Preprints
https://doi.org/10.5194/egusphere-2025-4566
https://doi.org/10.5194/egusphere-2025-4566
01 Oct 2025
 | 01 Oct 2025
Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Abstract. We report basinal and global compilations of deep-water temperature and δ18Oseawater for the past 1.5 million years using tandem oxygen isotopic and Mg/Ca measurements of benthic foraminifera. Across the Middle Pleistocene Transition (MPT), interbasinal gradients suggest North Atlantic deep-water became colder and Pacific deep-water saltier during glacial periods after ~900 thousand years ago. Salinity in source areas increased in the marginal seas around Antarctica by decreased meltwater discharge from ice sheets and increased sea ice extent, which led to increased density stratification of the abyssal ocean. The deep ocean became a more effective carbon trap and lowered glacial atmospheric carbon dioxide, leading to expansion of continental ice sheets and longer glacial cycles. Results support a physical role for abyssal ocean stratification in explaining the MPT. Collectively, our deep ocean stacks lend support to hypotheses proposing that the MPT resulted from a progressive drawdown in glacial atmospheric pCO2, a conclusion that awaits verification from the Beyond EPICA–Oldest Ice core from Antarctica.

Competing interests: At least one of the authors is a member of the editorial board of Climate of the Past.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

Status: open (until 26 Nov 2025)

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Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell
Nicola C. Thomas, Heather L. Ford, Mervyn Greaves, and David A. Hodell

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Short summary
We reconstruct interbasinal temperature and salinity gradients using stacked Mg/Ca and benthic δ¹⁸O records for the past 1.5 Myr. Across the Middle Pleistocene Transition, the deep Atlantic cooled and the Pacific became more saline, increasing deep ocean density stratification. The glacial ocean became a more effective carbon trap, which lowered atmospheric pCO2, and led to the growth of larger ice sheets. Results support a physical role for abyssal ocean stratification in explaining the MPT.
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