18 Nov 2021

18 Nov 2021

Review status: this preprint is currently under review for the journal BG.

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

Rebecca Chmiel1,2, Nathan Lanning3, Allison Laubach4, Jong-Mi Lee4, Jessica Fitzsimmons3, Mariko Hatta5, William J. Jenkins2, Phoebe J. Lam4, Matthew McIlvin2, Alessandro Tagliabue6, and Mak A. Saito2 Rebecca Chmiel et al.
  • 1MIT/WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering, Woods Hole, MA, 02543, USA
  • 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
  • 3Department of Oceanography, Texas A & M University, College Station, TX, 77843, USA
  • 4Department of Ocean Sciences, University of California Santa Cruz, Santa Cruz, CA, 95064, USA
  • 5Institute of Arctic Climate and Environmental Research, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
  • 6School of Environmental Sciences, University of Liverpool, Liverpool, L3 5DA, UK

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.

Rebecca Chmiel et al.

Status: open (until 30 Dec 2021)

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Rebecca Chmiel et al.

Data sets

Dissolved Cobalt and Labile Cobalt from Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018 Mak A. Saito

Dissolved Cobalt and Labile Cobalt from Leg 2 (Hilo, HI to Papeete, French Polynesia) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1815) on R/V Roger Revelle from October to November 2018 Mak A. Saito

Rebecca Chmiel et al.


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Short summary
Dissolved cobalt is present in trace amounts in seawater and is a necessary nutrient for marine life. On a transect from the Alaskan coast to Tahiti, we measured seawater concentrations of dissolved cobalt. Here, we describe several interesting features of the Pacific cobalt cycle including cobalt sources along the Alaskan coast and Hawaiian vents, deep ocean particle formation, cobalt activity in low oxygen regions, and how our samples compare to a global biogeochemical model’s predictions.