The acceleration of dissolved cobalt's ecological stoichiometry due to biological uptake, remineralization, and scavenging in the Atlantic Ocean

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Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and 232Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs (238U:234Th ;210Pb:210Po; 228Ra:228Th; and 234U:230Th) at stations co‐located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and 232Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes.

Section: Resultsmentioning

confidence: 97%

Flux of Particulate Elements in the North Atlantic Ocean Constrained by Multiple Radionuclides

Hayes

Black

Anderson

et al. 2018

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“…Although the export of Co in the upper 200 m of this region mostly reflects nutrient-like properties, Co can be considered a hybrid element based on the scavenging-like export patterns observed in the Gyre (Saito et al, 2017). Key patterns emerge within the three biogeochemical zones of the transect for the fluxes of each component.…”

Section: Discussionmentioning

confidence: 99%

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

Black

Lam

Lee

et al. 2019

Better constraints on the magnitude of particulate export and the residence times of trace elements are required to understand marine food web dynamics, track the transport of anthropogenic trace metals in the ocean, and improve global climate models. While prior studies have been successful in constructing basin‐scale budgets of elements like carbon in the upper ocean, the cycling of particulate trace metals is poorly understood. The 238U‐234Th method is used here with data from the GP‐16 GEOTRACES transect to investigate the upper ocean processes controlling the particulate export of cadmium, cobalt, and manganese in the southeastern Pacific. Patterns in the flux data indicated that particulate cadmium and cobalt behave similarly to particulate phosphorus and organic carbon, with the highest export in the productive coastal region and decreasing flux with depth due to remineralization. The export of manganese was influenced by redox conditions at the low oxygen coastal stations and by precipitation and/or scavenging elsewhere. Residence times with respect to export (total inventory divided by particulate flux) for phosphorus, cadmium, cobalt, and manganese in the upper 100 and 200 m were determined to be on the order of months to years. These GEOTRACES‐based synthesis efforts, combining a host of concentration and tracer data with unprecedented resolution, will help to close the oceanic budgets of trace metals.

“…Co loss is generally controlled by biological uptake in oligotrophic regions (Moffett & Ho, ). Although the factors controlling Mn oxidation remain poorly understood (Lee & Fisher, ), Mn oxidation is the likely vector for Co scavenging given (1) the known ability for Co to be co‐oxidized by Mn‐oxidizing bacteria (Lee & Fisher, ; Moffett & Ho, ); (2) similar redox potentials and ionic radii of Co and Mn (Moffett & Ho, ; Swanner et al, ); (3) extensive covariation between Co and Mn contents of solid‐phase marine sediments, manganese nodules, and ferromanganese crusts (Krishnaswami, ; Manheim, ), which accumulate Co scavenged from the water column; and (4) covariation of particulate Co and Mn phases in the mesopelagic (Saito et al, ). In addition to oxygen‐related cycling of Mn‐oxides in sediments, the absence of particulate Mn has been long noted in offshore oxygen minimum zones of the North and South Pacific (Johnson et al, ; Landing & Bruland, ; Ohnemus et al, ) and attributed to slow Mn oxide formation at low O 2 and in situ reduction.The specific rate of scavenging (Λ) is based on a minimum (ΛCo min ) and maximum scavenging rate (ΛCo) that is modulated by oxygen, bacterial activity (itself affected by nutrient and dissolved organic matter limitation), and light:

normalΛ = Λ_{MIN} + ΛCo \times normalQ \times normalk O_{2} \times kBACT \times ((), 1 - kPAR)

…”

Section: Methodsmentioning

confidence: 99%

“…In contrast, Co/P ratios are greatest (>150 μmol/mol) in the tropical Atlantic and Indian Oceans. Notably this is without including any Co substitution within alkaline phosphatase of the dominant cyanobacteria populations in the model, which is suggested by observations of this metalloenzyme within regions of “accelerating” dCo:PO 4 stoichiometries (Saito et al, ). Finally, it is noteworthy that the Co/P ratios increase strongly with depth due to the production of additional particulate Co from the interior ocean scavenging of dCo by Mn‐oxidizing bacteria (Figure f).…”

Section: Toward Quantifying the Biological Role Of Cobaltmentioning

confidence: 99%

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

Tagliabue

Hawco

Bundy

et al. 2018

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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.