. Hines et al., 2024; Pena and Goldstein 2014, Farmer et al., 2019; Lear et al., 2016) —more prominently at the start would help frame the topic, clarify existing knowledge gaps, and better motivate the study within its scientific context. Moreover, a concise overview of the main hypotheses proposed to explain the MPT—including e.g. the regolith hypothesis (e.g. Clark and Pollard, 1998, Clark et al., 2006), the expansion of North American and/or East Antarctic ice sheets (e.g. An et al., 2024; Bintanja and van de Wal, 2008; Raymo et al., 2006), or feedback mechanisms involving changes of the marine carbon cycle (e.g. Chalk et al., 2017; Willeit et al., 2019)—would provide important context and more clearly situate the study within the broader landscape of MPT research.

Overall, the figures would benefit from improved clarity and more explicit information to help the reader follow the comparisons and interpretations of the many timeseries presented. It would be helpful to include the location of each record directly in the figure, for instance by labelling the regions (e.g., Pacific, Southern Ocean, Atlantic…) in the figures. For the Iberian Margin record, it should be clearly indicated which data were generated in this study and which were compiled from previous publications (different symbols or colours). Finally, it would be beneficial to show key time intervals discussed in the manuscript at higher temporal resolution, rather than referring exclusively to the long composite records. This would allow for a clearer evaluation of the main features and transitions emphasised in the text. If feasible, reducing the overall number of figures may improve the readability of the manuscript, as the large number of time series and figures can be somewhat overwhelming for the reader.

Salinity on the Practical Salinity Scale (PSS-78) is a dimensionless quantity, and the use of the unit “PSU” (Practical Salinity Unit) is therefore incorrect. The authors should avoid using “PSU” throughout the manuscript. Instead, it should be stated upon first mention that all salinity values are reported on the Practical Salinity Scale (PSS-78) and are thus unitless. It would be preferable to adopt the TEOS-10 (Thermodynamic Equation of Seawater, since 2009) scale and thus report Absolute Salinity in g/kg. However, given that global datasets such as GLODAP and WOA report salinity on the PSS-78 scale, calculated from conductivity, temperature and pressure, clarification in the text that values are dimensionless and follow PSS-78 are sufficient.

Detailed comments:
Line 13, 14: The study would benefit from more cautious wording, as it presents no direct evidence for changes in the carbon storage capacity.
Line 23: One of the main conclusions of the study concerns a salinity change across the entire deep Pacific after 900 ka based on two Sites. However, the text states that δ¹⁸O_sw only reflects variations in global ice volume and local hydrographic effects. This phrasing is somewhat ambiguous and should be clarified.
Line 32: This statement is too vague. Since this appears to be one of the main motivations of the present study, the divergent interpretations should be clearly introduced in the Introduction.
Line 40-43: Please specify what specific concerns are being referred to here and clarify whether “previous findings” refers to Yu & Broecker (2010) or Sosdian & Rosenthal (2009).
Table 1: Please use a consistent coordinate format. The authors should also cite the sources or datasets from which present-day temperature and salinity data were obtained.
Fig. 1: Please indicate in panel (a) where the transects shown in panels (b) and (c) were taken. Given potential differences between the eastern and western Atlantic, this spatial context is important. It would also be useful to include a single transect (covering the Pacific, Southern Ocean, and Atlantic) directly in the main manuscript rather than in the Supplement, as temperature gradients between these basins are a key aspect of the study.
Fig. 3: Please use a consistent capitalisation for “Marine Isotope Stage” throughout the manuscript (either always “Marine isotope stage” as in line 81 or “Marine Isotope Stage,” as in line 103). In addition, clarify the basis of the 2σ error bars mentioned in lines 82-83; are these derived from replicate measurements or represent analytical uncertainty or both?
Line 89 and 92: The phrase “and have similar seawater physical properties” is somewhat vague. Please specify which properties are referred to (e.g., temperature, salinity, density) or include a reference to Table 1 for clarification.
Line 107-109: Do all studies use the same approach for their age model?
Line 122-123: To my knowledge the MD01-2444 record spans ~50–35 ka, which does not include the LGM.
Line 123-124: Please Specify whether this offset is based on overlapping intervals between the cores or on modern temperature differences (which are not listed in Table 1).
Line 127: I cannot fully understand this sentence. If the text refers to a specific taxonomic level, please use the term “species” to avoid confusion.
Line 128: Please clarify how comparable the results are between the two genera when both were measured.
Line 143-144: Please specify which error the authors are referring to. No corresponding uncertainty is shown in Fig. 2a, so it is unclear what uncertainty estimate is referred to or how it was derived.
Line 146: It seems likely that the intended method was inductively coupled plasma–optical emission spectrometry (ICP OES), rather than "spectrophotometry".
Line 166: I think the intention was to refer to the excluded samples when using the term “the remaining samples.” However, lines 166–169 read as if this refers to the samples actually used in the dataset. Please replace “remaining” with “excluded” to avoid confusion.
Line 168: Please remove the “<” before 0.03 mol/mol⁻¹ or instead use “< 0.1 mol/mol⁻¹,” as Barker et al. (2003) only rejected samples with Fe/Mg < 0.1 mol/mol⁻¹, whereas the 0.03 mol/mol⁻¹ value cited here represents the typical range of their samples rather than an exclusion criterion.
Line 171: units are missing.
Line 172-177: The reasoning in this paragraph is difficult to follow. The use of “rather” and “more likely” is confusing. "Ferromanganese oxide overgrowths" and "authigenic Mn–Fe-oxide coatings" refer to the same type of contamination.
Line 174: What is meant by elevated? Please quantify.
Line 175: Please rephrase "this contamination" as not all Mn in foraminiferal calcite necessarily indicate contamination. It could also reflect (partly) natural incorporation of Mn during biomineralisation.
Line 191: This phrasing is ambiguous. Do the authors mean a 0.32 mmol/mol correction for Mg/Mn_coating within formula 1, or a constant correction for all foraminiferal Mg/Ca data?
Equation 2: How do the core-top Mg/Ca-derived temperatures compare with modern and/or pre-industrial deep-water temperatures at the study site? Please also justify the use of the Elderfield et al. (2012) calibration (originally introduced in Elderfield et al., 2010). The Elderfield et al., 2010 calibrations lower end is ~1°C. Roberts et al. (2016) specifically adjusted the Elderfield calibration for temperatures below 0 °C.
Line 225: Is this uncertainty based on replicate measurements of the same standard material, or of multiple aliquots of the same sample?
Figure 4: Is there a reason why some interglacials and glacials are missing their labels in panel (a)? For example, MIS 13, 14, 17, and 18 are not indicated. Please clarify or add the missing MIS numbers for consistency.
Line 283: Please ensure consistent use of time units throughout the manuscript. Earlier, “Ma/ka” was used for specific ages or time periods in the past, whereas “Myr/kyr” was used for durations. Here, “1.5 Myrs” is inconsistent with that convention. The same issue appears in line 285. Please standardise the terminology.
Line 284 - 285: This statementis difficult to follow. Is the observed cooling occurring during interglacials, glacials, or consistently across both climate states? Are uncertainties associated with this trend quantified? How are these numbers calculated?
Line 313-316:The comparison is difficult to follow, as it mixes different quantities: interbasin glacial–interglacial temperature differences for 1.5–0.9 Ma with glacial-only temperatures before and after 0.9 Ma. This makes the intended conclusion unclear. Please clarify the rationale for comparing these metrics and include uncertainties.
Line 319-321: If the authors already write “We interpret the greater increase …,” this should strictly not appear in the Results section.
Figure 9: The purpose of the dashed lines in Figure 9 is unclear. In panel 9b, it appears that the upper line might represent the mean interglacial δ¹⁸Oₛ_w across all interglacials and the lower line the maximum values—but this is not explained. Were these lines calculated from the data, or were they arbitrarily chosen? Similarly, in panel 9c, it is not evident how the dashed line is intended to represent “constant deep-ocean temperatures (line 343).” It appears to be placed at ~0 °C rather than reflecting a calculated mean. If this line is meant to represent an average, please show the associated variability (e.g., 2 SD).
Line 351-353: Please add numbers for the Temperature decrease
Line 357-360: There may also be an important interbasin difference between the western and eastern North Atlantic, which challenges the assumption that Iberian Margin records can be treated as representative of the entire North Atlantic basin (e.g., Chalk et al., 2019). This aspect should be discussed, especially in relation to the statement in lines 287–290 that the carbonate ion effect cannot fully explain the temperature differences between Sites 607 and U1385. If the carbonate ion effect is not the sole driver of interbasin- and species Mg/Ca temperature offsets, the authors should explicitly consider and discuss alternative explanations, including east–west basin-scale hydrographic differences, which are currently not addressed anywhere in the manuscript.
Section 4.1: I would appreciate it if this entire section were given more structure. It is difficult to follow the connections between the different hypotheses and literature references and to identify the main message of the section. Throughout the paragraph, several mechanisms are mentioned (e.g., standing volume effect, NADW cooling, SCW salinity increase, ice sheet/shelf melting, and a stratification inversion in the North Atlantic), but they are not sufficiently integrated into a coherent narrative or overarching context in my opinion. For example, it is not clear to me whether the authors are arguing that the North Atlantic was bathed in NCW, in SCW, or favouring some other scenario.
Line 400: Please remove strong, as it is relative.
Figure 11c: Please ensure that your own data points are clearly visible. They are currently obscured by the Grimmer et al. (2025) dataset, which prevents a proper comparison between the two records.
Line 425: The period mentioned (2.75–2.15 Ma, according to the time frame defined in line 38) is not covered by the dataset. Please rephrase.
Line 427-429: Please clarify which MDOT dataset you are referring to and indicate the corresponding figure.
Line 431: See comment on line 400
Line 431-433: I would strongly recommend being more cautious with the argument that the records from Sites 607 and U1313 are complicated by multi-species measurements, since the record presented in this study (U1385) is also a derived from three different species.
Figure 12: Please indicate uncertainties
Line 453-456: Are there any similar approaches applied to more recent, data-rich intervals? A comparison would help assess uncertainties.
Conclusion: The conclusion includes a substantial amount of important and relevant literature. I would appreciate it if these key studies were introduced in more detail in the Introduction, allowing the conclusion to focus more clearly on the study’s own findings.
Line 465-467: Most of the studies cited in this context examine glacial–interglacial changes—often limited to the last glacial cycle—and do not address variability across the MPT. It is therefore unclear how these references support the conclusions drawn here or how they relate to the long-term patterns discussed in the study.
References cited in this review that do not appear in the manuscript:
Roberts et al.: Evolution of South Atlantic density and chemical stratification across the last deglaciation, Proc. Natl. Acad. Sci. U.S.A., 113 (3) 514-519, https://doi.org/10.1073/pnas.1511252113 (2016).
B. Chalk et al.: Dynamic storage of glacial CO₂ in the Atlantic Ocean revealed by boron [CO₂^3-] and pH records, Earth Planet. Sc. Lett., 510, 1–11, https://doi.org/10.1016/j.epsl.2018.12.022, (2019).
Lamy et al.: Five million years of Antarctic Circumpolar Current strength variability. Nature, 627, 789–796, https://doi.org/10.1038/s41586-024-07143-3, (2024).
An et al.: Mid-Pleistocene climate transition triggered by Antarctic Ice Sheet growth. Science, 385, 560-565, https://doi.org/10.1126/science.abn4861, (2024).
U. Clark & D. Pollard: Origin of the middle Pleistocene transition by ice sheet erosion of regolith. Paleoceanography, 13, 1–9. https://doi.org/10.1029/97PA02660, (1998).
Willeit et al.: Mid-Pleistocene transition in glacial cycles explained by declining CO₂ and regolith removal. Sci. Adv., 5, 7337, https://doi.org/10.1126/sciadv.aav7337, (2019).
B. Qin et al.: Sustained Deep Pacific Carbon Storage After the Mid‐Pleistocene Transition Linked to Enhanced Southern Ocean Stratification, Geophys. Res. Lett., 49, (4), 1944-8007, https://doi.org/10.1029/2021GL097121, (2022).
Citation: https://doi.org/10.5194/egusphere-2025-4566-RC3
- AC1: 'Reply on RC3', Nicola Thomas, 26 Jan 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4566/egusphere-2025-4566-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-4566-AC1

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19 Aug 2026

Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Clim. Past, 22, 1559–1584, https://doi.org/10.5194/cp-22-1559-2026,https://doi.org/10.5194/cp-22-1559-2026, 2026

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Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

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Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

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We reconstruct interbasinal temperature and salinity gradients using stacked Mg/Ca and benthic δ¹⁸O records for the past 1.5 Myr. Across the Middle Pleistocene Transition, the deep Atlantic cooled and the Pacific became more saline, increasing deep ocean density stratification. The glacial ocean became a more effective carbon trap, which lowered atmospheric pCO₂, and led to the growth of larger ice sheets. Results support a physical role for abyssal ocean stratification in explaining the MPT.


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Increased abyssal ocean density stratification across the Middle Pleistocene Transition

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