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

Trace Metal Chemistry in the Water Column of the Angola Basin - A Contribution to the International GEOTRACES Program - Cruise No. M121, November 22, – December 27, 2015, Walvis Bay (Namibia) – Walvis Bay (Namibia)

Meteor Cruise M121 was dedicated to the investigation of the distribution of dissolved and particulate trace metals and their isotopic compositions (TEIs) in the full water column of the Angola Basin and the northernmost Cape Basin. A key aim was to determine the driving factors for the observed distributions, which includes the main external inputs, as well as internal cycling and ocean circulation. The research program of the cruise is official part of the international GEOTRACES program (www.geotraces.org) and cruise M121 corresponds to GEOTRACES cruise GA11. Subject of the cruise was the trace metal clean and contamination-free sampling of waters and particulates for subsequent analyses of the TEIs in the home laboratories of the national and international participants. Besides a standard rosette for the less contaminant prone metals, trace metal clean sampling was realized by using for the first time a new dedicated, coated trace metal clean rosette equipped with Teflon-coated GO-FLO bottles operated via a plastic coated cable from a mobile winch of GEOMAR Kiel. The particulate samples were collected under trace metal clean conditions using established in-situ pump systems operated from Meteor’s Aramid line. The cruise track led from Walvis Bay northwards along the West African margin until 3°S, then turned west until the Zero Meridian, which was followed southwards until 30°S. Then the cruise track turned east again until the Namibian margin was reached and then completed the near shore track northwards until Walvis Bay. The track crossed areas of major external inputs including dust from the Namib Desert and exchange with the west African continental margin and with the oxygen depleted shelf sediments of the Benguela upwelling, as well as with the plume of the Congo outflow, that was followed from its mouth northwards. Our investigations of internal cycling included the extremely high productivity associated with the Benguela Upwelling and the elevated productivity of the Congo plume contrasting with the extremely oligotrophic waters of the southeastern Atlantic Gyre. The links between TEI biogeochemistry and the nitrogen cycle forms an important aspect of our study. The major water masses contributing the Atlantic Meridional Overturning Circulation were sampled in order to investigate if particular TEI signatures are suitable as water mass tracers, in particular near the ocean margin and in the restricted deep Angola Basin. A total of 51 full water column stations were sampled for the different dissolved TEIs, which were in most cases accompanied by sampling for particulates and radium isotopes using the in-situ pumps. In addition, surface waters were continuously sampled under trace metal clean conditions using a towed fish and aerosol and rain samples were continuously collected

Neodymium isotopes from water bottle samples measured during METEOR cruise M121 (GEOTRACES cruise GA08)

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 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 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 waters of a coastal plume originating near 13 °S, 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 -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. The data include station numbers, coordinates, depth, pot. temperatures, salinities, Nd concentrations in pmol/kg, epsilon Nd values and their 2SD

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

Highlights • First systematic dissolved neodymium isotope distributions in Angola and Cape Basins. • Deep water neodymium isotopes dominated by conservative mixing in study area. • Neodymium isotopes trace surface waters from Mozambique Channel in Angola Basin. • Cape Basin bottom water neodymium isotopes are unaffected by neodymium from sediments. • Glacial Cape Basin bottom waters may show effects of sedimentary neodymium inputs. 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

Neodymium and hafnium isotopes and rare earth element concentrations from water bottle samples measured during METEOR cruise M121 along the Congo River plume (GEOTRACES cruise GA08)

To better understand the inputs of riverine REEs and Hf to the ocean and their impact on the open ocean we collected filtered seawater samples from 16 full water column profiles along the Congo River plume at the coast of Congo and Gabon and along an offshore section at 3°S, complemented by three freshwater samples collected directly from the Congo River. The water samples from the Congo River plume were collected during GEOTRACES cruise GA08 with German RV Meteor (M121) in November and December 2015 with a stainless steel CTD rosette and a towed fish for surface waters. River water end-member samples from the Congo River at zero salinity were taken upstream by boat in May, July and October 2017 near the center of the river at ~6°S and ~12.5°E (stations 12.1–12.3). We determine for the first time REE and Hf concentrations and isotope compositions within the Congo River plume and quantify Nd and Hf fluxes. Based on these data we evaluate how far the Congo River plume is traceable by these geochemical parameters, how it contributes to their budgets in the South East Atlantic Ocean, and to which degree the Nd and Hf isotope and REE distributions can be used as water mass tracers in areas of high continental inputs

Dissolved neodymium and hafnium isotopes and rare earth elements in the Congo River Plume: Tracing and quantifying continental inputs into the southeast Atlantic

Highlights • First dissolved Nd/Hf isotope and REE data from the Congo River Plume. • High REE and Hf fluxes from the Congo River to the southeast Atlantic. • Conservative river and surface seawater mixing of Nd/Hf isotopes and REEs/Hf at S > 23. • Congo River particles impact intermediate and deep water signals of NE Angola Basin. Abstract The Congo River is the second largest river by discharge in the world and a major source of element inputs into the South Atlantic Ocean. Yet, the element fluxes and transport mechanisms across and beyond its estuary and their impacts on the marine distribution and cycling of many major and trace elements are not well understood. We present the first combined dissolved neodymium (Nd) and hafnium (Hf) isotope and rare earth element (REE) concentration distributions following the Congo River plume along its flow path off the West African coast and along a connected offshore latitudinal section at 3°S. The Congo River freshwater itself is characterized by extraordinarily high Nd and Hf concentrations of up to 4000 pmol/kg and 54 pmol/kg, and by Nd (εNd) and Hf (εHf) isotope compositions that range between −15.6 and −16.4 and between 0.35 and −1.4, respectively. Our near- and offshore data indicate that at salinities above 23 conservative mixing of Congo-derived Nd and Hf concentrations and isotopic signatures with ambient surface seawater occurs for at least 1000 km to the northwest of the river mouth. This demonstrates a large spatial extent of the influence of the Congo plume on trace metal distributions in the eastern south Atlantic surface waters. A comparison between dissolved Nd and Hf fluxes from the Congo River and the shelf zone estimated based on radium isotope compositions indicate that release from Congo-derived particulate phases likely balances strong estuarine REE and Hf removal in the low salinity zone. The combined riverine and shelf zone flux for Nd is almost twice as high as that estimated for the Amazon River, despite that the Amazon discharge is about five times higher than that of the Congo River. Even the offshore Nd flux estimated for the 3 °S transect based on radium isotope compositions still corresponds to ∼40% of the Congo-shelf-zone flux and reaches 150 ± 50 Mg/year for Nd. Moreover, intermediate waters below the plume are strongly affected by exchange with particulate inputs from the Congo River given that Nd isotope signatures are inconsistent with values expected from large-scale water mass mixing and instead support unradiogenic Nd release either from sinking or deposited Congo-derived detrital material. Deep and bottom water isotopic signatures are also slightly affected by interaction with particles and benthic Nd release