Deep Ocean Density Shift May Explain Ice Age Rhythm Change a Million Years Ago

Roughly one million years ago, Earth’s ice ages underwent one of the most dramatic transformations in the planet’s climatic history. For millions of years beforehand, glaciers had advanced and retreated on a metronomic 41,000-year rhythm set by the tilt of Earth’s axis. Then, during an interval scientists call the Middle Pleistocene Transition, the cycles lengthened to roughly 100,000 years and the ice sheets themselves grew far larger. Crucially, this happened without any corresponding change in Earth’s orbital geometry, which means the explanation must lie in processes internal to the climate system. A new study published in the journal Climate of the Past by Nicola Thomas of the University of Cambridge and colleagues now points to a surprising suspect: a fundamental reorganisation of the density structure of the deep ocean, driven by changes in temperature and salinity in the abyssal waters of the Atlantic and Pacific basins.

The team’s approach relied on a clever piece of geochemical detective work. Benthic foraminifera, single-celled organisms that live on the seafloor, build their calcium carbonate shells using the chemistry of the water around them. The oxygen isotope composition of these shells has long been the workhorse of palaeoclimate science, but it carries an awkward ambiguity: it records both the temperature of the deep water and the oxygen isotope composition of the seawater itself, which in turn reflects both global ice volume and local salinity. To untangle these signals, the researchers measured magnesium-to-calcium ratios in the shells alongside the oxygen isotopes, allowing them to calculate temperature independently and then subtract its influence to isolate the seawater signal.

The centrepiece of the study is a new 1.5-million-year record from IODP Site U1385 on the Iberian Margin off Portugal, a location famous among palaeoceanographers as the Shackleton Site for its exceptional sedimentation rates of roughly 11 centimetres per thousand years. The team analysed more than a thousand samples of two infaunal foraminiferal species, Uvigerina peregrina and Globobulimina affinis, at resolutions as fine as 500 years in key intervals. Rigorous quality control was essential: the researchers screened every sample for contamination from clays and from manganese-iron oxide coatings that form on shells buried in sediment, applying a correction based on the well-characterised magnesium-to-manganese ratio of such coatings to avoid biasing their temperature estimates.

With the new Atlantic data in hand, the team stacked their record with published reconstructions from the North Atlantic, the North and Southwest Pacific, and the Southern Ocean, interpolating everything to a common 3,000-year grid and weighting the basins by their true volumes of deep water. The result is one of the first globally representative pictures of how abyssal temperature and salinity evolved together across the transition. What emerged was a striking asymmetry. Before about 930,000 years ago, deep North Atlantic water was on average about 2.5 degrees Celsius warmer than the deep Pacific. Across the transition, the glacial Atlantic cooled while the Pacific barely changed, narrowing that gap to roughly 1.1 degrees.

The salinity story proved even more consequential. The oxygen isotope composition of seawater rose in both basins across the transition, as expected from the growth of continental ice sheets, which lock up isotopically light water on land and leave the ocean isotopically heavier. But the increase was significantly larger in the Pacific than in the Atlantic. Because ice volume affects the whole ocean uniformly, that asymmetry must reflect a regional change: the researchers conclude that Southern Component Water, the cold dense water formed around Antarctica that feeds the deep Pacific, became substantially saltier and therefore denser during glacial periods after about 930,000 years ago. A statistically significant step-like jump in the volume-weighted mean seawater isotope stack at 930,000 years ago marks this reorganisation.

What could have made Antarctic-sourced waters saltier? The authors propose that freshwater input to the marginal seas around Antarctica, such as the Weddell and Ross Seas, declined beginning around 930,000 years ago. Several mechanisms could have contributed: reduced melting of the Antarctic Ice Sheet and its floating ice shelves, a transition of the ice sheet toward a marine-based configuration with less terrestrial melting, increased export of freshwater away from Antarctica by icebergs and sea ice, and expanded sea ice formation, which concentrates salt in the underlying water through brine rejection. Notably, sea ice formation does not appreciably alter the oxygen isotope composition of seawater, which helps explain why the salinity signal appears in the isotope records without a corresponding freshwater isotope fingerprint.

The climatic payoff of this densification lies in the physics of ocean stratification. When the salinity and density of Southern Component Water increased, the deep ocean became more strongly layered, with dense bottom waters sitting beneath less dense waters above and mixing between them suppressed. A more stratified abyssal ocean is a more effective carbon trap: organic carbon sinking from the surface is remineralised at depth, consuming oxygen and releasing dissolved inorganic carbon that then accumulates in the isolated deep reservoir rather than being vented back to the atmosphere. The effect was amplified, the authors argue, by increased iron fertilisation of Southern Ocean productivity from wind-blown dust and by a strengthened surface halocline that reduced the deep-to-surface exchange through which the ocean releases carbon dioxide around Antarctica.

Independent lines of evidence bolster this picture. Records of neodymium isotopes and carbon isotopes from the deep Atlantic indicate a greater contribution of southern-sourced water during glacials after the transition, consistent with a denser Southern Component Water shoaling over its northern counterpart. Studies of deep-sea nutrients, carbonate ions, and oxygenation in both the Atlantic and Pacific point to enhanced carbon storage in the abyss across the same interval. Some atmospheric carbon dioxide reconstructions suggest glacial concentrations dropped modestly, by around 20 parts per million, across the transition, and lower carbon dioxide would have allowed ice sheets to grow large enough to survive modest rises in summer insolation that would previously have triggered deglaciation, thereby lengthening the cycles to the familiar 100,000-year beat.

Importantly, the study also constrains what did not change. The volume-weighted mean deep ocean temperature record shows minimal cooling across the transition, because the Pacific, which dominates the global deep ocean by volume, was already near freezing during glacials before 900,000 years ago. This finding aligns with noble gas-based estimates of mean ocean temperature from Antarctic ice cores and challenges reconstructions that infer pronounced global deep-ocean cooling, which the authors attribute to over-reliance on a single North Atlantic site. Similarly, their seawater isotope stack supports a substantial increase in glacial ice volume across the transition, in line with sea-level reconstructions and process-based estimates.

The authors are careful to frame their sequence of events as a working hypothesis rather than a settled verdict. With only five long paired magnesium-calcium and oxygen isotope records spanning the interval, the global stacks remain sparsely constrained, and the propagated uncertainties on the seawater isotope reconstructions are considerable. Testing the hypothesis will require additional records from under-sampled basins, numerical model simulations of the coupled ocean-cryosphere-carbon system, and, perhaps most eagerly awaited, high-resolution carbon dioxide and mean ocean temperature measurements from the Beyond EPICA Oldest Ice core, which extends continuously through the transition. If the abyssal stratification mechanism holds, it would mark a major step toward solving a puzzle that has occupied palaeoclimate science for half a century, and a reminder that the deepest layers of the ocean can quietly rewrite the rhythm of the ice ages.

Subject of Research: Abyssal ocean density stratification and carbon storage changes across the Middle Pleistocene Transition

Article Title: Increased abyssal ocean density stratification across the Middle Pleistocene Transition

Article References: Thomas, N. C., Ford, H. L., Greaves, M., & Hodell, D. A. (2026). Increased abyssal ocean density stratification across the Middle Pleistocene Transition. Climate of the Past, 22(8), 1559-1584. https://doi.org/10.5194/cp-22-1559-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1559-2026

Keywords: Middle Pleistocene Transition, abyssal ocean, density stratification, Mg/Ca paleothermometry, benthic foraminifera, oxygen isotopes, deep ocean circulation, Southern Component Water, carbon storage, atmospheric CO2, ice sheets, palaeoclimate