Cobalt, manganese, and iron near the Hawaiian Islands: A potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals

Abstract

The vertical distributions of cobalt, iron, and manganese in the water column were studied during the E-Flux Program (E-Flux II and III), which focused on the biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were sampled for cobalt (n =147), all of which demonstrated nutrient-like depletion in surface waters. During E-Flux III, two depth profiles collected from within a mesoscale coldcore eddy, Cyclone Opal, revealed small distinct maxima in cobalt at ~100m depth and a larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt concentrating effect within the eddy, where upwelled cobalt was subsequently associated with sinking particulate organic carbon (POC) via biological activity and was released at a depth coincident with nearly complete POC remineralization (Benitez-Nelson et al. 2007). There is also evidence for the formation of a correlation between cobalt and soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we suggest is due to increased productivity, implying a minimum threshold of primary production below which cobalt-phosphate coupling does not occur. Dissolved iron was measured in E-Flux II and found in somewhat elevated concentrations (~0.5nM) in surface waters relative to the iron depleted waters of the surrounding Pacific (Fitzwater et al. 1996), possibly due to island effects associated with the iron-rich volcanic soil from the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total dissolved cobalt were observed at 400 to 600m depth, suggestive of the release of metals from the shelf area of comparable depth that surrounds these islands.

A seasonal study of dissolved cobalt in the Ross Sea, Antarctica: micronutrient behavior, absence of scavenging, and relationships with Zn, Cd, and P

“…Recent studies have found that micronutrient drawdown can be an important process in controlling the distribution of Co in ocean surface waters (Ellwood and van den Berg, 2001;Saito et al, 2004Saito et al, , 2005Ellwood et al, 2005;Noble et al, 2008). This contrasts with earlier reviews that characterized Co as controlled by scavenging removal processes (Whitfield and Turner, 1987;Nozaki, 1997).…”

“…This abundance of labile cobalt is likely important in providing a bioavailable form of Co to eukaryotic phytoplankton (Saito et al, , 2004. Moreover, the presence of measurable labile cobalt throughout the water column of the Ross Sea contrasts with observations in tropical and subtropical regions, where cobalt is often completely bound by strong ligands in the upper euphotic zone (Saito and Moffett, 2001;Saito et al, 2004Saito et al, , 2005Noble et al, 2008). Exceptions to this appear to be related to significant sources of cobalt, such as in the Peru Upwelling Region, where a large flux of cobalt is thought to enter the system as labile cobalt, and where large phytoplankton community shifts occurred concurrently with changes in cobalt speciation (Saito et al, 2004).…”

“…Upwelling should result in a diagonal vector with a lower slope than the biotic vectors due to the effect of scavenging on removal of hybrid-type metal relative to phosphate. After Noble et al (2008). 2010), suggesting they are not identical, but rather are linked via chemical transformation. If cobalt ligands are indeed a degradation product of vitamin B 12 , we hypothesize that the lack of cyanobacteria and low abundance of heterotrophic bacteria in the Ross Sea in the austral spring results in a low production rate of vitamin B 12 and perhaps a commensurate low cobalt ligand production rate in the Ross Sea relative to temperate and tropical oceans.…”

“…In general, deviation from the seawater metal:P relationship is expected to result from processes other than biological uptake and remineralization, such as aeolian deposition, particle scavenging, and oxidation/precipitation by Mn-oxidizing bacteria (Tebo et al, 1984), which should produce vectors in metal:phosphate space that are vertical rather than diagonal, as indicated in Fig. 7 (Noble et al, 2008). This type of vector analysis in Co:PO 3− 4 space appears to be useful in identifying sporadic and/or subtle influences by their deviation from the coherent uptake and remineralization signal within large datasets such as this.…”

“…This contrasts with earlier reviews that characterized Co as controlled by scavenging removal processes (Whitfield and Turner, 1987;Nozaki, 1997). Elements such as Co, Fe, Cu, and (at high latitudes) Mn are considered hybrid-type metals, due to their oceanic distributions being strongly affected by both micronutrient uptake and scavenging processes (Bruland and Lohan, 2003;Noble et al, 2008). In addition, organic ligands have been shown to bind Co very strongly and are believed to provide an important control on Co biological availability as well as potentially protecting Co from scavenging in the ocean (Ellwood and van den Berg, 2001;Saito and Moffett, 2001;Ellwood et al, 2005).…”