Abstract
Over the past decade, the GEOTRACES and wider trace metal geochemical community have made substantial contributions towards constraining the marine cobalt (Co) cycle and its major biogeochemical processes. However, few Co speciation studies have been conducted in the North and equatorial Pacific Ocean, a vast portion of the world’s oceans by volume and an important endmember of deep thermohaline circulation. Dissolved Co (dCo) samples, including total dissolved and labile Co, were measured at-sea during the GEOTRACES Pacific meridional transect (GP15) along the 152° W longitudinal from 56° N to 20° S. Along this transect, upper ocean dCo was linearly correlated to dissolved phosphate (slope = 82 ± 2 µM:M) due to phytoplankton uptake and remineralization. As depth increased, dCo concentrations became increasingly decoupled from phosphate concentrations due to co-scavenging with manganese oxide particles in the mesopelagic. The transect revealed an organically-bound coastal source of dCo to the Alaskan Stream associated with low salinity waters. An intermediate-depth hydrothermal flux of dCo was observed off the Hawaiian coast at the Loihi Seamount, and the elevated dCo was correlated to estimated xs3He at and above the vent site; however, the Loihi Seamount likely did not represent a major source of Co to the Pacific basin. Elevated concentrations of dCo within oxygen minimum zones (OMZs) in the equatorial North and South Pacific were consistent with the suppression of oxidative scavenging, and we estimate that future deoxygenation could increase the OMZ dCo inventory by 13–28 % over the next century. In North Pacific Deep Water (NPDW), a fraction of elevated ligand-bound dCo appeared protected from scavenging by the high biogenic particle flux in the North Pacific basin. This finding is counter to previous expectations of low dCo concentrations in the deep Pacific due to scavenging over thermohaline circulation. Compared to a Co global biogeochemical model, the observed transect displayed more extreme inventories and fluxes of dCo than predicted by the model, suggesting a highly dynamic Pacific Co cycle.
Highlights
Upper ocean Dissolved Co (dCo) was linearly correlated to dissolved phosphate due to phytoplankton uptake and remineralization
We examine the distribution of both total dissolved and labile dCo along the GEOTRACES GP15 Pacific meridional transect through the North and equatorial Pacific Ocean
This paper presents dissolved Fe (dFe) and dissolved Mn (dMn) data determined by flow injection analysis (FIA) for the Ocean Data Facility (ODF) cast and inductively coupled plasma mass spectrometry (ICP-MS) for the GEOTRACES Carousel” (GTC) cast, as described in Jenkins et al, 2020
Summary
Cobalt (Co) is a necessary inorganic micronutrient for many phytoplankton and other forms of marine life but is one of the scarcest essential bioactive metals in the surface ocean. Many cyanobacteria, such as Synechococcus and 35 Prochlorococcus, have an absolute requirement of Co (Saito et al, 2002; Sunda and Huntsman, 1995), while other phytoplankton require vitamin B12 for methionine synthesis, which contains Co as a metallic cofactor (Bertrand et al, 2013). Some of Co’s biological influence is through vitamin B12, which has been found to be a co-limiting nutrient in regions 45 like the Antarctic coastal seas (Bertrand et al, 2007; Gobler et al, 2007; Sanudo-Wilhelmy et al, 2006)
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