Paleogene

The Paleogene is a geochronological and chrono-stratigraphic unit of the Cenozoic era/erathem. It is a period in the geochronological scale and a system in the chronostratigraphic scale, and it is placed between the Cretaceous and Neogene periods. It began 65 million years ago, at the Cretaceous-Paleogene (K-Pg or K-T) boundary, and ended 23 million years ago, at the Oligocene-Miocene (O-M) boundary; thus, the Paleogene lasted 42 million years. It consists of three epochs and/or series: Paleocene, Eocene, and Oligocène.

The Paleogene followed the Cretaceous period and began with the Cretaceous-Paleogene mass extinction event. There is paleontological evidence of abrupt changes in flora and fauna in this event (most often referred to as the K-T boundary mass extinction), including the total extinction of dinosaurs, ammonites, belemnites, cephalopods, and rudist molluscs; and the catastrophic mass extinction of plank-tic foraminifers, calcareous nannofossils, corals, bivalves, brachiopods, fishes, mammals, and other reptile groups. The Paleogene is most notable as being the period in which mammals and birds were diversified, exploiting ecological niches untouched by the previously extinct dinosaurs. Both groups evolved and came to dominate the land. Mammals evolved considerably into large forms in terrestrial and marine environments; birds evolved into roughly modern forms in an airborne environment.

During the Paleogene, global tectonic processes continued that had begun during the Mesozoic era, with the continents drifting toward their present positions. Although these were gradual processes, the drifting of the continents caused significant paleoclimatic and paleoceanographic turnovers during the Paleogene. The former components of the old supercontinent Gondwana continued to split apart, with South America, Africa, and Antarctica-Australia pulling away from each other. Africa moved north toward Europe, slowly closing the occidental Tethys Ocean until it disappeared during the Eocene, and uplifting the Alps during the Oligocène. Similarly, India initiated its rapid migration toward the north, until it collided with Asia, narrowing the oriental Tethys Ocean, folding the Himalayas, and forming the Indian Ocean. The Tethys Ocean vanished during the Paleogene, becoming today's Mediterranean Sea, the remnant of that old ocean. The northern supercontinent Laurasia began to break up during the Eocene, with Europe, Greenland, and North America drifting apart. The tectonic splitting of the Greenland and Norwegian seas increased the submarine volcanic and hydrothermal activity (North Atlantic flood basalts) during the Paleocene-Eocene transition. Antarctica and Australia began to split in the late Eocene, and South America and Antarctica in the Oligocène, which allowed the formation of the circumantarctic current.

The climate of the earliest Paleogene was slightly cooler than that of the preceding Cretaceous. Nevertheless, the temperature rose again in the late Paleocene, reaching its highest point at the Paleocene-Eocene (P-E) boundary. A sudden and extreme global warming event occurred in the P-E [Page 956]boundary, 55.8 million years ago, called the Paleocene-Eocene Thermal Maximum (PETM). It was an episode that lasted less than 100,000 years, very rapid in geologic terms, and it caused an intense warming of the high latitudes (up to 7° C) and a mass extinction in the benthonic fauna of the bathyal and abyssal oceanic environments (mainly benthic foraminifera). It is hypothesized that PETM was caused by runaway greenhouse effect due to a sudden release of methane from oceanic hydrates. This methane flux and its oxidation product carbon dioxide could be of a magnitude similar to that from present-day anthropogenic sources, creating the sudden increase of greenhouse warming. The main cause of this short-term change may be related to the reorganization of tectonic plates that produced an increase of volcanic activity (mainly in the North Atlantic) as well as significant paleogeo-graphic and paleoceanographic turnovers. Among these last, the most important was the closing of the Tethys Ocean with the formation of vast areas of shallow epicontinental seas. This may have been responsible for the shift in the locus of ongoing deep-water formation from cold and nutrient-depleted deep waters produced in the polar (Artie and Antarctic) regions to warm, saline, and oxygen-deficient deep waters formed in Tethyan evaporative basins. The stability of these methane hydrates depends on temperature, and, therefore, it is possible that the abrupt deep sea warming induced a shift in sediment geotherms.

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