| dc.description.abstract | The last 3 Ma is a crucial period for the Earth’s climate evolution as it went through the transition from a warm mid-late Pliocene (~3.2 Ma) into the colder late Pliocene- early Pleistocene (~2.5 Ma), with larger ice sheets and high amplitude glacial to interglacial cycles. During the Mid Pleistocene Transition (MPT), in the field of palaeoceanography, the Southern Ocean has been proposed as one of the key regions of the global ocean, as during the Quaternary, subantarctic waters of the Southern Ocean cooled by 4-9oC, as a result of a northward movement of the fronts in the Indian Ocean sector, the Great Australian Bright and the Tasmanian regions. The sedimentary record around Tasmania thus represents a perfect archive to record the oceanographic conditions in this region and especially the latitudinal position of the Subtropical Front (STF), which emerged as a key parameter in the global climate due to its influence on Atlantic Meridional Overturning Circulation (AMOC) and the Indian and Atlantic Oceans. We here present the continuous BAYSPAR and BAYSPLINE-based SST record of the Subtropical Front in the Southern Ocean at Ocean Drilling Program (ODP) Site 1168 in western Tasmania, to identify the consequences of the SST changes observed at this site, for the oceanographic conditions, specifically the strength and the position of the subtropical front. This extra record of SST change is a long-term high-resolution analysis of organic geochemistry proxies and yields a better understanding of the influence of Antarctica and the Southern Ocean on Late Pliocene and Quaternary based on temperature proxies. Previous studies (Bostock et al., 2015; Sikes et al., 2009) in this area, focused on the isotope signals which are significantly influenced by other factors, such as seasonality or rainfall, presenting a short-term analysis, however, here we present a long-term continuous high-resolution analysis of temperature records based on organic geochemistry proxies for the late Pliocene and Quaternary.
Lower reflectance values, at Site 1168, were reported during interglacial periods while higher values were depicted during glacial, indicating that glacial sediments mainly consist of foraminifera ooze while interglacial sediments mainly contain nannofossil ooze. SST records decrease from 3 to approximately 1.3 Ma, on average 6 degrees to 4 degrees, and then increase until modern, presenting a constant variability in temperature of 9 to 18 degrees throughout the record. Subtropical Front (STF) faced an extremely low value of 3-4oC during MIS16, at approximately 0.6 Ma and after this period STF SST eventually increases to the modern levels of about 16 to 23 oC. However, our results suggest that non-thermal factors, such as water column oxygenation and nutrient supply, may influence GDGT cyclization which is linked to some cold bias in the TEX86-based SST records. Site 1168 presents a temperature difference of 9-18 degrees throughout the records, except the early Pleistocene period (~2.3 Ma); this variability is higher than the equatorial sites which is a sign of polar amplification and it is closely related to glacial-interglacial migration of the subtropical front, resulting in a northward movement of STF and a shift of the Subtropical Convergence to ~42oS west of Tasmania, at approximately 0.02 Ma. | |
