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

The study, led by Chia-Te Chien of GEOMAR Helmholtz Centre for Ocean Research Kiel and National Taiwan University, together with Markus Pahlow, Christopher J. Somes, Markus Schartau, and Andreas Oschlies, is published in the journal Earth System Dynamics. Rather than running a single simulation with one best-guess set of model parameters, the team built an ensemble of 480 simulations of the Last Glacial Maximum, systematically varying both the environmental boundary conditions of the glacial world and the physiological parameters of the plankton ecosystem model embedded within an Earth system model. The design was aimed squarely at a blind spot that has haunted glacial carbon cycle modeling: most previous studies used a single parameter set calibrated against modern observations and simply assumed those values would hold in a radically different ocean.

The technical foundation of the work is the optimality-based plankton ecosystem model, or OPEM, coupled to the University of Victoria Earth system model of intermediate complexity. Unlike conventional models that fix the elemental composition of marine organic matter to the canonical Redfield ratio of 106 carbon to 16 nitrogen to 1 phosphorus, OPEM allows phytoplankton to flexibly adjust their carbon-to-nitrogen and carbon-to-phosphorus ratios in response to nutrient stress and light. This variable stoichiometry matters enormously for carbon sequestration, because if glacial plankton built organic matter with more carbon per unit of nutrient, the same nutrient supply could export more carbon into the deep ocean and pull down atmospheric carbon dioxide more efficiently.

The ensemble architecture was deliberately layered. The researchers first generated 600 parameter sets, each representing a unique combination of values for 19 parameters governing plankton physiology, detritus remineralization, and particle sinking. Each of the 600 was spun up for more than 10,000 years under pre-industrial conditions with a prescribed atmospheric carbon dioxide level of 284.3 parts per million. A likelihood-based cost function then scored each simulation against observations of phosphate, nitrate, and oxygen from the World Ocean Atlas across 17 biomes, penalizing mismatches in both mean values and spatial and temporal variability. The 20 parameter sets with the lowest costs, all within 1.3-fold of the best, were carried forward. Each was then run under 24 different combinations of glacial boundary conditions, producing the 480-simulation ensemble.

Those boundary conditions captured the major hypothesized differences between the glacial and pre-industrial ocean. On the physical side, the team applied Last Glacial Maximum orbital parameters, wind stress patterns drawn from PMIP3 models featuring a northward shift and intensification of Southern Ocean westerlies, and a halving of meridional moisture diffusivity over the Southern Ocean, which stratifies the surface and weakens deep water formation. On the biogeochemical side, the 120-meter drop in sea level exposed continental shelves, cutting benthic denitrification by roughly 40 percent and slashing sedimentary iron input by about 80 percent, while terrestrial erosion boosted the global phosphate inventory by 15 percent and glacial dust quadrupled atmospheric iron deposition to 6.1 gigamoles of iron per year.

The results were unambiguous about which lever mattered most. Iron supply, whether arriving as dust or released from sediments, exerted the most profound influence on marine biogeochemistry and atmospheric carbon dioxide. Quadrupling atmospheric iron deposition lowered carbon dioxide by about 26 parts per million on its own, while cutting sedimentary iron raised it by roughly 60 parts per million, a swing so large it dwarfed every other forcing tested. By contrast, increasing the phosphate inventory or reducing benthic denitrification, the two classic macronutrient hypotheses, produced only marginal effects. The reason lies in co-limitation: adding one major nutrient simply aggravates limitation by the others, and the model’s flexible stoichiometry partially offsets the gain by allowing plankton to incorporate relatively less carbon per unit of nutrient when nutrients become abundant.

The sedimentary iron finding carries a provocative implication. Because falling sea level stripped away shallow seafloor sources, the total dissolved iron inventory in the glacial ocean was actually about 1 percent lower than before the ice age, even as dust deposition soared. Lower temperatures also reduced productivity and therefore the organic carbon flux reaching the seafloor, further starving sedimentary iron release. Yet atmospheric carbon dioxide still fell, driven primarily by a spatial redistribution of surface dissolved inorganic carbon and altered air-sea fluxes rather than by a wholesale increase in oceanic iron fertilization. Earlier modeling studies that considered only enhanced dust deposition, without the sedimentary decline, may have substantially overestimated the glacial iron effect.

The ensemble also exposed a sobering uncertainty. Under full glacial conditions, atmospheric carbon dioxide decreased by 36 to 58 parts per million across the 20 parameter sets, a spread equal to roughly half the mean drawdown of 43.5 parts per million, even though all 20 sets reproduced pre-industrial biogeochemistry about equally well. Statistical analysis traced the variance to parameters governing phytoplankton nitrogen subsistence, temperature-dependent mortality, and zooplankton grazing, and revealed that strongly correlated parameter pairs could mask or amplify the carbon dioxide response depending on how their values co-varied. Notably, the drawdown achieved by each simulation was unrelated to its cost function score, meaning that a model fitting modern data well tells you almost nothing about how it will behave in a glacial ocean.

The full ensemble accounted for only about half of the observed 90 parts per million glacial drawdown, leaving room for processes outside the model’s scope, such as brine-induced stratification around Antarctica, air-sea carbon disequilibrium, and a larger terrestrial carbon transfer than the simulations produced. Still, the physical forcings alone, colder temperatures plus altered winds and moisture diffusivity, contributed about 35 parts per million, with the biological and stoichiometric effects adding the rest. The team estimates that flexible carbon-to-nutrient ratios contributed an additional 16 to 17 parts per million of drawdown when iron deposition alone is considered, consistent with earlier theoretical estimates.

Beyond solving an ice age puzzle, the findings speak directly to one of the most debated proposals for engineered carbon removal: ocean iron fertilization. The glacial ocean serves as a natural experiment in what happens when iron delivery to the sea surges, and the model’s behavior, including the nutrient-robbing effect in which fertilized regions deplete macronutrients downstream, mirrors the central criticism of iron fertilization as a climate intervention. Intriguingly, the variable stoichiometry feedback, which sustains productivity even as macronutrients decline, does not exist in fixed-ratio models and could partially buffer that limitation. The authors argue that the same parameter and boundary condition uncertainties that complicate glacial simulations apply equally to assessing carbon dioxide removal strategies, making ensemble approaches like this one essential for any credible accounting of what the ocean can, and cannot, be persuaded to store.

Subject of Research: Ensemble modeling of Last Glacial Maximum marine biogeochemistry and the role of the soft-tissue biological carbon pump in glacial atmospheric CO2 drawdown

Article Title: Ensemble simulation of the Last Glacial Maximum marine biogeochemistry and atmospheric pCO2 drawdown due to the soft-tissue biological carbon pump

Article References: Chien, C.-T., Pahlow, M., Somes, C. J., Schartau, M., & Oschlies, A. (2026). Ensemble simulation of the Last Glacial Maximum marine biogeochemistry and atmospheric p CO 2 drawdown due to the soft-tissue biological carbon pump. Earth System Dynamics, 17(5), 1277-1297. https://doi.org/10.5194/esd-17-1277-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1277-2026

Keywords: Last Glacial Maximum, biological carbon pump, iron fertilization, marine biogeochemistry, atmospheric CO2, paleoceanography, Earth system modeling, variable stoichiometry, ocean circulation, nutrient limitation, carbon sequestration, ensemble simulation

The scale of sea ice’s fingerprints on the Southern Ocean is staggering. The annual freeze-melt cycle moves more freshwater than precipitation minus evaporation and glacial melt combined, and it does so across a zone where the atmosphere and deep ocean exchange heat, salt and carbon. During the Last Glacial Maximum, roughly 19,000 to 23,000 years ago, winter sea ice expanded to approximately twice its modern extent while sea surface temperatures ran about 3.9 degrees Celsius colder than today. Conversely, during the last interglacial around 130,000 to 116,000 years ago, ice core records suggest winter sea ice contracted by as much as 36 percent relative to the Holocene, with models indicating reductions of 40 to 60 percent. These two climate states — one much icier, one much warmer — provide natural experiments for testing how sea ice couples to the rest of the planet.

The first and perhaps most fundamental coupling involves ocean circulation. Deep water from the world’s basins, collectively known as Circumpolar Deep Water, upwells in the Southern Ocean and splits into two branches. The northern branch is freshened by sea-ice melt and subducts to form Antarctic Intermediate Water, while the southern branch loses buoyancy through cooling and brine rejection during ice formation, ultimately producing the dense Antarctic Bottom Water that ventilates the abyss. Proxy evidence shows that during glacial periods Antarctic Intermediate Water shoaled in the southwest Pacific, consistent with expanded sea ice, while the deep Atlantic was filled to a greater volume by poorly ventilated southern-sourced water. Intriguingly, manganese peaks in sediments offshore major bottom-water production sites suggest that the rate of Antarctic Bottom Water formation may actually have been lower during the Last Glacial Maximum, even as its volume expanded — a paradox that highlights how much remains unknown.

Sea ice also acts as a physical guardian of the Antarctic ice sheet itself. A sea-ice barrier dampens ocean swell that would otherwise batter ice shelf fronts, and landfast ice can bond the debris-laden melange that stabilises calving glacier fronts. The disintegration of ice shelves over recent decades, driven by warm currents and rising air temperatures, has drawn attention to this protective role because ice shelves buttress the grounded glaciers behind them and thus regulate sea level rise. The review documents a darker feedback loop: declining summer sea ice exposes ocean surfaces to solar heating, and that warmed water can be advected beneath ice shelves, accelerating basal melt. Reduced winter ice formation likewise weakens stratification in ways that allow warm Circumpolar Deep Water to intrude onto the shelf. In the paleo record, a mid-Holocene expansion of sea ice off Wilkes Land has even been linked to the retreat of the Ross Sea grounding line, whose meltwater outflow cooled downstream waters and promoted ice growth downstream.

The atmosphere is not spared. Sea ice sits directly beneath the polar frontal zone, so even modest shifts in its edge can reorganise atmospheric circulation. Most modelling studies find that shrinking sea ice weakens and shifts the Southern Hemisphere westerly winds equatorward, because a warmer polar region flattens the temperature gradient that drives the jet. This sea-ice effect runs opposite to the poleward wind shift expected from greenhouse warming itself, meaning the two influences partially offset each other. So-called ghost flux experiments, which inject artificial heat fluxes to isolate sea-ice impacts, reveal responses that ripple pole to pole — warming the Antarctic interior, altering katabatic winds, slowing the Antarctic Circumpolar Current, warming equatorial Pacific sea surface temperatures and even thinning Arctic sea ice.

Yet the paleo evidence complicates this tidy picture. Reconstructions of past winds suggest that during the last deglaciation the westerlies shifted poleward by more than four degrees of latitude — more than any model predicts — implying that the sea-ice effect on winds may be overcompensating in simulations. Even more strikingly, millennial-scale Antarctic warming events recorded in ice cores show abrupt sea-ice declines that were not matched by corresponding wind shifts; instead, the winds responded to Northern Hemisphere warming. The review concludes that over millennial timescales, the direct influence of sea ice on atmospheric circulation appears to be a second-order process compared with other climate feedbacks — a humbling result for a variable often assumed to be climatically dominant.

Biology adds another layer of complexity. Sea ice structures Southern Ocean ecosystems into distinct zones, seeding phytoplankton blooms at the retreating ice edge and releasing a springtime pulse of iron that triggers algal growth. Sea ice hosts its own microbial communities, and its melt releases dimethyl sulphide, a sulphur compound that seeds cloud formation and influences albedo. But the glacial record delivers conflicting messages. Sediment cores south of the Antarctic Polar Front show lower productivity during ice ages, which might seem like straightforward light limitation under expanded ice. Yet nitrogen isotope data reveal more complete nutrient consumption during glacials — the opposite of what light limitation would produce. The proxies instead point to reduced upwelling of nitrate-rich deep water, meaning sea ice influenced productivity indirectly through stratification and circulation rather than simply by blocking sunlight.

The carbon cycle is where the stakes are highest. Sea ice caps the ocean, suppressing the release of carbon dioxide from carbon-rich deep waters that upwell in the Southern Ocean. Early box models suggested this capping alone could explain the roughly 80 parts per million drop in glacial atmospheric CO2, but subsequent work showed the mechanism requires implausibly permanent ice cover and is offset by reduced biological uptake. The emerging consensus is subtler: sea ice matters most when it acts in concert with circulation changes. Idealised models show that atmospheric cooling can lower CO2 by about 40 parts per million, but only because expanded ice forces upwelling carbon-rich water to reach the surface beneath the ice, where gas exchange is suppressed — a strengthening of the so-called disequilibrium pump. Proxy timing supports this coupling: during the glacial inception around 115,000 years ago, Antarctic temperatures and sea ice shifted roughly 3,000 years before CO2 began to fall, suggesting that sea ice alone cannot drive atmospheric carbon change without accompanying circulation reorganisation.

The review’s most sobering conclusion is that the relationships are bidirectional, state-dependent and incompletely captured by models. Most global ocean models cannot resolve the coastal polynyas where dense shelf water forms today, instead representing bottom-water formation as an open-ocean process. Summer sea-ice extent during glacial periods remains poorly constrained, and no sediment records exist south of the current summer ice limit. If the net effect of Antarctic sea ice is to enhance ocean carbon storage, its ongoing decline could weaken the ocean’s sink for anthropogenic carbon and shrink humanity’s remaining carbon budget. The past 130,000 years show that sea ice has repeatedly coordinated sweeping changes across the cryosphere, atmosphere, ocean and biosphere — and that understanding its demise requires reading the deep archive of Earth’s climate history with far greater resolution than we currently possess.

Subject of Research: The role of Antarctic sea ice in ocean circulation, ice shelves, atmospheric circulation, marine productivity and the carbon cycle over the past 130,000 years

Article Title: Antarctic sea ice over the past 130 000 years – Part 2: A review of its role in the Earth system

Article References: Chase, Z., Kohfeld, K. E., Leventer, A., Lund, D., Crosta, X., Menviel, L., Bostock, H. C., Chadwick, M., Jaccard, S. L., Jones, J., Marzocchi, A., Meissner, K. J., Sikes, E., Sime, L. C., & Skinner, L. (2026). Antarctic sea ice over the past 130 000 years – Part 2: A review of its role in the Earth system. Climate of the Past, 22(10), 1881-1911. https://doi.org/10.5194/cp-22-1881-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1881-2026

Keywords: Antarctic sea ice, Southern Ocean, paleoclimate, Last Glacial Maximum, last interglacial, ocean circulation, Antarctic Bottom Water, ice shelves, carbon cycle, atmospheric CO2, westerly winds, climate models

The research, led by Fang Qian of Tulane University and the Woods Hole Oceanographic Institution together with Yi Wang, Kassandra Costa, Sarah Shackleton, Sophia Hines, Ingrid Hendy and Sune Nielsen, hinges on an elegant but underused geochemical detective tool: thallium isotopes. Thallium is a trace metal whose two stable isotopes are sorted, ever so slightly differently, by chemical reactions in seawater. When oxygen-rich conditions prevail, manganese oxides form on the seafloor and preferentially scavenge the heavy isotope, thallium-205, leaving the water enriched in the lighter isotope. When oxygen dwindles, manganese oxide burial falters, and the seawater isotopic signature shifts in the opposite direction. Because manganese oxide burial happens across the entire ocean, the thallium isotopic composition recorded in sediments acts as a globally integrated gauge of marine oxygenation rather than a purely local thermometer of one basin’s health.

To capture that global signal, the team analyzed two sediment cores from opposite sides of the planet: core TN041-8JPC from the Arabian Sea, one of the ocean’s most pronounced oxygen minimum zones, and Ocean Drilling Program Site 1017E and its companion Site 1017B on the Southern California Margin. Both locations sit beneath waters that are sensitive to changes in oxygen supply, and both archives delivered what the authors describe as high-resolution records, with average temporal resolutions of roughly 300 to 400 years. That is fine enough to resolve changes that unfold over centuries to millennia, a pace directly relevant to how today’s ocean is responding to warming. The two independently derived records track each other closely, with a Pearson correlation of r = 0.73 and no time lag between them, a strong indication that they are recording a shared, planet-wide signal rather than regional noise.

The story the isotopes tell begins during Marine Isotope Stage 6, the penultimate glacial period that preceded the interglacial. During that icy stretch, global oceanic oxygen was substantially lower than today, consistent with a long-standing picture in which glacial oceans, cut off from vigorous ventilation and loaded with respired carbon, stagnated and lost oxygen. Then came Termination II, the abrupt deglaciation that ended the ice age. As the planet warmed and ice sheets retreated, oceanic oxygen climbed, broadly in step with rising atmospheric carbon dioxide recorded in Antarctic ice cores. By the early part of Marine Isotope Stage 5e, the reconstructed oxygen content of the global ocean peaked at levels close to what we measure today.

Here is where the record takes its surprising turn. Rather than settling into a stable plateau for the remainder of the interglacial, as a simple temperature-driven view of oxygen solubility would predict, the thallium isotope curves show a clear and sustained decline after that early peak. The ocean’s oxygen content dropped even though the climate remained warm, indicating that the interglacial state was transient rather than steady. The decline parallels reconstructed variability in Antarctic temperature and sea-ice extent, pointing the finger at the Southern Hemisphere as the dynamic control knob. When Antarctic sea ice expanded and Southern Ocean ventilation weakened, the deep ocean’s connection to the atmosphere was throttled back, and the global oxygen budget sagged in response.

The Southern Ocean’s outsized role is reinforced by a comparison the authors carried out across the two most recent deglaciations, Termination II and the more recent Termination I that ended the last ice age about 18,000 years ago. In both cases, the timing and shape of the global oxygen recovery line up with changes in Southern Ocean ventilation, suggesting that this high-latitude region, where deep waters rise, exchange gases with the atmosphere and are transformed by sea ice, is the primary driver of both global oceanic oxygen and atmospheric carbon dioxide swings. This builds on the team’s earlier work published in Science Advances in 2024, which reached a similar conclusion for the most recent deglaciation and now appears to be a repeatable feature of Earth’s climate system.

The mechanics behind that control are worth unpacking. The Southern Ocean is the main window through which the ocean’s deep interior communicates with the atmosphere. Around Antarctica, sea ice forms and melts, brine rejection and buoyancy fluxes drive the transformation of surface waters into deep and intermediate waters, and the westerly winds modulate how much old, oxygen-poor water is upwelled to the surface. When sea ice caps the region, that window effectively closes: the deep ocean becomes isolated, accumulates respired carbon dioxide, and loses its oxygen resupply. When the cap retreats, ventilation resumes, carbon dioxide escapes to the atmosphere, and oxygen floods back into the abyss. Ice core records of carbon dioxide and Antarctic temperature, together with sedimentary proxies for sea ice and productivity, all shift in concert with the thallium isotope signal, weaving a coherent mechanistic picture across two glacial cycles.

Not every candidate explanation survives scrutiny in the new data. Changes in the export of organic carbon from the surface ocean, which consumes oxygen as it decays at depth, have long been proposed as a driver of glacial-interglacial oxygen changes. But the compiled productivity records from the Southern Ocean and the deep sea show divergent trends during Marine Isotope Stage 5e compared with the preceding glacial, leaving the net effect of export productivity on global oxygenation ambiguous. The authors also ran a transient, non-steady-state thallium mass balance model, validated with 100,000 Monte Carlo simulations, to convert the isotope record into quantitative estimates of manganese oxide burial and oxygen concentration. The model confirms that the observed isotope shifts require real, time-dependent changes in global redox conditions, not artifacts of sedimentation or local diagenesis, and it brackets the magnitude of the oxygen swing from the glacial minimum toward the early interglacial peak.

Why should a warming world care about an interglacial that ended 115,000 years ago? Because Marine Isotope Stage 5e is one of the best natural analogues for the near-term future: global temperatures and sea levels were higher than preindustrial values, and atmospheric carbon dioxide, while lower than today’s anthropogenic levels, was rising out of a glacial baseline. The new record demonstrates that in such a warm climate, ocean oxygen does not simply follow temperature-dependent solubility, the factor most emphasized in current projections of ocean deoxygenation. Circulation and ventilation, particularly in the Southern Ocean, can dominate the global budget and produce oxygen losses even under warm, high-carbon-dioxide conditions. Since the 1960s, the ocean has already lost measurable oxygen, and oxygen minimum zones have expanded, squeezing the habitat of tropical pelagic fishes and threatening marine ecosystems worldwide.

The implications for climate models are direct. Earth system models that project ocean deoxygenation through the end of this century and beyond need to capture Southern Ocean ventilation processes, including sea ice dynamics, westerly wind shifts and deep water formation, with fidelity, because those processes, not solubility alone, appear to set the pace of global oxygen change. The study’s data and modeling code are openly available through Zenodo, and the authors acknowledge support from the U.S. National Science Foundation. What the ancient sediments ultimately whisper is a caution: the ocean’s oxygen supply is a living, breathing system with a hair trigger, and the last time Earth ran this warm experiment naturally, the breathing faltered mid-interglacial. Whether the modern ocean follows the same script, with humanity’s carbon emissions as the new variable, is now one of the most consequential open questions in climate science.

Subject of Research: Reconstruction of transient global oceanic oxygen variability during the Last Interglacial Period using sedimentary thallium isotopes

Article Title: Transient variability in global oceanic oxygen during the Last Interglacial Period

Article References: Qian, F., Wang, Y., Costa, K. M., Shackleton, S., Hines, S. K. V., Hendy, I. L., & Nielsen, S. G. (2026). Transient variability in global oceanic oxygen during the Last Interglacial Period. Nature Geoscience. https://doi.org/10.1038/s41561-026-02120-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02120-z

Keywords: ocean deoxygenation, thallium isotopes, Last Interglacial, Marine Isotope Stage 5e, Southern Ocean ventilation, paleoceanography, manganese oxide burial, Antarctic sea ice, atmospheric CO2, glacial-interglacial cycles, Nature Geoscience, climate change

Silicate weathering is one of the fundamental thermostats of the Earth system. When rainwater, charged with atmospheric carbon dioxide in the form of carbonic acid, attacks silicate minerals in rocks, the carbon is converted into dissolved bicarbonate and carried by rivers to the ocean, where much of it is ultimately buried in carbonate sediments. Over geological timescales, this process acts as a long-term sink for atmospheric CO2, and its intensity is conventionally assumed to track temperature and runoff: hot, humid climates accelerate chemical denudation, while cold, dry glacial climates slow it down. That assumption underpins much of how scientists model the carbon cycle over glacial-interglacial cycles, which is precisely why the new findings are so consequential. The study suggests that during the very intervals when the conventional model predicts weakened weathering, an enormous and previously overlooked weathering engine was running at full tilt on the exposed mid-latitude shelves of East Asia.

The evidence comes from Site CSDP-1, a core recovered by the Chinese Continental Shelf Drilling Program in the South Yellow Sea. The team conducted a comprehensive analysis of the mineralogical and geochemical properties of the sediments, layer by layer, building a continuous archive that spans six hundred millennia of alternating glacial and interglacial conditions. Among the key indicators they examined was the Chemical Index of Alteration, or CIA, a widely used proxy that measures the degree to which feldspars and other primary silicate minerals have been chemically transformed into clay minerals through hydrolysis. They also tracked the ratio of potassium to aluminum, K/Al, which decreases as labile potassium-bearing minerals are leached away during weathering, and the ratio of smectite to illite plus chlorite, which captures the balance between newly formed alteration products and relatively unweathered detrital minerals transported from the river basin.

All of these proxies told the same story. The weathering intensity recorded in glacial shelf sediments was consistently and markedly higher than that recorded in interglacial sediments. In quantitative terms, the team calculated that the rate of carbon consumption through silicate weathering during glacial periods was approximately 1.5 times that of interglacial periods. This inversion of the expected pattern, in which colder climates should yield weaker chemical denudation, is the heart of the discovery. The researchers interpret it as evidence that glacial weathering on the shelf was decoupled from climate in the conventional sense. It was not temperature or precipitation over the source basin that controlled the signal, but rather the physical state of the sediment itself and the amount of time it spent exposed at the surface.

The mechanism hinges on sea level. During glacial periods, global sea level fell by well over a hundred meters as water was locked into continental ice sheets. This regression exposed the loosely consolidated, unconsolidated sediments that had accumulated on the continental shelf during earlier highstands. Rather than being rapidly buried beneath the ocean, these fine-grained deposits were left stranded on the emergent plain, where they remained for thousands of years, subjected to repeated wetting and drying, oxidation, and infiltration by CO2-charged meteoric water. Because the sediments were fine-grained, freshly deposited, and largely unweathered when first delivered by the Yellow River, they presented an enormous reactive surface area to chemical attack. The long residence time of these materials on the exposed shelf gave weathering reactions the opportunity to proceed far beyond what would have been possible had the sediment been quickly sequestered on the seafloor.

Interglacial conditions reversed this arrangement. As sea level rose, the river mouth retreated landward and the previously exposed shelf was flooded once more. Fine-grained material eroded from the basin was then rapidly transported through the estuary and dispersed across the shelf and into the deep sea, sharply reducing both the time and the space available for reweathering of the sediment. In other words, the same sediment that lingered and weathered during glacial lowstands was efficiently exported and buried during highstands. The sediment routing system, rather than the climate system, emerged as the dominant control on chemical weathering intensity in this basin-shelf configuration, a conclusion that challenges how weathering signals in marine sediments are commonly interpreted.

To appreciate the scale of the overlooked sink, the researchers extended their quantitative estimates beyond the Yellow Sea. Taking into account the exposed shelves of the Yellow Sea and the East China Sea, along with comparable low-latitude marine areas that experience similar sea-level-driven exposure, they calculated that the increased silicate weathering of shelf sediments during glacial periods may account for approximately 9.3 percent of the total reduction in atmospheric CO2 during glacial times. For a mechanism that had previously gone essentially unrecognized in glacial carbon budgets, nearly a tenth of the glacial drawdown is a substantial contribution. It implies that the mid-latitude shelf region functioned as an active and underestimated silicate weathering carbon sink precisely during the intervals when the global climate was coldest and atmospheric CO2 was at its lowest.

This finding also reframes comparisons with earlier work in low-latitude seas, where enhanced glacial weathering had been documented but attributed to specific tropical conditions. The new study demonstrates that the phenomenon is not confined to the tropics. Mid-latitude shelves fed by major sediment-discharging rivers, of which the Yellow River is one of the largest on Earth, can host equally vigorous weathering during lowstands. Given the abundance of such river-shelf systems around the world, from the Sunda Shelf of Southeast Asia to the shelves of other major river depocenters, the implication is that shelf weathering may be a globally significant and systematically underestimated term in glacial carbon cycle accounting.

Methodologically, the study is notable as the first quantitative reconstruction of the Late Quaternary history of silicate weathering carbon consumption in the Yellow River basin-continental shelf system. By combining multiple independent weathering proxies with calculated carbon consumption fluxes, the team was able to move beyond qualitative statements about alteration intensity and place firm numbers on the carbon sink, both through time at the study site and extrapolated across the broader shelf systems of the western Pacific margin. The 600,000-year record captures numerous glacial-interglacial cycles, giving the researchers the statistical confidence to distinguish a systematic glacial enhancement of weathering from episodic or local effects.

The broader significance of the work lies in the perspective it opens for understanding the mechanisms of glacial-interglacial climate evolution. If exposed continental shelves acted as a meaningful carbon sink during ice ages, then models of the glacial carbon cycle must incorporate sea-level-driven shelf exposure as a first-order process, alongside the ocean’s biological pump, changes in ocean circulation, and carbonate compensation dynamics that have traditionally dominated the discussion. The study, authored by Zhang, Wan, and colleagues in SCIENCE CHINA Earth Sciences, thus adds a continental margin process to the roster of mechanisms that helped the planet cool into glacial states and, by implication, helped release that carbon as shelves were reflooded during deglaciation. For a field that has long focused its attention on the open ocean and the terrestrial biosphere, the message from the muddy floor of the South Yellow Sea is that some of the most important carbon chemistry of the ice ages may have happened in plain sight, on land that is now underwater.

Subject of Research: Enhanced silicate weathering on the South Yellow Sea continental shelf during glacial periods and its role in the glacial carbon cycle

Article Title: Enhanced silicate weathering on the South Yellow Sea shelf during glacial periods and its influence on the glacial carbon cycle

Article References: Enhanced silicate weathering on the South Yellow Sea shelf during glacial periods and its influence on the glacial carbon cycle. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: silicate weathering, South Yellow Sea, continental shelf, glacial periods, carbon cycle, atmospheric CO2, sea level, Chemical Index of Alteration, Yellow River, CSDP-1, Science China Earth Sciences, Quaternary climate