Eocene–Oligocene transition
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| Eocene–Oligocene transition | |
|---|---|
 | |
| Name | Eocene–Oligocene transition |
| Start | ~34 Ma |
| End | ~33.6 Ma |
| Preceding | Eocene |
| Following | Oligocene |
| Notable events | Antarctic glaciation; Grande Coupure |
Eocene–Oligocene transition The Eocene–Oligocene transition marks a major global shift from the warm Eocene world to the cooler Oligocene climate and the onset of large-scale Antarctic glaciation, occurring near 34 million years ago during the Paleogene. This interval is associated with widespread faunal turnover including the European Grande Coupure, major changes recorded in marine sections such as the Walvis Ridge, and stratigraphic signals used by projects like Deep Sea Drilling Project and Ocean Drilling Program.
Background and paleoclimate
In the latest Paleocene and through the Eocene, high-latitude warmth prevailed, evidenced by proxies from sites like Green River Formation, Fossil Lake, and Messel Pit where floras and faunas show greenhouse conditions. Atmospheric evolution inferred from Vostok-era ice studies and comparisons with later records such as EPICA and Mauna Kea volcanism contextualize why the transition stands out versus earlier events like the Paleocene–Eocene Thermal Maximum and later shifts linked to Middle Miocene Climate Transition. Paleobotanical data from Florissant Fossil Beds, Rhineland Basin, and Bighorn Basin indicate declining tropical elements and expanding temperate assemblages concurrent with tectonic rearrangements associated with the Alpine orogeny and Andean orogeny.
Causes and mechanisms of cooling
Multiple interacting drivers are proposed, including orbital forcing documented in Milankovitch cycles, declining concentrations of greenhouse gases inferred from comparisons to Keeling Curve data analogues, and tectonic processes such as the opening of the Southern Ocean gateways including the Tasmanian Gateway and the nascent Drake Passage. Volcanic and weathering fluxes tied to the Deccan Traps and the uplift of the Himalayas and Tibetan Plateau altered carbon dioxide sinks, while ocean circulation reorganizations linked to the formation of the Antarctic Circumpolar Current and changes at the Agulhas Current and Gulf Stream modulated heat transport. Hypotheses invoking extraterrestrial forcing reference impacts discussed alongside the Chicxulub framework, though consensus favors combined tectonic, greenhouse gas, and orbital mechanisms.
Biotic responses and extinctions
The transition coincides with faunal turnovers such as the European Grande Coupure and shifts in mammalian assemblages documented at Quarry A, Quarry B, and North American localities including Willwood Formation. Marine biota show extinctions and radiations in groups recorded from Foraminifera assemblages at Blake Nose and Shatsky Rise, while terrestrial plants shift from Nothofagus-dominated floras to more temperate elements seen in Florence Lake and McAbee Fossil Beds. Bird and reptile records from Messel and Green River Formation reveal turnover coincident with mammalian changes tracked by researchers associated with institutions like the Smithsonian Institution and Natural History Museum, London.
Marine and terrestrial sedimentary records
High-resolution marine records come from cores recovered by the Deep Sea Drilling Project, Ocean Drilling Program, and Integrated Ocean Drilling Program, with key sites on Walvis Ridge, Kerguelen Plateau, and Shatsky Rise preserving foraminiferal and nannofossil evidence. Continental records from the Bighorn Basin, Brule Formation, and Rupelian successions preserve paleosols, leaf fossils, and mammalian faunas that correlate with marine signals; tectonostratigraphic studies reference the Ebro Basin and Paris Basin for European correlations. Lithologic markers include shifts from carbonate-rich to siliceous ooze facies and the appearance of glendonites and dropstones indicative of cooling and iceberg activity analogous to features studied in the Antarctic Peninsula.
Chronology and stratigraphic markers
The transition is often bracketed by magnetostratigraphic chrons such as C13r/C13n boundaries and by biostratigraphic turnovers in calcareous nannofossils like Discoaster and Reticulofenestra species changes. Isotopic excursions in oxygen and carbon recorded in benthic and planktonic foraminifera provide correlation points used by stratigraphers working with conventions from the International Commission on Stratigraphy and regional stage names like Rupelian and Chattian. Key localities for chronostratigraphic calibration include Oamaru Basin, Adelaide Geosyncline, and sections in the Paris Basin and Sierra Nevada used by geologists affiliated with universities such as University of Cambridge and University of California, Berkeley.
Modeling and isotope evidence
Climate models ranging from early general circulation models employed by groups at National Center for Atmospheric Research to coupled atmosphere–ocean models used by teams linked to Max Planck Institute for Meteorology reproduce temperature drops when CO2 is reduced to levels inferred from stomatal and boron isotope proxies developed in studies associated with Lamont–Doherty Earth Observatory and Scripps Institution of Oceanography. Oxygen isotope (δ18O) shifts in benthic foraminifera from cores at Site 522 and ODP Site 738 integrate with carbon isotope (δ13C) records and mercury anomalies to test hypotheses involving weathering fluxes, volcanic input, and biospheric carbon redistribution, with model intercomparisons overseen by consortia including the Paleoclimate Modelling Intercomparison Project.