Eocene-Oligocene extinction event

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The transition between the end of the Eocene and the beginning of the Oligocene, called the Grande Coupure in Europe, occurring 33.9 ± 0.1 Ma, is marked by large-scale extinction and floral and faunal turnover (although minor in comparison to the largest mass extinctions).

Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient cetaceans, the Archaeoceti.

This was a time of major climatic change, especially cooling, not obviously linked with any single major impact or any major volcanic event. One cause of the extinction event is speculated to be volcanic activity. Another speculation is that the extinctions are related to several meteorite impacts that occurred about this time. One such event happened near present-day Chesapeake Bay, and another in Central Siberia, scattering debris perhaps as far as Europe. The leading scientific theory on climate cooling at this time is decrease in atmospheric CO2, which slowy declined in the mid to late Eocene and possibly reached some threshold approximately 34 million years ago.

This boundary is closely linked with the Oligocene Oi-1 event, an oxygen isotope excursion that marks the beginning of ice sheet coverage on Antarctica.

http://www.ucmp.berkeley.edu/education/events/cowen1a.html