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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.

 

Despite the Pacific being the location of the earliest seawater Cd studies, the processes which control Cd distributions in this region remain incompletely understood, largely due to the sparsity of data. Here, we present dissolved Cd and δ¹¹⁴Cd data from the US GEOTRACES GP15 meridional transect along 152°W from the Alaskan margin to the equatorial Pacific. Our examination of this region's surface ocean Cd isotope systematics is consistent with previous observations, showing a stark disparity between northern Cd‐rich high‐nutrient low‐chlorophyll waters and Cd‐depleted waters of the subtropical and equatorial Pacific. Away from the margin, an open system model ably describes data in Cd‐depleted surface waters, but atmospheric inputs of isotopically light Cd likely play an important role in setting surface Cd isotope ratios (δ¹¹⁴Cd) at the lowest Cd concentrations. Below the surface, Southern Ocean processes and water mass mixing are the dominant control on Pacific Cd and δ¹¹⁴Cd distributions. Cd‐depleted Antarctic Intermediate Water has a far‐reaching effect on North Pacific intermediate waters as far as 47°N, contrasting with northern‐sourced Cd signatures in North Pacific Intermediate Water. Finally, we show that the previously identified negative Cd* signal at depth in the North Pacific is associated with the PO₄maximum and is thus a consequence of an integrated regeneration signal of Cd and PO₄at a slightly lower Cd:P ratio than the deep ocean ratio (0.35 mmol mol⁻¹), rather than being related to in situ removal processes in low‐oxygen waters.

 

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.

 

Reversible scavenging, the oceanographic process by which dissolved metals exchange onto and off sinking particles and are thereby transported to deeper depths, has been well established for the metal thorium for decades. Reversible scavenging both deepens the elemental distribution of adsorptive elements and shortens their oceanic residence times in the ocean compared to nonadsorptive metals, and scavenging ultimately removes elements from the ocean via sedimentation. Thus, it is important to understand which metals undergo reversible scavenging and under what conditions. Recently, reversible scavenging has been invoked in global biogeochemical models of a range of metals including lead, iron, copper, and zinc to fit modeled data to observations of oceanic dissolved metal distributions. Nonetheless, the effects of reversible scavenging remain difficult to visualize in ocean sections of dissolved metals and to distinguish from other processes such as biological regeneration. Here, we show that particle-rich “veils” descending from high-productivity zones in the equatorial and North Pacific provide idealized illustrations of reversible scavenging of dissolved lead (Pb). A meridional section of dissolved Pb isotope ratios across the central Pacific shows that where particle concentrations are sufficiently high, such as within particle veils, vertical transport of anthropogenic surface–dissolved Pb isotope ratios toward the deep ocean is manifested as columnar isotope anomalies. Modeling of this effect shows that reversible scavenging within particle-rich waters allows anthropogenic Pb isotope ratios from the surface to penetrate ancient deep waters on timescales sufficiently rapid to overcome horizontal mixing of deep water Pb isotope ratios along abyssal isopycnals. 

The micronutrient iron plays a major role in setting the magnitude and distribution of primary production across the global ocean. As such, an understanding of the sources, sinks, and internal cycling processes that drive the oceanic distribution of iron is key to unlocking iron's role in the global carbon cycle and climate, both today and in the geologic past. Iron isotopic analyses of seawater have emerged as a transformative tool for diagnosing iron sources to the ocean and tracing biogeochemical processes. In this review, we summarize the end-member isotope signatures of different iron source fluxes and highlight the novel insights into iron provenance gained using this tracer. We also review ways in which iron isotope fractionation might be used to understand internal oceanic cycling of iron, including speciation changes, biological uptake, and particle scavenging. We conclude with an overview of future research needed to expand the utilization of this cutting-edge tracer. 

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.