Ocean Oxygen Swung Wildly During Earth’s Last Warm Interglacial, Sediments Reveal

More than 100,000 years ago, long before humans burned a single barrel of oil, the global ocean went through a dramatic rollercoaster of oxygen loss and recovery during a warm period that scientists often treat as a template for our own climate future. A new study published in Nature Geoscience shows that the Last Interglacial, the balmy stretch between roughly 130,000 and 115,000 years ago known as Marine Isotope Stage 5e, was not the stable, uniformly well-oxygenated ocean basin that many models have assumed. Instead, the world’s seawater oxygen content rose to a peak near modern levels early in the interval and then slid into a sustained decline, revealing that even a natural warm climate can push the ocean’s breathing apparatus out of rhythm within a few thousand years.

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