Marking the start of the Eocene, the planet heated up in one of the most rapid and extreme global warming events recorded in geologic history, currently being identified as the 'Paleocene-Eocene Thermal Maximum' or the 'Initial Eocene Thermal Maximum' (PETM or IETM). Sea surface temperatures rose almost 15oF over a period of a few thousand years. In 1990, marine scientists James Kennett and Lowell Stott, both then at the University of California in Santa Barbara, reported analysis of marine sediments showing that, not only had the surface of the Antarctic ocean heated up about 20 degrees at the beginning of the Eocene, but that the entire depth of the ocean had warmed, and its chemistry changed disastrously. There was severely reduced oxygen in deep sea waters, and 30 to 40% of deep sea foraminifera suddenly went extinct. Geologist Jim Zachos of the University of California in Santa Cruz has connected the Eocene heat wave to drastic changes in ocean chemistry that caused the massive worldwide die-off. More recently a synchronous drop in carbon isotope ratios has been identified in many terrestrial environment.
Tracking the ratio of carbon isotopes in marine calcium carbonate sediments, Kennett and Stott found a sharp decrease in the amount of heavy carbon in 55-million-year-old marine fossils, a decline that caused the relative ratio of 13C to 12C to plunge. A gas with very low amounts of heavy 13C must have literally flooded the atmosphere. In 1995, Gerry Dickens, University of Michigan, argued that only methane gas had enough light carbon to produce the early Eocene plunge. He proposed that a belch of methane escaped from ice in seafloor sediments as the Earth warmed during the latest Paleocene. The methane escaped from submarine clathrates, ice crystals that trap methane hydroxide, a form of methane 'ice' that forms in cold bottom water under great pressures and is widely distributed and plentiful in sediments on the outer edges of continental margins. Methane is ten times as effective as carbon dioxide as a 'greenhouse' gas. The massive sublimation and release of sedimentary methane hydrates into the ocean-atmosphere reservoir upset the global carbon cycle and led to runaway global warming.
In the atmosphere, methane breaks down and releases carbon dioxide. According to Zachos and Dickens, methane combined with oxygen in the air and water, forming carbon dioxide and essentially suffocating marine life. But whether volcanic activity or a methane belch was the culprit, the greenhouse gas locked in the sunís warmth, sending global temperatures soaring.
Dissolved in the oceans, the added carbon dioxide also increased the overall acidity of seawater. This, in turn, would increase the dissolution of calcite shells of microplankton, which are the dominant component of seafloor sediments, leaving behind only nonsoluble clays. A documented change in colors of the sediment, from bright white carbonate to deep red clays, marks the Paleocene-Eocene event. Normal deposition of microscopic carbonate foram shells on the deeper reaches of the seafloor did not resume for at least 50,000 years, and the total recovery time to a "normal state" took as long as 100,000 years.
At the start of the Eocene, the Earth remained warm for about 80,000 to 200,000 years. On land, there was a massive turnover of mammals, as most of the primitive mammals that had developed since the end of the Cretaceous were suddenly replaced by the ancestors of most of the surviving modern mammal groups, all of them in small versions, adapted to Eocene heat.