Liu, Te, Hopwood, Mark J. , Krisch, Stephan , Vieira, Lucia Helena and Achterberg, Eric P. (2023) Trace metal fluxes of Cd, Cu, Pb and Zn from the Congo River into the South Atlantic Ocean are supplemented by atmospheric inputs. Geophysical Research Letters, 50 (24). e2023GL107150. DOI 10.1029/2023GL107150.

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Abstract

The Congo River supplies vast quantities of trace metals (TMs) to the South Atlantic Ocean, but TM budgets for the Congo plume derived using radium isotopes for GEOTRACES cruise GA08 suggest additional input other than the river outflow. Considering the tight correlations between most dissolved TMs and salinity in the plume and the high rainfall during the wet season over the Congo shelf, we hypothesized that wet atmospheric deposition is a TM source to the Congo plume. Observed TM concentrations in rainwaters across the Congo shelf were mostly comparable to values from previous work in the North Atlantic and Mediterranean Sea. Wet deposition contributed the equivalent of 43% dCd, 21% dCu, 20% dPb and 68% dZn of the Congo River fluxes. Our findings show an important role of wet deposition in supplying TMs to the South Atlantic overlapping with the region that receives substantial TM fluxes from the Congo River.

Key Points

The Congo River is an important source of trace metals (TMs) to the South Atlantic Ocean revealed by data from GEOTRACES cruise GA08

Wet deposition (rainfall) is identified as an additional TM source to the Congo plume by concurrently considering river and rain data

Rainfall supplies anthropogenic dTMs (Cd, Cu, Pb and Zn) with fluxes equivalent to 20%–68% of those from the Congo River on the Congo shelf

Plain Language Summary

The Congo River has the second largest freshwater discharge volume globally and creates an extensive near-equatorial plume into the Atlantic Ocean. The Congo plume constitutes an important source of trace metals (TMs) to the ocean, which impacts biogeochemical cycles in the tropical and subtropical ocean. However, existing work suggests a discrepancy within the TM budgets in the Congo plume and points to unknown source other than the Congo River or shelf sediments. Most TM concentrations across the Congo plume remain tightly correlated with salinity, suggesting that any additional sources are likely also freshwater-derived or enter the ocean at the river mouth coincidently with direct riverine TM inputs. Here, TM concentrations in ocean, river and rainwater collected during the GEOTRACES GA08 cruise are combined to suggest that wet deposition augmented some Congo TM fluxes to the ocean. Fluxes of anthropogenic Cd, Cu, Pb and Zn to the Congo shelf from wet deposition are of the same order of magnitude as the Congo River. Concentrations of these elements in rainwater are similar to prior observations reported for the North Atlantic and Mediterranean Sea, suggesting that a large fraction of the global range of rainwater concentrations over the ocean has been captured in our observations.

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