Reconstructing South Atlantic Ocean Circulation changes across the Eocene-Palaeocene

The Atlantic meridional overturning circulation (AMOC) is a system of surface and deep ocean currents, which constitutes a major component of modern global ocean circulation. The AMOC distributes heat, salt, and bio-essential nutrients around the global oceans and has a fundamental influence on global and regional climates. Yet, we do not know how stable the current AMOC is and how it may change owing to ongoing climate change. To help us improve our understanding of how the AMOC might change in a warmer climate, we can investigate intervals in the geological past to determine how ocean circulation may have operated during warmer-than-modern climates. The Eocene and Palaeocene (34 to 65 million years ago) epochs were intervals of time when the climate was considerably warmer than modern, and similar to projections for the level of warmth for our medium- to long-term future. Because the Eocene-Paleocene are useful approximations for our future climate, we propose to reconstruct the ocean circulation during these epochs to provide invaluable insights into how the modern AMOC might change its operation with ongoing global warming.

To reconstruct past ocean circulation, we use geochemical techniques such as measurements of the carbon isotopes (d13C) of fossil shells of benthic foraminifera (microscopic sea-floor dwelling organisms). The carbon isotopes of fossil benthic foraminifera record the dissolved carbon of the ambient seawater at the time they grew their shells. In the ocean, microscopic plankton in the uppermost ocean take up nutrients and carbon, preferentially removing the light carbon isotope (12C). As these plankton die and settle through the ocean, remineralization of their organic remains in deeper waters releases nutrients and 12C-enriched carbon, imparting a relatively low d13C signature at depth. Along the main deep ocean flow-path, progressive organic carbon remineralisation gives rise to a progressive decrease in d13C, which is termed water mass 'ageing'. Carbon isotopes can thereby tell us how recently the deep waters were last at the ocean surface i.e. a rough approximation of the water-mass' relative 'age'. Oxygen isotopes (d18O) of the same fossil benthic foraminifera will be used to provide a record of deep ocean temperature and salinity during the Eocene-Palaeocene. A third technique utilises neodymium isotopes (eNd) measured on fossil fish teeth to identify and trace the flow directions of deep water masses. Ocean basins acquire distinctive eNd signatures from the weathering of isotopically distinct material from the surrounding continents, with the water masses bathing these ocean basins reflecting these unique eNd signatures. eNd data also allow the determination of any mixing between water masses. These three methods will be applied to samples obtained during the International Ocean Discovery Program (IODP) Expedition 390/393 in the South Atlantic. This will allow the following objectives to be accomplished:

1. Assess the spatial structure of ocean circulation across the South Atlantic during the Eocene-Palaeocene. New records from Expedition 390/393 will be compared to similar datasets generated from the south-east Atlantic (Expedition 208).
2. Reconstruct the evolution of the AMOC structure through the Eocene-Palaeocene, with the objective to investigate the timing of when northern sourced waters first extended into the South Atlantic, similar to today's AMOC. This will be achieved by combining new records generated in this project with previously generated Eocene-Paleocene eNd and d13C records from the North Atlantic and Tropical Atlantic.
3. Determine the role of Atlantic ocean circulation, particularly the South Atlantic, in regulating global climate change during the climatic transition from the extreme greenhouse warmth of the Paleocene and early Eocene to the cooler icehouse of the latest Eocene.