the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Oct 2025
Increased abyssal ocean density stratification across the Middle Pleistocene Transition
Abstract. We report basinal and global compilations of deep-water temperature and δ18Oseawater 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.
Competing interests: At least one of the authors is a member of the editorial board of Climate of the Past.
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Status: open (until 08 Dec 2025)
- CC1: 'Comment on egusphere-2025-4566', Peter U. Clark, 22 Oct 2025 reply
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RC1: 'Review', Anonymous Referee #1, 27 Oct 2025
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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 -
RC2: 'Short addition to my review', Anonymous Referee #1, 28 Oct 2025
reply
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
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It would be helpful to add uncertainties to all time series shown in Figure 12.