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The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by 7 to 11 °C (13 to 20 °F) in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene paleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this.
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]
The global climate entered icehouse conditions at the Eocene-Oligocene boundary and the present day Late Cenozoic ice age began. [ 33 ] The Paleogene began with the brief but intense " impact winter " caused by the Chicxulub impact , which was followed by an abrupt period of warming.
The Eocene-Oligocene Boundary 33.9 million years ago was the transition from the last greenhouse period to the present icehouse climate. [ 17 ] [ 18 ] [ 10 ] At this point, when ~25% more of Antarctica's surface was above sea level and able to support land-based ice sheets relative to today, [ 19 ] CO 2 levels had dropped to 750 ppm. [ 20 ]
Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene–Oligocene transition at 34 Ma. During this decrease, ice began to reappear at the poles, and the Eocene–Oligocene transition is the period of time when the Antarctic ice sheet began to rapidly expand. [52]
A brief marine incursion marks the early Oligocene in Europe. There appears to have been a land bridge in the early Oligocene between North America and Europe since the faunas of the two regions are very similar. During the Oligocene, South America was finally detached from Antarctica and drifted north toward North America.
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The onset of South Asian monsoon is poorly constrained since limited paleoclimatic data is available. It is generally accepted to have occurred during the Eocene-Oligocene climate transition (33.9 Ma onwards). [46] The onset mechanism has long been debated and remained poorly understood.