Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Abstract. The cause of the fundamental reorganisation of the climate system during the Middle Pleistocene Transition (MPT; ∼ 1.25–0.65 million years ago (Ma)), when glacial cycles intensified and lengthened from 41 thousand year (kyr) to quasi-100 kyr periodicity, remains one of the enduring unsolved questions in palaeoclimate science. Increasing attention has focused on the role of the marine carbon cycle, with enhanced carbon storage in the deep sea linked to changes in deep-water temperature and salinity that increase abyssal ocean density stratification. Here we report new high-resolution reconstructions of deep-water temperature and the isotopic composition of seawater (δ18Oseawater), a proxy for ice volume and salinity, from North Atlantic Site U1385, using paired benthic foraminiferal Mg/Ca and oxygen isotope (δ18O) analyses. Together with published records, we present basin-scale and globally distributed compilations of deep-water temperature and δ18Oseawater to assess changes in abyssal ocean stratification for the past 1.5 million years (Myr). Across the MPT, interbasinal gradients suggest North Atlantic deep-water became colder while Pacific deep-water became more saline during glacial periods after ∼ 930 thousand years ago (ka). Deep-ocean δ18Oseawater increased in both the Atlantic and Pacific across the MPT indicating increased continental ice volume. But, the increase was greater in the Pacific than the Atlantic, which we suggest reflects increased salinity of Southern Component Water (SCW). We propose the following sequence of changes during the MPT as a working hypothesis: (1) freshwater input to the marginal seas around Antarctica was reduced beginning at 930 ka by decreased melting of the Antarctic Ice Sheet and/or marine ice shelves and increased sea ice formation in the Southern Ocean; (2) the salinity and density of SCW increased, resulting in enhanced abyssal ocean density stratification and rendering the deep ocean a more effective carbon trap; (3) together with increased export production of organic matter and reduced deep-to-surface water exchange in the Southern Ocean, carbon storage in the deep ocean increased, lowering glacial atmospheric pCO2; (4) lower pCO2 permitted the growth of larger continental ice sheets, which reached a critical size and lengthened the glacial cycles. Our hypothesis supports an important role for abyssal ocean density stratification in the MPT, and requires further testing with additional benthic Mg/Ca-δ18O records, numerical model simulations, and forthcoming atmospheric pCO2 and mean ocean temperature results from the Beyond EPICA–Oldest Ice core from Antarctica.