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, although the global surface temperature difference compared to MIS 7e is within uncertainty of the reconstruction (around 0.4 ± 0.4 °C warmer). MIS 7c is a period when atmospheric CO2 concentrations are 30 ppm lower than during MIS 7e. This result illustrates a transient decoupling between radiative forcing and the global surface temperature response during periods of CO2 overshoot, after which global temperature re-equilibrate with CO2 concentrations. 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 likely caused by 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.