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Abstract. Over the past decade, the GEOTRACES and wider trace metalgeochemical community has made substantial contributions towardsconstraining the marine cobalt (Co) cycle and its major biogeochemicalprocesses. However, few Co speciation studies have been conducted in theNorth and equatorial Pacific Ocean, a vast portion of the world's oceans byvolume and an important end-member of deep thermohaline circulation.Dissolved Co (dCo) samples, including total dissolved and labile Co, weremeasured at-sea during the GEOTRACES Pacific Meridional Transect (GP15) expedition along the 152∘ W longitudinal from 56∘ N to20∘ 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 fromphosphate concentrations due to co-scavenging with manganese oxide particlesin the mesopelagic. The transect revealed an organically bound coastalsource of dCo to the Alaskan Stream associated with low-salinity waters. Anintermediate-depth hydrothermal flux of dCo was observed off the Hawaiiancoast at the Loihi Seamount, and the elevated dCo was correlated withpotential xs3He at and above the vent site; however, the Loihi Seamountlikely did not represent a major source of Co to the Pacific basin. Elevatedconcentrations of dCo within oxygen minimum zones (OMZs) in the equatorialNorth and South Pacific were consistent with the suppression of oxidativescavenging, and we estimate that future deoxygenation could increase the OMZdCo inventory by 18 % to 36 % over the next century. In Pacific Deep Water(PDW), a fraction of elevated ligand-bound dCo appeared protected fromscavenging by the high biogenic particle flux in the North Pacific basin.This finding is counter to previous expectations of low dCo concentrationsin the deep Pacific due to scavenging over thermohaline circulation.Compared to a Co global biogeochemical model, the observed transectdisplayed more extreme inventories and fluxes of dCo than predicted by themodel, suggesting a highly dynamic Pacific Co cycle. 

Atmospheric deposition represents a major input for micronutrient trace elements (TEs) to the surface ocean and is often quantified indirectly through measurements of aerosol TE concentrations. Sea spray aerosol (SSA) dominates aerosol mass concentration over much of the global ocean, but few studies have assessed its contribution to aerosol TE loading, which could result in overestimates of “new” TE inputs. Low‐mineral aerosol concentrations measured during the U.S. GEOTRACES Pacific Meridional Transect (GP15; 152°W, 56°N to 20°S), along with concurrent towfish sampling of surface seawater, provided an opportunity to investigate this aspect of TE biogeochemical cycling. Central Pacific Ocean surface seawater Al, V, Mn, Fe, Co, Ni, Cu, Zn, and Pb concentrations were combined with aerosol Na data to calculate a “recycled” SSA contribution to aerosol TE loading. Only vanadium was calculated to have a SSA contribution averaging >1% along the transect (mean of 1.5%). We derive scaling factors from previous studies on TE enrichments in the sea surface microlayer and in freshly produced SSA to assess the broader potential for SSA contributions to aerosol TE loading. Maximum applied scaling factors suggest that SSA could contribute significantly to the aerosol loading of some elements (notably V, Cu, and Pb), while for others (e.g., Fe and Al), SSA contributions largely remained <1%. Our study highlights that a lack of focused measurements of TEs in SSA limits our ability to quantify this component of marine aerosol loading and the associated potential for overestimating new TE inputs from atmospheric deposition.

 

Ocean time‐series sites are influenced by both temporal variability, as in situ conditions change, as well as spatial variability, as water masses move across the fixed observation point. To remove the effect of spatial variability, this study made sub‐daily Lagrangian observations of trace elements and isotopes (Al, Sc, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb,²³²Th, and²³⁰Th) in surface water over a 12‐day period (July–August 2015) in the North Pacific near the Hawaii Ocean Time‐series Station ALOHA. Additionally, a vertical profile in the upper 250 m was analyzed. This dataset is intercalibrated with GEOTRACES standards and provides a consistent baseline for trace element studies in the oligotrophic North Pacific. No diel changes in trace elements could be resolved, although day‐to‐day variations were resolved for some elements (Fe, Cu, and Zn), which may be related to organic matter cycling or ligand availability. Pb concentrations remained relatively constant during 1997–2015, presenting a change from previous decreases. Nutrient to trace element stoichiometric ratios were compared to those observed in phytoplankton as an indication of the extent of biological trace element utilization in this ecosystem, providing a basis for future ecological trace element studies.