Lipid-biomarker-based sea surface temperature record offshore Tasmania over the last 23 million years

The Neogene (23.04-2.58 Ma) is characterised by progressive buildup of ice volume and climate cooling in the Antarctic and the Northern Hemisphere. Heat and moisture delivery to Antarctica is, to a large extent, regulated by the strength of meridional temperature gradients. However, the evolution of the Southern Ocean frontal systems remains scarcely studied in the Neogene. Here, we present the first long-term continuous sea surface temperature (SST) record of the subtropical front area in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 off western Tasmania. This site is, at present, located near the subtropical front (STF), as it was during the Neogene, despite a 10ffi northward tectonic drift of Tasmania. We analysed glycerol dialkyl glycerol tetraethers (GDGTs - on 433 samples) and alkenones (on 163 samples) and reconstructed the paleotemperature evolution using TEX86 and Uk0 37 as two independent quantitative proxies. Both proxies indicate that Site 1168 experienced a temperate similar to 25 ffiC during the early Miocene (23-17 Ma), reaching similar to 29 ffiC during the mid-Miocene climatic optimum. The stepwise similar to 10 ffiC cooling (20-10 ffi C) in the mid-to-late Miocene (12.5-5.0 Ma) is larger than that observed in records from lower and higher latitudes. From the Pliocene to modern (5.3-0 Ma), STF SST first plateaus at similar to 15 ffiC (3 Ma), then decreases to similar to 6 ffiC (1.3 Ma), and eventually increases to the modern levels around similar to 16 ffi C (0 Ma), with a higher variability of 5ffi compared to the Miocene. Our results imply that the latitudinal temperature gradient between the Pacific Equator and the STF during late Miocene cooling increased from 4 to 14 ffi C. Meanwhile, the SST gradient between the STF and the Antarctic margin decreased due to amplified STF cooling compared to the Antarctic margin. This implies a narrowing SST gradient in the Neogene, with contraction of warm SSTs and northward expansion of subpolar conditions.

Automated summary

This study reconstructs the paleotemperature evolution of the subtropical front area in the Southern Ocean using two independent proxies. The results show that the temperature in the area was high during the early Miocene, decreased in the mid-to-late Miocene, and then increased again in the Pliocene to modern times. These findings reveal the changes in temperature gradient near the Antarctic and expansion of subpolar conditions during the Neogene.