The Role of External Inputs and Internal Cycling in Shaping the Global Ocean Cobalt Distribution: Insights From the First Cobalt Biogeochemical Model

Abstract

Cobalt is an important micronutrient for ocean microbes as it is present in vitamin B 12 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.

Insights From the238U‐234Th Method Into the Coupling of Biological Export and the Cycling of Cadmium, Cobalt, and Manganese in the Southeast Pacific Ocean

“…Therefore, long-term sources of the dissolved Co pool in the Gyre most likely originate from coastal fluxes and upwelling, which is consistent with recent global modeling results (Hawco et al, 2016;Saito et al, 2004;Sanial et al, 2018;Tagliabue et al, 2018). The dust input to the Gyre ranges from 0.004 to 0.01 nmol·m À2 ·day À1 which is, at most, 0.8% of export if the station with the lowest particulate Co flux is used.…”

Insights Into the Major Processes Driving the Global Distribution of Copper in the Ocean From a Global Model

“…PISCES reproduces the biogeochemical cycling of various macronutrients (phosphate PO 4 , nitrate NO 3 , ammonium NH 4 , and silicate Si), planktons (nanophytoplankton, diatoms, microzooplankton, and mesozooplankton), and one trace element (iron [Fe]). Recent developments allowed representing new trace elements such as manganese (Mn; Hulten et al, ) and cobalt (Co; Tagliabue et al, ) and gave new insights on the processes governing Fe distributions in the ocean (Tagliabue & Resing, ). Seven new tracers were implemented in PISCES in order to represent copper biogeochemical cycling: dissolved copper (DCu); scavenged copper, which represents copper adsorbed on the surface of particles (SCu, divided into small and large scavenged particles, SCup and SCug), this copper can be desorbed to the dissolved phase at depth; copper associated with biogenic particles (CuPart, divided into small and large copper particles, pCuPart and gCuPart) that can only be resupplied via bacterial activity; and copper in phytoplankton cells (Cu ϕ , with ϕ = N for nanophytoplankton or ϕ = D for diatoms).…”

“…The general equation for scavenged Cu is presented in equation : with Scav Cu as copper scavenging and sinking as SCu sinking rate, increasing with depth following the same equations as C, Fe, and Co (Tagliabue et al, ).…”

“…The biogeochemical parameter values are based on the PISCES general equations valid for macronutrients (Aumont et al, ), Fe (Tagliabue & Resing, ), and Co (Tagliabue et al, ).…”

“…Cu uptake by phytoplankton is modeled following the Co model from Tagliabue et al () and is represented using an evolving Cu:P ratio in the planktonic cells (equation ). …”