Assessing Agulhas Leakage using Terrigenous Sediment Provenance at IODP Site U1479: Initial Results

At the western boundary of the Indian Ocean, warm, saline waters are carried southward by the Agulhas Current until they retroflect, shedding rings and filaments of tropical Indian Ocean water into the South Atlantic ("Agulhas Leakage"), and mixing with Southern Ocean waters at the Subtropical Front. Variations in Agulhas Leakage are positively associated with North Atlantic Deep Water flow on various timescales, and may be important for triggering glacial terminations. Because of the dynamic and turbulent nature of this region, few sedimentary archives exist beyond a few hundred thousand years, and the characteristics of the Agulhas Leakage are unknown beyond about 1 Ma. IODP Expedition 361 Site U1479 (35°S, 17°'E; 2615 mbsl), drilled in 2016, provides a new continuous sedimentary sequence from the Late Pleistocene back to the late Miocene. We will present the initial results of a geochemical provenance study of these sediments. Because the Agulhas Current has its source near where very old continental crust is exposed on the eastern coast of Africa, it carries sediment with a distinctive geochemical signature. This distinctive provenance acts like a 'fingerprint' or a 'dye', allowing us to identify the Agulhas Current source in sediments deposited in the deep ocean. We measured major and trace element concentrations, and radiogenic isotopes of Sr, Nd, Ar, and Pb from the fine grained (<2 micron) terrigenous clays. Over the last glacial cycle, geochemistry of the clays reflect an older terrigenous sediment provenance during warmer periods, indicating more Agulhas Leakage during those times. Conversely, a younger provenance is observed for colder periods, especially MIS2, indicating reduced Agulhas Leakage. We see a good correlation between sediment provenance and δ¹³C, consistent with strong relationship between Agulhas Leakage and NADW. Future work will extend this record to cover the Mid-Pleistocene Transition, and compare our data with other proxies for Agulhas Leakage (faunal assemblages) and NADW (ɛNd).