Oligocene Epoch

What happened during this time?

Biological

Flora

• Angiosperms continued their expansion throughout the world 

• Tropical and sub-tropical forests were replaced by temperate deciduous woodlands

  • The Antarctic forests of the early Paleogene had been replaced by thick continental ice sheets, similar to today's conditions (Lauretano et al. 2021)

• Open plains and deserts became more common

  • Indirect evidence for the first grasslands around 32.5 Ma.

• Major evolution and dispersal of modern types of angiosperms occurred

  • Grasses, which appeared for the first time as plants of water margins, became more common in open habitats

  • In North America was mixture of subtropical elements, such as cashews and lychee trees, with temperate trees such as roses, beech, and pine

  • Leguminous plants of the pea and bean family were common, as were sedges, bulrushes, and a variety of ferns

Geophysical

• Sudden cooling and global increase in ice volume as well as dramatic decrease in sea level with closely related temperature depression

• Carbon dioxide level decline to ~470 ppm from a high of 900 ppm at the start of the epoch

• Oxygen levels hover around 22.5%

• "Mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary" (Lauretano et al. 2021)

  • The long-term cooling trend is due to reduced carbon dioxide in the atmosphere

• Paratethys Sea formed about 34 Ma when the northern region of the Tethys Ocean (Peri-Tethys) was separated from the Mediterranean region of the Tethys realm due to the formation of the Alps, Carpathians, Dinarides, Taurus and Elburz mountains

  • Due to poor connectivity with the global ocean, the Paratethys became stratified and turned into a giant anoxic sea.

  • The Eastern Paratethys basin, holding most of the water of Paratethys, was anoxic for almost 20 million years (34-15 Ma), and during this time Paratethys acted as an enormous carbon sink trapping organic matter in its sediments (Palcu & Krijgsman 2023).