The research, published in Nature Communications, turns to the Paleocene-Eocene Thermal Maximum, or PETM, a hyperthermal event about 55.9 million years ago that is widely regarded as the best deep-time analog for modern climate change. During the PETM, between 3,000 and 10,000 gigatons of isotopically light carbon were injected into the ocean-atmosphere system within roughly 10,000 years, producing a negative carbon isotope excursion of 3 to 5 per mil. Seawater temperatures climbed by 5 to 8 degrees Celsius, and the ocean suffered acidification, deoxygenation, and major turnovers among bottom-dwelling foraminifera and calcareous nannoplankton. Although the carbon release rate was about an order of magnitude slower than today’s fossil fuel emissions, the total amount of carbon released is comparable to what high-emission scenarios project for 2100. That makes the PETM a rare natural experiment in what happens to marine ecosystems under sustained greenhouse forcing.

The team, led by Xiuwen Zhou and Weiqi Yao of the Southern University of Science and Technology in Shenzhen, tackled a long-standing contradiction in the PETM literature. Marine barite accumulation rates, considered a reliable tracer of carbon export from the surface ocean, show an average threefold increase in export productivity during the event, likely driven by enhanced nutrient input from land. But evidence from benthic foraminifera and nannofossil assemblages suggests nutrient supply declined in the South Atlantic, Southern Ocean, and North Pacific after the PETM onset, and Earth system models indicate a warmer, more stratified ocean should have suppressed productivity in the tropics. To resolve the puzzle, the researchers needed an independent record of productivity higher up the food chain, and they found one in an unlikely place: fossilized fish teeth.

Ichthyoliths, the microscopic remains of fish teeth and shark dermal denticles, rain down onto the seafloor as fish naturally shed teeth or as predators excrete the indigestible parts of their prey. Most ichthyoliths in marine sediments come from small pelagic species, including juvenile fishes and shark taxa, which sit at mid-trophic levels and whose abundance is sensitive to both productivity and the efficiency with which energy moves through the food web. By measuring ichthyolith accumulation rates, the researchers could reconstruct fish productivity directly. They analyzed 105 samples from five sediment cores spanning the North Pacific, Eastern Equatorial Pacific, North Atlantic, South Atlantic, and Southeast Indian Ocean, and combined these with previously published data from a sixth equatorial Pacific site.

The results were striking. At the Pacific and North Atlantic sites, ichthyolith accumulation rates rose dramatically during the PETM, with peak values five to eleven times higher than background levels. The highest rates appeared in the Eastern Equatorial Pacific, reaching 121 ichthyoliths per square centimeter per thousand years at Site 1221, while the North Pacific and North Atlantic sites peaked at 51 and 64 respectively. Crucially, the rise was not an artifact of sedimentation rates, since ichthyolith abundance showed negligible correlation with the total mass of sediment accumulating. Nor was it a sign of a community shift: most ichthyoliths were smaller than 63 micrometers, tooth-type diversity remained stable across the event, and rare tooth types were evenly distributed throughout the sequence, all pointing to persistent fish communities dominated by the same conical and triangular tooth morphotypes before, during, and after the warming.

With community composition ruled out as the driver, the evidence converged on a genuine increase in fish productivity. Warmer waters can accelerate fish growth and shorten generation times, which might boost tooth flux on its own, but the researchers found a nonlinear relationship between ichthyolith rates and temperature or latitude, suggesting trophic effects alone could not explain the geographic pattern. The missing piece came from the paired barite records. At the equatorial Pacific sites, barite-derived carbon export climbed to 187 grams of carbon per square meter per year within the main carbon isotope excursion, and its two prominent peaks, one at the onset of the event and another roughly 45,000 years later, coincided precisely with the two peaks in the fish record. In the North Atlantic, export reached 166 grams of carbon per square meter per year, far above what the region supports today.

Globally, the correlation between the two proxies was statistically robust: pooled carbon export data from all sites showed a significant positive relationship with ichthyolith accumulation, with a Pearson correlation coefficient of 0.55 and a Spearman coefficient of 0.61, both significant at the 0.001 level. The spatial pattern matched too, with the strongest responses in the Eastern Equatorial Pacific and North Atlantic, regions close to upwelling zones where nutrient-rich deep waters reach the surface, and the weakest in the mid- to high-latitude Southern Hemisphere, where both export and fish productivity remained low. The implication is that enhanced carbon export from the surface ocean fueled the boom in fish productivity, delivering more food to the mid-trophic-level consumers whose teeth preserve the signal.

To understand why export productivity rose despite the stratifying effects of warming, the team ran a series of simulations with the cGENIE Earth system model, using early Eocene paleogeography and progressively switching on individual mechanisms. Doubling atmospheric carbon dioxide from 1,112 to 2,224 parts per million on its own produced only a negligible long-term increase in organic carbon export, far smaller than the proxy records demand. The decisive factor was enhanced weathering on land. Lithium isotope evidence indicates terrestrial weathering intensified globally by 50 to 60 percent during the PETM, and when the model incorporated temperature-dependent phosphorus weathering, the ocean-wide average export flux rose from 0.7 to 1.2 moles per square meter per year, with a 1.9-fold weathering enhancement pushing the flux to about 1.6 moles, consistent with the two- to threefold rise inferred from the barite records. Regional iron fertilization from volcanic ash of the North Atlantic Igneous Province and enhanced dust production in North America likely amplified productivity near those sources, which the model, lacking regional iron input, underestimates. Notably, adding redox-dependent phosphorus recycling under low-oxygen conditions contributed almost nothing, because a deeper thermocline in the warmer, more stratified ocean limited the upward transport of recycled nutrients into the sunlit zone.

The findings carry a provocative message for the future. The fertilizing effect of terrestrial nutrient input operates on longer timescales than the decadal trends captured by satellites, and the PETM record shows it can outweigh the suppressive effects of stratification, sustaining higher ocean productivity under sustained greenhouse warming. Rising export productivity may even have helped the Earth system recover from the hyperthermal by burying organic carbon and drawing down carbon dioxide, a negative feedback visible in the elevated organic carbon contents of equatorial Pacific and North Atlantic sediments from the event. The researchers caution that higher export in the water column does not necessarily mean more organic matter reaches the seafloor, since warmer, acidified waters accelerate microbial degradation of sinking particles, and that PETM dynamics cannot be applied directly to decadal forecasts. Still, the study raises the possibility that today’s declining productivity trends could be moderated, or even reversed, over the long arc of a warming century and beyond, a nuance that current climate models, with their divergent projections, have yet to fully capture.

Subject of Research: Marine productivity and fish abundance during the Paleocene-Eocene Thermal Maximum

Article Title: Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum

Article References: Zhou, X., Zhang, R., Tsang, M.-Y., Li, M., & Yao, W. (2026). Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum. Nature Communications, 17(1), Article 10030. https://doi.org/10.1038/s41467-026-77863-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77863-9

Keywords: Paleocene-Eocene Thermal Maximum, marine productivity, ichthyoliths, marine barite, carbon export, paleoceanography, ocean warming, nutrient supply, cGENIE model, fish fossils, ocean stratification, climate change

The controversy began in 2021, when researchers reported widespread intervals in Arctic sediments that were barren of excess thorium-230, an isotope produced in seawater from the decay of dissolved uranium, alongside strikingly low concentrations of the cosmogenic isotope beryllium-10, which rains down from the atmosphere. The original team interpreted these signals as evidence of a freshwater-filled Arctic Ocean: without salty, uranium-bearing Atlantic water, little thorium-230 would be produced, and a permanent ice cover would starve the sea of atmospheric beryllium-10. Critics countered that sudden pulses of sediment could simply have diluted both isotopes, leaving the ocean connected after all. The debate has raged ever since, because the interpretation carries enormous implications for how the global climate system behaved during ice ages.

The new research, led by Agathe Ollive of the Alfred Wegener Institute in Bremerhaven together with colleagues from Germany and Norway, sidesteps this dispute by turning to a cleverer tracer: the ratio of two beryllium isotopes, beryllium-10 and beryllium-9, measured in the reactive fraction of seafloor sediments. The beauty of this ratio lies in its resilience. While the concentrations of each isotope individually can be skewed by changing sedimentation rates and particle scavenging, normalizing beryllium-10 against beryllium-9 largely cancels out those effects. What remains is a fingerprint of the water masses themselves—essentially a chemical tag that records which waters bathed the seafloor when the sediments were deposited.

The logic of the test is elegantly simple. Today, the Arctic Ocean is fed by two very different beryllium sources. Atlantic water carries a dissolved beryllium-10 to beryllium-9 ratio of roughly 5 to 7 times ten to the minus eight atoms per atom, while Arctic rivers deliver beryllium with ratios below 1.5 times ten to the minus eight. If North Atlantic inflow had continued during the glacial intervals in question, the sedimentary ratio should have stayed near Atlantic values. But if the Arctic Mediterranean was truly cut off and gradually freshened by river discharge, the ratio should have collapsed toward riverine signatures. The team measured 84 samples from sediment cores spanning the central Arctic Ocean, the Lomonosov Ridge, the Yermak Plateau, the Barents Sea slope, the Morris Jessup Rise, the Nordic Seas and the North Atlantic, at water depths from about 1,073 to 2,802 meters.

The result was unambiguous. During the low-thorium glacial intervals, reactive beryllium-10 to beryllium-9 ratios fell to between 0.06 and 0.64 times ten to the minus eight—uniformly low values that are not merely riverine but even below the range of modern Arctic rivers. Remarkably, this signature appeared at every site, across all the major basins of the Arctic Mediterranean, despite huge differences in water depth, sedimentation rate and sediment source. Such basin-wide homogeneity is exactly what an isolated, freshened sea would produce, and it is utterly incompatible with an active Atlantic inflow, which would have delivered beryllium with ratios one to two orders of magnitude higher. During the warm interglacial of Marine Isotope Stage 5e, by contrast, the ratios rebounded to around 4 times ten to the minus eight in the Nordic Seas and about 1.4 times ten to the minus eight in the central Arctic, matching the signatures of Atlantic-influenced waters seen in modern deep Arctic sediments.

The team then extended the story into the open Atlantic by analyzing core MD95-2016, a site today bathed in dense overflow waters spilling across the Greenland-Scotland Ridge. Because this core contains a continuous oxygen isotope record, it could be aligned precisely to global stratigraphy. Both the Atlantic core and the Nordic Seas core recorded nearly identical beryllium ratios of about 5 times ten to the minus eight during the interglacial, confirming an open connection. But during the MIS6 glaciation, both records plunged. The researchers interpret the low value in the North Atlantic core as marking the moment of reconnection, when water with an extremely low beryllium isotope ratio—stored up in the isolated Arctic during millennia of isolation—suddenly overflowed the Greenland-Scotland Ridge and flooded into the Atlantic.

Crucially, the beryllium evidence converges with entirely independent tracers. In a central Arctic core, the glacial drop in beryllium-10 to beryllium-9 coincides with shifts in neodymium, hafnium and strontium isotopes away from North Atlantic endmember values and toward compositions characteristic of Siberian rivers draining the Putorana Basalts into the Kara Sea. The absence of dinocysts and planktic foraminifera in cores from the same intervals further points to a shutdown of Atlantic water and harsh surface conditions. When multiple unrelated proxy systems all swing in the same direction at the same time, the case for a genuine circulation reorganization becomes very difficult to dismiss.

The authors also ran a steady-state mass balance model to test whether anything other than isolation could explain the numbers. Under modern conditions, the North Atlantic inflow supplies roughly 70 percent of the beryllium-10 entering the Arctic Ocean, and the model reproduces the observed water-column ratio of about 5.14 times ten to the minus eight. Reducing atmospheric deposition under permanent sea ice lowers the ratio by only about 20 percent—nowhere near enough. Closing the Bering Strait actually raises the modeled ratio slightly, because the Pacific inflow carries a relatively low signature. Even catastrophic meltwater outbursts from ice-dammed lakes such as Lake Komi and Lake Yamal, with combined volumes exceeding 30,000 cubic kilometers, could push the ratio down only to about 1.56 times ten to the minus eight. Only a complete shutdown of Atlantic inflow reproduces the measured glacial values of around 0.85 times ten to the minus eight. The team likewise ruled out hyperpycnal sediment flows, nepheloid plumes, brine rejection and benthic fluxes from seafloor diagenesis as primary drivers, each on quantitative grounds.

The combined weight of the thorium and beryllium systems points to a single coherent picture: during parts of MIS6 and MIS4, the Arctic Mediterranean was largely cut off from the global ocean, its salts progressively replaced by river freshwater within perhaps less than 10,000 years, all sealed beneath a grounded ice shelf that blocked exchange across the shallow straits of the Greenland-Scotland Ridge. Such a state would have fundamentally altered deep-water renewal, strengthened stratification and limited oxygen penetration to the deep basins—conditions consistent with geochemical hints of past anoxia in both the Canadian and Eurasian Basins. It would also have meant that a key nursery of today’s Atlantic Meridional Overturning Circulation simply switched off.

Perhaps the most tantalizing implication lies at the end of each isolation episode. When the Arctic ice shelf collapsed and the straits reopened, the enormous reservoir of low-salinity water accumulated over millennia would have been released through Fram Strait in short order. Whether such pulses were powerful enough to disrupt North Atlantic thermohaline circulation and trigger the abrupt cooling events recorded in ice cores remains an open question, and one the authors flag as a priority for future work. What is already clear is that the Arctic Ocean of the ice ages was not simply a colder version of today’s sea. It was, at times, a fundamentally different world—a giant, dark, freshwater basin sealed beneath ice, waiting for the lid to come off.

Subject of Research: Geochemical evidence for episodic isolation and freshening of the Arctic Mediterranean from the North Atlantic during late Pleistocene glaciations

Article Title: Episodic isolation of the Arctic Mediterranean from the North Atlantic during late Pleistocene glacials

Article References: Ollive, A., Geibert, W., Matthiessen, J., Loftfield, J., Lachner, J., Koll, D., Patton, H., Vogt, C., & Adolphi, F. (2026). Episodic isolation of the Arctic Mediterranean from the North Atlantic during late Pleistocene glacials. Communications Earth & Environment, 7(1), Article 821. https://doi.org/10.1038/s43247-026-04133-3

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04133-3

Keywords: Arctic Ocean, North Atlantic, beryllium isotopes, thorium-230, Pleistocene glaciations, ice shelf, paleoceanography, freshwater, AMOC, Greenland-Scotland Ridge, marine sediments, climate

The finding matters far beyond paleoclimate circles. The Miocene Climatic Optimum, abbreviated MCO and centered around fifteen million years ago, is one of the best available analogues for understanding how Earth’s climate system behaves when greenhouse gas concentrations and temperatures sit well above those of the modern era. If the MCO was warmer than previously believed, then the climate system’s long-term sensitivity to carbon dioxide, a quantity known as Earth system sensitivity, is likely higher than reconstructions based on the same interval have implied. That has direct consequences for how scientists project the planet’s response to ongoing fossil fuel emissions.

The central obstacle has long been the gap between what deep-sea sediments record and what climate models predict. The workhorse of deep-time temperature reconstruction is the oxygen isotope ratio, denoted δ18O, measured in the calcium carbonate shells of benthic foraminifera, single-celled organisms that live on or within seafloor sediments. Because foraminifera incorporate oxygen isotopes from seawater into their shells in a temperature-dependent way, the ratio preserved in fossil shells encodes information about the temperature of the deep ocean where the organisms lived. But the signal is confounded: the isotopic composition of seawater itself changes with global ice volume and with the hydrological cycle, so a single δ18O measurement cannot cleanly separate temperature from water-mass chemistry.

Earlier benthic δ18O-based reconstructions of the MCO typically handled this ambiguity with simplified assumptions, often subtracting an ice-volume correction from the isotope record and converting the remainder into temperature using linear calibrations. Those approaches suggested a Miocene world that was warm but, by some estimates, only modestly hotter than preindustrial conditions at the global mean. Surface proxy records, however, including those based on organic molecules and other temperature-sensitive indicators, frequently painted a picture of much greater warmth, particularly at high latitudes. The tension between deep-ocean and surface estimates has been one of the persistent puzzles of Miocene science.

Zhu and his coauthors, drawn from the NSF National Center for Atmospheric Research, Brown University, the University of Arizona, George Mason University, and the University of Southampton, attacked the problem by refusing to treat models and data as separate enterprises. Instead, they built a probabilistic inference framework that fuses the two. On the modeling side, they ran long simulations of the Community Earth System Model, or CESM, configured for Miocene boundary conditions and, crucially, run long enough for the deep ocean to reach full equilibrium with the imposed forcing. Deep-ocean equilibration is a technical but decisive requirement: the abyssal ocean adjusts to climate change on timescales of many thousands of years, and simulations that stop short of equilibrium can misstate deep-ocean temperatures by large margins, corrupting any subsequent comparison with benthic records.

The second modeling innovation was water isotope capability. The CESM simulations tracked the full cycle of oxygen isotopes through evaporation, atmospheric transport, precipitation, and ocean mixing, allowing the researchers to predict not just temperature fields but the isotopic composition of seawater at every location and depth. This matters because the δ18O of a benthic foraminiferal shell reflects both the temperature of the surrounding water and the δ18O of that water. By simulating both quantities explicitly, the model can generate synthetic benthic isotope values that can be compared, like for like, against the real sedimentary measurements, without relying on the simplifying assumptions that plagued earlier conversions.

On the data side, the team assembled a global compilation of benthic foraminiferal δ18O spanning the Miocene Climatic Optimum, drawing on deep-sea sediment cores from multiple ocean basins. The probabilistic framework then asked a deceptively simple question: given the ensemble of model simulations and their uncertainties, which combination of global mean surface temperature and other climate parameters is most likely to have produced the observed pattern of benthic isotope values across the globe? Because different climate states produce distinct spatial fingerprints in both deep-ocean temperature and seawater isotopes, the global pattern of measurements carries far more information than any single core, and the statistical framework exploits that structure to narrow the range of plausible climates.

The answer that emerged was striking. The maximum likelihood estimate placed MCO global mean surface temperature 7.5 degrees Celsius above preindustrial, a value significantly warmer than previous benthic δ18O-based reconstructions had delivered. The corresponding surface temperature field, the full geographic map of Miocene warmth implied by that best-fit climate state, ranked among the best agreements with independent surface temperature proxies, the very records that had previously seemed at odds with the deep-sea data. In other words, the framework did not merely split the difference between conflicting datasets; it found a climate state consistent with both, resolving the apparent contradiction by showing that earlier deep-ocean temperature estimates had been biased cold.

The implications ripple outward in two directions. First, a warmer MCO implies a higher MCO-derived estimate of Earth system sensitivity, the total warming the planet eventually delivers in response to a given change in radiative forcing once ice sheets, vegetation, and other slow feedbacks have fully adjusted. Earth system sensitivity inferred from warm paleoclimate intervals is one of the key benchmarks used to evaluate the long-term tail of future warming projections, and a higher Miocene value suggests that slow feedbacks may amplify greenhouse warming more strongly than some estimates allow. Second, the study demonstrates a methodological template. The authors emphasize that deep-ocean equilibration and proxy-model integration are both essential for accurately estimating deep-ocean and surface temperatures, and they present their inference framework as an approach applicable to improving global climate reconstructions across other time intervals, from older greenhouse episodes to the more recent glacial cycles.

The research also reflects the growing maturity of isotope-enabled climate modeling as a bridge between the geologic record and the physics of the climate system. Computing support came from the Derecho system at the NSF National Center for Atmospheric Research, and the work was funded by the U.S. National Science Foundation and the Heising-Simons Foundation, with additional support to coauthor David Evans from the Royal Society. As climate scientists confront a future that is pushing the Earth system toward states not seen in millions of years, studies like this one sharpen the only real experiments the planet has ever run on itself. Fifteen million years ago, the deep ocean was recording the signature of a world far hotter than the sediments had seemed to admit. Reading that signature correctly, with models and data speaking the same isotopic language, suggests the Miocene Climatic Optimum was warmer than we thought, and that the climate system’s memory of high carbon dioxide runs deeper than previous reconstructions allowed.

Subject of Research: Reconstructing the global warmth of the Miocene Climatic Optimum by integrating isotope-enabled climate model simulations with benthic foraminiferal oxygen isotope records

Article Title: Model-data synthesis of benthic isotopes suggests a warmer Miocene Climatic Optimum

Article References: Zhu, F., Zhu, J., Si, W., Nirenberg, J. E., Herbert, T., Tierney, J. E., Acosta, R. P., Burls, N. J., & Evans, D. (2026). Model-data synthesis of benthic isotopes suggests a warmer Miocene Climatic Optimum. Nature Communications. https://doi.org/10.1038/s41467-026-77980-5

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77980-5

Keywords: Miocene Climatic Optimum, paleoclimate, benthic foraminifera, oxygen isotopes, Earth system sensitivity, climate modeling, CESM, deep-ocean equilibration, proxy-model integration, Nature Communications, global mean surface temperature, paleoceanography

The study, published in the Journal of Micropalaeontology, focuses on the Cerro Azul section in the Neuquén Basin of Argentina, a sedimentary succession that straddles the Cretaceous–Paleogene (K–Pg) boundary, the geological dividing line between the Mesozoic and Cenozoic eras. The researchers analyzed a 1.10-meter interval of calcareous mudstone at 10-centimeter resolution, combining classical micropaleontology with a modern organic geochemical thermometer known as TEX86H. Their results paint a vivid picture of an ocean in crisis, dominated by a handful of hardy survivor species, followed by the first faint signals of climatic recovery roughly 45 centimeters above the extinction horizon.

Planktic foraminifera are ideal witnesses to the catastrophe because they live in the upper layers of the ocean and are highly sensitive to temperature, salinity, and water-column structure. The team processed roughly 10 grams of sediment from each sample, breaking it down mechanically, soaking it in 29 percent hydrogen peroxide for 24 hours, and washing the residue through a 38-micrometer sieve. They then picked approximately 300 specimens per sample and identified them under a stereomicroscope using established taxonomic schemes, with key specimens imaged by scanning electron microscopy and deposited in the paleontological collections of the University of Buenos Aires.

What they found in the upper Maastrichtian, the final stage of the Cretaceous, was striking for what was missing. Typical open-marine taxa of that time, such as the ornate genera Abathomphalus and Globotruncana, are entirely absent from the section. Instead, the assemblage is composed almost exclusively of opportunistic, long-ranging species: Muricohedbergella monmouthensis, Planoheterohelix globulosa, and above all the tiny microperforate genus Guembelitria. At one horizon, Guembelitria species together account for a staggering 98.1 percent of the assemblage, with Guembelitria cretacea alone making up 66.4 percent. The researchers attribute this unusual composition to the shallow, semi-restricted nature of the Neuquén Basin, which imposed environmental stress on marine life even before the asteroid struck.

The absence of standard biostratigraphic markers posed a challenge for dating the sequence. The late Maastrichtian index species Plummerita hantkeninoides, which defines the very top of the Cretaceous at low-latitude sites such as the Poty quarry and Campos Basin in Brazil, never appears at Cerro Azul. Nor does the classic earliest Danian marker Parvularugoglobigerina eugubina, the zonal species used worldwide to subdivide the first moments of the Paleogene. Faced with this problem, the team turned to a high-latitude zonal scheme developed for Antarctic and circum-Antarctic sections, which relies on species that thrive at cooler southern latitudes and can be correlated reliably across vast distances of the Southern Ocean.

Using that scheme, the researchers identified two early Danian biozones at Cerro Azul. The lowermost 45 centimeters above the K–Pg boundary belong to the AP0 zone, defined by the partial range of Turborotalita nikolasi, a species whose base is marked by the last appearances of the Cretaceous survivors Planoheterohelix globulosa and Muricohedbergella monmouthensis. Above that, the first occurrence of Globoconusa daubjergensis defines the base of subzone AP1a, which extends to the top of the studied interval. The recognition of these zones confirms that the Cerro Azul section preserves a continuous record across the extinction, a rarity in South America, where many purported K–Pg sections, including the nearby Bajada del Jagüel locality, were truncated by a disconformity that erased part of the critical interval.

Perhaps the most significant biogeographic discovery concerns Antarcticella pauciloculata, a species previously known only from the Austral Biogeographic Province, encompassing the circum-Antarctic region and New Zealand, with records from Seymour Island, Maud Rise, and the Mentelle Basin off southwestern Australia. At Cerro Azul, the species appears immediately after the K–Pg boundary and becomes the most abundant Danian taxon, reaching 42.3 percent of the assemblage. This marks the first time A. pauciloculata has been reported outside the Austral Province, extending the northern boundary of its known range to mid-latitudes of roughly 45 degrees south in the South Atlantic Ocean and demonstrating that cool-water faunas pushed much farther equatorward in the extinction’s aftermath than previously recognized.

The geochemical half of the study provided an independent temperature record to test these biological signals. The TEX86H proxy is based on isoprenoid glycerol dialkyl glycerol tetraethers, membrane lipids produced by archaea living in the surface ocean, whose relative abundances shift predictably with temperature. The team measured these compounds using high-performance liquid chromatography coupled to mass spectrometry and converted the TEX86H index into sea surface temperature using the calibration of Kim and colleagues. Crucially, they ran a battery of quality checks: BIT index values below 0.2 indicated genuinely marine conditions with little terrestrial contamination, low GDGT-0/cren ratios ruled out methanogen inputs, low GDGT-2/GDGT-3 ratios confirmed surface-water production of the lipids, and methane index values between 0.13 and 0.33 showed no interference from methane-loving archaea. Together, these diagnostics indicate the temperature signal is robust.

The temperature record tells a compelling story. In the first 45 centimeters of the Danian, within biozone AP0, sea surface temperatures hovered between 28.6 and 29.3 degrees Celsius. Then, right at the transition to subzone AP1a, temperatures jumped by approximately 1.5 degrees Celsius, reaching values between 29.8 and 30.7 degrees Celsius with an average near 30.3, before declining slightly toward the top of the section. This warming coincides precisely with the appearance and gradual increase of Woodringina claytonensis, a species whose stable isotope signatures indicate a preference for warm, near-surface waters and which is abundant at low-latitude open-ocean sites but rare or absent at high latitudes. No Woodringina species has ever been reported from Southern Hemisphere high-latitude sections, making its presence at 45 degrees south a genuine anomaly.

The authors suggest that the warming spike may correspond to the Dan-C2 event, a short-lived hyperthermal episode recognized in the earliest Danian at sites ranging from the Blake Nose Plateau, where oxygen isotopes record a warming of about 4 degrees Celsius, to the Tethyan Contessa Highway section in Italy and the Walvis Ridge in the South Atlantic. If the correlation holds, it would mean that even a mid-latitude, semi-restricted basin felt the pulse of this global warming event, and that tropical species such as W. claytonensis migrated poleward in response. The slight decrease in the abundance of the cool-water Antarcticella pauciloculata at the same horizon fits this interpretation neatly. Taken together, the Cerro Azul record captures the full arc of the extinction story: a stressed Late Cretaceous ocean, a catastrophic collapse of diversity, a world of opportunistic survivors, and finally, within a few tens of thousands of years, the first warming-driven stirrings of recovery that would eventually give rise to all the Cenozoic lineages of plankton, and ultimately to the modern ocean ecosystem.

Subject of Research: Planktic foraminiferal biostratigraphy and sea surface temperature change across the Cretaceous–Paleogene boundary in the Neuquén Basin, Argentina

Article Title: Planktic foraminiferal distribution across the Cretaceous–Paleogene (K–Pg) boundary from the Neuquén Basin (Cerro Azul section): biostratigraphy and paleoenvironmental significance

Article References: Krahl, G., Concheyro, A., Bom, M. H. H., Guerra, R. M., Kochhann, K. G. D., Bauersachs, T., Schwark, L., Ceolin, D., Musso, T., & Fauth, G. (2026). Planktic foraminiferal distribution across the Cretaceous–Paleogene (K–Pg) boundary from the Neuquén Basin (Cerro Azul section): biostratigraphy and paleoenvironmental significance. Journal of Micropalaeontology, 45(1), 297-308. https://doi.org/10.5194/jm-45-297-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-297-2026

Keywords: planktic foraminifera, K–Pg boundary, mass extinction, Neuquén Basin, Cerro Azul, TEX86H, sea surface temperature, biostratigraphy, early Danian, Antarcticella pauciloculata, Guembelitria, paleoceanography

The research team, led by Fen Yang and Sen Li of the China University of Geosciences in Wuhan, together with Stephen Grasby of the Geological Survey of Canada, David Bond of the University of Hull, and colleagues, examined two exquisitely preserved rock successions that straddle the Permian-Triassic boundary. The first, at Chibi in South China, sat in equatorial waters of the Tethys Ocean during the Late Permian. The second, at Ursula Creek on the north shore of Williston Lake in British Columbia, lay some thirty degrees north along the northwestern margin of Pangaea, facing the vast Panthalassa Ocean that surrounded the supercontinent. By comparing these palaeogeographically distinct archives, the team could test whether the oxygen crisis was a local quirk or a global phenomenon, and whether its character changed as the extinction gave way to the hothouse world of the Early Triassic.

Deciphering the chemistry of ancient seawater requires proxies, and the study deployed three independent ones in concert. The first involves pyrite framboids, tiny raspberry-shaped clusters of iron sulfide that form in the water column before settling into sediment. Their size is a sensitive oxygen gauge: framboids forming in anoxic, sulfidic waters are typically smaller than five micrometers, while those forming under low-oxygen but not fully anoxic conditions grow larger, often between six and ten micrometers. Where bottom waters are well oxygenated, framboids are absent altogether. The researchers measured more than two thousand framboids from Ursula Creek using scanning electron microscopy, tracking how their size distributions shifted through the extinction interval.

The second and third proxies come from sediment geochemistry. Iron speciation separates the iron locked in pyrite and other reactive phases from the total iron pool, allowing researchers to distinguish between euxinic conditions, where hydrogen sulfide accumulates in the water, and ferruginous conditions, where the water is anoxic but rich in dissolved iron(II) and essentially sulfide-free. Meanwhile, the enrichment of the redox-sensitive trace elements molybdenum and uranium, and especially the ratio between their enrichment factors, provides an independent cross-check: strong enrichment of both metals with a high molybdenum-to-uranium ratio points toward euxinia, whereas moderate enrichment with a low ratio signals oxygen-depleted but non-sulfidic waters. Applying all three methods to ninety-seven samples for iron analysis and one hundred and three for trace elements gave the team an unusually robust picture of redox evolution.

The results tell a two-act story. During the late Changhsingian, the final stage of the Permian, bottom waters at both Chibi and Ursula Creek were predominantly dysoxic, meaning oxygen was present but persistently low. At Ursula Creek, abundant framboids with mean diameters between roughly six and ten micrometers, combined with low molybdenum-to-uranium enrichment ratios, painted a picture of a seafloor limping along under chronic oxygen stress. Intriguingly, an interval in the middle of the Late Permian succession showed intense bioturbation, trace fossils burrowed by organisms that need oxygen to survive, alongside a near-total absence of framboids, indicating that oxygenated refugia existed at least episodically even as the crisis approached. The picture was not one of uniform suffocation but of a patchy, oscillating ocean.

Then, at the extinction horizon itself, conditions at Ursula Creek deteriorated dramatically. The framboid populations shrank, with mean diameters dropping to between 5.3 and 6.5 micrometers, and molybdenum and uranium enrichment factors surged, with molybdenum concentrations in the earliest Triassic Grayling Formation reaching mean values of twenty parts per million, more than twenty-five times the Late Permian average. These signals, the authors argue, implicate seafloor anoxia as a key trigger for the marine extinctions recorded in this Panthalassan margin section. The killing mechanism, in other words, was not a slow decline but an abrupt intensification of oxygen loss synchronized with the biotic collapse.

The most surprising twist came in the aftermath. In the earliest Triassic, both sections recorded a shift to persistently ferruginous conditions, anoxic waters charged with dissolved iron but lacking hydrogen sulfide. At Ursula Creek, this showed up as a striking gap in the pyrite framboid record, a phenomenon the researchers link to sulfide-poor waters, combined with high ratios of highly reactive iron to total iron and elevated sums of oxide- and pyrite-bound iron that exceeded the threshold for ferruginous deposition. At Chibi, the Lower Triassic Daye Formation similarly lacked framboids in its basal beds and showed iron speciation signatures and molybdenum-to-uranium ratios well below one-tenth of modern seawater values, all consistent with iron-rich, oxygen-limited bottom waters. A global compilation of iron speciation data from sections spanning Panthalassa, the Tethys realm, and the Pangaea margin confirmed that this was no regional accident: ferruginous conditions expanded significantly across epicontinental seas in the earliest Triassic, even as euxinic and ferruginous states continued to trade places in a spatially patchy ocean.

Why did the oceans swing toward iron instead of sulfide? The authors propose a combination of mechanisms. Extensive deposition of Late Permian evaporites and intensified pyrite burial drew down the ocean’s sulfate reservoir, starving the microbial processes that generate hydrogen sulfide. At the same time, extreme greenhouse warming stratified the ocean, suppressing vertical mixing and nutrient upwelling, which curtailed primary productivity and reduced the organic carbon flux that fuels sulfate reduction in sediments. Finally, intense chemical weathering of iron-bearing silicate rocks under the hot, carbon-dioxide-rich Early Triassic atmosphere delivered a flood of reactive iron to the sea, tipping the balance decisively toward ferruginous chemistry.

That ferruginous aftermath may hold the key to one of the great puzzles of the extinction: why recovery took so long. Ferruginous waters are nutrient traps. Dissolved iron(II) binds phosphorus, the limiting nutrient for marine productivity, sequestering it into authigenic minerals such as vivianite and green-rust phases and locking it away in sediments. This reduces the recycled phosphorus flux back into the water column, weakening the feedback loop that normally sustains rich plankton blooms. The study points to covariation between redox state and productivity across the extinction interval: where ferruginous conditions prevailed, productivity collapsed, while euxinic settings sometimes sustained higher organic carbon fluxes. In South China, total organic carbon concentrations and accumulation rates plummeted across the extinction, and phosphorus recycling was restricted, even as enhanced continental weathering delivered more detritus to the sea than ever. The contradiction between abundant eroded material and starving seas is resolved, the authors suggest, by recognizing that thermal stratification, disrupted nutrient recycling, redox instability, and lethal heat stress on primary producers combined to keep the marine food web from restarting.

The broader implication is sobering. Oxygen depletion alone may not fully explain either the killing or the delay; instead, the end-Permian crisis may have been a compound disaster in which expanding ferruginous seas imposed simultaneous oxygen and nutrient stress on marine ecosystems. As the modern ocean warms and loses oxygen in response to rising greenhouse gases, the Permian-Triassic record offers a deep-time warning: deoxygenation does not merely asphyxiate marine life, it can rewire ocean chemistry in ways that sabotage the very nutrient cycles life depends on, turning recovery from a sprint into a marathon of millions of years.

Subject of Research: Ocean redox conditions and ferruginous anoxia across the Permian-Triassic mass extinction and recovery

Article Title: Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas

Article References: Yang, F., Li, S., Grasby, S. E., Bond, D. P. G., Pan, M., & Sun, Y. (2026). Permian-Triassic redox shift and its ferruginous aftermath in epicontinental seas. Climate of the Past, 22(8), 1481-1497. https://doi.org/10.5194/cp-22-1481-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1481-2026

Keywords: end-Permian mass extinction, ocean anoxia, ferruginous conditions, pyrite framboids, iron speciation, molybdenum-uranium proxies, Early Triassic, epicontinental seas, phosphorus cycling, biotic recovery, paleoceanography, Climate of the Past

A new study by Boris-Theofanis Karatsolis, Joseph D. Asanbe, and Jorijntje Henderiks, published in the Journal of Micropalaeontology, tackles that puzzle by reading the fine print written by the algae themselves. Rather than relying solely on bulk carbonate accumulation rates, the team compared the species composition of calcareous nannofossil assemblages at two astronomically tuned deep-sea archives: ODP Site 1264 on Walvis Ridge in the South Atlantic and IODP Site U1463 on the northwest Australian shelf in the Indian Ocean. Their conclusion is striking. The timing of the biogenic bloom’s demise was not dictated by a single global trigger acting uniformly everywhere, but by which fast-growing coccolithophore species happened to dominate the ecosystem in each region.

The backdrop to this work is a genuine scientific tension. A 2022 meta-analysis of multiple deep-sea records, led by Karatsolis, pointed to a synchronous termination of the biogenic bloom in low latitudes between 4.6 and 4.4 million years ago, an abrupt step toward reduced paleoproductivity that coincided with a phase of decreasing eccentricity and low-amplitude obliquity in Earth’s orbital cycles. That orbital configuration, the authors proposed, may have weakened monsoon systems and cut the nutrient supply to the surface ocean, throttling biogenic sedimentation across the tropics. Yet an independently tuned carbonate accumulation record from South Atlantic Site 1264 told a different tale: productivity there declined around 4 million years ago, but elevated carbonate burial persisted in the southern mid-latitudes until roughly 3.3 million years ago. Something regional, not global, seemed to be controlling the schedule.

To investigate, the team analyzed 21 sediment samples from Site 1264 spanning 5 to 3 million years ago, taken at depths aligned with the astronomical tuning ties of the site’s high-resolution age model. Using a dilution-based drop technique to prepare microscopy slides, they counted at least 300 specimens per sample at 1000-times magnification, classifying the dominant Noelaerhabdaceae coccolithophores, the genera Reticulofenestra and Gephyrocapsa, into small, medium, and large size categories using a 3 and 5 micrometer cutoff. From these counts they calculated relative and absolute abundances, nannofossil accumulation rates, and, crucially, the carbonate mass contributed by each taxon, converting coccolith size and shape into estimates of the actual calcite each group delivered to the seafloor.

The results reveal a carbonate factory in transition. At Site 1264, small Reticulofenestra, opportunistic, bloom-forming algae with coccoliths under 3 micrometers, climbed steadily to around 90 percent of the assemblage by about 4.4 million years ago, then began a long decline toward roughly 20 percent by the late Pliocene. Small Gephyrocapsa, the species that stole the show in the Indian Ocean, was consistently present at Site 1264 between 4.2 and 3.3 million years ago but never exceeded about 8 percent of the assemblage. Meanwhile, medium-sized Reticulofenestra faded early, then rebounded after 3.8 million years ago to reach about 60 percent by 3.2 million years ago, likely reflecting the appearance of the species Reticulofenestra minutula. Absolute abundances peaked spectacularly, with concentrations exceeding 30 billion coccoliths per gram of sediment around 4.4 and 3.5 million years ago.

When the team compared these ecological shifts against the carbonate mass accumulation rates, a two-step pattern emerged. Median carbonate fluxes at Site 1264 dropped significantly after 4.1 million years ago, falling to levels resembling those before the bloom even began, and then declined again at 3.3 million years ago. The first step coincided with reductions in the abundance and carbonate mass contribution of small and large Reticulofenestra, while the second matched a broader decline in sedimentation rates that pushed small Reticulofenestra nearly to disappearance. Throughout, nannofossils, not foraminifera, drove the long-term carbonate trends, confirming that these microscopic algae were the engine of carbonate burial at the site.

The contrast with the northwest Australian shelf could hardly be sharper. At IODP Site U1463, the end of the biogenic bloom was marked by an abrupt collapse of small Reticulofenestra between 4.6 and 4.4 million years ago, followed by the rise of small Gephyrocapsa, which by 4.2 million years ago had become the dominant species and, by mass, the most effective carbonate producer, sometimes contributing more than 45 percent of the nannofossil-derived calcite. At Site 1264, no such takeover ever occurred. Instead, medium-sized Reticulofenestra filled the ecological space left vacant by their shrinking relatives. The same species turnover that looked like a universal biostratigraphic marker in the Indian Ocean simply never materialized in this corner of the South Atlantic.

The explanation, the authors argue, lies in regional oceanography. Small Gephyrocapsa acmes, short-lived intervals of dramatic abundance, have now been documented across the Indian Ocean, in the Tasman Sea gateway, and at Atlantic sites bathed by Indian Ocean water, including ODP Site 1088 on the Agulhas Ridge, where the species exceeded 60 percent of the small coccolith assemblage. Sites within the South Atlantic Gyre, like Site 1264, never saw the acme. Modern descendants of Gephyrocapsa prefer warm, stratified, tropical-to-subtropical waters, and the early Pliocene distribution of their ancestors appears to have followed the same rules, tracking the reach of Indian Ocean-influenced surface currents. The ecological dominance of small Gephyrocapsa, in other words, was an adaptation to the warmer, more stratified post-bloom ocean, but only where those conditions actually prevailed.

Why does this matter beyond the Miocene-Pliocene boundary? Because coccolithophores remain one of the planet’s most important carbonate pumps, and this study demonstrates that the species-level identity of the dominant bloom-formers, not just total productivity, controls how much calcite reaches the seafloor and when. A global climate perturbation that lowers nutrient supply will produce different sedimentary signatures depending on whether the local ecosystem is built around Reticulofenestra or Gephyrocapsa, species that likely differed in growth rates, calcification rates, and ecological tolerances. The delayed termination of the biogenic bloom at mid-latitude Site 1264, the authors conclude, is best explained by the regional ecological success of the coccolithophores that controlled carbonate export there. Bridging the gap between short-term phytoplankton dynamics and long-term biogeochemical change, they suggest, will require exactly this kind of high-resolution, basin-to-basin comparison, along with morphometric and geochemical work to pin down what made these ancient opportunists tick.

Subject of Research: Coccolithophore assemblage changes across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans

Article Title: Coccolithophore signatures across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans

Article References: Karatsolis, B.-T., Asanbe, J. D., & Henderiks, J. (2026). Coccolithophore signatures across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans. Journal of Micropalaeontology, 45(1), 475-486. https://doi.org/10.5194/jm-45-475-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-475-2026

Keywords: coccolithophores, biogenic bloom, Pliocene, Miocene, calcareous nannofossils, paleoproductivity, carbonate burial, Reticulofenestra, Gephyrocapsa, ODP Site 1264, IODP Site U1463, paleoceanography

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 team examined 10 outcrop sections spread across five regions of the Yangtze Platform: western Hunan, northeastern Guizhou, eastern Hubei, southern Shaanxi, and northern Sichuan. These strata, assigned to Cambrian Series 2 Stages 3 and 4, include both classic archaeocyath sponge reefs and quieter non-reefal carbonate settings. The scale of the work is staggering: 13,094 thin sections were prepared and scrutinized under a polarizing microscope, yielding more than 4,000 photomicrographs. Fossils were identified using diagnostic features such as filament diameter, length, branching pattern, sheath thickness, and overall morphology, with classifications cross-referenced against both fossil material and living cyanobacteria.

Among the headline discoveries are two new species: Streptubularia robustus, a robust curved tube with a thick micritic wall reaching external diameters of 85 to 125 micrometers, and Xianella mollis, an unbranched, wavy filament forming distinctive U-shaped mat-like bundles up to 3.3 millimeters long. The researchers interpret Xianella mollis as the calcified sheath of a cyanobacterium comparable to living Phormidiaceae such as Phormidium, whose modern bundles curve in strikingly similar ways. Streptubularia, by contrast, is compared with oscillatoriacean sheaths, though its sheer size and Y-shaped branching set it apart from better-known genera like Girvanella.

Perhaps the most consequential single finding concerns Acuasiphonoria, a genus of long, straight, needle-tipped filaments previously known only from Late Ordovician rocks of the Tarim Basin in northwest China. Its presence in the Qingxudong Formation of Hunan Province pushes the fossil record of this genus back roughly 40 million years, from the Late Ordovician into Cambrian Series 2. The study also reports the first occurrences in South China of six additional genera: Razumovskia, Subtifloria, Xianella, Bija, Botomaella, and Acuasiphonoria itself, substantially closing a biogeographic gap between the famously well-studied Cambrian floras of Siberia and the more fragmentary records elsewhere.

The systematic framework assigns the fossils to two cyanobacterial orders. Girvanella, Subtifloria, Razumovskia, Acuasiphonoria, Xianella, and Streptubularia belong to the Oscillatoriales, filamentous forms whose calcified sheaths tangle, bundle, or anastomose into cable-like strands. Kordephyton, Bija, Hedstroemia, and Botomaella are placed in the Nostocales, bushy, fan-like, radially branching forms resembling modern rivulariaceans such as Rivularia. The associated Microproblematica, including the dendritic Epiphyton, the chambered Renalcis, and the cleft-walled Izhella, remain of uncertain affinity, though some evidence, including direct indications of photosynthesis reported in earlier work, points toward a cyanobacterial or bacterial origin for at least some of them.

Why do these microbes calcify at all? The answer lies in their physiology. Cyanobacterial photosynthesis raises pH within and around the extracellular polysaccharide sheath, promoting the precipitation of calcium carbonate on or within the sheath itself. Because this process depends on seawater carbonate saturation state and atmospheric carbon dioxide levels, the abundance and diversity of calcified cyanobacteria through geological time can serve as a proxy for ancient ocean and atmosphere chemistry. Riding’s 1992 concept of cyanobacterial calcification episodes, or CCEs, formalizes this idea: intervals of enhanced marine cyanobacterial calcification alternate with episodes of reduced calcification, tracking secular shifts in the carbon cycle.

The Cambrian and Early Ordovician have long been recognized as major CCEs, and the new South Chinese data reinforce that picture with unprecedented regional detail. The assemblages from the Yangtze Platform closely resemble those of the Siberian Platform and North China, establishing that at least 15 genera of calcified cyanobacteria had achieved a genuinely global distribution in shallow marine waters by the early Cambrian. This is a striking contrast with North America and Europe, where early Cambrian calcified microfossil diversity is comparatively depauperate, commonly limited to Girvanella and Botomaella.

The study also reveals how these microbes built and organized ecosystems through time. During deposition of the Xiannüdong Formation in Stage 3, microbial-archaeocyath reefs developed, but calcified cyanobacteria were scarce, with Epiphyton and Renalcis merely encrusting sponge frameworks amid substantial terrigenous clastic input. In the Tianheban Formation of Stage 4, archaeocyaths still formed the reef skeleton, but calcified cyanobacteria such as Kordephyton, Streptubularia, and Bija took on important encrusting and binding roles. Then, when archaeocyaths went extinct, reefs in the Qingxudong Formation were constructed entirely by calcimicrobes, whose diversity and abundance surged. Filamentous mats reduced water flow and trapped sediment, while chambered Epiphyton and Renalcis clusters reinforced the framework and filled interstitial spaces, demonstrating a complete ecological handover from animals to microbes.

Notably, the research documents for the first time that calcified cyanobacteria flourished not only in reefs but also in non-reefal shallow-marine carbonate facies in South China, where they occur in low-energy settings, though in depauperate assemblages. The taxonomic composition tracks sedimentary environment closely, with diversity and abundance peaking in reefal facies whose character was set by the other reef builders present. The authors suggest that the gradual evolution from simple oscillatoriacean-dominated assemblages to more diverse communities including Phormidiaceae and Rivulariaceae may have been driven by ecological competition with archaeocyaths, by declining terrigenous detrital input, or by both factors together.

Beyond its taxonomic contributions, the study carries implications for understanding the dawn of the Phanerozoic Eon, the interval during which nearly all major animal phyla appeared. While the Cambrian explosion transformed animal life, the microbial world was undergoing its own quiet revolution, with calcifying cyanobacteria proliferating across reefs and open shelves alike. Because cyanobacterial calcification responds to atmospheric carbon dioxide and oceanic carbonate saturation, these fossil assemblages offer an independent window into the environmental conditions that accompanied the rise of complex animal ecosystems. The authors argue that continued study of these globally distributed floras will sharpen our understanding of cyanobacterial evolution, biogeography, and their environmental significance at one of the most pivotal moments in Earth history, when the chemistry of the oceans and atmosphere set the stage for half a billion years of animal-dominated life.

Subject of Research: Diversity and systematics of early Cambrian calcified cyanobacteria and associated microfossils in shallow-marine carbonates of the Yangtze Platform, South China

Article Title: Diversity and systematics of calcified cyanobacteria and associated microfossils in Cambrian Series 2 shallow-marine carbonates, Yangtze Platform, South China

Article References: Shen, J., Zheng, J., Liu, L., Wang, R., & Riding, R. (2026). Diversity and systematics of calcified cyanobacteria and associated microfossils in Cambrian Series 2 shallow-marine carbonates, Yangtze Platform, South China. Journal of Micropalaeontology, 45(2), 513-546. https://doi.org/10.5194/jm-45-513-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-513-2026

Keywords: calcified cyanobacteria, Cambrian Series 2, Yangtze Platform, South China, microfossils, cyanobacterial calcification episode, archaeocyath reefs, Girvanella, Acuasiphonoria, Microproblematica, paleoceanography, systematic paleontology

The problem that confronted Helen Coxall of Stockholm University and her international team was deceptively simple but had festered for five decades. Two distinct spinose morphotypes occur in central Arctic cores at different stratigraphic levels. The younger, shallower horizons, potentially including the Last Interglacial around 120,000 years ago, contain a small form generally assigned to Turborotalita quinqueloba, a well-known subpolar species that today thrives along the sea-ice edge of the Nordic Seas. The deeper, older horizons contain a larger, more loosely coiled form that has been variously called Globigerina exumbilicata or Turborotalita egelida. Without a reliable taxonomic framework, researchers could not confidently correlate these bioevents between cores, let alone interpret what they meant for past climate. Compounding the confusion, the name egelida was virtually unknown to most foraminifera specialists and had never appeared in the last great taxonomic synthesis of Neogene and Quaternary planktonic foraminifera.

To resolve the mess, the team combined classical micropalaeontology with modern quantitative rigor. They imaged hundreds of specimens using both light microscopy and high-resolution scanning electron microscopy, drawing on sediment cores spanning the Eurasian and Amerasian basins as well as living plankton collected south of Iceland during a 2002 research cruise. Crucially, they also tracked down and imaged, many for the first time, the original type specimens housed in the Smithsonian’s Cushman Collection, including the holotypes of exumbilicata, egelida and quinqueloba itself. Around 300 individuals from each of three key populations were measured for traits including maximum test diameter, proloculus size, final chamber dimensions and apertural lip thickness, then subjected to multivariate analysis of variance and linear discriminant analysis.

The statistics delivered a verdict with unusual clarity for microfossil work. The three populations occupy significantly different regions of morphospace, with a Pillai’s Trace statistic of 1.18 and a p-value below 0.001. Leave-one-out cross-validation of the discriminant model achieved an overall classification accuracy of nearly 89 percent, and the younger Arctic morphotype was correctly assigned in 98.3 percent of cases. The first discriminant axis, explaining just over 70 percent of between-group variance, was driven mainly by final chamber geometry and overall shell size, while the second axis was dominated by proloculus diameter, the size of the tiny initial chamber around which the whole shell is built.

On this evidence, the team confirmed that the younger Arctic invader is indeed Turborotalita quinqueloba, but with a twist. The central Arctic specimens are true dwarfs, averaging just 114 micrometres across compared with 190 micrometres for their Nordic Sea cousins, and they lack the thick gametogenic calcite crust that characterises sedimented shells from the subpolar North Atlantic. Yet their proloculus sizes are almost identical to those of classic quinqueloba, and they retain the species’ diagnostic tear-drop-shaped final chamber that overhangs the umbilicus. The researchers interpret the size difference as an environmental response, a dwarfing effect seen before in polar populations near the species’ modern northern limit, rather than evidence of a separate species. In other words, when conditions briefly warmed enough, quinqueloba pushed into the central Arctic and shrank under the stress.

The older morphotype is a different story altogether. It builds larger shells, averages fewer chambers, coils more loosely to leave a wide open umbilicus, and sports a globular final chamber that never overhangs the aperture. Its proloculus is strikingly larger, averaging nearly 22 micrometres against roughly 14 in both quinqueloba populations, and its wall texture is unique: broad, flat spine bases and, in spiral view, rows of pustules aligned into costellae-like radiating ridges, a feature never observed in Turborotalita. The team concludes that this form deserves recognition as a distinct morphospecies, and likely a distinct biological taxon, rather than a growth stage or ecophenotypic variant of quinqueloba.

Then comes the taxonomic detective work that gives the study its bite. The name egelida, coined by Richard Cifelli and R. K. Smith in 1970 from living plankton netted in the North Atlantic, turns out to rest on immature specimens. The team’s own genetically confirmed, living quinqueloba pumped from Icelandic waters lack the elongate final chamber that develops only at full maturity, and they look almost exactly like Cifelli and Smith’s egelida type material. Since the Arctic fossils are mature, sedimented shells, egelida cannot be the right name. Instead, the authors reinstate Globigerina exumbilicata, established by the pioneering Arctic oceanographer Yvonne Herman in 1974 from Alpha Ridge cores, whose holotype they imaged for the first time. Lacking the defining ampullate final chamber of Turborotalita, the species is provisionally placed in the genus Globigerina, with the generic name held in quotation marks to signal uncertainty.

The study also exposes a long-running case of mistaken identity with implications for anyone counting foraminifera under a light microscope. A smooth-walled, shiny, five-chambered morphotype of the polar species N. pachyderma, known as Nps-5, can look remarkably like small quinqueloba or even exumbilicata. Herman herself conflated this form with egelida in Baffin Bay plankton samples in the 1980s, an error the new SEM imagery decisively corrects: Nps-5’s walls bear solid, irregularly distributed pustules with no spine holes, whereas true spinose taxa show distinctive spine bases with central cavities. The authors argue that scanning electron microscopy should be treated as primary data in foraminiferal taxonomy, because the characters that define species often emerge only in the final stages of life.

Perhaps the most tantalising implication concerns timing. The latest biostratigraphic constraints, anchored by the last occurrence of the nannofossil Pseudoemiliania lacunosa around 440,000 years ago and dinoflagellate cyst events older than 1.8 million years, place the exumbilicata acme somewhere in the early to middle Pleistocene, not in the MIS 11 interglacial to which it had long been assigned. If the Globigerina affiliation holds, this would mean a member of the Globigerina lineage attempted to colonise the polar Arctic before N. pachyderma achieved its ecological dominance, an experiment in polar occupation that modern Globigerina bulloides, confined today to the southern Arctic gateways, has never repeated. The exumbilicata zone also appears to mark a pivotal transition from agglutinated to calcareous-dominated assemblages across the entire central Arctic.

Why does this matter beyond the world of shell taxonomy? Because these tiny fossils are among the few direct archives of past Arctic Ocean conditions, and their abundance peaks are used as markers of warm, seasonally ice-free intervals that may be analogues for where today’s rapidly warming Arctic is heading. A consistent taxonomic framework means cores from the Amerasian and Eurasian basins can finally be correlated on common ground, and paleoenvironmental reconstructions can be built on solid species definitions rather than a tangle of synonyms and misidentifications. After half a century of confusion, the ghost invaders of the Arctic now have names, and with names comes the ability to tell their story properly, one chamber at a time.

Subject of Research: Taxonomic revision of Quaternary subpolar spinose planktonic foraminifera from the central Arctic Ocean

Article Title: Quaternary Arctic planktonic foraminifera: morphological differentiation and taxonomic reassessment of subpolar spinose taxa

Article References: Coxall, H. K., Cronin, T. M., Darling, K., Husum, K., Handl, T., Huber, B. T., Kanvinde, M. M., Razmjooei, M., Vermassen, F., Voelker, A. H. L., Weitkamp, T. M., & O'Regan, M. (2026). Quaternary Arctic planktonic foraminifera: morphological differentiation and taxonomic reassessment of subpolar spinose taxa. Journal of Micropalaeontology, 45(2), 581-622. https://doi.org/10.5194/jm-45-581-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-581-2026

Keywords: planktonic foraminifera, Arctic Ocean, Quaternary, paleoceanography, taxonomy, Turborotalita quinqueloba, Globigerina exumbilicata, sea ice, morphometrics, scanning electron microscopy, Pleistocene, micropalaeontology

The research hinges on coccolithophores, single-celled calcifying phytoplankton that build minute plates called coccoliths around their cells. These organisms occupy a privileged position in marine carbon cycling because they participate in both the organic pump, fixing carbon dioxide through photosynthesis, and the carbonate pump, producing calcium carbonate that sinks toward the seafloor. When coccolithophores die, their plates rain down onto the ocean floor, where they are exceptionally well preserved. Because different species thrive at different depths and nutrient levels, the composition of fossil assemblages encodes a detailed record of past upper-ocean conditions. In tropical waters, the deep-dwelling species Florisphaera profunda is particularly valuable: it lives below the euphotic zone near the nutricline, the depth at which nutrient concentrations rise sharply, and its relative abundance in sediments tracks how deep that nutrient reservoir sits.

To exploit this proxy, the team analyzed piston core YDY05, a 2.39-meter sediment core retrieved from the southern Bay of Bengal at a water depth of 3,300 meters during a research cruise of the R/V Shiyan 3. The core was subsampled at 2-centimeter intervals, yielding 120 samples that were processed with a standard drop technique and examined under a polarizing-light microscope at 1,000-fold magnification. At least 300 coccoliths were counted per sample, and the chronological framework was anchored by six accelerator mass spectrometry radiocarbon dates on mixed planktonic foraminifera, calibrated with the Marine20 dataset and refined through Bayesian age-depth modeling. The resulting record extends back roughly 52,680 years, spanning Marine Isotope Stages 3, 2, and 1, with sedimentation rates ranging from about 3.7 to 8.6 centimeters per thousand years.

The assemblages turned out to be abundant and beautifully preserved throughout, dominated by three ecologically telling species: Florisphaera profunda, Gephyrocapsa oceanica, and Emiliania huxleyi, which together account for 85 to 97 percent of the community. From the relative abundance of F. profunda, the researchers computed primary productivity using an updated calibration for the tropical Indian Ocean derived from a global core-top dataset and satellite-based productivity estimates, a relationship with a correlation coefficient squared of 0.74 and a residual standard error of 1.5 grams of carbon per square meter per year. The reconstructed productivity ranged from roughly 108 to 412 grams of carbon per square meter per year, and the mean value of about 192 grams closely matches modern satellite estimates for the southern Bay of Bengal, a reassuring sign that the proxy is capturing real oceanographic signal rather than artifact.

The temporal pattern is striking. Between about 53,000 and 27,000 years ago, productivity averaged around 166 grams of carbon per square meter per year, then dipped to a minimum of roughly 108 grams during the Last Glacial Maximum. As the deglaciation unfolded, productivity climbed steadily, surging from about 146 grams at 18,000 years ago to a remarkable peak of 412 grams at around 11,000 years ago in the Early Holocene, before easing back to about 239 grams during the mid-Holocene. Species-level trends mirror this arc: F. profunda dominated before the glacial maximum, at one point spiking to about 80 percent of the assemblage between 27,000 and 25,000 years ago, while the upper-euphotic zone taxa E. huxleyi and G. oceanica surged during the deglaciation, reaching pronounced maxima near 11,000 years ago. Total coccolith fluxes followed suit, bottoming out before 27,000 years ago and peaking between 18,000 and 11,000 years ago.

Interpreting these numbers requires understanding why the Bay of Bengal behaves so differently from its neighbor across the Indian subcontinent, the Arabian Sea. Both basins feel the full force of the Indian Summer Monsoon, yet the Bay of Bengal is consistently less productive. The reason lies in freshwater. Summer monsoon precipitation there can reach about 20 millimeters per day, compounded by enormous discharge from the Ganges, Brahmaputra, Meghna, and Irrawaddy river systems. This deluge lowers surface salinity, creating a low-density lid over saltier subsurface water. The resulting halocline and barrier layer suppress vertical mixing, so nutrient-rich waters below the nutricline never reach the sunlit euphotic zone. During the spring intermonsoon, modern measurements show primary productivity between roughly 13 and 174 millimoles of carbon per square meter per day, but particulate organic carbon export stays low, between zero and 7.7 millimoles, a signature of chronic nutrient limitation imposed by stratification.

The glacial story that emerges from core YDY05 is one in which this freshwater control was amplified. During the Last Glacial Maximum, global sea level stood about 120 meters lower than today, restricting the Indonesian Throughflow and reorganizing Indo-Pacific circulation. But in the southern Bay of Bengal, the decisive factor was the regional freshwater balance: reduced precipitation, diminished river discharge, and disrupted shelf-basin connectivity kept the upper ocean strongly stratified, deepened the nutricline, and starved surface waters of nutrients. The persistently high abundance of the deep-dwelling F. profunda during glacial intervals, including its elevated absolute abundance during the glacial maximum itself, therefore reflects enhanced upper-ocean stability rather than enhanced productivity, a subtle distinction that only an assemblage-based proxy can resolve. As the planet deglaciated, rising seas restored shelf connectivity, and processes such as cyclonic eddies, island-induced upwelling near the Maldives, and equatorial circulation associated with the Wyrtki Jet periodically shoaled the thermocline, delivering nutrients upward and driving the Early Holocene productivity peak.

Perhaps the most consequential finding comes from the basin-wide comparison. The team compiled productivity proxy records from six sediment cores spanning the Indian Ocean, including coccolith-derived productivity, biogenic barium, total organic carbon accumulation, and coccolith strontium-to-calcium ratios, interpolated them to a common 1,700-year grid, and ran a principal component analysis. The first two components explain 80 percent of the variance. The dominant mode, PC1, tracks the global benthic oxygen isotope stack almost inversely, confirming that glacial-interglacial climate change is the primary driver of basin-scale productivity. Yet the spatial pattern of loadings reveals something deeper: northeastern Indian Ocean sites load positively while western and eastern tropical Indian Ocean sites load negatively. In the upwelling-dominated western and eastern basins, wind-driven systems produced the opposite productivity history, with glacial intervals often more productive than interglacial ones. The same planetary-scale climate forcing thus produced regionally opposite biological responses, depending on whether wind or freshwater governed nutrient supply.

The secondary mode, PC2, aligns with the Indian Monsoon Stack and separates the monsoon’s two faces: its atmospheric expression, seen in wind-driven upwelling in the Arabian Sea, and its hydrological expression, seen in precipitation, runoff, and salinity proxies from the Bay of Bengal. Site YDY05 loads only weakly on this wind-driven component and clusters instead with the hydrological records, demonstrating that in the southern Bay of Bengal, monsoon rains and river discharge, not monsoon winds, held the upper hand. Even during the intensified summer monsoons of the mid-Holocene, increased freshwater input reinforced stratification and offset the benefits of stronger wind-driven nutrient supply, which is why productivity declined after the Early Holocene peak. For scientists projecting how the ocean’s carbon pump will respond to ongoing warming and changing monsoon rainfall, the message from this 53,000-year record is sobering and clear: in stratified, river-influenced basins, the fate of carbon sequestration may rest less on how hard the winds blow and more on how much fresh water falls from the sky.

Subject of Research: Reconstruction of biological pump and primary productivity variability in the northeastern Indian Ocean since the last glaciation using coccolithophore assemblages

Article Title: Biological pump variations in the northeastern Indian Ocean since the last glaciation: evidence from coccolithophores

Article References: Liza, N. Y., Su, X., Luo, C., Zhang, L., Wan, S., & Xiang, R. (2026). Biological pump variations in the northeastern Indian Ocean since the last glaciation: evidence from coccolithophores. Journal of Micropalaeontology, 45(2), 699-716. https://doi.org/10.5194/jm-45-699-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-699-2026

Keywords: biological pump, coccolithophores, Bay of Bengal, Indian Ocean, primary productivity, monsoon, last glacial maximum, paleoceanography, Florisphaera profunda, carbon cycle, ocean stratification, sediment core