Unprecedented Fe delivery from the Congo River margin to the South Atlantic Gyre

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.

Radium-228-derived ocean mixing and trace element inputs in the South Atlantic

“…where FTE-a is the offshore advective flux of TE, w is the net offshore advection velocity (0.6 ± 0.3 cm s -1 ) along the SAC in (Charette et al, 2016;Sanial et al, 2018;Vieira et al, 2020). This method provides the integrated net fluxes of TEs, considering all the possible inputs (e.g.…”

“…Comparing the shelf-ocean TE fluxes from this study with previous studies, all the TE fluxes are normalised to the area of the shelf-ocean cross section. The areas of shelf-ocean cross section from the reference regions are summarised in Table D2 ( Emery, 1966;Windom et al, 2006;Hooker et al, 2013;Vieira et al, 2020). In this study, the 228 Ra-derived shelf-ocean Co fluxes range from 3.8 to 22 × 10 3 nmol m -2 d -1 in the Argentine margin and from 4.3 to 6.2 × 10 3 nmol m -2 d -1 in the Cape margin of the 40 o S transect in the South Atlantic.…”

“…In this study, the 228 Ra-derived shelf-ocean Co fluxes range from 3.8 to 22 × 10 3 nmol m -2 d -1 in the Argentine margin and from 4.3 to 6.2 × 10 3 nmol m -2 d -1 in the Cape margin of the 40 o S transect in the South Atlantic. In comparison, previous studies have applied the TE/ 228 Ra approach to estimate the shelf Co fluxes in several continental margins: the western North Atlantic (1.6 × 10 5 nmol m -2 d -1 , Charette et al, 2016), the Peruvian shelf (1.4 × 10 5 nmol m -2 d -1 , Sanial et al, 2018) and the Congo-offshelf 3 o S (2.8 × 10 6 nmol m -2 d -1 , Vieira et al, 2020). Although these fluxes are about one and two orders of magnitude respectively higher than the estimates in the South Atlantic, these regions are also associated with low oxygen which is prone to result in higher Co fluxes.…”

“…Previous work has assessed TE input to the wider South Atlantic from rivers (Vieira et al, 2020), atmospheric dust (Gaiero et al, 2013), shelf sediments (Graham et al, 2015), the Agulhas current (Paul et al, 2015) and hydrothermal vents . There are also studies of TE distributions in some areas of the South Atlantic (e.g.…”

“…Subsequent downward mixing from the surface can also be used to assess vertical mixing in the upper water column (Charette et al, 2007;Sarmiento et al, 1976;van Beek et al, 2008). Radium-228 has also been used as a conservative tracer to estimate submarine groundwater discharge (SGD) (Windom et al, 2006;Moore et al, 2008;Kwon et al, 2014;Rodellas et al, 2015;Le Gland et al, 2017), river inputs (Vieira et al, 2020), continental shelf (van der Loeff et al, 1995;Charette et al, 2016;Sanial et al 2018;Kipp et al, 2018a) and hydrothermal inputs (Kipp et al, 2018b).…”