South Atlantic Nd isotope distribution and water mass geometry during the Holocene and LGM

The climate system and the meridional overturning circulation (MOC) are closely linked. The detailed nature of this coupling, however, is still elusive. Paleo-records of ocean circulation and climate can yield information on past changes and provide powerful constraints on ocean-climate interactions. Documenting past MOC changes is therefore essential for a detailed understanding of the ocean-climate system. Neodymium isotope ratios behave quasi-conservatively in the ocean that is, they approximately reflect the mixture of different water masses without being affected by changes nutrient cycling or carbonate chemistry. Their application to paleoceanography therefore facilitates direct tracing of water masses in the past ocean and has the potential to give insight into changes in volume transport of the MOC. In order to improve the basis for the use of Nd isotopes as paleo-water mass tracer and for robust quantifications of past circulation changes, we have analyzed Nd isotopes on the authigenic ferromanganese oxide fraction of core-top sediments from the Atlantic Ocean with a focus on the Southern Hemisphere part of the basin. We collected samples from intermediate to bottom water depths and from 43°S to 30°N in the Atlantic, significantly adding to existing data from the South Atlantic from Piotrowski et al. (EPSL, 2008). Eighty percent of our new authigenic Nd isotope data match the seawater isotopic distribution well. This confirms the value of Nd isotopes as a water mass tracer and strongly supports its application and merit for the reconstruction of past ocean circulation changes. Nd isotope ratios diverge from the seawater isotopic composition at some margin sites, where the data suggest overprints from local terrigenous Nd sources, and at some sites in the Southern Ocean, where the seawater integrity of Nd isotopes is compromised by Nd from volcanic material. Nd isotope values of last glacial maximum (LGM) Atlantic sediments show a northward invasion of Southern Ocean-like Nd isotope signatures at depth, consistent with the increased northward expansion of Antarctic Bottom Water and the reduction of southward export of North Atlantic Deep Water at this time.