Climatological implications of Australian-Antarctic separation

Abstract

We face a major challenge in learning how to quantify the impact of the anticipated global warming before it occurs. It is widely recognized that the application of coupled ocean-atmosphere global circulation models (GCMs) offers a sophisticated method of predicting climate. However, while GCMs accurately reproduce present-day conditions, their long-term forecasting accuracy can be compromised by the possibility that crucial climatic feedback elements are not realistically reproduced, or not included in the models at all. Provided that the derived 'hindcasts' can be calibrated and verified against proxy paleoenvironmental data preserved in the sedimentary record, an effective approach to examine the robustness of GCM predictions is to use the models to 'predict the past' in essentially the same way as they predict the future. It is commonly accepted that the Antarctic ice cap and the cold Antarctic Circumpolar Current (ACC) in the adjacent Southern Ocean act together to constitute a highly influential component of the climate system. However, our knowledge of the Antarctic climate is still limited. One of the most fundamental problems is the causation of the onset of Antarctic glaciation about 34 million years ago. The initial development of Antarctic ice sheets marks the transition from the so-called 'hothouse' to an 'icehouse' world. In the early 1970s, it was proposed that Antarctic glaciation was related to the breaking up of the Supercontinent Pangea. Australia drifted to the north, away from Antarctica, opening an ocean channel known as the Tasmanian Gateway. It was hypothesized that this tectonic event resulted in the initiation of the cold (proto-)ACC. The hypothesis stated that the presence of this current prevented the warm East Australian Current to reach Antarctica, hereby causing the onset of 'thermal isolation' of Antarctica and the concomitant development of extensive ice sheets. Due to the lack of high-quality palaeoenvironmental records the (widely accepted) 'thermal isolation' hypothesis was never tested or refined since it was initially proposed. However, in 2000, excellent sedimentary data sets became available from Ocean Drilling Program (ODP) Leg 189 Sites 1170-1172 off Tasmania. In order to test the 'thermal isolation' hypothesis, these records were analyzed in detail and compared with sedimentary information from other circum-Antarctic sites, as well as with the output of GCM experiments. In this dissertation it is definitively concluded that the onset of massive glaciation of the Antarctic continent during the Eocene-Oligocene transition cannot be related the formation of the Tasmanian Gateway. Integration of phytoplankton (dinoflagellates, diatoms) records and results of coupled GCM experiments provides conclusive evidence that deepening of the Gateway caused significant changes in circulation patterns and surface temperatures of the Southern Ocean around Antarctica. However, these changes did not include the development of a (proto-)ACC. This cold current and concomitant thermal isolation of Antarctica did not come into existence before Late Oligocene times. In order to explain the earlier ice expansion, Antarctic climate responses to decreasing atmospheric CO2 concentrations and orbital forcing should be further investigated.