Imprint of a dissolved cobalt basaltic source on the Kerguelen Plateau

Abstract

Abstract. Processes of cobalt (Co) entrainment from shelf sediments over the Kerguelen Plateau were studied during the KEOPS (Kerguelen Ocean Plateau compared Study) in order to explain the exceptionally high dissolved cobalt concentrations that have been measured in the surface waters above the Kerguelen Plateau, and in intermediate and deep waters above its eastern slope. Lateral advection and dissolution of Co contained in basalt sediments around Heard Island, a main source of lithogenic Co in the study area, were shown to imprint the process of surface enrichment over the plateau. Dissolved Co enrichment was strongest at the intercept of the eastern slope with intermediate and deep waters, probably due to more efficient mobilisation of the sediments in the slope current, in addition to advection of Co-enriched and low-oxygenated ocean water masses.In surface waters, the strong sedimentary Co inputs were estimated to be much higher than biological Co uptake in phytoplankton blooms, underlining the potential use of dissolved cobalt as tracer of the natural iron fertilization above the Kerguelen Plateau. Based on a simple steady-state balance equation of the external input of dissolved iron over the plateau, the fertilization of iron inferred by using dissolved Co as a tracer of basalt sources is estimated to be 28 × 10 2 ± 21 × 10 2 t yr −1 in surface waters of the Kerguelen Plateau. This estimate is consistent with preceding ones (Zhang et al., 2008;Chever et al., 2010), and the calculated iron supply matches with the phytoplankton demand (Sarthou et al., 2008).

Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03

“…E. Noble et al: Coastal sources, sinks and strong organic complexation of dissolved cobalt imum zones (OMZs) of the South Atlantic and South Pacific Noble et al, 2012), and from more limited datasets from the North Pacific (Ahlgren et al, 2014;Saito et al, 2004Saito et al, , 2005, are likely due to reductive dissolution and advection of sedimentary sources in regions with low-oxygen bottom water sediment-water interfaces (Heggie and Lewis, 1984). Coastal and island sources in oxygenated environments have also been observed, for example, off the North American continental shelf (Saito and Moffett, 2002) and near the Kerguelen Islands (Bown et al, 2012a). While there is limited information regarding the riverine and coastal fluxes of particulate and dissolved cobalt to the oceans, earlier datasets show a significant "desorbable" load of particulate cobalt as well as estuarine sources of organic cobalt complexes (Kharkar et al, 1968;Zhang et al, 1990).…”

“…Cobalt was also observed to be scavenged more slowly than other hybrid-type metals like Fe and Mn, likely due to its slower oxidation kinetics and lower oxygen abundances in the OMZ (Noble et al, 2012). Additional datasets have explored the distribution and speciation of cobalt in the Atlantic and Pacific sectors of the Southern Ocean (Bown et al, 2011;Ellwood, 2008); the Ross Sea and McMurdo Sound of Antarctica (including seasonal variability and under ice early spring conditions) Saito et al, 2010); the eastern tropical North Pacific and Costa Rica Dome (Ahlgren et al, 2014); the eastern tropical North Atlantic (Baars and Croot, 2015); near the Bermuda, Hawaiian, and Kerguelen islands (Bown et al, 2012a;Noble et al, 2008;Shelley et al, 2012); and throughout meridional transects of the western Atlantic Ocean (Dulaquais et al, 2014a, b;Shelley et al, 2016). The establishment of these high-throughput sampling and analytical methods for cobalt, largely in response to the GEOTRACES program, has greatly improved our ability to assess and monitor the biogeochemistry of this key micronutrient throughout the global oceans.…”

The cobalt cycle in the Tropical Pacific Ocean

“…In the tropical South Pacific, where growth rates are low due to iron limitation, Prochlorococcus may be able to accumulate cobalt while waiting for bioavailable iron to diffuse to the cell surface (Mann and Chisholm, 2000;Moore et al, 2013b;. If basin-scale iron limitation is relieved near islands via terrigenous input or by dust deposition events without also adding cobalt, temporary cobalt limitation may be possible as growth rates increase (Bown et al, 2012a;Shiozaki et al, 2014).…”

Author response ro Reviewer #2

“…E. Noble et al: Coastal sources, sinks and strong organic complexation of dissolved cobalt imum zones (OMZs) of the South Atlantic and South Pacific Noble et al, 2012), and from more limited datasets from the North Pacific (Ahlgren et al, 2014;Saito et al, 2004Saito et al, , 2005, are likely due to reductive dissolution and advection of sedimentary sources in regions with low-oxygen bottom water sediment-water interfaces (Heggie and Lewis, 1984). Coastal and island sources in oxygenated environments have also been observed, for example, off the North American continental shelf (Saito and Moffett, 2002) and near the Kerguelen Islands (Bown et al, 2012a). While there is limited information regarding the riverine and coastal fluxes of particulate and dissolved cobalt to the oceans, earlier datasets show a significant "desorbable" load of particulate cobalt as well as estuarine sources of organic cobalt complexes (Kharkar et al, 1968;Zhang et al, 1990).…”

“…In the photic zone and upper water column, saturating concentrations of cobalt-binding ligands are often observed, particularly in oligotrophic regimes where cyanobacteria are well represented (Saito and Moffett, 2001;Saito et al, 2005). These ligands are extraordinarily strong, with con-ditional stability constants on the order of > 10 16.8 (Bown et al, 2012b;Saito et al, 2005), which is significantly higher than those for other transition metals such as iron (FeL 1 ), for which measured stability constants are on the order of 10 13.1 (Rue and Bruland, 1997;Buck et al, 2015). To achieve stability constants in this range, the cobalt-ligand complexes, measured by the difference between the total cobalt and the labile cobalt, almost certainly have a redox state of Co(III), and are inert to back reaction with added competitive ligands (Baars and Croot, 2015;Saito and Moffett, 2001;Saito et al, 2005).…”