Comment on egusphere-2025-6068

Abstract. Biogenic and authigenic particulate iron (pFe) are key components of the marine iron cycle, influencing the fate of Fe in the upper ocean. However, their relative contributions to the total pFe pool are challenging to quantify. The chemical leach commonly used to operationally define ‘labile’ pFe is thought to extract both the biogenic and authigenic phases. To independently determine biogenic pFe, we conducted Fe uptake experiments in the surface mixed layer on 12 cruises in the subtropical North Pacific Ocean. Bulk Fe uptake rates varied ~2.5-fold throughout the year, increasing with increasing Prochlorococcus and picoeukaryotes abundances. We used particulate carbon and phosphorus as biomass estimates in combination with Fe:C uptake ratios, finding that both led to overestimations of the biogenic pFe pool (>200% of labile pFe in the surface mixed layer). Using the nucleotide adenosine-5’-triphosphate (ATP) as an alternate estimate of living biomass instead suggested that biogenic pFe comprised ~60% of labile pFe in the mixed layer. The remainder of the labile pFe pool, defined as ‘nonliving’, dominated the labile pFe pool below the euphotic zone, capturing contributions from detrital organic matter, authigenic minerals, and dust. A comparison of Fe phases between Station ALOHA and the subtropical North Atlantic revealed similar concentrations of dissolved Fe and biogenic pFe, but higher nonliving labile pFe concentrations in the North Atlantic, likely reflecting greater dust deposition. The greater role of biogenic pFe in the North Pacific may enable high efficiency Fe recycling, which rivals that observed in Fe-limited ecosystems.