UKIODP Moratorium Award - IODP Expedition 361, Southern African Climates - Ian Hall

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The South African Climates (SAFARI) Expedition 361 will investigate the interaction between climate and the Agulhas Current during the Pliocene/Pleistocene. Six drill sites in the southern Indian Ocean and Mozambique Channel will determine (1) the sensitivity of the Agulhas Current, (2) the dynamics of Indian-Atlantic gateway circulation, and (3) the connection between Agulhas Leakage and shifts of the Atlantic Meridional Overturning Circulation (AMOC) during major ocean and climate reorganizations of the past 5 Ma. The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm and saline surface waters from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rain-fall patterns. Recent ocean models and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the AMOC during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that act as a control mechanism on the basin-wide AMOC, with implications for convective activity in the North Atlantic and Northern Hemisphere climate. Expedition 361 aims to extend this work to periods of major ocean and climate restructuring during the Pliocene/Pleistocene to assess the role that the Agulhas Current and ensuing (interocean) marine heat and salt transports have played in shaping the regional- and global-scale ocean and climate development.Within this moratorium proposal a specific focus will be on the The Middle Pleistocene Transition (MPT), circa 1250 and 700 thousand years (kyr) before present (BP) which represents a key interval of Pleistocene climate history. During this time the periodicity of glacial-interglacial cycles altered from a 40 kyr rhythm to a less frequent, more intense and asymmetric 100 kyr pattern. This shift occurred in the absence of significant changes in orbital forcing, suggesting that internal feedbacks in the climate system played a considerable role in bringing about this major climatic reorganisation. Most hypotheses for the origin of the MPT invoke a response to a long-term cooling, driven by decreasing atmospheric pCO2. Ice core records extending over the past 800 kyr indicate glacial-interglacial pCO2 fluctuations of ~100 ppm. Various mechanisms of carbon sequestration and deep ocean storage have been proposed over the past three decades. One of these mechanisms invokes an increase in the efficiency of preformed nutrients utilisation in the Subantarctic Zone of Southern Ocean as a result of increased dust flux. Glacial dust fertilisation in the iron-limited conditions of this High Nutrient, Low Chlorophyl (HNLC) region, could have driven an increase in primary productivity and increased export of carbon from the surface ocean. Given the potential of this mechanism as a driver of Late Pleistocene pCO2 variability, it is conceivable that a similar process may have operated across the MPT. I will use use material from Exp 361 Site on the Agulhas Plateau to reconstruct export production in the Subantarctic Zone across the MPT. This will allow me to investigate the response of the soft-tissue biological pump to the increased iron deposition observed during the onset of the MPT and will ultimately contribute to our understanding of the processes that resulted in lower glacial pCO2 in the post-MPT world.