Tracing water masses and terrestrial inputs with radiogenic neodymium and hafnium isotopes and rare earth elements in the southeastern Atlantic Ocean
The southeastern Atlantic Ocean is a key region to investigate large-scale ocean circulation as the water masses passing this region comprise important parts of the return flow of the Atlantic Meridional Overturning Circulation (AMOC). This doctoral thesis presents the first detailed investigation of water mass mixing processes and the distribution and fluxes of trace elements in the Angola and Cape Basins based on dissolved radiogenic neodymium (εNd) and hafnium (εHf) isotopes as well as rare earth element (REE) concentrations. Near surface water εNd signatures reaching −17 in the uppermost 200 m of the Angola Basin are mainly a consequence of the admixture of an unradiogenic coastal plume originating from the dissolution of Fe-Mn coatings of particles in the oxygen minimum zone. In contrast, εNd signatures of up to -17.6 in the upper water column of the northern Cape Basin are a consequence of advection of shallow waters via the Agulhas current, which originates from the Mozambique Channel. The Nd isotope compositions of the deep water masses in both basins primarily reflect conservative water mass mixing, while Nd isotope signatures of deep and bottom waters of the central Angola Basin are significantly overprinted by terrestrial inputs. Bottom waters of the Cape Basin show excess Nd concentrations likely originating from resuspended bottom sediments and/or dissolution of dust, but without changing the Nd isotopic composition of the bottom waters significantly due to similar εNd values of particles and seawater. The Congo River is the second largest river by discharge in the world and its plume carries extraordinarily high Nd and Hf concentrations of up to 4000 pmol/kg and 54 pmol/kg into the northern Angola Basin. Its freshwater is characterized by εNd and εHf values ranging between -15.6 and -16.4 and between 0.35 and -1.4, respectively. Rapid scavenging and removal of light rare earth elements (LREEs) and middle rare earth elements (MREEs) by coagulation processes form typical seawater REE patterns at salinities between 0 and 23. However, particle-seawater interactions in the low salinity zone may also result in Nd release from or exchange with Congo-derived particulate phases, as is indicated by elevated Congo-shelf-zone REE and Hf fluxes. Yet, this process is not reflected by changes in Nd and Hf isotopic compositions given that the latter are identical in the dissolved and particulate pools of the Congo River. Having passed the estuary, the Nd and Hf concentrations and isotopic signatures in the Congo freshwater plume are mixed conservatively for up to 1000 km northwest of the river mouth. Intermediate and deep waters below the plume and in the open northern Angola Basin are strongly affected by inputs from the Congo River resulting in less radiogenic signatures compared to what is expected from water mass mixing only. Surface waters of the Angolan coast near the Angola Benguela Front (ABF) are characterized by unradiogenic εNd signatures of up to -21 and elevated REE concentrations (35 pmol/kg for Nd), which are likely caused by dissolution of Fe-Mn oxide coatings of coastal sediment particles in the prevailing oxygen minimum zone. Decreasing REE concentrations in intermediate and deep waters suggest removal via scavenging due to high vertical particle fluxes, while Nd signatures of about -15 indicate release of unradiogenic Nd. Surface waters off the Namibian coast are more radiogenic (εNd -12) and have lower REE concentrations (12 pmol/kg for Nd) suggesting minor terrestrial inputs from the Namib and Kalahari or the Orange River. Waters off the Namibian coast are strongly influenced by upwelling and resuspension of particles from shelf sediments into the oxygen minimum zone, where REEs and distinct Nd isotope signatures are released and advected into the surface waters. The investigations support the application of Nd isotopes as a quasi-conservative tracer of present and past deep water mass mixing in the southeastern Atlantic Ocean. Nd isotopes and REE concentrations trace advection of near surface water masses with distinct isotope composition such as the Agulhas current but also serve to monitor non-conservative terrestrial inputs. In coastal areas, Nd isotopes can be used to examine seawater-particle exchange processes and in combination with Hf isotope compositions and element distributions serve to quantify riverine fluxes of elements to the open ocean, such as across the Congo estuary. Overall, this study demonstrates that combined REE concentrations and Nd and Hf isotopes help to better understand the mixing and advection of the water masses of the AMOC in the SE Atlantic Ocean and the importance of local terrestrial inputs, which has implications for the application of these geochemical tools in other parts of the world ocean and for the reconstruction of past ocean circulation.