Assessing neodymium isotopes as an ocean circulation tracer in the Southwest Atlantic

The global overturning ocean circulation plays a key role in global climate by distributing heat around the Earth and by triggering or amplifying major climate changes. Neodymium (Nd) isotopes are widely used to trace present-day and past ocean circulation changes; however, their value as a circulation tracer has been increasingly challenged by studies that have focused on processes that can modify seawater Nd isotope ratios (for example, seawater interaction with particles, pore water fluxes from sediments). The Southwest Atlantic represents an excellent test bed for investigating the integrity of Nd isotopes as an ocean circulation tracer, as it includes the major Atlantic northern and southern hemisphere-sourced end-member water masses associated with the global overturning ocean circulation, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water (AAIW, NADW, and AABW, respectively) and potential regional sources of Nd that could impact that integrity. This study reports Nd isotope data on the GEOTRACES GA02 Southwest Atlantic Meridional Transect, spanning the equator to ∼50°S. Below the pycnocline, substantial non-conservative behavior is observed only in samples dominated by AAIW in the northern portion of the transect (∼25°S to the equator); this appears to reflect addition of Nd from the cratons of South America or Africa from above the pycnocline. This effect is not observed at depths directly below dominated by NADW. Otherwise, Nd isotopes behave as a near-conservative water mass tracer along the transect, with 48% of samples within analytical error of the predicted value from water mass mixing, and 84% within 0.9 εNd-units (∼3 times the analytical error), thus confirming its potential at most depths and locations in the Southwest Atlantic to reconstruct past ocean circulation changes.