Major processes of the dissolved cobalt cycle in the North and equatorial Pacific Ocean

Abstract

Abstract. Over the past decade, the GEOTRACES and wider trace metal geochemical community has 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 end-member 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) expedition along the 152∘ W longitudinal from 56∘ N to 20∘ S. Along this transect, upper-ocean dCo (σ0<26) was linearly correlated with dissolved phosphate (slope = 82±3, µmol : mol) 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 with potential 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 18 % to 36 % over the next century. In Pacific Deep Water (PDW), 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.

Low Cobalt Inventories in the Amundsen and Ross Seas Driven by High Demand for Labile Cobalt Uptake Among Native Phytoplankton Communities

“…Two-way regressions are ideal for stoichiometric ratios because they allow for error in both the x and y parameters and do not assume dependence between the x and y axes. The two-way regression function used in this study was rewritten to Python from a MATLAB file (lsqfitma.m) originally written by Ed Pelzer circa 1995 (Chmiel et al, 2022) (Stations 22,27,41,57,60). The top of the grey box marks the location of the seafloor.…”

“…CSV analysis was conducted using a Metrohm 663 VA and µAutolabIII systems equipped with a hanging mercury drop working electrode. All reagents were prepared as described in Chmiel et al (2022). Most samples were analyzed at sea within 3 weeks of sample collection, and stations 57 and 60 were analyzed for labile dCo at sea and their duplicate preserved samples were analyzed for total dCo in November 2019 in the laboratory.…”

“…While additional preserved dCo samples were analyzed in the laboratory in November 2019, triplicate GSC2 GEOTRACES community intercalibration standards were carefully neutralized to a pH of ~8 using negligible volumes of ammonium hydroxide (NH4OH) and analyzed for dCo. This is the same intercalibration batch originally reported in Table 1 of Chmiel et al (2022), as analysis for both expeditions overlapped temporally. The GSC2 standard was determined to have a dCo concentration of 80.2 ± 6.2 (n = 3), a value that is very similar to the one reported by Hawco et al, (2016) (77.7 ± 2.4).…”

Response of Dissolved Trace Metals to Dust Storms, Sediment Resuspension, and Flash Floods in Oligotrophic Oceans

“…Dissolved Co presents a scavenged‐type distribution, depicting surface enrichment due to leaching from atmospheric particles and decreasing concentrations with depth brought on by scavenging induced by sinking particles (Dulaquais et al., 2017; Hawco et al., 2018). Co scavenging is known to be associated with Mn oxides, as Co is microbially co‐oxidized with Mn (Chmiel et al., 2022; Hawco et al., 2018; Moffett & Ho, 1996). Indeed, Co and Mn distributions are similar (Figure 2e) and display a strong covariance (Figure S8 and Table S3 in Supporting Information S1), suggesting that on long time scales, Mn oxides are a prominent scavenger of Co in the GoA.…”

Does Sea Spray Aerosol Contribute Significantly to Aerosol Trace Element Loading? A Case Study From the U.S. GEOTRACES Pacific Meridional Transect (GP15)

“…Samples for dissolved Co were collected and stored in acid‐washed low‐density polyethylene (LDPE) bottles (Nalgene) at 4°C prior to analysis. Analysis was carried out by cathodic stripping voltammetry, following an established 1h UV oxidation procedure (Chmiel et al., 2022; Hawco et al., 2016; M. A. Saito & Moffett, 2001). All data are archived at BCO‐DMO (M. A. Saito, 2021; M. Saito, 2020).…”