Decoupling of barium and silicon at the Congo river-dominated Southeast Atlantic Margin: combined Barium and silicon isotope evidence

9 May 2023

Zhang and co-authors (2023, see reference below) conducted a comprehensive study to investigate the biogeochemical cycles of barium (Ba) and silicon (Si) and their relationship during transport from the Congo River mouth along the southwestern African shelf to the open northern Angola Basin, as part of the GEOTRACES eastern South Atlantic Ocean GA08 cruise (Meteor 121). The study employed a combination of stable Ba (δ138Ba) and Si (δ30Si) isotopes, along with Ba and Si fluxes estimated based on radioactive radium isotopes (228Ra). The authors have observed and demonstrated:

Stronger enrichment of dissolved barium (dBa) than silicon (dSi) in the shelf-zone of the Congo plume
Diatom silica production has negligible effect on dissolved Ba isotopic compositions in large river plumes
Marked dBa enrichment (up to 24 nM) in the deep water of the northern Angola Basin likely originating from high benthic inputs

Their findings reveal a pronounced decoupling of Ba and Si at the Congo River-dominated Southeast Atlantic margin and highlight the strong influence of the margin on marine Ba cycling. Furthermore, their research suggests that the association between concentrations of dissolved Ba (dBa) and Si (dSi) across the global ocean is primarily a consequence of the large-scale ocean circulation.

**Figure 1:** Surface distributions of (a) salinity, (b) dSi, (c) δ³⁰Si, (e) dBa, and (f) δ¹³⁸Ba following the Congo River plume along its flow path off the West African coast to a connected off-shelf latitudinal section at 3°S in the northern Angola Basin. (d) shows the sampling stations.

**Figure 2:** This conceptual figure illustrates the decoupling of Ba and Si at the Congo River-dominated Southeast Atlantic margin. The thickness of the solid line arrows represents the changes in the fluxes of dBa and dSi from the Congo River to the Congo-shelf-zone and the off-shelf transect. The arrows representing dBa and dSi fluxes from the Congo River are normalized to have equal thickness. The vertical profile of Ba* (the deviation of dBa from the dBa-dSi correlation in a defined oceanic area) reflects the decoupling of Ba and Si. The data used for this profile were collected from a station in the northern Angola Basin close to the margin.

Reference:

Zhang, Z., Yu, Y., Hathorne, E. C., Vieira, L. H., Grasse, P., Siebert, C., Rahlf, P., Frank, M. (2023). Decoupling of barium and silicon at the Congo River-dominated Southeast Atlantic margin: Insights from combined barium and silicon isotopes. Global Biogeochemical Cycles, 37, e2022GB007610, Access the paper: https://doi.org/10.1029/2022GB007610

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