Pliocene-Pleistocene evolution of Benguela upwelling and Agulhas Leakage in the SE Atlantic

Abstract

Understanding the impacts of the transition from the warmth of the middle Pliocene to the large amplitude, 100 ka glacial-interglacial cycles of the late Pleistocene helps us to better interpret both the local forcings and global impacts of possible future climate changes. In this thesis, changes in ocean circulation over the last 3.5 million years (Ma) are investigated using a marine sediment core recovered from the SE Atlantic Ocean, a region often described as an ocean gateway because it includes the transfer of heat and salt from the Indian to the Atlantic Ocean between Antarctica and South Africa (the “Agulhas Leakage”). However, the response of this region to Pliocene-Pleistocene climate evolution remains unclear. This thesis analyses the climate information recorded at Ocean Drilling Program (ODP) Site 1087 (31º28’S, 15º19’E, 1374m water depth) to investigate the history of Agulhas Leakage and associated ocean circulation changes including the Antarctic Circumpolar Current to the south, and the productive Benguela upwelling system to the north.. This thesis presents the results generated using several organic geochemistry proxies and foraminiferal analyses to reconstruct the climate history at ODP 1087. These include the UK37’ index (for sea surface temperatures, SSTs), the TEX86 index (for ocean temperatures and an upwelling indicator), pigment analysis (for productivity changes), foraminifera assemblages (as water mass indicators), and dinoflagellate assemblages (for SSTs and water mass indicators). During the Pliocene and early Pleistocene, ODP 1087 was dominated by the Benguela Upwelling system, which had shifted south relative to today, and three prominent cold periods punctuate the overall warmth (during the M2 and KM2 stages, and at 2.8 Ma). From 2.2 Ma a longer term cooling trend begins, and further cooling occurs at 1.6 Ma, both of which are interpreted to represent periods of intensification of the Benguela Upwelling. The start of modern Agulhas Leakage occurs at ~0.9 Ma, marked by the start of early warming at the site ahead of the terminations. Finally, from 0.6 Ma there is an intensification of Agulhas Leakage which has led to an overall warming of SSTs which span both glacial and interglacial stages. Overall, the ODP 1087 record shows that this region is more reactive to southern hemisphere and local forcings such as changes in the southern wind field and ice expansion around Antarctica, rather than to northern hemisphere forcing.