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1. Bulk Aerosol Trace Element Concentrations and Deposition Fluxes During the U.S. GEOTRACES GP15 Pacific Meridional Transect

   https://doi.org/10.1029/2021GB007122  

   Marsay, Chris M.; Kadko, David; Landing, William M.; Buck, Clifton S. (February 2022, Global Biogeochemical Cycles)

   Abstract Atmospheric deposition of aerosols transported from the continents is an important source of nutrient and pollutant trace elements (TEs) to the surface ocean. During the U.S. GEOTRACES GP15 Pacific Meridional Transect between Alaska and Tahiti (September–November 2018), aerosol samples were collected over the North Pacific and equatorial Pacific and analyzed for a suite of TEs, including Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb. Sampling coincided with the annual minimum in dust transport from Asia, providing an opportunity to quantify aerosol TE concentrations and deposition during the low dust season. Nevertheless, peak concentrations of “crustal” TEs measured at ∼40–50°N (∼145 pmol/m³Fe) were associated with transport from northern Asia, with lower concentrations (36 ± 14 pmol/m³Fe) over the equatorial Pacific. Relative to crustal abundances, equatorial Pacific aerosols typically had higher TE enrichment factors than North Pacific aerosols. In contrast, aerosol V was more enriched over the North Pacific, presumably due to greater supply to this region from oil combustion products. Bulk deposition velocity (V_bulk) was calculated along the transect using the surface ocean decay inventory of the naturally occurring radionuclide,⁷Be, and aerosol⁷Be activity. Deposition velocities were significantly higher (4,570 ± 1,146 m/d) within the Intertropical Convergence Zone than elsewhere (1,764 ± 261 m/d) due to aerosol scavenging by intense rainfall. Daily deposition fluxes to the central Pacific during the low dust season were calculated using V_bulkand aerosol TE concentration data, with Fe fluxes ranging from 19 to 258 nmol/m²/d. 

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2. Surface Ocean Biogeochemistry Regulates the Impact of Anthropogenic Aerosol Fe Deposition on the Cycling of Iron and Iron Isotopes in the North Pacific

   https://doi.org/10.1029/2022GL098016  

   König, D.; Conway, T. M.; Hamilton, D. S.; Tagliabue, A. (July 2022, Geophysical Research Letters)

   Abstract Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle processes modulate oceanic dissolved Fe (dFe) signatures (δ⁵⁶Fe_diss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ⁵⁶Fe_diss. Using the North Pacific as a natural laboratory, the response of dFe, δ⁵⁶Fe_diss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ⁵⁶Fe_dissdoes trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources. 

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3. ²¹⁰ Pb and ⁷ Be as Coupled Flux and Source Tracers for Aerosols in the Pacific Ocean

   https://doi.org/10.1029/2022GB007378  

   Wei, Ziran; Cochran, J. Kirk; Horowitz, Evan; Fitzgerald, Patrick; Heilbrun, Christina; Kadko, David; Stephens, Mark; Marsay, Chris M.; Buck, Clifton S.; Landing, William M. (August 2022, Global Biogeochemical Cycles)

   Abstract Deposition of aerosols to the surface ocean is an important factor affecting primary production in the surface ocean. However, the sources and fluxes of aerosols and associated trace elements remain poorly defined. Aerosol²¹⁰Pb,²¹⁰Po, and⁷Be data were collected on US GEOTRACES cruise GP15 (Pacific Meridional Transect, 152°W; 2018).²¹⁰Pb fluxes are low close to the Alaskan margin, increase to a maximum at ∼43°N, then decrease to lower values. There is good agreement between²¹⁰Pb fluxes and long‐term land‐based fluxes during the SEAREX program (1970–1980s), as well as between GP15 and GP16 (East Pacific Zonal Transect, 12°S; 2013) at adjacent stations. A normalized fractionf(⁷Be,²¹⁰Pb) is used to discern aerosols with upper (highf) versus lower (lowf) troposphere sources. Alaskan/North Pacific aerosols show significant continental influence while equatorial/South Pacific aerosols are supplied to the marine boundary layer from the upper troposphere. Lithogenic trace elements Al and Ti show inverse correlations withf(⁷Be,²¹⁰Pb), supporting a continental boundary layer provenance while anthropogenic Pb shows no clear relationship withf(⁷Be,²¹⁰Pb). All but four samples have²¹⁰Po/²¹⁰Pb activity ratios <0.2 suggesting short aerosol residence time. Among the four samples (²¹⁰Po/²¹⁰Pb = 0.42–0.88), two suggest an upper troposphere source and longer aerosol residence time while the remaining two cannot be explained by long aerosol residence time nor a significant component of dust. We hypothesize that enrichments of²¹⁰Po in them are linked to Po enrichments in the sea surface microlayer, possibly through Po speciation as a dissolved organic or dimethyl polonide species. 

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4. Does Sea Spray Aerosol Contribute Significantly to Aerosol Trace Element Loading? A Case Study From the U.S. GEOTRACES Pacific Meridional Transect (GP15)

   https://doi.org/10.1029/2022GB007416  

   Marsay, Chris M.; Landing, William M.; Umstead, Devon; Till, Claire P.; Freiberger, Robert; Fitzsimmons, Jessica N.; Lanning, Nathan T.; Shiller, Alan M.; Hatta, Mariko; Chmiel, Rebecca; et al (August 2022, Global Biogeochemical Cycles)

   Abstract 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. 

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5. The Importance of Reversible Scavenging for the Marine Zn Cycle Evidenced by the Distribution of Zinc and Its Isotopes in the Pacific Ocean

   https://doi.org/10.1029/2022JC019419  

   Sieber, M.; Lanning, N. T.; Bian, X.; Yang, S. ‐C.; Takano, S.; Sohrin, Y.; Weber, T. S.; Fitzsimmons, J. N.; John, S. G.; Conway, T. M. (April 2023, Journal of Geophysical Research: Oceans)

   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. 

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