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