Tracing water mass mixing and continental inputs in the southeastern Atlantic Ocean with dissolved neodymium isotopes

Abstract

In contrast to the vigorous deep ocean circulation system of the north- and southwestern Atlantic Ocean, no systematically sampled datasets of dissolved radiogenic neodymium (Nd) isotope signatures exist to trace water mass mixing and provenance for the more restricted and less well ventilated Angola Basin and the Cape Basin in the southeastern Atlantic Ocean, where important parts of the return flow of the Atlantic Meridional Overturning Circulation are generated. Here, to improve our understanding of water mass mixing and provenance, we present the first full water column Nd isotope (expressed as εNd values) and concentration data for a section across the western Angola Basin from 3° to 30° S along the Zero Meridian and along an E-W section across the northern Cape Basin at 30° S sampled during GEOTRACES cruise GA08. Compared with the southwestern Atlantic basin we find overall less radiogenic εNd signatures reaching −17.6 in the uppermost 200 m of the Angola and Cape basins. In the western Angola Basin these signatures are the consequence of the admixture of a coastal plume originating near 13° S and carrying an unradiogenic Nd signal that likely resulted from the dissolution of Fe-Mn coatings of particles formed in river estuaries or near the West African coast. The highly unradiogenic Nd isotope signatures in the upper water column of the northern Cape Basin, in contrast, originate from old Archean terrains of southern Africa and are introduced into the Mozambique Channel via rivers like the Limpopo and Zambezi. These signatures allow tracing the advection of shallow waters via the Agulhas and Benguela currents into the southeastern Atlantic Ocean. The Nd isotope compositions of the deep water masses in both basins primarily reflect conservative water mass mixing with the only exception being the central Angola Basin, where the signatures are significantly overprinted by terrestrial inputs. Bottom waters of the Cape Basin show excess Nd concentrations of up to 6 pmol/kg (20%), originating from resuspended bottom sediments and/or dissolution of dust, but without significantly changing the isotopic composition of the waters due to similar εNd values of particles and bottom waters ranging between −9.6 and −10.5. Given that bottom waters within the Cape Basin today are enriched in Nd, non-conservative Nd isotopic effects may have been resolvable under past glacial boundary conditions when bottom waters were more radiogenic.