Eocene

The Eocene (IPA: EE-ə-seen, EE-oh-) is a geological epoch that began about 56 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The end point of the Eocene is the Eocene-Oligocene transition (EOT), between 33.9 and 33.4 Ma. Precise boundaries between the epochs are sometimes given at 33.9 or 33.7 Ma. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain.