The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition (EOT) or Grande Coupure (
French for "great cut"), is the transition between the end of the
Eocene and the beginning of the
Oligocene, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago.[1] It was marked by large-scale extinction and floral and faunal turnover, although it was relatively minor in comparison to the
largest mass extinctions.[2]
Causes
Glaciation
The boundary between the Eocene and Oligocene epochs is marked by the glaciation of Antarctica and the consequent beginning of the
Late Cenozoic Ice Age.[3] This enormous shift in climatic regime is the leading candidate for the extinction event's cause. Though ephemeral ice sheets may have existed on the Antarctic continent during parts of the Middle and Late Eocene,[4] this interval of severe global cooling marked the beginning of permanent
ice sheet coverage of
Antarctica,[5][6] and thus the end of the greenhouse climate of the Early Palaeogene.[7] The global cooling also correlated with marked drying conditions in low-latitudes Asia,[8] though a causal relationship between the two has been contradicted by some research.[9]
A leading model of climate cooling at this time predicts a decrease in atmospheric
carbon dioxide, which slowly declined over the course of the Middle to Late Eocene.[10][11][12] Significant cooling took place in the final hundreds of thousands of years prior to the start of major Antarctic glaciation.[13] This cooling reached some threshold approximately 34 million years ago,[14][15][4] precipitating the formation of a large ice sheet in
East Antarctica in response to falling carbon dioxide levels.[16][17] The cause of the drop in
pCO2 was the drift of the
Indian Subcontinent into equatorial latitudes, supercharging the
silicate weathering of the
Deccan Traps.[18] Another factor was the opening of the
Drake Passage and the creation of the
Antarctic Circumpolar Current (ACC), which had the effect of creating
ocean gyres that promote
upwelling of cold bottom waters and diminishing heat transport to Antarctica by isolating the water around it.[19] Likewise, the
Tasmanian Gateway also opened up around the time of the EOT.[20] Ocean circulation changes were, however, not as significant in engendering cooling as the decline in pCO2.[21] On top of that, the timing of the creation of the ACC is uncertain.[22]
Evidence points to the glaciation of Antarctica occurring in two steps, with the first step, the less pronounced and more modest step of the two, taking place at the Eocene-Oligocene boundary itself. This first step is referred to as EOT-1.[4] Carbon dioxide concentrations dropped from about 885 ppm to about 560 ppm.[23] The Oligocene Oi-1 event, an
oxygen isotope excursion that occurred around 33.55 million years ago,[24] was the second major pulse of Antarctic ice sheet formation.[4]
These large climate changes have been linked to biotic turnovers. Even before the Eocene-Oligocene boundary itself, during the early Priabonian, extinction rates went up in connection with falling global temperatures.[23] Radiolarians suffered major losses thanks to a decrease in nutrient availability in deep and intermediate waters.[25] In the
Gulf of Mexico, marine turnover is associated with climatic change, though the ultimate cause according to the study was not the drop in average temperatures themselves but colder winters and increased seasonality.[2]
On land, the increased seasonality brought on by this abrupt cooling caused the Grande Coupure faunal turnover in Europe.[26] In the
Ebro Basin, major aridification occurred amidst the Grande Coupure, suggesting causality.[1] The remarkable cooling period in the ocean is correlated with pronounced mammalian faunal replacement within continental Asia as well. The Asian biotic reorganization events are comparable to the Grande Coupure in Europe and the Mongolian Remodeling of mammalian communities.[27]
Extraterrestrial impact
Another speculation points to several large
meteorite impacts near this time, including those of the
Chesapeake Bay crater 40 km (25 mi) and the
Popigai impact structure 100 km (62 mi) of central
Siberia, which scattered debris perhaps as far as Europe. New dating of the Popigai meteor strengthens its association with the extinction.[28] However, other studies have failed to find any association between the extinction event and any impact event.[29]
Solar activity
Imprints of sunspot cycles from the Bohai Bay Basin (BBB) show no evidence that any significant change in solar activity occurred across the EOT.[30]
Extinction patterns
Terrestrial biota
Grande Coupure
The Grande Coupure, or 'great break' in
French,[31] with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the
Priabonian, and the arrival in Europe of Asian species. The Grande Coupure is characterized by widespread extinctions and
allopatric speciation in small isolated
relict populations.[32] It was given its name in 1910 by the
Swiss palaeontologist
Hans Georg Stehlin, to characterise the dramatic turnover of European mammalian fauna, which he placed at the Eocene–Oligocene boundary.[33] A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling".
The Grande Coupure marks a break between
endemic European faunas before the break and mixed faunas with a strong Asian component afterwards. J. J. Hooker and his team summarized the break:[34]
"Only the
marsupial family
Herpetotheriidae, the artiodactyl family
Cainotheriidae, and the rodent families Theridomyidae and Gliridae (
dormice) crossed the faunal divide undiminished."
An element of the paradigm of the Grande Coupure was the apparent
extinction of all European primates at the Coupure. However, the 1999 discovery of a
mouse-sized early Oligocene
omomyid, reflecting the better survival chances of small mammals, undercut the Grand Coupureparadigm.[35]
Additionally, a second dispersal event of Asian taxa into Europe, known as the Bachitherium dispersal event, occurred later, around 31 Ma. Unlike the Grande Coupure, which took place via Central and Northern Asia, this later dispersal occurred via a southern corridor.[36]
It has been suggested that this was caused by
climate change associated with the earliest polar glaciations and a major fall in sea levels, or by competition with taxa dispersing from Asia. However, few argue for an
isolated single cause. Other possible causes are related to the impact of one or more large bolides in northern hemisphere at
Popigai,
Toms Canyon and
Chesapeake Bay.[37] Improved correlation of
northwest European successions to global events confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350
millennia prior to the first record of post-Grande Coupure Asian immigrant taxa.[34] Research suggests that in the Ebro Basin of Spain, the turnover lagged the Eocene-Oligocene boundary by at most 500 kyr.[1]
Marine biota
In the marine realm, the frequency of drilling in recovery faunas, especially among bivalves, was drastically higher than in assemblages before the extinction event, a phenomenon attributed to a high extinction rate among escalated prey taxa with highly evolved defences against predators.[38] Veneroid bivalves experienced a short-term size increase during the biotic recovery.[39] Orthophragminid foraminifera (late Paleocene–early Eocene larger benthic foraminifera of two families, Discocyclinidae and Orbitoclypeidae) disappeared in the extinction event; in Alpine carbonates,
bryozoan facies show an expansion in response to the loss of orthophragminids.[40]
Some sites contain evidence that the Eocene–Oligocene extinction was not a sudden event but a prolonged biotic transition drawn out over as much as 6 million years. Localities near
Eugene, Oregon, record a plant extinction 33.4 Ma and a marine invertebrate turnover 33.2 Ma; both of these turnovers post-date the supposed extinction event by hundreds of thousands of years.[41]
^Barker, Peter F.; Filippelli, Gabriel M.; Florindo, Fabio; Martin, Ellen E.; Scher, Howard D. (October–November 2007).
"Onset and role of the Antarctic Circumpolar Current". Deep Sea Research Part II: Topical Studies in Oceanography. 54 (21–22): 2388–2398.
doi:
10.1016/j.dsr2.2007.07.028. Retrieved 1 January 2024 – via Elsevier Science Direct.
^Molina, Eustoquio; Gonzalvo, Concepción; Ortiz, Silvia; Cruz, Luis E. (2006-02-28). "Foraminiferal turnover across the Eocene–Oligocene transition at Fuente Caldera, southern Spain: No cause–effect relationship between meteorite impacts and extinctions". Marine Micropaleontology. 58 (4): 270–286.
Bibcode:
2006MarMP..58..270M.
doi:
10.1016/j.marmicro.2005.11.006.
^A major cooling event preceded the Grande Coupure, based on pollen studies in the Paris Basin conducted by Chateauneuf (
J.J. Chateauneuf, 1980. "Palynostratigraphie et paleoclimatologie de l'Éocene superieur et de l'Oligocene du Bassin de
Paris (
France)" in Mémoires du Bureau de Recherches Géologiques et Minières, 116 1980).