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 Fe in biomass, 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 higher abundances of Prochlorococcus and picoeukaryotes. 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 biotic pFe comprised ∼ 60 % of labile pFe in the mixed layer. The remainder of the labile pFe pool, defined as “authigenic + detrital', 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 in the North Pacific Subtropical Gyre and the BATS station in the North Atlantic Subtropical Gyre revealed similar concentrations of dissolved Fe and biotic pFe, but higher authigenic pFe concentrations in the North Atlantic, likely reflecting greater dust deposition. The greater role of biotic pFe in the subtropical North Pacific may enable high efficiency Fe recycling, which rivals that observed in Fe-limited ecosystems.
Bates et al. (Fri,) studied this question.