Eocene

The Eocene is a significant geological epoch within the Paleogene Period of the Cenozoic Era. It lasted from approximately 56 million years ago (Ma) to 33.9 Ma. The name comes from Ancient Greek words meaning the "dawn of recent life."

This epoch is vital for understanding Earth's history. It represents a massive climatic transition. The planet shifted from a "hot house" state to a "cold house" state. This period saw the rise of modern biological systems.

The Eocene began with the Paleocene-Eocene Thermal Maximum (PETM). This was a short period of intense global warming. It occurred around 55.8 Ma due to a massive release of carbon. This carbon entered the atmosphere and ocean systems. The sudden change caused ocean acidification. It also triggered a major mass extinction. Between 30% and 50% of benthic foraminifera died out. These are single-celled organisms used as bioindicators of marine health.

Climate patterns during the Eocene were highly dynamic. The Early Eocene was characterized by extreme warmth. This reached a peak during the Eocene Optimum around 49 Ma. During this time, there was little to no polar ice. Consequently, sea levels were about 150 meters higher than today. The middle Eocene saw a cooling trend starting around 47.8 Ma. This was briefly interrupted by the Middle Eocene Climatic Optimum (MECO). The MECO lasted about 400,000 years. It caused a 4° to 6°C warming in both surface and deep oceans.

Greenhouse gases were the primary drivers of these temperature shifts. Carbon dioxide and methane played massive roles. During the early Eocene, carbon dioxide levels were very high. Some proxies suggest levels of 700–900 ppm. Other models suggest concentrations as high as 1,680 ppm. For comparison, modern levels are around 400 ppm. Methane also had a drastic effect. Methane has a global warming potential 29.8 times higher than carbon dioxide over 100 years. Much of this methane came from expanding wetlands and forests.

Plate tectonics reshaped the world during this epoch. At the start, Australia and Antarctica were still connected. This connection allowed warm equatorial currents to circulate. These currents helped maintain high global temperatures. Around 45 Ma, Australia split from Antarctica. This created a cold water channel. This isolation likely contributed to the cooling of the Antarctic region. In the north, the supercontinent Laurasia began to fragment. Europe, Greenland, and North America drifted apart.

Significant geological events also occurred across different continents. In Asia, India collided with the Asian continent. This collision began the formation of the Himalayas. Around 35 Ma, an asteroid impact occurred near North America. This created the Chesapeake Bay impact crater. In western North America, the Kishenehn Basin was uplifted. This uplift led to the formation of huge lakes. These lakes deposited the Green River Formation lagerstätte. This is a famous site of well-preserved fossils.

Geologists divide the Eocene into three distinct subdivisions. These are the early, middle, and late Eocene. They correspond to specific rock layers called the lower, middle, and upper Eocene. The early Eocene includes the Ypresian Stage. The middle Eocene includes the Lutetian and Bartonian stages. The late Eocene is defined by the Priabonian Stage.

Scientists use these stages to organize the complex history of the period.

Our understanding of the Eocene comes from many researchers. In 1833, Charles Lyell divided the Tertiary Epoch. In 1840, John Phillips proposed the Cenozoic name. In 1853, Moritz Hörnes introduced the Paleogene term. Today, the International Commission on Stratigraphy standardizes these boundaries. The end of the Eocene is marked by the Eocene–Oligocene extinction event. This is also called the Grande Coupure, or the "Great Break." This event is linked to the disappearance of Hantkeninidae foraminifera. It also may be related to bolide impacts in Siberia and Chesapeake Bay.