Abstract
Rivers are a major supplier of particulate and dissolved material to the ocean, but their role as sources of bio-essential dissolved iron (dFe) is thought to be limited due to rapid, efficient Fe removal during estuarine mixing. Here, we use trace element and radium isotope data to show that the influence of the Congo River margin on surface Fe concentrations is evident over 1000 km from the Congo outflow. Due to an unusual combination of high Fe input into the Congo-shelf-zone and rapid lateral transport, the Congo plume constitutes an exceptionally large offshore dFe flux of 6.8 ± 2.3 × 108 mol year−1. This corresponds to 40 ± 15% of atmospheric dFe input into the South Atlantic Ocean and makes a higher contribution to offshore Fe availability than any other river globally. The Congo River therefore contributes significantly to relieving Fe limitation of phytoplankton growth across much of the South Atlantic.
Highlights
Rivers are a major supplier of particulate and dissolved material to the ocean, but their role as sources of bio-essential dissolved iron is thought to be limited due to rapid, efficient Fe removal during estuarine mixing
Elevated dissolved trace element concentrations in coastal regions are derived from riverine inputs[1,2], benthic pore-water and resuspended sediment supply[3], atmospheric deposition[4], and submarine groundwater discharge (SGD)[5]
On the shelf where Congo waters first encounter the Atlantic Ocean, hereafter the Congo-shelf-zone (Fig. 1), the mean dissolved Fe (dFe) concentration was ~15% of the Congo River concentration, indicating low apparent dFe removal compared with Congo River freshwater
Summary
Rivers are a major supplier of particulate and dissolved material to the ocean, but their role as sources of bio-essential dissolved iron (dFe) is thought to be limited due to rapid, efficient Fe removal during estuarine mixing. Conservative mixing between the Congo-shelf-endmember and offshore waters (Fig. 2b) indicates a riverine 228Ra effective-zerosalinity-endmember concentration of 85 ± 4 dpm 100 L−1, including both dissolved and desorbed Ra. Together with the river discharge (1.3 × 1012 m3 year−1)[31], this suggests a fluvial 228Ra flux of 4.8 ± 0.4 × 1021 atoms year−1, which is similar to the
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