Abstract
Cobalt is an important micronutrient for ocean microbes as it is present in vitamin B12 and is a co‐factor in various metalloenzymes that catalyze cellular processes. Moreover, when seawater availability of cobalt is compared to biological demands, cobalt emerges as being depleted in seawater, pointing to a potentially important limiting role. To properly account for the potential biological role for cobalt, there is therefore a need to understand the processes driving the biogeochemical cycling of cobalt and, in particular, the balance between external inputs and internal cycling. To do so, we developed the first cobalt model within a state‐of‐the‐art three‐dimensional global ocean biogeochemical model. Overall, our model does a good job in reproducing measurements with a correlation coefficient of >0.7 in the surface and >0.5 at depth. We find that continental margins are the dominant source of cobalt, with a crucial role played by supply under low bottom‐water oxygen conditions. The basin‐scale distribution of cobalt supplied from margins is facilitated by the activity of manganese‐oxidizing bacteria being suppressed under low oxygen and low temperatures, which extends the residence time of cobalt. Overall, we find a residence time of 7 and 250 years in the upper 250 m and global ocean, respectively. Importantly, we find that the dominant internal resupply process switches from regeneration and recycling of particulate cobalt to dissolution of scavenged cobalt between the upper ocean and the ocean interior. Our model highlights key regions of the ocean where biological activity may be most sensitive to cobalt availability.
Highlights
When compared to typical phytoplankton requirements, cobalt (Co) emerges as being relatively depleted in seawater (Moore et al, 2013; Saito et al, 2008), and in some ocean regions, there is evidence that Co is the primary or secondary limiting nutrient (Bertrand et al, 2007, 2015; Browning et al, 2017; Sañudo-Wilhelmy et al, 2006)
The basin-scale distribution of cobalt supplied from margins is facilitated by the activity of manganese-oxidizing bacteria being suppressed under low oxygen and low temperatures, which extends the residence time of cobalt
Our model does a good job in reproducing the growing data set of dissolved cobalt (dCo) measurements arising from the GEOTRACES and CLIVAR efforts and allows for some of the first global-scale estimates of Co fluxes
Summary
When compared to typical phytoplankton requirements, cobalt (Co) emerges as being relatively depleted in seawater (Moore et al, 2013; Saito et al, 2008), and in some ocean regions, there is evidence that Co is the primary or secondary limiting nutrient (Bertrand et al, 2007, 2015; Browning et al, 2017; Sañudo-Wilhelmy et al, 2006). Co is better described as a “hybrid-type” element, with external inputs from continental margins (in particular within the major oxygen minimum zones), as well as riverine, and dust sources (Noble et al, 2012; Saito & Moffett, 2002; Saito et al, 2004; Shelley et al, 2012; Zhang et al, 1990). In addition to these external inputs, Co is strongly removed from the dissolved fraction via scavenging (Bruland et al, 2014; Moffett & Ho, 1996). The scavenging of Co is catalyzed by manganese (Mn) oxidizing bacteria, due to the similar ionic radii and redox potentials of Mn and Co (Cowen & Bruland, 1985; Moffett & Ho, 1996; Sunda & Huntsman, 1988)
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