Surface and deep hydrography across the mid-pleistocene transition; multi-proxy paleoceanographic reconstructions from the Southwest Indian Ocean

Studies of the Last Glacial Maximum have demonstrated a vital role for the carbon cycle in regulating climate on glacial-interglacial timescales, in particular emphasising the effects of storage or exhalation of CO2 from the world’s oceans. However the utility of proposed mechanisms over longer timescales, such as Early Pleistocene glacial cycles, remains unclear. The aim of this thesis is to investigate how changes in surface hydrography, deep ocean ventilation and flow, and marine biogeochemistry evolved across the Pleistocene, focusing on the Southwest Indian Ocean. New multi-proxy paleoceanographic records are generated from sediments cores in the Southwest Indian Ocean, at the boundary between the Subantarctic Southern Ocean and the subtropical Indian and Atlantic Oceans. These records, from International Ocean Discovery Program (IODP) Site U1475, span the Early to Late Pleistocene and include: Measurements of benthic C. wuellerstorfi and the deep-dwelling planktic G. truncatulinoides foraminifera stable isotopes; the accumulation of ice-rafted debris (IRD) deposited by Antarctic icebergs; and the ‘Sortable Silt’ mean grain-size proxy for near-bottom flow speed. The records are combined with published data from a nearby core (MD02-2588) and presented on the ‘Agulhas Plateau composite’. The results show that during glacial intervals, IRD deposition and near-bottom flow speeds were high, deep chemical ventilation was reduced, and the mode-water to deep-water chemical gradient was stronger relative to interglacial times. The glacial Agulhas Plateau was characterised by a northerly expansion of Southern Ocean waters accompanied by a stronger or more proximal Antarctic Circumpolar Current, a stronger chemocline, and a reduction in northern-sourced deep water presence. In these records, the Mid-Pleistocene Transition begins around 1.25 Ma with a stepwise increase in IRD deposition and higher amplitude cycles in near-bottom flow speed. The records also demonstrate that key interglacial periods are characterised by exceptionally sluggish deep-flow and a breakdown in vertical carbon stratification. Finally, the temporal phasing of the records describe a sequence of events whereby Southern Ocean surface conditions appear to lead deep circulation and global ice volume into ice ages; potential mechanisms to explain this phasing are outlined, involving shifts in the distribution of Antarctic iceberg melt