Late-Paleocene paleoenvironmental and temperature changes at Point Margaret, South-East Australia, derived from dinoflagellate cysts, isotope and organic biomarker analyses

Abstract

A good understanding of the climate dynamics of the transition from the cold mid-Paleocene to the early Eocene hothouse is essential. This entails being able to recreate the reconstructed hothouse conditions in climate models. However, there is a mismatch between reconstructed and modelled sea surface temperatures for hothouse conditions at the Southern Ocean. An outcrop section of Late Paleocene age called Point Margaret, located in the Otway Basin, South-East Australia – paleolatitude ~56° S, was investigated to check whether the discrepancy between models and reconstructed SSTs is also present at the Australo-Antarctic Gulf (AAG). Branched and isoprenoidal GDGT concentrations were measured, elemental and δ13C analyses were performed to and dinoflagellate cysts were counted to reconstruct the paleoenvironment. During the Late Paleocene, Point Margaret was located near shore close to a river which provided a high nutrient load. The FCO of Apectodinium occurred ~300 kyr before the PETM, this early acme is not related to the PETM but to changing conditions of the Proto-Leeuwin Current and/or AAG. Point Margaret and ODP Site 1172, Tasman Plateau, show a very similar response from ~100 kyr before to the onset of the PETM. This means that the sites must have been more connected than is possible with a closed Tasmanian Gateway and the ocean configuration related to this. Differences between SST and MAT records in the Southern Ocean, concerning especially absolute changes during the PETM, are extremely large. Thus MAT proxies with higher calibration ranges are needed to enable the reconstruction of higher absolute temperatures during hothouse conditions and to improve the comparison between MAT and SST. The relative temperature increase during the PETM at all Southern Ocean sites is in accordance with the increase obtained from models (~4 °C) and the maximal global mean surface temperature anomaly (~5 °C). However, models underestimate absolute temperatures of the entire Southern Ocean during the Late Paleocene and PETM with differences that cannot only be explained by calibration errors. Problems with models concerning the climate sensitivity and feedback mechanisms but also the application of a low latitudinal gradient and a greater polar amplification at more acceptable CO2 conditions must be solved before they can be applied to the present day ‘greenhouse world’.