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Abstract Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition. 

Abstract The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient‐rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer (UHL)) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient‐poor) Atlantic‐derived water below. Biogeochemical signals entrained into the UHL result from Pacific Waters modified by sediment and river influence on the shelf. In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi‐tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf‐influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, dissolved organic carbon), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation.