The research, led by Victoria E. Taylor of the University of Bergen together with Alison M. Piasecki, Eivind O. Straume and A. Nele Meckler, applied a technique called carbonate clumped isotope thermometry to fossil foraminifera from two of the world’s great ocean basins. Unlike conventional oxygen isotope measurements, which conflate temperature with the isotopic composition of seawater, clumped isotope thermometry exploits the tendency of heavy isotopes of carbon and oxygen to bond with one another in carbonate minerals. The abundance of these carbon-13–oxygen-18 bonds, quantified as the Δ47 value, depends on temperature alone at the time the shell formed. That independence makes it possible to reconstruct deep-sea temperatures directly, without having to guess how salty or isotopically heavy the ancient ocean was.

The team assembled temperature records spanning the Eocene–Oligocene transition from sediment cores recovered by the Integrated Ocean Drilling Program in the eastern equatorial Pacific, at Sites U1333, U1334 and 1218, and from the northwest North Atlantic, at IODP Site U1406. What they found was genuinely surprising. Although the oxygen isotope records from the two basins look remarkably similar, the actual temperatures behind them behaved in completely different ways. The deep Pacific began cooling earlier, with its temperature decline preceding the main phase of the oxygen isotope increase by roughly 100,000 years. The North Atlantic, by contrast, cooled later in the transition, even though its isotope signal shifted at much the same time as the Pacific’s.

This decoupling is the kind of result that forces a rethink of a foundational dataset. For decades, the globally coherent rise in benthic oxygen isotope values at the Eocene–Oligocene transition has been treated as evidence that the deep ocean cooled everywhere in step as the Antarctic ice sheet expanded. The new clumped isotope data reveal that the isotope signal can be misleading. Because oxygen isotope values in foraminiferal calcite depend on both temperature and the isotopic composition of seawater, which itself tracks salinity, two water masses can record identical isotope shifts while experiencing entirely different thermal histories. The authors argue that contrasting salinity gradients, required to maintain similar water densities between the basins, probably masked the divergent temperature behaviour in the traditional records.

Perhaps the most striking finding concerns the direction of the temperature gradient between the basins. Today, the deep North Atlantic is bathed in relatively warm water formed in the northern high latitudes, while the deep Pacific receives colder water sourced from the Southern Ocean. In the early Oligocene, the situation appears to have been reversed: the deep North Atlantic was, on the whole, cooler than the equatorial Pacific. In fact, between about 33.52 and 33.29 million years ago, deep northwest Atlantic temperatures dropped to values approaching those of the modern ocean, levels that, within the limits of the available records, were not reached again until the Pleistocene ice ages tens of millions of years later. That is a remarkable early glimpse of ice-age-like cold in a basin usually associated with warmth.

The explanation the researchers propose is a deep ocean organised in a fundamentally different way from today’s. Rather than a single dominant overturning circulation, the Eocene–Oligocene deep ocean may have hosted two distinct southern-sourced deep-water masses with divergent properties. The compartmentalised Southern Ocean of the time, still partially cut off by the unfinished opening of the Drake Passage and the Tasmanian Gateway, offered greater potential to form cold deep waters in its Atlantic sector than in its Pacific sector. The two basins were therefore effectively decoupled, each receiving deep water whose characteristics depended on where and how it last contacted the surface.

This picture carries a broader implication: the growing Antarctic cryosphere was not merely a passive response to cooling, but an active participant in shaping the ocean’s circulation. The spatially and temporally variable early ice sheet would have altered brine rejection, freshwater fluxes and the buoyancy of surface waters around the Antarctic margin, directly influencing the properties of the deep waters forming in adjacent sectors. The divergent behaviour of the Pacific and Atlantic basins at the transition may thus record the fingerprint of an ice sheet learning, so to speak, how to be an ice sheet, with each phase of its growth leaving a distinct imprint on the water masses it helped create.

The study builds on a growing body of work applying clumped isotope thermometry to the deep past. A 2022 analysis in Science by Meckler and colleagues established the long-term Cenozoic evolution of deep ocean temperature using the same method, and subsequent studies have documented transient deep cooling in the eastern equatorial Pacific at the transition. By extending the approach to the North Atlantic and comparing basins directly, the new work transforms what was a single global narrative into a two-basin story with genuinely different thermal trajectories. The methodological rigour is considerable: the team used long-integration dual-inlet mass spectrometry, community standardisation protocols and careful error propagation, and their Atlantic temperatures reproduce independently published measurements from nearby Expedition 342 sites.

Why does this matter beyond the Eocene? The Eocene–Oligocene transition is one of the best-studied natural experiments in how a greenhouse world tips into an icehouse one, and it is frequently used to benchmark climate models and to frame expectations about future change. If the deep ocean’s response to that tipping was spatially heterogeneous rather than uniform, then reconstructions and models that assume a globally coherent deep ocean may need revision. It also matters for how scientists interpret the oxygen isotope archives that underpin so much of Cenozoic climate science: the same isotope shift can hide different combinations of cooling and salinity change in different places, and clumped isotope thermometry offers a way to untangle them.

The deep ocean, covering most of the planet and storing the vast majority of its accessible heat, is the flywheel of the climate system. What this study reveals is that during one of Earth’s great climatic reorganisations, that flywheel was not turning as a single machine but as two, with the Pacific and Atlantic spinning along divergent paths even as the ice advanced on Antarctica. The samples and data underpinning the work, provided through the Integrated Ocean Drilling Program and archived on Zenodo, ensure that other researchers can now interrogate this pivotal interval further. As the Antarctic ice sheet assembled itself 34 million years ago, it did not simply chill the world’s oceans in unison; it reorganised them from the bottom up, leaving a signature that is only now coming into focus.

Subject of Research: Deep-sea temperature evolution in the Pacific and Atlantic oceans during the Eocene–Oligocene onset of Antarctic glaciation

Article Title: Divergent deep-sea temperatures in the Pacific and Atlantic during the onset of Antarctic glaciation

Article References: Taylor, V. E., Piasecki, A. M., Straume, E. O., & Meckler, A. N. (2026). Divergent deep-sea temperatures in the Pacific and Atlantic during the onset of Antarctic glaciation. Nature Geoscience. https://doi.org/10.1038/s41561-026-02106-x

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02106-x

Keywords: Eocene–Oligocene transition, Antarctic glaciation, clumped isotope thermometry, benthic foraminifera, deep-ocean circulation, palaeoceanography, oxygen isotopes, North Atlantic Deep Water, Southern Ocean, IODP, paleoclimate, ocean gateways

Mesoscale eddies are rotating structures in the ocean, typically spanning tens to hundreds of kilometres and lasting from weeks to several months. Some resemble atmospheric storms, with swirling currents that trap and transport water over long distances. In the Southern Ocean, these rotating features form as powerful winds, fronts and large-scale currents interact with the complex seafloor and steep density gradients surrounding Antarctica. Although eddies are smaller than the major ocean currents, their cumulative influence can be enormous because they constantly redistribute heat, salt, nutrients and carbon.

The new research focuses on the deep Southern Ocean, a region far below the surface where the movement of heat is especially important but notoriously challenging to observe. Much of the heat absorbed by the ocean from the atmosphere eventually reaches deeper layers, where it can remain isolated from the atmosphere for years or even centuries. If that heat is transported toward Antarctica, it can influence sea-ice conditions, ice shelves and the stability of the Southern Ocean’s layered structure. The study’s central finding is that this deep heat movement is not driven only by broad, steady currents. Mesoscale eddies make a measurable poleward contribution.

For decades, scientists have recognized that eddies can act as the ocean’s “weather,” stirring and mixing water in ways that are difficult to represent in global climate models. Their effects are often estimated indirectly because the structures evolve rapidly and can be missed by widely spaced observations. By identifying an observed poleward heat transport associated with deep-ocean eddies, the study provides evidence that these features are not merely sources of turbulence. They are organized vehicles that move thermal energy across the Southern Ocean.

The physics behind the process is subtle. Ocean water becomes denser when it is colder or saltier, and differences in density create pressure gradients that drive currents. Earth’s rotation then deflects moving water through the Coriolis effect, helping produce large-scale fronts and rotating eddies. Inside an eddy, water masses can be displaced vertically and horizontally, carrying their temperature properties with them. When warm deep water is shifted poleward, the eddy transports heat across latitude even if the average flow appears to move mainly eastward around Antarctica.

This distinction matters because the Southern Ocean is dominated by the Antarctic Circumpolar Current, the world’s strongest ocean current and the only current that circles the planet without being blocked by continents. The current links the Atlantic, Pacific and Indian oceans, while winds drive a broad eastward flow. Eddies superimposed on that circulation can redirect part of the energy northward or southward, including toward Antarctica. Their heat transport may therefore help connect distant regions of the global ocean and influence how rapidly climate signals propagate into the deep sea.

The discovery also carries implications for Antarctica’s future. The waters surrounding the continent are already experiencing changes in temperature, stratification and circulation. In some regions, relatively warm water at depth can approach the continental shelf and come into contact with the undersides of floating ice shelves. Increased basal melting can weaken these ice shelves, which normally act as buttresses that slow the movement of inland glaciers toward the ocean. The study does not by itself determine how eddy-driven heat transport will affect any particular ice shelf, but it highlights a mechanism that climate projections must capture if they are to simulate Antarctic change accurately.

The findings are equally important for the ocean’s role in regulating atmospheric warming. The ocean has absorbed most of the excess heat associated with human-driven greenhouse-gas emissions, but the timing and location of that uptake are not uniform. Deep transport by eddies can redistribute heat away from the surface, temporarily reducing the amount of warming visible in the atmosphere while increasing the thermal content of the ocean. This does not eliminate global warming; it changes where and when the energy appears. Better measurements of these pathways could help explain short-term variations in the pace of surface warming and improve estimates of Earth’s climate sensitivity.

For climate scientists, the result is a reminder that small-scale processes can shape planetary-scale outcomes. Global models cannot resolve every eddy individually, so they use mathematical parameterizations to approximate the collective effects of unresolved turbulence. If those approximations underestimate the poleward movement of heat in the deep Southern Ocean, projections of ocean warming, sea-ice change and ice-sheet vulnerability could be biased. Continued observations will be needed to determine how the transport varies with seasons, storms, wind patterns and long-term changes in the Antarctic Circumpolar Current.

The study ultimately transforms the image of the deep Southern Ocean from a passive reservoir into an active, highly dynamic heat-distribution network. Mesoscale eddies may be difficult to see from the surface, but their rotating cores and shifting boundaries can influence the fate of heat on a continental scale. As the climate system absorbs more energy, understanding these hidden ocean pathways will become increasingly urgent. The Southern Ocean’s eddies are not just stirring the deep; they are helping decide where the planet’s excess heat goes next.

Subject of Research: Mesoscale eddies and their role in transporting heat poleward through the deep Southern Ocean.

Article Title: Observed poleward heat transport by mesoscale eddies in the deep Southern Ocean.

Article References: Liu, T., Zhai, X., He, Q. et al. “Observed poleward heat transport by mesoscale eddies in the deep Southern Ocean.” Nature Communications (2026). https://doi.org/10.1038/s41467-026-76674-2

Image Credits: AI Generated

DOI: 10.1038/s41467-026-76674-2

Keywords: Southern Ocean, mesoscale eddies, poleward heat transport, deep ocean, ocean circulation, climate change, Antarctic ice shelves, ocean heat uptake, marine climate dynamics

The AMOC is one of Earth’s largest climate-regulating systems. It transports warm surface water northward, releases heat to the atmosphere in the North Atlantic, and returns colder, denser water toward the deep ocean. This circulation influences temperatures and precipitation patterns across the North Atlantic region and Europe. Because seawater becomes denser when it is colder and saltier, scientists have proposed that the salt carried from the Indian Ocean through the Agulhas system helps sustain the formation of North Atlantic Deep Water, a key component of the AMOC’s returning deep branch.

The new research, led by Suning Hou of Utrecht University with collaborators in the Netherlands, the United States, China and the United Kingdom, examined a major climate transition during the late Pliocene, between approximately 3.6 and 2.6 million years ago. This interval included a pronounced glacial event followed by the mid-Piacenzian Warm Period, when global temperatures exceeded modern preindustrial conditions. The ancient climate offered the researchers a natural experiment: they could investigate how ocean circulation responded when temperatures, ocean fronts and the exchange of water between basins changed substantially.

To reconstruct conditions south of Africa, the team analysed a sediment core collected from International Ocean Discovery Program Site U1475 on the Agulhas Plateau, roughly 500 kilometres south of the South African coast. The sediment preserved microscopic biological and chemical signals from the overlying ocean. Fossil dinocysts—planktonic organisms whose species distributions reflect water temperature—were used to track movement of the Southern Ocean subtropical front. The researchers also analysed organic lipid biomarkers, molecules that can record past sea-surface temperatures and provide an independent indication of changing ocean conditions.

The evidence showed that the subtropical front shifted northward from about 3.4 million years ago into the glacial interval. As the front moved north, the region around the Agulhas Plateau cooled by approximately 3 degrees Celsius and developed subpolar conditions. Because the position of this front helps determine how easily Indian Ocean water can flow around southern Africa and enter the Atlantic, the reconstruction indicates that Agulhas Leakage weakened sharply and may have nearly shut down. Under the traditional salt-transport hypothesis, such a reduction should have weakened the AMOC.

The Atlantic records told a different story. To determine how the circulation responded, the researchers produced temperature reconstructions from Ocean Drilling Program Site 625 in the northern Gulf of Mexico and compared them with published records from the equatorial and North Atlantic and the Caribbean Sea. They also tested the geological evidence against numerical climate-model simulations of the same late Pliocene transition. Together, the data and simulations indicated that North Atlantic Deep Water formation and lower-latitude overturning intensified even as the Agulhas contribution declined.

The circulation did not simply become stronger everywhere. The simulations showed that the North Atlantic Current reached less far into the high northern latitudes during the glacial event, while overturning intensified farther south. This reorganization produced a shallower thermocline across much of the Atlantic. The thermocline is the layer separating relatively warm surface water from colder deep water, and its depth strongly influences heat storage, nutrient distribution and the exchange of energy between the ocean and atmosphere. The result suggests that a basin-wide adjustment of the Atlantic water column compensated for the reduced southern salt input.

The pattern initially appeared in an isolated sediment record and was difficult to interpret. When a similar signal was found in sediments from the Agulhas Plateau, the researchers recognized that it was unlikely to be a local anomaly. Instead, the records pointed to a broad restructuring of the Atlantic thermocline and overturning system. Climate-model experiments supported that interpretation, showing that changes in high-latitude deep-water formation and the internal structure of the Atlantic could override the direct influence expected from Agulhas salt transport.

The findings do not mean that Agulhas Leakage is irrelevant to the AMOC, nor do they provide a simple forecast of how the modern circulation will respond to global warming. The late Pliocene had different continental configurations, ice sheets, atmospheric conditions and freshwater pathways from those of today. In particular, Arctic freshwater input—an important influence on modern deep-water formation—was not identical to the present-day system. The study instead shows that the controls on ocean overturning are not fixed: the same ocean connection can have very different consequences under different climate and boundary conditions.

That conclusion has major implications for climate research. Models and observations often treat the exchange of salt between ocean basins as a direct influence on the AMOC, but the new evidence emphasizes the importance of local processes in the North Atlantic, including cooling, mixing, water-column structure and the precise location of deep-water formation. A future southward shift of the subtropical front could increase salt transport into the Atlantic, while increased Arctic freshwater could counteract that influence. Understanding which effect dominates will require combining geological records, modern observations and climate simulations rather than relying on a single controlling mechanism. The research was conducted as part of the OceaNice project, funded by the European Research Council.

Subject of Research: Ocean circulation and paleoclimate

Article Title: Disconnection of the late Pliocene Agulhas Leakage from Atlantic Meridional Overturning Circulation

News Publication Date: 3-Aug-2026

Web References: https://doi.org/10.1038/s41561-026-02055-5

References: Nature Geoscience, DOI: 10.1038/s41561-026-02055-5

Image Credits: NASA’s Goddard Space Flight Center

Keywords: Agulhas Leakage, Atlantic Meridional Overturning Circulation, AMOC, ocean circulation, climate change, paleoclimate, late Pliocene, North Atlantic Deep Water, Southern Ocean subtropical front, Utrecht University