Abstract. The role and climatic impact of the opening of the Drake Passage and how it affected both marine and terrestrial environments across the Eocene–Oligocene transition (EOT ∼34 Ma) period remains poorly understood. Here we present new terrestrial palynomorph data compared with recently compiled lipid biomarker (n-alkane) data from Ocean Drilling Program (ODP) Leg 113, Site 696, drilled on the margin of the South Orkney Microcontinent (SOM) in the Weddell Sea, to investigate changes in terrestrial environments and palaeoclimate across the late Eocene and early Oligocene (∼37.6–32.2 Ma). Early late Eocene floras and sporomorph-based climate estimates reveal Nothofagus-dominated forests growing under wet temperate conditions, with mean annual temperature (MAT) and precipitation (MAP) around 12 ∘C and 1802 mm respectively. A phase of latest Eocene terrestrial cooling at 35.5 Ma reveals a decrease in MAT by around 1.4 ∘C possibly linked to the opening of the Powell Basin. This is followed by an increase in reworked Mesozoic sporomorphs together with sedimentological evidence indicating ice expansion to coastal and shelf areas approximately 34.1 Myr ago. However, major changes to the terrestrial vegetation at Site 696 did not take place until the early Oligocene, where there is a distinct expansion of gymnosperms and cryptogams accompanied by a rapid increase in taxon diversity and a shift in terrestrial biomarkers reflecting a change from temperate forests to cool temperate forests following 33.5 Ma. This surprising expansion of gymnosperms and cryptogams is suggested to be linked to environmental disturbance caused by repeat glacial expansion and retreat, which facilitated the proliferation of conifers and ferns. The timing of glacial onset at Site 696 is linked to the global cooling at the EOT, yet the latest Eocene regional cooling cannot directly be linked to the observed vegetation changes. Therefore, our vegetation record provides further evidence that the opening of the Drake Passage and Antarctic glaciation were not contemporaneous, although stepwise cooling in response to the opening of ocean gateways surrounding the Antarctic continent may have occurred prior to the EOT.

Abstract. The role and climatic impact of the opening of the Drake Passage and how it affected both marine and terrestrial environments across the Eocene-Oligocene Transition (EOT ~ 34 Ma) period remains poorly understood. Here we present new terrestrial palynomorph data compared with recently compiled lipid biomarker (n-alkane) data from Ocean Drilling Program (ODP) Leg 113 Site 696 drilled on the margin of the South Orkney Microcontinent in the Weddell Sea, to investigate changes in terrestrial environments and paleoclimate across the late Eocene and early Oligocene (~ 37.6–32.2 Ma). Early late Eocene floras and sporomorph-based climate estimates reveal Nothofagus-dominated forests growing under wet temperate conditions, with mean annual temperature (MAT) and precipitation (MAP) around 13 °C and 1660 mm, respectively. A phase of latest Eocene terrestrial cooling at 35.5 Ma reveals a decrease in MAT by around 2 °C possibly linked to the opening of the Powell Basin. This is followed by an increase in Mesozoic sporomorphs together with a shift in terrestrial biomarkers and sedimentological evidence indicating ice expansion to coastal and shelf areas approximately 34.1 million years ago. However, major changes to the terrestrial vegetation at Site 696 did not take place until the early Oligocene, where there is a distinct expansion of gymnosperms and cryptogams accompanied by a rapid increase in taxa diversity following 33.5 Ma. This unusual expansion of gymnosperms and cryptogams is suggested to be linked to environmental disturbance caused by repeat glacial expansion and retreat, which facilitated the expansion of conifer and ferns. We conclude that the timing of glacial onset rather suggests that the event at site 696 is linked to the global cooling at the EOT and that latest Eocene regional cooling cannot directly be linked. Therefore, confirming that the opening of ocean gateways alone did not trigger Antarctic glaciation, even if ocean gateways may have played a role in stepwise cooling before the EOT.

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.

Abstract. Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene Transition (EOT; ca. 34.44–33.65) is characterised by global cooling and the first major glacial advance on Antarctica. While in the southern high-latitudes, the EOT cooling is primarily recorded in the marine realm, the extent and effect on terrestrial climate and vegetation is poorly documented. Here, we present a 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 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 37.97–37.52 Ma; (ii) cool-temperate Nothofagus dominated rainforest with secondary Podocarpaceae rapidly expanding and taking over regions previously occupied by the warmer taxa between 37.306–35.60 Ma; (iii) fluctuation between warm temperate – paratropical taxa and cool temperate forest from 35.50–34.49 Ma, followed by a cool phase across the EOT (34.30–33.82 Ma); (iv) post-EOT (earliest Oligocene) recovery characterised by a warm-temperate forest association from 33.55–33.06 Ma. Coincident with changes in stratification of water masses and sequestration of carbon from surface water in the Southern Ocean, our sporomorph-based temperature estimates between 37.52 Ma 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 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 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, but a series of regional and global events, including a change in stratification of water masses, sequestration of carbon from surface waters, and changes in pCO2 may have played vital roles.

<p>Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene (E/O) Transition (ca. 33.9-33.5 Ma) is characterised by global cooling coupled with glacial advance on Antarctica. Combined micropalaeontological (diatom and dinoflagellate) and sedimentological evidence hint of regional reorganisation of ocean currents around Antarctica, in association with the Eocene-Oligocene transition. The late Eocene to early Oligocene deepening of the Tasman Gateway resulted in the flow of warm surface waters from the Australo-Antarctic gulf into the southwestern Pacific Ocean.  However, the extent and effect of these changes in ocean circulation on regional terrestrial climate and vegetation across the E/O Transition is not readily known. Here, we present new well-dated, high resolution palynological (sporomorph) data from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation dynamics from the late Eocene through to the early Oligocene. Results from our sporomorph data point to four vegetation communities occupying Tasmania under different precipitation and temperature regimes: (i) Paratropical rainforest along the coastlines and temperate rainforests at higher altitude of the hinterlands from 37.97-37.52 Ma; (ii) cool temperate forest expanding into areas previously occupied by the paratropical forests between 37.306-35.60 Ma; (iii) a complex mix of paratropical associations coexisting with frost-tolerant taxa, followed by a period of relative stability shown in the dominance of cold-temperate taxa from 35.50-33.36 Ma; (iv) a warm temperate forest present in the early Oligocene (33.25-33.06 Ma). Our sporomorph record showed a general cooling trend from the latest-middle Eocene to the late Eocene (37.97-35.60 Ma), fluctuations between warm and cold climates (35.50 – 34.19 Ma), a period of relative stable cooling across the E/O transition (33.94-33.5 Ma), and a rather unusual rapid warming right after the E/O transition (earliest Oligocene; 33.36 - 33.06 Ma). Our quantitative estimates of terrestrial temperature change and palaeoecological reconstructions show a close link with previously published dinoflagellate cyst data from this same study site, suggesting a possible vegetation and climate response to tectonic changes (most likely the tectonic opening and deepening of the Tasman Gateway ca. 35.5 Ma) and relative short-term regional reorganisation of ocean currents.</p><p><strong>Keywords: Antarctica, Eocene-Oligocene Transition, sporomorph, temperate rainforest, Tasman Gateway</strong></p>