The Eocene-Oligocene extinction event, also known as the Eocene-Oligocene Transition (EOT) or Grande Coupure, occurred approximately 33.9 to 33.4 million years ago. This period marked a profound global shift from a warm, ice-free world to a cooler, more glaciated state, significantly influencing the evolution and distribution of life.
A Changing World
The Eocene epoch concluded with a dramatic transformation of Earth’s climate, shifting from a pervasive “greenhouse” state to cooler “icehouse” conditions. This involved a substantial decline in global temperatures. This cooling trend culminated in the formation of extensive ice sheets, particularly across Antarctica, which had previously been covered by lush forests.
Antarctic glaciation unfolded in at least two major steps, significantly lowering global sea levels by tens of meters. Concurrently, tectonic movements opened major ocean gateways, including the Drake Passage and the Tasmanian Gateway. These openings facilitated the development of the Antarctic Circumpolar Current, which thermally isolated Antarctica and contributed to global cooling.
Potential Triggers
The Eocene-Oligocene extinction event was driven by a complex interplay of environmental factors, with rapid global cooling and glaciation considered a primary cause. A significant decline in atmospheric carbon dioxide levels played a substantial role in this climatic shift. This reduction in greenhouse gases enabled the widespread formation of ice sheets. The resulting colder temperatures and changes in ocean circulation profoundly impacted marine and terrestrial ecosystems.
Another proposed trigger involves extraterrestrial impacts. Large impact craters, such as Popigai in Siberia, formed around the Eocene-Oligocene transition. These impacts may have scattered debris across vast distances, potentially contributing to environmental disruption. While their direct link to extinctions is debated, they represent a potential source of short-term environmental stress.
Volcanic activity has also been considered as a contributing factor, though evidence is less conclusive for this specific event. Many researchers conclude that climatic changes, primarily driven by declining CO2 and altered ocean currents, were the main drivers of the biotic turnover. These factors likely acted in concert.
Biological Impact
The Eocene-Oligocene extinction event had widespread consequences for life forms, both in marine and terrestrial environments. Marine organisms were particularly affected, with significant declines observed in groups such as larger benthic and planktonic foraminifera. Ancient cetaceans also experienced significant losses. Cooling ocean temperatures and changes in nutrient availability contributed to the reduction in diversity among these marine groups.
On land, the impact was substantial for mammalian faunas. In Europe, a dramatic faunal turnover, the Grande Coupure, occurred, characterized by widespread extinctions of archaic mammal groups and the immigration of new species from Asia. A mass extinction event was identified in Africa and the Arabian Peninsula, where many mammal species went extinct. This event, linked to the cooling climate, converted swampy habitats into drier environments.
Despite the extinctions, the Eocene-Oligocene transition paved the way for the diversification of new life forms. Surviving mammal lineages adapted to the changing conditions, leading to the rise of modern mammal orders. The shift from lush Eocene forests to more open grasslands facilitated the expansion of grazing animals. This period represents a selective filter, where species better equipped for cooler, more variable climates survived and flourished.
Scientific Discovery
Scientists investigate the Eocene-Oligocene extinction event through analysis of Earth’s geological record. Sedimentary rock layers, particularly those found in deep-sea sediment cores, provide a continuous archive of past environmental conditions. These cores contain fossil remains and chemical signatures that reveal changes over millions of years.
Paleontological evidence offers direct insights into the species that thrived, declined, or went extinct during this period. Researchers also rely on geochemical analyses, such as studying oxygen isotope ratios in ancient shells of marine organisms like foraminifera. These isotopic variations act as proxies for past ocean temperatures and global ice volume, helping to reconstruct the cooling event.