Southern high latitude vegetation change across the Drake Passage region linked to prolonged intervals of climate cooling during the early Oligocene

The possible causes of the onset of Antarctic glaciation around the Eocene-Oligocene Transition (EOT), approximately 34 million years ago (~34Ma), are poorly understood. Uncertainties particularly remain over the role of the Drake Passage opening on the development of the Antarctic Circumpolar Current (ACC), and how this affected both marine and terrestrial environments. A major obstacle in understanding the role of the opening Drake Passage and ACC in Cenozoic climate changes has been the lack of continuous records spanning the EOT from the region. Here we present new palynomorph data from ODP Leg 113 Site 696 Hole B, recording changes in terrestrial environments and paleoclimate across the EOT. The sporomorph assemblage reveals the presence of Nothofagus-dominated forests with secondary Podocarpaceae and an understory of angiosperms and cryptogams growing across much of the Northern Antarctic Peninsula and South Orkney Microcontinent during the late Eocene (~37.60-34.95 Ma). Palaeoclimate reconstructions show that these forests grew under wet temperate conditions, with mean annual temperature and precipitation around 12°C and 1650mm, respectively. Today, similar temperate Nothofagus-dominated mixed-podocarp forests occur in the temperate Valdivian region of southern Chile. At the onset of the EOT, the palynomorph assemblage indicates an unusual expansion of gymnosperms and cryptogams, accompanied by a rapid increase in taxa diversity between ca. 34 and 32 Ma. Sporomorph based climate reconstructions do not provide evidence for an abrupt cooling at the EOT but reveal the onset of prolonged cooling phases throughout the early Oligocene. A contemporaneous increase in reworked Mesozoic sporomorphs at the EOT is likely to be linked to frequent glacial advances from the Antarctic Peninsula and South Orkney Microcontinent, although iceberg-rafted debris from Antarctica cannot be ruled out. We conclude that climate instability and glacial related disturbance at the onset of the EOT facilitated the suppression of Nothofagus and the expansion of a more diverse vegetation with many pioneer taxa that were able to quickly colonise during glacial retreat cycles.