Eocene

Eocene
Scotese, Christopher R.; Vérard, Christian; Burgener, Landon; Elling, Reece P.; · CC BY 4.0 · source
Name	Eocene
Color	Eocene
Time	56.0 – 33.9 million years ago
Caption	Paleogeography of the Earth in the early Eocene, ~50 Ma.
Usage	Global (ICS)
Timescales used	ICS Time Scale
Chrono unit	Epoch
Strat unit	Series
Timespan formality	Formal
Lower boundary def	Magnetic base of Chron C24r
Lower gssp location	Dababiya, Luxor, Egypt
Lower gssp approved	2003
Upper boundary def	LAD of the Haptophyte Reticulofenestra umbilicus
Upper gssp location	Massignano, Ancona, Italy
Upper gssp approved	1992

Eocene. The Eocene is a geological epoch that constitutes the middle part of the Paleogene Period, spanning from approximately 56 to 33.9 million years ago. It follows the Paleocene and precedes the Oligocene, representing a critical interval of profound global change. This epoch witnessed the emergence of most modern mammalian orders and experienced dramatic climatic shifts from a "greenhouse" world to the beginnings of modern "icehouse" conditions.

Etymology and history

The term Eocene derives from the Ancient Greek words *ēṓs* (dawn) and *kainós* (new), coined by the Scottish geologist Sir Charles Lyell in his seminal work, *Principles of Geology*. Lyell introduced the name in 1833 to denote the "dawn of recent life," reflecting the early appearance of modern fauna in the fossil record compared to the underlying strata. The epoch's formal definition and global correlation were solidified much later through the designation of GSSP locations, with the base defined at the Dababiya quarry section in Egypt and the top at the Massignano section in Italy. Key figures in its study include Mary Anning, whose discoveries in the Jurassic Coast cliffs, though often from older periods, helped pioneer the paleontological methods applied to Cenozoic deposits.

Geology and climate

The Eocene is marked by distinctive lithological sequences globally, including the extensive London Clay in England and the Green River Formation in the United States. The epoch began with a brief but intense period of global warming known as the Paleocene-Eocene Thermal Maximum, an event linked to a massive release of carbon into the oceans and atmosphere, possibly from methane clathrate dissociation or volcanic activity in the North Atlantic Igneous Province. Following this peak, the climate remained exceptionally warm, with high-latitude regions like Antarctica and the Arctic supporting temperate forests and alligators, as evidenced by fossils from Ellesmere Island. The epoch concluded with a significant cooling trend, driven by declining atmospheric carbon dioxide levels and the opening of key ocean gateways like the Drake Passage, which isolated Antarctica and permitted the formation of the Antarctic Circumpolar Current.

Flora and fauna

Terrestrial ecosystems saw the dramatic radiation of modern mammals following the extinction of non-avian dinosaurs. Iconic early forms included the giant, flightless bird Gastornis, the early horse Hyracotherium, and massive terrestrial mammals like Uintatherium. Primates, including early ancestors of simians, diversified in the forests of North America and Europe. The warm, wet climates supported extensive rainforests that reached into polar regions, with fossil evidence of breadfruit and palm trees found in Geodetic Hills of the Canadian Arctic. Marine environments were dominated by new groups of cetaceans like Basilosaurus, which evolved from terrestrial ancestors, while the seas also teemed with nummulite foraminifera, whose massive limestone deposits built the pyramids of Giza.

Paleogeography

Continental configurations during the Eocene were in transition towards their modern positions. The supercontinent Gondwana was in its final stages of breakup, with Australia and South America separating from Antarctica. The Tethys Ocean, a vast equatorial seaway, was gradually closing due to the northward movement of the African Plate and Indian Plate towards Eurasia, initiating the Alpine orogeny and the rise of the Himalayas. Shallow epicontinental seas covered parts of Europe and central North America, as recorded in the fossil-rich Paris Basin and White River Group. The connection between the Atlantic and Pacific oceans via the Central American Seaway remained open, allowing extensive faunal interchange.

Major events

The most significant climatic event was the aforementioned Paleocene-Eocene Thermal Maximum, a geologically rapid warming episode associated with a major excursion in carbon isotope ratios and a deep-sea benthic extinction event. Another pivotal occurrence was the Azolla event in the middle Eocene, where massive blooms of the freshwater fern Azolla in the Arctic Ocean may have drawn down atmospheric carbon dioxide, contributing to long-term cooling. The epoch ended with the Eocene-Oligocene extinction event, a period of significant turnover in both marine and terrestrial life linked to global cooling and the expansion of Antarctic ice sheets. This transition is marked in the marine record by the extinction of many foraminifera and the rise of cetaceans like Dorudon. Category:Geological epochs Category:Cenozoic