The Paleocene–Eocene thermal maximum (PETM), alternatively "Eocene thermal maximum 1 (ETM1)" and formerly known as the "Initial Eocene" or "Late Paleocene thermal maximum", was a geologically brief time interval characterized by a 5–8 °C (9–14 °F) global average temperature rise and massive input of carbon into the ocean and atmosphere. The event began, now formally codified, at the precise time boundary between the Paleocene and Eocene geological epochs. The PETM lasted about 200,000 years, and occurred about 56 million years ago. The PETM arguably represents our best past analogue by which to understand how global warming and the carbon cycle operate in a greenhouse world. The time interval is marked by a prominent negative excursion in carbon stable isotope (δ13C) records from around the globe; more specifically, a large decrease in the 13C/12C ratio of marine and terrestrial carbonates and organic carbon has been found and correlated across hundreds of locations. The magnitude and timing of the PETM (δ13C) excursion, which attest to the massive past carbon release to the ocean and atmosphere, and the source of this carbon remain topics of considerable current geoscience research. What has become clear over the last few decades is that stratigraphic sections across the PETM reveal numerous changes beyond warming and carbon emission. Consistent with an Epoch boundary, fossil records of many organisms show major turnovers. In the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion taxa, including within planktic foraminifera and calcareous nannofossils, all occurred during the beginning stages of the PETM. On land, many modern mammal orders (including primates) suddenly appear in Europe and in North America.
Setting The configuration of oceans and continents was somewhat different during the early Paleogene relative to the present day. The Panama Isthmus did not yet connect North America and South America, and this allowed direct low-latitude circulation between the Pacific and Atlantic Oceans. The Drake Passage, which now separates South America and Antarctica, was closed, and this perhaps prevented thermal isolation of Antarctica. The Arctic was also more restricted. Although various proxies for past atmospheric carbon dioxide concentrations across the Cenozoic do not agree in absolute terms, all suggest that levels in the early Paleogene before and after the PETM were much higher than at present-day. In any case, significant terrestrial ice sheets and sea-ice did not exist during the late Paleocene through early Eocene. Earth surface temperatures gradually increased by about 6 °C (11 °F) from the late Paleocene through the early Eocene. Superimposed on this long-term, gradual warming were at least three (and probably more) "hyperthermals". These can be defined as geologically brief (<200,000 year) events characterized by rapid global warming, major changes in the environment, and massive carbon addition. Though not the first within the Cenozoic, the PETM was the most extreme hyperthermal, and stands out as a major change in the lithologic, biotic and geochemical composition of sediment in hundreds of records across Earth. Other hyperthermals clearly occurred around 53.7 million years ago (now called ETM-2 and also referred to as H-1, or the Elmo event) and around 53.6 million years ago (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 million years ago (informally called K, X or ETM-3). The number, nomenclature, absolute ages, and relative global impact of the Eocene hyperthermals remain a source of current research. Whether they only occurred during the long-term warming, and whether they are causally related to apparently similar events in older intervals of the geological record (e.g. the Toarcian turnover of the Jurassic) are open issues.
Global warming
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