Late Eocene to late Oligocene terrestrial climate and vegetation change in the western Tasmanian region

Abstract

While many palaeoclimate studies have focussed on the Eocene-Oligocene transition (EOT), little is known about the timing and drivers of the post-EOT climate recovery. To better understand and reconstruct terrestrial climate and vegetation dynamics from the late Eocene to late Oligocene (35.5–27.46 Ma), we use a new, high-resolution palynological record and quantitative sporomorph-based climate estimates recovered from ODP Site 1168 on the western Tasmanian margin. Late Eocene (35.50–34.81 Ma) floras reveal Nothofagus-dominated temperate forests with secondary Gymnostoma and minor thermophilic plant elements growing under wet conditions, with mean annual temperatures (MATs) of ∼13 °C. This is followed by a small decrease in terrestrial temperatures across the EOT by ∼2 °C. Apart from a slight decline in abundance of Gymnostoma, increases in the Fuscospora and Lophozonia-type Nothofagus, and the disappearance of palms (Arecaceae), vegetation remained relatively stable across the EOT. However, a prolonged interval of warm-temperate conditions after 33.0 Ma, independent of fluctuations in the current pCO2 record, imply additional regional controls on local climate. Changes in surface oceanographic currents, due to sustained deepening of the Tasmanian Gateway, may have played a significant role in sustaining warm-temperate vegetation in southern Australia post-EOT. The early Oligocene (PZ 3; 30.5–27.46 Ma) vegetation record still maintains the Nothofagus-dominated forest with a recovery in Gymnostoma. Gymnosperms (especially Araucariaceae, Dacrydium, and Podocarpus) and cryptogams expanded alongside an increase in overall species diversity. Sporomorph-based MATs averaged ∼11 °C in this interval. The expansion of cool-temperate forest (sustained cool-temperate climate conditions in our terrestrial records) matches the general declining pCO2 concentrations in the early Oligocene. The synchroneity between terrestrial and marine temperatures (MATs and SSTs gradually decline) and atmospheric pCO2 highlight the importance of pCO2 in driving terrestrial climate and vegetation change in the Tasmanian region during the early Oligocene.