Dissolved cobalt speciation along the GEOTRACES Pacific meridional transect (GP15)

Rebecca Chmiel, Woods Hole Oceanographic Institution, Woods Hole, MA, United States, Karen L Casciotti, Stanford University, Stanford, United States, Gregory A Cutter, Old Dominion University, Norfolk, VA, United States, Phoebe J Lam, University of California Santa Cruz, Department of Ocean Sciences, Santa Cruz, CA, United States and Mak A Saito, WHOI, Woods Hole, MA, United States
Cobalt is present at picomolar concentrations in the oceans and is a necessary nutrient for marine life, but the distribution of both labile and ligand-bound dissolved cobalt in the global oceans is not fully understood. The distribution, sources, and sinks of marine cobalt have not been widely characterized in the North and Equatorial Pacific, leaving a vast section of the ocean understudied with regards to cobalt cycling and dynamics. During the GEOTRACES GP15 expedition (September to November 2018), 719 dissolved trace metal samples were collected along the 152⁰W longitudinal from 56⁰N to 20⁰S and analyzed at sea for cobalt speciation by cathodic stripping voltammetry. The Pacific transect suggests a substantial source of cobalt to the North Pacific basin along the Alaskan Shelf associated with a low salinity waters. Elevated concentrations of total and labile cobalt within oxygen minimum zones in the Equatorial North and South Pacific are likely due to the reduction of manganese oxide particles and release of incorporated cobalt. Additionally, increased cobalt concentrations above the Lō’ihi Seamount suggests a local hydrothermal source of dissolved cobalt. Low concentrations of cobalt are found in the surface ocean where cobalt is used as a nutrient for phytoplankton, particularly in the oligotrophic gyres. At depth, cobalt is depleted relative to phosphate throughout the transect, revealing a strong scavenging removal process in the deep Pacific. The GP15 transect captures many relevant sources and sinks within the cobalt cycle, providing a representative model of cobalt biogeochemical processes in the North and Equatorial Pacific Ocean.