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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 Atmospheric deposition is an important source of iron (Fe) and perhaps zinc (Zn) to the oceans. We present total and water‐soluble aerosol Fe and Zn isotopic compositions, size‐fractionated aerosol Fe isotopic compositions, and aerosol enrichment factors from the North Pacific GEOTRACES GP15 section (Alaska‐Tahiti) during the low dust season. We found distinct bulk aerosol provinces along this latitudinal transect: Asian aerosols (especially crustal dust) dominate at higher latitudes (52–32°N) while North American heavier‐than‐crustal wildfire aerosols dominate in Equatorial Pacific deployments (20°N to 20°S). Soluble aerosol Fe was isotopically lighter‐than‐crustal along the full transect, strongly indicative of a pervasive anthropogenic Fe contribution to the Pacific. Comparison to a global aerosol deposition model corroborates that an isotopically heavy endmember is required for wildfire Fe, attributed to pyroconvective entrainment of soil particles. For Zn, the entire GP15 section is dominated by non‐crustal anthropogenic sources, reflected by light isotopic compositions (bulk: −0.12 ± 0.08‰ and soluble: −0.17 ± 0.14‰).