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.