Nicholas J. Hawco

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.

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.

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.

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.

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.

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.

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.

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.

The seasonal variability of dissolved iron (dFe), an essential phytoplankton micronutrient, was examined over three years on 21 cruises to Station ALOHA in the North Pacific Subtropical Gyre. Mixed layer concentrations and euphotic zone inventories of dFe were highest in winter and spring and decreased throughout the year. We estimated a mean dFe residence time in the euphotic zone (0–150 m) for each season, which was consistent at 5 ± 1 months despite 2‐fold seasonal variability in dFe inventories. The sub‐annual residence time highlights the importance of recycling winter and spring dFe sources to sustain productivity and nitrogen fixation through the summer and fall. Coincident increases in dissolved titanium and elevated particulate Ti:Al suggest dFe inputs from basalt weathering on Hawaiʻi, particularly during increased rainfall and runoff in winter. The source of dFe to Station ALOHA appears to shift from island input in winter to Asian dust deposition in spring.