The biogeochemical cycles of barium and silicon in the Congo River-influenced continental margin: insights from combined silicon and barium isotopes 

Dr Zhouling Zhang, GEOMAR

Abstract: Dissolved barium (DBa) concentrations are closely correlated to those of silicon (DSi) in the ocean, however, the mechanism behind their linkages and hence processes controlling ocean Ba cycling remain enigmatic. We provide the first combined measurement of stable Ba (δ¹³⁸Ba) and Si (δ³⁰Si) isotopes on seawater samples from the Congo River-influenced continental margin. In the surface water, Ba and Si flux estimated based on ²²⁸Ra reveal non-conservative addition of both Ba and Si into the Congo shelf zone, attributable to shelf inputs via submarine groundwater discharge and/or porewater efflux and river-borne particle desorption/dissolution. The addition of Ba is particularly high, equivalent to 5.9 ± 4.9% of the global freshwater Ba input, but it contributes Ba with δ¹³⁸Ba signatures indistinguishable from those of the dissolved Ba pool. Contrarily, the addition of Si contributes lighter δ³⁰Si signatures to the shelf waters. In addition, by comparing the Congo plume to the Amazon plume, we found that δ¹³⁸Ba is not significantly influenced by diatom productivity in large river plumes. Moreover, we also observe a large enrichment of DBa in the deep water (below 2000m) of the northern Angola Basin, likely associated with high benthic inputs from the Congo submarine canyon and deep-sea fan sediments, which are dominated by massive terrigenous deposits composed of labile organic and inorganic materials. Our results reveal fundamentally different mechanisms controlling the biogeochemical cycling of Ba and Si. As such, the close association of dissolved Si and Ba concentrations in the global ocean is rather a result of common hydrodynamic control. Our work also highlights a strong margin influence on the Ba cycle.