| 28 Nov 2025
The Marine Isotopic Stage 7: a relic of the "41-ka world"? Perspectives from a global-scale sea-surface temperature synthesis
Abstract. The Marine Isotope Stage 7 (MIS 7, ~ 245–190 ka) displays an unusual morphology compared to the other interglacials of the late Pleistocene. It comprises two major warm periods (MIS 7e and MIS 7c) each preceded by multi-millennial-scale warming intervals (Termination III (TIII) and TIIIa, respectively) and separated by a brief return to glacial conditions (MIS 7d). When considered as two distinct warm phases, MIS 7 has been compared to the 41-ka obliquity-driven climate cycles of the pre-mid-Pleistocene transition (MPT) world. However, a coherent spatio-temporal picture of MIS 7 surface temperature remains lacking to enable a comprehensive comparison with other interglacials. Here we compiled 132 high-resolution (better than 4 ka) sea surface temperature (SST) records derived from 85 marine sites over the time interval 260–190 ka. In order to provide a spatio-temporal comparison of these records, we (i) align them on a common temporal framework relying on the AICC2023 reference ice core chronology and (ii) recompute SSTs using a homogenized proxy-calibration, both steps applying Bayesian and Monte Carlo approaches to quantify the attached uncertainty. Finally, we produce global and regional stacks of SST anomalies relative to the pre-industrial covering TIII and the following MIS 7.
Our results evidence that global mean surface temperature remains below pre-industrial (PI) values over both MIS 7e (-1.4 ± 0.3 °C) and MIS 7c (-1.0 ± 0.3 °C) periods. The warmest phase across MIS 7 occurs during the MIS 7c substage, a period when atmospheric CO2 concentrations are 30 ppm lower than during MIS 7e, highlighting a decoupling between radiative forcing and the global surface temperature response. In addition, TIII exhibits a greater warming amplitude than TIIIa, both globally and regionally. The spatial and temporal dynamics of the two terminations differ markedly. TIII follows a "classic" sequential deglaciation pattern, with an early warming initiated in the Southern Hemisphere, which then gradually propagates toward the Northern Hemisphere. In contrast, TIIIa displays near-synchronous warming across all latitudes, lacking the interhemispheric pattern typical of classical terminations. This suggests that TIIIa is not a standard glacial termination, but rather a distinct climatic transition. Supporting this, correlation analyses between orbital parameters and regional SST stacks evidence the role of obliquity in shaping MIS 7 temperature records, likely due to the most extreme obliquity values of the Pleistocene occurring over this period. We therefore propose that TIIIa is the result of a self-sustained climatic oscillation that temporarily re-synchronised to the 41-ka cycles because of an exceptional orbital context. As a result, MIS 7 represents a hybrid interglacial, embedded within the post-MPT 100-ka framework, yet shaped by obliquity-driven forcing such as during the early Pleistocene.
“The Marine Isotope Stage 7: A relic of the 41-ka world?” by Legrain et al provides a valuable compilation and summary of sea surface temperature (SST) spatio-temporal variability during MIS 7, which after revision would make an excellent contribution to Climate of the Past. The comparison with the Clark et al (2024) SST compilation is particularly interesting. Below are some details I would like to see clarified in the manuscript before publication.
Major comments:
Methods:
Line 151: The LR04 benthic d18O curve is affected both by ice volume and deep water temperature. Due to a lag between temperature and ice volume, benthic d18O is not synchronous with ice volume change. The Spratt & Lisiecki (2016) sea level stack or the new Clark et al sea level reconstruction would be more suitable to use for ice-volume correction.
Lines 241-245: Similarity between the results of “selected” and “all” records is not a particularly good argument in favor of using the selected records compilations because the set of all records is acknowledged to be impacted by bias, particularly from the fact that many planktonic d18O records may be impacted by salinity changes. The compilation that excludes all planktonic d18O records produces a noticeably sharper and warmer peak for MIS 7e, especially in the SH stack. Is there anything particular about the spatial distribution of records that have been removed that makes you consider the d18O_p-excluded compilation inaccurate? Would using the smaller compilation alter any of your main conclusions?
Chronologies:
To what extent is the relative timing of NH SST change and SH SST change set directly by the Greenland and Antarctic alignment targets? To help readers evaluate this, please add the GLT_syn time series to right panel of Fig. 1 and summarize the assumptions inherent in its construction. Are similar regional shifts in SST timing observed when SST and benthic d18O are compared in individual cores? (The timing of benthic d18O changes can vary by 2-4 kyr during terminations also, which would be useful to also mention.)
Are cores from the North Pacific aligned to the synthetic Greenland record? If so, the authors should discuss the degree to which North Pacific SST is expected to be synchronous with Greenland air temperature. For example, do they vary synchronously with one another during T1 and the Holocene?
Fig. S4 – Alignments to core U1429: I have concerns that U1429 is used as an alignment target for other cores given that it has a large gap immediately before (during?) TIII. Could the authors align those cores to a different core? Also, Fig S4 suggests that the age model for U1429 is based on alignment to Sanbao, which isn’t mentioned as an age model target in the main text.
Interpretation:
Line 446: Is the time resolution of the data sufficient to evaluate the SST response to a 2-kyr CO2 overshoot? Low temporal resolution in many records and the smoothing effect of bioturbation may prevent you from observing a short-lived warming response to the CO2 overshoot.
Lines 580-585: The manuscript discusses whether the mode of glacial climate dynamics shifted during MIS 7 due to the particularly strong obliquity forcing at the time. One way to evaluate whether the double peak in MIS 7 actually implies a change in dynamics is to analyze the outputs of simple models that fit the pattern of glacial cycles using a single set of equations and parameter values for the entire late Pleistocene. For example, is a double peak for MIS 7 but not other glacial cycles predicted by the Parrenin & Paillard (2012) model or others? It might also be useful to compare the performance of models that are forced by 65N summer insolation with those that separately optimize sensitivity to precession and obliquity orbital parameters.
Minor comments:
Line 206: Rather than “inverse” (which often means 1/x), it would be better to say “multiplied by -1”
Line 256: awkward phrase – “display a very pronounced regional variability of the MIS 7”
Line 258: I think “low amplitude” would be clearer than “weakly amplified”
Line 307-308: “North of 23N” or “23-90N” would be clearer than “North to 23N” (same for south)
Line 309: This should be “Southern Hemisphere stack”
Line 564-565: Missing words? “a global synthesis evidence warmer deep ocean temperature during MIS 7C, similar to what observed during Holocene”
Line 565: Insert a paragraph break before “Regarding termination mechanisms…”
Line 614-624: English usage needs revision