Temperate Oligocene surface ocean conditions offshore of Cape Adare, Ross Sea, Antarctica

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.

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach

“…At DSDP Site 274, acritarch assemblages (mainly skolochorate, similar to our Acritarch sp. 1) are relatively low (<30 %) throughout the lower Oligocene but increase towards the upper Oligocene (Hoem et al, 2021a). Acritarch blooms in each of the various records analysed seem to be linked to a different environmental condition.…”

“…In the simulation (Sauermilch et al, 2021), depth changes from 300 to 600 m led to prominent surface water cooling offshore of Antarctica, whilst the subpolar gyres (Weddell and Ross gyres) weakened and shrank significantly and an eastward proto-Antarctic Circumpolar Current that was weaker than the present day current could be established. Model-data comparison suggests further gateway deepening (>600 m) occurring from 33.6 to 30 Ma, likely leading to stronger bottom-water currents passing through the Tasmanian Gate- is presented along with that from IODP Site U1356 (Bijl et al, 2018), DSDP Site 274 (Hoem et al, 2021a), Cape Roberts Project (CRP) (Clowes et al, 2016), ODP Site 1172 (Sluijs et al, 2003), Site 1128 and Browns Creek (Houben et al, 2019) (Table S3 in the Supplement). The dinocyst taxa and/or ecological groups are indicated by different colours (see legend), following interpretations by Röhl et al (2004) and Frieling and Sluijs.…”

“…This likely indicates generally cold temperate, nutrient-rich, productive waters where seasonal sea ice melts. The more distal locations, DSDP sites 269 and 274 and IODP Site U1356, are bathed by temperate waters (Bijl et al, 2018;Sangiorgi et al, 2018;Hartman et al, 2018b;Salabarnada et al, 2018;Evangelinos et al, 2020;Hoem et al, 2021a) and represented open-ocean high-nutrient conditions of the Antarctic divergence. Our study shows that ocean waters at the Australian margin in the late Eocene-early Miocene resembled those of modern subtropical water near-shore sites north of New Zealand (Prebble et al, 2013), despite the more southerly position of Australia.…”

“…Dinocyst biogeographic patterns in the Tasmanian sector in three time slices in the late Eocene-early Miocene: (a) 37-30.5 Ma (31 data points), (b) 30.5-25 Ma (42 data points) and (c) 25-20 Ma (50 data points). The dinocyst assemblage of ODP Site 1168 (this study)is presented along with that from IODP Site U1356(Bijl et al, 2018), DSDP Site 274(Hoem et al, 2021a), Cape Roberts Project (CRP)(Clowes et al, 2016), ODP Site 1172(Sluijs et al, 2003), Site 1128 and Browns Creek(Houben et al, 2019) (TableS3in the Supplement). The dinocyst taxa and/or ecological groups are indicated by different colours (see legend), following interpretations byRöhl et al (2004) andFrieling and Sluijs.…”

Late Cenozoic Sea Surface Temperature evolution of the South Atlantic Ocean

“…Final breakup between Australia and Antarctica started around ~35.5 Ma, with ocean crust formation between southwestern Tasmania and Wilkes Land, Antarctica, and onset of bottom-water currents around 35.5-33.5 Ma (Stickley et al, 2004;Houben et al, 2019), although the strength of this socalled 'proto-ACC' during the Oligocene remains debated. New field data reconstructing sea surface temperatures (SST) and water properties (Bijl et al, 2018;Hartman et al, 2018;Salabarnada et al, 2018;Evangelinos et al, 2020Evangelinos et al, , 2022Sauermilch et al, 2021;Hoem et al, 2021aHoem et al, , b, 2022Hou et al, 2022) allow tracing the migration of frontal systems, which may be reconducted to opening of gateways. Furthermore, our understanding of the effect of the opening of the Southern Ocean gateways on the Australian-Antarctic Gulf oceanographic conditions has been improved through modelling exercises (England et al, 2017;O'Brien et al, 2020;Sauermilch et al, 2021;Nooteboom et al, 2022).…”