Basin‐scale inputs of cobalt, iron, and manganese from the Benguela‐Angola front to the South Atlantic Ocean

Abstract

We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N 2 O, and O 2 , as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Diagnosing oceanic nutrient deficiency

“…Data on the distributions of the macronutrient elements N, P, Si [44,[62][63][64][65][66][67], the micronutrient elements Fe, Mn, Ni, Zn and Cd [64,[68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84] and the non-nutrient element Al [67,85,86] were downloaded from the IDP2014 collection [20] and imported into a Matlab™ environment for analysis. For each sampling location and/or seawater volume/water mass, a ranking of the relative deficiencies of multiple nutrient elements was calculated by dividing observed dissolved concentrations by an assumed 'typical' stoichiometric ratio within newly formed organic (biological) material (figure 1) [11,18].…”

“…In addition to providing confirmation of the ubiquity of sub-surface Fe deficiency within the ocean, results from the performed simple analysis can be further considered in the context of physical processes which return interior waters to the productive surface ( figure 5). Thus, in the absence of other local/regional sources/factors, such as interactions with sediments on the continental shelves [35,36], dust deposition [34] or specific sub-surface conditions including marked oxygen minima [84], any upwelled or upwardly mixed waters are likely to be deficient in Fe throughout the majority of the oceans. Development of Fe limitation in approximately 30% of the surface waters of the open ocean [11,19] should hence perhaps not be surprising.…”

Processes and patterns of oceanic nutrient limitation

“…Subsequent expansion of oxygen minimum zones could decrease the ocean inventory of fixed nitrogen species by increasing microbial denitrification and/or anaerobic ammonium oxidation. Expansion of oxygen minimum zones could also increase trace metal 66 and phosphorus inventories by increasing the release of these nutrients from sediments, as may have occurred over glacial-interglacial cycles (see Supplementary Information). Such changes at depth could influence surface waters on timescales of decades or longer (Fig.…”

“…These waters are deficient in those trace metals that have short oceanic residence times due to scavenging losses (Fig. 1a,d) 6,66 , contributing to the tendency for iron limitation in the Southern Ocean (Fig. 3).…”

Overview of the mechanisms that could explain the ‘Boundary Exchange’ at the land–ocean contact

“…Actually, distinguishing the riverine solid load from the deposited sediments is not straightforward in coastal environments, owing to the fact that they essentially refer to the freshness of the material. However, recent works identified that submarine weathering of deposited material can act as a source of trace elements and isotopes to the ocean, allowing the present distinction of this later pathway [13][14][15][16][17][18][19]. -Submarine groundwater discharges (SGD).…”