Deciphering glacial-interglacial Southern Ocean dynamics with deep-sea corals

Abstract

Recent observations and past reconstructions have highlighted the significance of the Southern Ocean for modern and past ocean circulation patterns. Deep wind-induced upwelling renders the Southern Ocean unique, such that deep waters are brought directly to the surface where they can exchange with the atmosphere. Moreover, the Antarctic Circumpolar Current (ACC) is the dominant feature of Southern Ocean circulation linking all ocean basins and facilitating the inter-basin exchange of ocean properties. Hence, Southern Ocean dynamics act to (partly) moderate both, zonal and meridional transports, and both were shown to be sensitive to atmospheric forcing on different time scales. However, due to the remoteness and harsh conditions, direct evidence of past Southern Ocean circulation is scarce. This work uses neodymium (Nd) isotopes extracted from the aragonitic skeletons of deep-sea corals collected in the Drake Passage and at the Tasmanian margin. The fidelity of the skeletal Nd isotope signature was tested in a calibration effort and the nature of Nd in coralline aragonite was explored. Based on the promising results, the approach was then confidently applied to uranium-series dated deep-sea corals in order to decipher Southern Ocean water mass mixing across intervals of past climate perturbations. The results show that Drake Passage and Tasmanian margin corals record complimentary features of Southern Ocean circulation. Intervals of high sampling resolution reveal unexpectedly dynamic and abrupt changes of water mass mixing oscillating on millennial to (sub) centennial time scales during the past ~40,000 years. This thesis explores the nature of the recorded signal in the light of available literature, focusing on phases of abrupt change and fitting the processes considered to drive the Southern Ocean Nd isotope signal into a global framework.