The new study, published in the journal Climate of the Past, focuses on a nearly 200-meter-thick stack of sedimentary rocks exposed near the village of Büyükteflek in the Çankırı Basin, at the northern edge of the Kırşehir Massif in central Anatolia. This region sits within Balkanatolia, a semi-continuous strip of land that stretched from the Alps to the Lesser Caucasus during the Paleogene and that biogeographers now recognize as a critical stepping stone between Asia and western Europe. During the Eocene, Balkanatolia was a mosaic of islands isolated by the Paratethys Sea to the north and east, the Neotethys Ocean to the south, and narrower seaways to the northwest. That isolation bred endemism: for millions of years, the province hosted its own distinctive cast of mammals found nowhere else. Only as tectonic shortening and falling sea levels stitched the islands together, first in the east through Anatolia and later in the west, did land connections to the rest of Laurasia begin to re-emerge.

To pin down the age of the Büyükteflek section, the researchers combined magnetostratigraphy, the record of ancient reversals of Earth’s magnetic field preserved in the rocks, with uranium–lead dating of interbedded volcanic ash layers and marine microfossil biostratigraphy. They drilled and oriented 110 core samples from red clays, silts, and paleosol carbonates, demagnetized them step by step in alternating fields and at progressively higher temperatures, and isolated the characteristic remanent magnetization carried by magnetite and hematite grains. Six distinct magnetic polarity zones emerged from the section. Correlating these with the geomagnetic polarity time scale, anchored by ash beds dated at about 32.1 and 32.6 million years ago, showed that the lower part of the section spans the Priabonian stage of the late Eocene, beginning around 37.7 million years ago, while the upper part records the early Rupelian stage of the Oligocene, including the Eocene–Oligocene Glacial Maximum at about 33.65 million years ago.

The sedimentology tells its own vivid story. The lower part of the section consists of fine-grained red floodplain deposits, channelized sandstones, and paleosols, representing a broad alluvial plain drained by southward-flowing rivers. Two prominent unconformities punctuate the sequence. The first, marked by an erosional surface cut into a well-developed caliche, records the uplift of the Çiçekdağı anticline to the south, a reorganization of the drainage network, and the birth of a northward-flowing system feeding a new lake in the basin. Above the second unconformity, a striking change appears: the soil carbonates switch from pure calcite to dolomite. Because dolomite typically precipitates in hypersaline settings where evaporation outpaces freshwater input, its arrival signals rising lake salinity and a shift toward markedly drier conditions, a conclusion reinforced by the thick Oligocene evaporite deposits found elsewhere in the Çankırı Basin.

The most striking results come from clumped isotope thermometry, a geochemical technique that measures the tendency of heavy carbon and oxygen isotopes to bond together within carbonate minerals. Because this bonding is temperature-dependent, the abundance of these isotope clumps in pedogenic carbonates, the nodules and caliches that form in ancient soils, provides a direct gauge of the temperature at which the carbonate crystallized. The team analyzed 14 carefully screened samples on a Thermo 253+ mass spectrometer, running ten to twenty replicates per sample across 28 analytical sessions to achieve a repeatability of about 30 parts per million. Petrographic examination confirmed that the carbonates retained their original micritic textures with no evidence of burial recrystallization, and the reconstructed temperatures all fell within plausible Earth-surface values, giving confidence that the primary climate signal had survived.

The temperature record divides cleanly into three phases. In the early to middle Priabonian, soil carbonate formation temperatures averaged a temperate 24.4 degrees Celsius. Then, beginning in the middle Priabonian around 37 million years ago, temperatures jumped by roughly 9 degrees Celsius, with samples between the two unconformities averaging 34 degrees. Finally, above the second unconformity, temperatures fell by about 7 to 8 degrees, settling at an average of 27.4 degrees as the Eocene–Oligocene Glacial Maximum approached. Statistical tests confirmed that all three groups differ significantly. Because soil carbonates in mid-latitude seasonal climates tend to form during warm, dry periods of soil dewatering, the researchers interpret these values as tracking warm-season, or summer, temperatures rather than annual means, a reading consistent with pollen-based temperature estimates from slightly older Lutetian deposits in Anatolia.

The warming phase is the headline discovery. Marine records from the Southern Ocean and the North Atlantic have long hinted at a Late Eocene Warming between about 37.5 and 36.5 million years ago, but this is the first quantitative terrestrial record of the event in western Eurasia. Its cause remains debated: some evidence points to rising atmospheric carbon dioxide late in the Eocene, while other work implicates a reorganization of North Atlantic ocean circulation, including the possible onset of a subtropical gyre or an early version of the Atlantic meridional overturning circulation around 36 million years ago. Crucially, the warming in Anatolia coincided with the first of two aridification steps. Reconstructed soil water oxygen isotope values rose by about 1.7 per mil, a pattern the authors attribute to progressive evaporative enrichment of soil moisture, and a modest rise in carbon isotope values hints at increasing plant water stress. Together, the data sketch a late Eocene Anatolia that was both hotter and drier than before.

The second aridification step, beginning around 35.2 million years ago, is the more consequential one. It coincides with the lake retreat recorded at the second unconformity, spans the Eocene–Oligocene Transition itself, and culminates in the cooling and salinity increase of the earliest Oligocene. Notably, this drying began at least 1.5 million years before the peak of Antarctic glaciation, which means it cannot be a simple consequence of the ice sheet’s growth. Instead, the authors suggest it reflects an earlier, spatially uneven cooling of the North Atlantic, where shifting ocean gyres responding to falling carbon dioxide levels redistributed heat and moisture well before the classic glacial maximum. The lake level drop, in other words, was decoupled from global sea level, pointing to a regional hydroclimatic crisis rather than a glacio-eustatic one.

These climatic swings map onto one of the most famous events in mammalian evolution: the Grande Coupure, the great faunal rupture that saw roughly 55 percent of western Europe’s placental mammal genera go extinct as mammals of Asian origin poured in. The timing is telling. The late Eocene warming and the first aridification step cover the mammalian biohorizons MP18 and MP19, exactly when the first Asia-derived taxa, including the anthracotheriid artiodactyl Elomeryx, the possible gelocid Phaneromeryx, and the amphicyonid carnivoran Cynodictis, trickled through Balkanatolia into western Europe. Environmental stress from heat and drought may have weakened Balkanatolia’s endemic fauna, which all but vanished during the Priabonian, while opening ecological space for Asian newcomers that had been present in the province since the Bartonian. Recent diversification modeling supports the idea that climate stress, rather than direct competition with the invaders, drove the decline of Europe’s native lineages.

Yet the story carries an ironic twist. During the second aridification step and the glacial maximum itself, faunal exchange between Balkanatolia and western Europe appears to have stopped, even though the sea-level fall at the glacial maximum fully connected the western Balkanatolian landmass to western Europe. The authors suggest that the drying and cooling created a persistent environmental barrier across central and southern Europe, one possibly sharpened by orographic effects on precipitation from the rising Alps, that blocked the westward march of Asian mammals until exchanges resumed later in the Rupelian. The last known endemic Balkanatolian mammal, the embrithopod Axainamasia sandersi, clung on until around the transition, showing that some island relics persisted even as their world grew hotter, drier, and then colder. What emerges from the hills of central Anatolia is a portrait of a continent in climatic flux, where successive pulses of warming, drying, and cooling did not merely accompany the fall of one fauna and the rise of another, but actively choreographed it.

Subject of Research: Terrestrial climate change in Anatolia across the Eocene–Oligocene Transition and its role in mammalian dispersal

Article Title: Anatolia (Türkiye) during the late Eocene and the Eocene–Oligocene Transition: successive warming and cooling, aridification, and implications for the westward dispersal of Asian terrestrial mammals

Article References: Botté, P., Licht, A., Jourdan, A.-L., Montheil, L., Demory, F., Kaya, M., Ocakoğlu, F., Akkiraz, M. S., İbilioğlu, D., Coster, P., Métais, G., Raynaud, B., & Beard, K. C. (2026). Anatolia (Türkiye) during the late Eocene and the Eocene–Oligocene Transition: successive warming and cooling, aridification, and implications for the westward dispersal of Asian terrestrial mammals. Climate of the Past, 22(9), 1585-1608. https://doi.org/10.5194/cp-22-1585-2026

Image Credits: AI Generated

DOI: 10.5194/cp-22-1585-2026

Keywords: Eocene–Oligocene Transition, Anatolia, clumped isotope thermometry, paleoclimate, aridification, mammal dispersal, Grande Coupure, Balkanatolia, magnetostratigraphy, pedogenic carbonates, Antarctic glaciation, Climate of the Past

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

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

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

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

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

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

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

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

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

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

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

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

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02106-x

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