Spatiotemporal trends of aerosol provenance and trace metal concentrations in the northeast subarctic Pacific Ocean

The long-range transport of naturally occurring and anthropogenic aerosols originating from Asian deserts and megacities, respectively, can have a significant impact on the biogeochemical cycling of metals in the Fe-limited, high nutrient-low chlorophyll (HNLC) region of the northeast (NE) subarctic Pacific Ocean. These aerosols can deposit essential (e.g., Fe) and possibly toxic (e.g., Cu) metals to surface waters; thereby affecting micronutrients' bioavailability, and ultimately primary productivity in this region. In this study, we aimed to determine the provenance and spatiotemporal trends of metal inputs from Asian aerosol outflows into the NE Pacific Ocean. To do so, we collected aerosols on six research cruises along the Line P transect (GEOTRACES GPpr07), across three seasons in four years. Lead isotopic composition signatures were less radiogenic (high ²⁰⁸Pb/²⁰⁶Pb; low ²⁰⁶Pb/²⁰⁷Pb) in winter and spring compared to summer, signifying a greater anthropogenic Asian source in the cooler seasons. Furthermore, aerosol metal content also revealed seasonality. For example, aerosols collected in March 2022 (winter) contained higher concentrations of the lithogenic metals Al, Ti, Fe, Mn and Co, the anthropogenic metals Cu, Cd and Pb, as well as the mixed-source metals Ni and V, in comparison to May 2021 (spring) and August 2021 (summer). The estimated annual atmospheric flux of Fe and Zn accounted for 13-27 % and 6-10 %, respectively, of the total flux to the surface mixed layer (i.e., vertical mixing plus atmospheric flux) at Ocean Station Papa (50°N, 145°W), while that of Cu accounted for 95-99 %. This study provides the first insight into the seasonal patterns and geochemical characterizations of long-range transported Asian aerosols along the Line P transect. Anthropogenic activities will likely increase the input of aerosol-derived metals from distant Asian sources into the NE Pacific Ocean in the coming years, influencing biogeochemical cycles in this Fe-limited oceanic region.