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The Paleocene–Eocene Thermal Maximum (PETM), alternatively (ETM1), and formerly known as the "Initial Eocene" or "" refers to a climate event that began at the temporal boundary between the Paleocene and Eocene epochs. The absolute age and duration of the event remain uncertain, but are thought to be close to 55.8 million years ago and about 170,000 years of duration The PETM has become a focal point of considerable geoscience research because it probably provides our best past analog by which to understand impacts of global warming and massive carbon input to the ocean and atmosphere, including ocean acidification. The onset of the PETM has been linked to an initial 5 °C temperature rise and extreme changes in Earth’s carbon cycle. The PETM is marked by a prominent negative excursion in carbon stable isotope () records from around the globe; more specifically, there was a large decrease in 13C/12C ratio of marine and terrestrial carbonates and organic carbon.〔 Numerous other changes can be observed in stratigraphic sections containing the PETM.〔 Fossil records for many organisms show major turnovers. For example, in the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion, planktic foraminifera and calcareous nanofossils all occurred during the beginning stages of PETM. On land, there was a sudden appearance of modern mammal orders (including primates) in Europe and North America. Sediment deposition changed significantly at many outcrops and in many drill cores spanning this time interval. Although it is now widely accepted that the PETM represents a “case study” for global warming and massive carbon input to Earth’s surface, the cause, details and overall significance of the event remain perplexing. ==Setting== The configuration of oceans and continents was somewhat different during the early Paleogene relative to 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. 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 levels in the Eocene do not agree in absolute terms, all suggest that levels then were much higher than at present. In any case, there were no significant ice sheets during this time. Earth surface temperatures increased by about 6 °C from the late Paleocene through the early Eocene, culminating in the "Early Eocene Climatic Optimum" (EECO).〔 Superimposed on this long-term, gradual warming were at least two (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. Of these, the PETM was the most extreme and perhaps the first (at least within the Cenozoic). Another hyperthermal clearly occurred at approximately 53.7 Ma, and is now called ETM-2 (also referred to as H-1, or the Elmo event). However, additional hyperthermals probably occurred at about 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3). The number, nomenclature, absolute ages, and relative global impact of the Eocene hyperthermals are the source of considerable 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. Acidification of deep waters, and the later spreading from the North Atlantic can explain spatial variations in carbonate dissolution. Model simulations show acidic water accumulation in the deep North Atlantic at the onset of the event. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Paleocene–Eocene Thermal Maximum」の詳細全文を読む スポンサード リンク
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