Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)
Studies of Earth's past climate and oceans are important for developing an understanding of the range of possible behaviors in the climate system. This project seeks to reconstruct the history of the "Agulhas Leakage" from the Indian Ocean into the Atlantic Ocean, and to understand how changes in this leakage are related to past climate and the ocean's circulation. The Agulhas Current is a large boundary current (similar to the Gulf Stream) that flows southward along the southeast African margin. When it gets to the southern tip of Africa, most of the current turns back to the east, but some rings spin off into the Atlantic. This leakage brings water that is saltier than Atlantic water into the Atlantic, and this salt is carried by surface currents, including the Gulf Stream, to the far North Atlantic where it contributes to the formation of North Atlantic Deep Water. Thus, the Agulhas Current system is a key part of the ocean's large-scale overturning circulation. Changes in climate factors in the past may have led to changes in this system, and the goal of this project is to investigate such changes by combining evidence for the surface and deep ocean circulation from sediment cores collected on International Ocean Discovery Program (IODP) Expedition 361. The location of Site U1479 is ideal for making proxy records for both the surface and deep ocean, so it will be possible to simultaneously examine the changes and linkages between Agulhas Leakage and NADW production using a broad suite of geochemical tools. The project involves extensive international collaboration with the scientists from the expedition and will include training and mentorship of a diverse research team that includes undergraduates, graduate students, a postdoctoral fellow and an early career faculty member.The project seeks to detail the relative timing of variability in Agulhas Leakage and North Atlantic Deep Water (NADW) through the last glacial cycle (0-150 kyr) and over the mid-Pleistocene transition (MPT, 0.7-1.3 Ma), an important time of reorganization of the climate system that took place in the absence of changes in the underlying orbital forcing. The variability of Agulhas Leakage will be studied using detailed terrigenous clay provenance analyses, while the NADW variations will be studied with Nd-isotopes in fish teeth and foraminifera coatings. The primary site proposed for the combined deep and surface signal is IODP Site U1479, which is currently bathed in NADW and located where the Agulhas rings are shed into the South Atlantic, comprising the Agulhas Leakage. Additionally, clay provenance proxies from Site U1474, which is located near the beginning of the fully-constituted Agulhas Current, will constrain any end-member changes to the terrigenous composition of the Agulhas endmember. Because the data for both sets of proxies will be collected from the same samples, and also from the same samples that are being processed for benthic isotope stratigraphy, the relative timing of changes in the composition of the Agulhas endmember, the amount of Agulhas leakage, and the changes in NADW will be co-registered. IODP Expedition 361 collected sedimentary records from the broader Agulhas Current System that provide an unparalleled opportunity to examine in detail linkages between the Agulhas Leakage and Atlantic Meridional Overturning Circulation (AMOC) as well as linkages between the Agulhas Current System and southern African climates.