Mean ocean temperature change and decomposition of the benthic δ ¹⁸O record over the past 4.5 million years

We use a recent reconstruction of global mean sea surface temperature change relative to preindustrial (1GMSST) over the last 4.5 Myr together with independent proxy-based reconstructions of bottom water (1BWT) or deep-ocean (1DOT) temperatures to infer changes in mean ocean temperature (1MOT). Three independent lines of evidence show that the ratio of 1MOT /1GMSST, which is a measure of ocean heat storage efficiency (HSE), increased from ∼ 0.5 to ∼ 1 during the Middle Pleistocene Transition (MPT, 1.5–0.9 Ma), indicating an increase in ocean heat uptake (OHU) at this time. The first line of evidence comes from global climate models; the second from proxy-based reconstructions of 1BWT, 1MOT, and 1GMSST; and the third from decomposing a global mean benthic δ ¹⁸O stack (δ ¹⁸O _b) into its temperature (δ ¹⁸O _T) and seawater (δ ¹⁸O _sw) components. Regarding the latter, we also find that further corrections in benthic δ ¹⁸O, probably due to some combination of a long-term diagenetic overprint and to the carbonate ion effect, are necessary to explain reconstructed Pliocene sea-level highstands inferred from δ ¹⁸O _sw. We develop a simple conceptual model that invokes an increase in OHU and HSE during the MPT in response to changes in deep-ocean circulation driven largely by surface forcing of the Southern Ocean. Our model accounts for heat uptake and temperature in the non-polar upper ocean (0–2000 m) that is mainly due to wind-driven ventilation, while changes in the deeper ocean (> 2000 m) 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 HSE. 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.