Paleocene

The Paleocene, ( /ˈpæl.i.əˌsn, -i.-, ˈp.li-, -li.-/ PAL-ee-ə-seen, -ee-oh-, PAY-lee-, -lee-oh-)[4] or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek παλαιός palaiós meaning "old" and the Eocene Epoch (which succeeds the Paleocene), translating to "the old part of the Eocene".

Paleocene
66.0 – 56.0 Ma
Chronology
Etymology
Name formalityFormal
Name ratified1978
Alternate spelling(s)Palaeocene
Usage information
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitEpoch
Stratigraphic unitSeries
First proposed byWilhelm Philipp Schimper, 1874
Time span formalityFormal
Lower boundary definitionIridium enriched layer associated with a major meteorite impact and subsequent K–Pg extinction event[2]
Lower boundary GSSPEl Kef Section, El Kef, Tunisia
36.1537°N 8.6486°E / 36.1537; 8.6486[2]
GSSP ratified1991[2]
Upper boundary definitionStrong negative anomaly in δ13C values at the PETM[3]
Upper boundary GSSPDababiya section, Luxor, Egypt[3]
25.5000°N 32.5311°E / 25.5000; 32.5311
GSSP ratified2003[3]

The epoch is bracketed by two major events in Earth's history. The K–Pg extinction event, brought on by an asteroid impact and possibly volcanism, marked the beginning of the Paleocene and killed off 75% of living species, most famously the non-avian dinosaurs. The end of the epoch was marked by the Paleocene–Eocene Thermal Maximum (PETM), which was a major climatic event wherein about 2,500–4,500 gigatons of carbon were released into the atmosphere and ocean systems, causing a spike in global temperatures and ocean acidification.

In the Paleocene, the continents of the Northern Hemisphere were still connected via some land bridges; and South America, Antarctica, and Australia had not completely separated yet. The Rocky Mountains were being uplifted, the Americas had not yet joined, the Indian Plate had begun its collision with Asia, and the North Atlantic Igneous Province was forming in the third-largest magmatic event of the last 150 million years. In the oceans, the thermohaline circulation probably was much different than it is today, with downwellings occurring in the North Pacific rather than the North Atlantic, and water density mainly being controlled by salinity rather than temperature.

The K–Pg extinction event caused a floral and faunal turnover of species, with previously abundant species being replaced by previously uncommon ones. In the Paleocene, with a global average temperature of about 24–25 °C (75–77 °F), compared to 14 °C (57 °F) in more recent times, the Earth had a greenhouse climate without permanent ice sheets at the poles, like the preceding Mesozoic. As such, there were forests worldwide—including at the poles—but they had low species richness in regards to plant life, and were populated by mainly small creatures that were rapidly evolving to take advantage of the recently emptied Earth. Though some animals attained great size, most remained rather small. The forests grew quite dense in the general absence of large herbivores. Mammals proliferated in the Paleocene, and the earliest placental and marsupial mammals are recorded from this time, but most Paleocene taxa have ambiguous affinities. In the seas, ray-finned fish rose to dominate open ocean and reef ecosystems.