the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2024
Mean ocean temperature change and decomposition of the benthic δ18O record over the last 4.5 Myr
Abstract. We use a recent compilation of global mean sea surface temperature changes (ΔGMSST) over the last 4.5 Myr together with independent proxy-based reconstructions of bottom water or deep ocean temperatures to infer changes in mean ocean temperature (ΔMOT). We find that the ratio of ΔMOT/ΔGMSST, which is also a measure of ocean heat storage efficiency, was around 0.5 before the Middle Pleistocene Transition (MPT, 1.5–0.9 Ma), but was 1 thereafter. This finding is also supported when using our ΔMOT to decompose a global mean benthic δ18O stack into its temperature and seawater components. However, further corrections in benthic δ18O, probably due to a long-term diagenetic overprint, are necessary to explain reconstructed Pliocene sea level highstands. Finally, we develop a theoretical understanding of why the ocean heat storage efficiency changed over the Plio-Pleistocene. According to our conceptual model, heat uptake and temperature in the non-polar upper ocean is mainly driven by wind, while changes in the deeper ocean in both polar and non-polar waters occur due to high-latitude deepwater formation. We propose that deepwater formation was substantially reduced prior to the MPT, effectively decreasing ΔMOT with respect to ΔGMSST. We attribute these changes in deepwater formation across the MPT to long-term cooling which caused a change starting ~1.5 Ma from a highly stratified Southern Ocean due to warm SSTs and reduced sea-ice extent to a Southern Ocean which, due to colder SSTs and increased sea-ice extent, had a greater vertical exchange of water masses.
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RC1: 'Comment on egusphere-2024-3010', Lorraine Lisiecki, 16 Nov 2024
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This manuscript is likely to be of great interest to the paleoceanography community because it makes significant progress in finding a self-consistent decomposition of global mean benthic d18O into temperature and seawater (ice volume) components in a way which is consistent with independent estimates of global mean sea surface temperature (GMSST) and sea level constraints. Overall, it is well written and well supported by evidence. However, the manuscript could be significantly improved with some additional clarification.
Major points:
1. The calculations of mean ocean temperature (MOT) change relies on a transition in the ocean heat storage efficiency (HSE) from ~0.5 before the MPT to ~1 after the MPT. While the need for such a transition is well justified by comparison with BWT measurements, the available proxy data before the MPT (particularly in the Pacific) are quite sparse with large scatter and uncertainties. Although the authors appropriately provide a large uncertainty estimate for HSE, they provide calculations for the decomposition of d18Osw using only one scenario, in which HSE changes linearly between 1.5-0.9 Ma. It would be enormously helpful for the interpretation of the d18Osw estimate if the authors also provided the d18Osw results of a few sensitivity tests in which the timing and amplitude of HSE change are varied within the range consistent with BWT estimates.
2. Similarly, the timing of the hypothesized diagenetic alteration of benthic d18O is not well constrained by proxy data. Although Raymo et al (2018) proposed a simple linear trend for this effect, one might alternatively hypothesize that the effect would covary with MOT or BWT change if the mechanism responsible for the effect is the cooling of BWT. Because the manuscript estimates that MOT cools most dramatically during the MPT, it would be informative to also show the results of a sensitivity test in which the rate of diagenesis is greater for d18O immediately preceding the MPT (keeping the same estimated total diagenetic contribution at 3 Ma).
3. These two sensitivity tests would be particularly helpful for interpreting the unexpected observation that smoothed d18Osw and glacial maxima d18Osw at ~1.5 Ma are similar to (or possibly more enriched than) the d18Osw of the Late Pleistocene. It’s important to clarify whether this finding is relatively robust to the specified timing and amplitude of HSE change and d18O diagenesis, neither of which is well constrained by the available proxy data.
4. An additional surprising result is the relative amplitudes of orbital-scale MOT variability and orbital-scale d18Osw variability in the pre-MPT time period. The pre-MPT MOT record contains very weak glacial-interglacial change compared to relatively large amplitude d18Osw changes from 2.6-1.5 Ma. The authors should add some discussion of the reliability of the amplitudes of the orbital-scale signal in GMSST change and MOT change. Are the resolution and age uncertainty of the SST records sufficient to accurately estimate orbital-scale changes in GMSST and, thus, its application to estimating orbital responses in MOT and d18Osw?
5. In Figure 13, the authors present a very interesting comparison of BWT and d18Osw estimates from two Pacific cores and their global compilation estimates. They make the compelling argument that the estimates from the two cores are unlikely to provide reliable global estimates because they imply that sea level would need to be ~50 m higher than PI for significant amounts of time between 1.4-1 Ma, suggesting that these sites may be affected by local salinity changes. Could the authors slightly expand upon this idea to explain how the locations of those Pacific cores could have significantly different bottom properties than the rest of the deep Pacific?
6. I really appreciated the section of the paper using model results to explore the mechanisms responsible for scaling between MOT and GMSST and why it might differ before the MPT. However, one question I have is about the authors’ apparent conclusion that AABW’s contribution to MOT was constant (and approximately equal to pre-industrial) from 4-1.5 Ma (Figure 16D). How can this be consistent with the PlioMIP2 findings that the deep Southern Ocean was 1.5-2.5 C warmer than pre-industrial and that increased stratification caused decreased AABW formation?
Minor points:
Line 546: The statement that 1123 records large ice sheets pre-MPT is unclear because most of the pre-MPT d18Osw record is significantly lighter than the post-MPT glacial values. I think the authors might be referring to one particularly large glacial maximum at ~1.5 Ma. Please clarify exactly what is referred to here and how it provides support for the new d18Osw record.
Lines 605-606: The same text is repeated on these two lines.
Lines 647-648: The meaning of this sentence isn’t clear. Ice sheets have enhanced the warming relative to what? How is this visible in Figure 14C?
Figure 1: Many of the individual records are partially/mostly hidden behind other data in this figure. Also, the caption suggests that there are two different orange lines in the figure, which seems like a problem.
Figure 10B: It’s very hard to see the light blue line (which is an important result to be able to see) due to overlap with the gray line. Maybe make the shade of blue darker or leave off the gray line.
Figure 16F: The caption doesn’t provide the color information for all the different records shown.
Citation: https://doi.org/10.5194/egusphere-2024-3010-RC1
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