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Abstract Nickel stable isotopes (δ⁶⁰Ni) provide insight to Ni biogeochemistry in the modern and past oceans. Here, we present the first Pacific Ocean high‐resolution dissolved Ni concentration and δ⁶⁰Ni data, from the US GEOTRACES GP15 cruise. As in other ocean basins, increases in δ⁶⁰Ni toward the surface ocean are observed across the entire transect, reflecting preferential biological uptake of light Ni isotopes, however the observed magnitude of fractionation is larger in the tropical Pacific than the North Pacific Subtropical Gyre. Such surface ocean fractionation by phytoplankton should accumulate isotopically lighter Ni in the deep Pacific, yet we find that North Pacific deep ocean δ⁶⁰Ni is similar to previously reported values from the deep Atlantic. Finally, we find that seawater dissolved δ⁶⁰Ni in regions with hydrothermal input can be either higher or lower than background deep ocean δ⁶⁰Ni, depending on vent geochemistry and proximity. 

One of the most exciting results from the GEOTRACES program’s zonal and meridional sections has been the recognition that hydrothermally sourced Fe may persist long enough to be upwelled along shoaling isopycnals and act as an essential micronutrient, stimulating primary productivity at high latitudes. In Aug-Sep 2023 our team used a combination of predictive plume dispersion modelling, real-time current meter data from the Ocean Networks Canada observatory, and in situ sensing and sampling from the AUV Sentry to guide biogeochemical sampling of dispersing hydrothermal plumes above the Juan de Fuca Ridge. A key motivation for this study was to investigate what sets the export flux of dissolved Fe and Mn away from ridge-axis venting. We specifically targeted hydrothermal vents in the NE Pacific for this study, at the far end of the thermohaline circulation, to maximize predicted Fe oxidation times within the dispersing plume and, hence, optimize our ability to reveal distinct processes that may contribute to regulating Fe flux as a function of time and distance down-plume. We also targeted an overlooked gap in the length-scale over which hydrothermal processes may regulate export fluxes, between the ≤1km range typical of submersible-based investigations and the ~100km spacing for GEOTRACES Section stations. Over 3 weeks on station we were able to use the Sentry AUV equipped with an in situ oxidation-reduction potential (ORP) sensor, an optical backscatter sensor (OBS) and two methane sensors (METS, SAGE) to track predicted plume dispersion trajectories and guide a telescopically-expanding program of water column sampling for dissolved, soluble, colloidal and particulate species of Fe, Mn and other metals, at <0.1, 0.25, 0.50, 1, 2, 5 and 10km down-plume from the High Rise and Main Endeavour vent-sites. We will present results from Sentry sensor data revealing length scales over which hydrothermal plume signatures attenuated, together with complementary TEI data, all set within the context of our dispersing plume model. Our approach will ultimately allow us to assign both effective distances down-plume from source, for each sample collected, and model dispersion ages. This will provide insights into both the processes active within a dispersing hydrothermal plume and the rates at which those processes occur. 

Abstract Hydrogen sulfide is produced by heterotrophic bacteria in anoxic waters and via carbonyl sulfide hydrolysis and phytoplankton emissions under oxic conditions. Apparent losses of dissolved cadmium (dCd) and zinc (dZn) in oxygen minimum zones (OMZs) of the Atlantic and Pacific Oceans have been attributed to metal‐sulfide precipitation formed via dissimilatory sulfate reduction. It has also been argued that such a removal process could be a globally important sink for dCd and dZn. However, our studies from the North Pacific OMZ show that dissolved and particulate sulfide concentrations are insufficient to support the removal of dCd via precipitation. In contrast, apparent dCd and dZn deficits in the eastern tropical South Pacific OMZ do reside in the oxycline with particulate sulfide maxima, but they also coincide with the secondary fluorescence maxima, suggesting that removal via sulfide precipitation may be due to a combination of dissimilatory and assimilatory sulfate reduction. Notably, dCd loss via precipitation with sulfide from assimilatory reduction was found in upper oxic waters of the North Pacific. While dissimilatory sulfate reduction may explain local dCd and dZn losses in some OMZs, our evaluation of North Pacific OMZs demonstrates that dCd and dZn losses are unlikely to be a globally relevant sink. Nevertheless, metal sulfide losses due to assimilatory sulfate reduction in surface waters should be considered in future biogeochemical models of oceanic Cd (and perhaps Zn) cycling. 

Abstract The North Pacific has played an important role in ongoing discussions on the origin of the global correlation between oceanic dissolved Zn and Si, while data in the North Pacific have remained sparse. Here, we present dissolved Zn and δ⁶⁶Zn data from the US GEOTRACES GP15 meridional transect along 152°W from Alaska to the South Pacific. In the south (<20°N) Zn and Si exhibit a tight linear correlation reflecting strong Southern Ocean influence, while in the north (>20°N) an excess of Zn relative to Si in upper and intermediate waters is due to regeneration of Zn together with PO₄. Using a mechanistic model, we show that reversible scavenging is required as an additional process transferring Zn from the upper to the deep ocean, explaining the deep Zn maximum below the PO₄maximum. This mechanism applied for reversible scavenging also provides an explanation for the observed isotope distribution: (a) fractionation during ligand binding and subsequent removal of residual heavy Zn in the upper ocean, drives the upper ocean toward lower δ⁶⁶Zn, while (b) release of heavy Zn then coincides with the PO₄maximum where carrier particles regenerate, causing a mid‐depth δ⁶⁶Zn maximum. In the upper ocean, seasonal physical stratification is an additional important process influencing shallow δ⁶⁶Zn signals. At the global scale, this mechanism invoking fractionation during ligand binding coupled with reversible scavenging offers a global explanation for isotopically light Zn at shallow depths and corresponding elevated mid‐depth δ⁶⁶Zn signals, seen dominantly in ocean regions away from strong Southern Ocean control. 

Abstract The biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol,r² = 0.58) that is higher than the global slope (0.059 nmol/μmol,r² = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries.