Abstract
Abstract. Antarctic continental ice masses fluctuated considerably during the Oligocene “coolhouse”, at elevated atmospheric CO2 concentrations of ∼600–800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7–24.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 ∘C ± 5.2 ∘C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5–24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states.
Highlights
The Southern Ocean plays a crucial role in global ocean circulation, stability of the Antarctic ice sheet and the carbon cycle
We investigated sediments recovered during Deep Sea Drilling Project (DSDP) Leg 28 at Site 274, located on the offshore continental rise in the Ross Sea, ∼ 250 km northwest of Cape Adare (Hayes et al, 1975), which is at an intermediate location between the aforementioned sites in the Ross Sea and offshore of Wilkes Land (Fig. 1)
Based on four new dinocyst-based first occurrence (FO) and last occurrence (LO) datums found in the DSDP Site 274 record, we provide additional age constraints for the age model upon which we correlate five new palaeomagnetic reversal results to specific magnetic chrons
Summary
The Southern Ocean plays a crucial role in global ocean circulation, stability of the Antarctic ice sheet and the carbon cycle. Southern Ocean temperatures at the earliest Oligocene oxygen isotope step (EOIS) cooled enough to sustain the marine-terminating ice sheets. Pälike et al, 2006; Galeotti et al, 2016; McKay et al, 2016; Liebrand et al, 2017; Levy et al, 2019), and the role Southern Ocean sea surface temperature (SST) conditions played in these fluctuations still remains poorly understood, mostly due to the sparse geographic coverage of reconstructions of Oligocene SSTs On orbital timescales, Oligocene Antarctic ice volume underwent major fluctuations in size (e.g. Pälike et al, 2006; Galeotti et al, 2016; McKay et al, 2016; Liebrand et al, 2017; Levy et al, 2019), and the role Southern Ocean sea surface temperature (SST) conditions played in these fluctuations still remains poorly understood, mostly due to the sparse geographic coverage of reconstructions of Oligocene SSTs
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