Eocene to Oligocene vegetation and climate in the Tasmanian Gateway region were controlled by changes in ocean currents and pCO2
Authors: Amoo Michael , Salzmann Ulrich , Pound Matthew J. , Thompson Nick , Bijl Peter K. 
Published: March 2022

Abstract. Considered one of the most significant climate reorganizations of the Cenozoic period, the Eocene–Oligocene Transition (EOT; ca. 34.44–33.65) is characterized by global cooling and the first major glacial advance on Antarctica. In the southern high latitudes, the EOT cooling
is primarily recorded in the marine realm, and its extent and effect on the
terrestrial climate and vegetation are poorly documented. Here, we present
new, well-dated, continuous, high-resolution palynological (sporomorph) data and quantitative sporomorph-based climate estimates recovered from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation
dynamics from the late Eocene (37.97 Ma) to the early Oligocene (33.06 Ma). Our results indicate three major climate transitions and four vegetation
communities occupying Tasmania under different precipitation and temperature regimes: (i) a warm-temperate Nothofagus–Podocarpaceae-dominated rainforest with paratropical elements from 37.97 to 37.52 Ma; (ii) a cool-temperate Nothofagus-dominated
rainforest with secondary Podocarpaceae rapidly expanding and taking over
regions previously occupied by the warmer taxa between 37.306 and 35.60 Ma;
(iii) fluctuation between warm-temperate–paratropical taxa and cool
temperate forest from 35.50 to 34.49 Ma, followed by a cool phase across the
EOT (34.30–33.82 Ma); and (iv) a post-EOT (earliest Oligocene) recovery
characterized by a warm-temperate forest association from 33.55 to 33.06 Ma.
Coincident with changes in the stratification of water masses and sequestration
of carbon from surface water in the Southern Ocean, our sporomorph-based
temperature estimates between 37.52 and 35.60 Ma (phase ii) showed
2–3 ∘C terrestrial cooling. The unusual fluctuation between
warm and cold temperate forest between 35.50 to 34.59 Ma is suggested to be linked to the initial deepening of the Tasmanian Gateway, allowing eastern Tasmania to come under the influence of warm water associated with the proto-Leeuwin Current (PLC). Further to the above, our terrestrial data show the mean annual temperature declining by about 2 ∘C across the
EOT before recovering in the earliest Oligocene. This phenomenon is
synchronous with regional and global cooling during the EOT and linked to
declining pCO2. However, the earliest Oligocene climate rebound along eastern Tasmania is linked to a transient recovery of atmospheric pCO2 and sustained deepening of the Tasmanian Gateway, promoting PLC throughflow. The
three main climate transitional events across the studied interval (late
Eocene–earliest Oligocene) in the Tasmanian Gateway region suggest that
changes in ocean circulation due to accelerated deepening of the Tasmanian
Gateway may not have been solely responsible for the changes in terrestrial
climate and vegetation dynamics; a series of regional and global events, including a change in the stratification of water masses, sequestration of carbon from surface waters, and changes in pCO2, may have also played vital roles.