Definition
The Gelasian was introduced in the geologic timescale in 1998. It is named after the Sicilian city of Gela in the south of the island. In 2009 it was moved from the Pliocene to the Pleistocene in order that the geologic time scale be more consistent with the key changes in Earth's climate, oceans, and biota that occurred 2.588 million years ago.
The base of the Gelasian is defined magnetostratigraphically as the base of the Matuyama (C2r) chronozone (at the Gauss-Matuyama magnetostratigraphic boundary), isotopic stage 103. Above this point there are notable extinctions of the calcareous nannofossils: Discoaster pentaradiatus and Discoaster surculus. The GSSP for the Gelasian is located at the Monte Sant Nicola near Gela.
The top of the Gelasian is defined magnetostratigraphically as the end of the Olduvai (C2n) chronozone, and faunally as the extinction level of the calcareous nannofossil Discoaster brouweri (base of biozone CN13). Above the Gelasian as the first occurrences of the calcareous nannofossil Gephyrocapsa sp. and the extinction level of the planktonic foraminifer Globigerinoides extremus.
During the Gelasian the ice sheets in the Northern Hemisphere began to grow, which is seen as the beginning of the Quaternary ice age.
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