Paleoceanographic variability on the AgulhasPlateau during the past 150 kyr BP

This thesis presents the results of a multi-proxy analysis of two sediment cores recovered from within the Indian-Atlantic Ocean Gateway (I-AOG). The main focus of this study was on sediment core MD02-2588 which was recovered from the southern Agulhas Plateau. This core was used to produce reconstructions of the paleoceanographic variability of the deep and surface ocean during the past 150 kyr BP. Preliminary paleoceanographic records spanning the past 50 kyr BP are also presented from sediment core CD154-23-16P, recovered off southern Africa. To reconstruct the history of mid-depth ocean circulation on the southern Agulhas Plateau during the past 150 kyr BP, a range of physical and chemical bottom water parameters were derived from the stable isotope, elemental ratio and grain size data from core MD02-2588. These data suggest that, during glacial stages, the southern Agulhas Plateau and the wider mid-depth South Atlantic Ocean came under increased influence of southern-sourced deep waters and experienced an increase in the storage of respired carbon, as CO2 was sequestered from the glacial atmosphere. The associated decrease in the relative volume of northern-sourced deep waters bathing the MD02-2588 core site appears to be counterbalanced by the lower nutrient content of northern-sourced water masses during glacial stages resulting in an overall reduction of nutrient concentrations in the mid-depth South Atlantic. The glacial lowering of seawater nutrient concentration in the mid-depth South Atlantic was possibly also affected by an increase in the formation of lower-nutrient mid-depth waters by open ocean convection in polynyas within the expanded circum-Antarctic sea ice zone. During glacial terminations, mid-depth nutrient concentrations within the I-AOG reach their highest values of the past 150 kyr BP. These increases are interpreted as resulting from the upward mixing of nutrient-rich bottom waters from the deepest and most isolated layers in the Southern Ocean following the deglacial breakdown of stratification in the ocean interior. The increases in mid-depth nutrient concentration recorded during deglaciations occur in tandem with increases in the bottom water carbonate saturation state on the southern Agulhas Plateau and increases of pCO2 in Antarctic ice cores. The covariation of these parameters supports the premise of increased out-gassing of carbon from the deep Southern Ocean during deglaciations. This thesis also examines how the position of the subtropical front (STF) within the I-AOG migrated over the past 150 ky BP and considers how these changes may have impacted the nutrient supply to the surface waters of the southern Agulhas Plateau. The record of bulk sediment nitrogen isotope composition (δ15NBulk) from sediment core MD02-2588 suggests that the northward migration of the STF during glacials is associated with increases in nutrient supply relative to interglacial levels. Enhanced nutrient supply to the surface waters of the southern Agulhas Plateau is thought to be caused by increased northward advection of relatively nutrient-rich Subantarctic surface waters to the MD02-2588 core site, along with weaker upper ocean stratification which facilitated vertical mixing of nutrients from the thermocline. The likeness of the δ15NBulk record from MD02-2588 with analogous records from the eastern equatorial Pacific may suggest that the temporal variability of the isotopic composition of ocean nitrate within these two regions is linked on glacial-interglacial timescales, possibly as a consequence of changes in the position of the Southern Ocean fronts. New data from a set of core-top samples collected around New Zealand were used to assess the utility of the deep-dwelling planktonic foraminifera species Globorotalia truncatulinoides as a recorder of the isotopic composition of dissolved inorganic carbon in seawater. The downcore record of carbon isotopes in this planktonic foraminifera from MD02-2588 displays a correlation with the Antarctic ice core records of the isotopic composition of carbon in the atmosphere during the past 150 kyr BP. The correlation of these records demonstrates the importance of carbon transfer between the Southern Ocean and the atmosphere. Preliminary results of the bulk elemental composition and foraminiferal stable isotope records spanning the past 50 kyr BP are presented from sediment core CD154-23-16P, recovered from the Mallory Seamount off southern Africa. These records are used to reconstruct the terrestrial hydroclimate of southeastern Africa and the hydrography of the Agulhas Current.