Distribution and isotopic signature of ligand-leachable particulate iron along the GEOTRACES GP16 East Pacific Zonal Transect

Abstract Particles play an important role in the marine biogeochemical cycling of iron, an essential micronutrient. Particles of biogenic, lithogenic and authigenic origin can act as sources or sinks of dissolved iron and therefore influence its distribution and availability to phytoplankton. However, not all particulate iron (pFe) is readily accessible to phytoplankton and various treatments are used to estimate the bioavailable, or labile fraction of pFe. Here we report concentration and iron stable isotope data for the “ligand-leachable” fraction of pFe, following the application of a pH 8 oxalate-EDTA leach to 0.8–51 μm-sized particles collected during the US GEOTRACES East Pacific Zonal Transect (GP16). Across the GP16 section, we identified two regions of high labile pFe concentrations and differing δ 56 Fe labile . Within a hydrothermal plume originating from the East Pacific Rise, nearly all of the pFe was labile, with concentrations as high as 42 nM. Particulate δ 56 Fe labile within the hydrothermal plume averaged − 0.26 ± 0.14‰ ( n  = 67), which is similar to both δ 56 Fe of hydrothermal plume dissolved Fe and the δ 56 Fe of primary vent fluids from previous studies, suggesting that vent fluid Fe is quantitatively precipitated as labile Fe oxyhydroxides and that the chemical form of hydrothermal dissolved and particulate Fe are the same, though they are found in different size classes. Elevated pFe concentrations were also observed over the Peruvian continental shelf and extending westwards, coincident with the secondary nitrite maximum at the top of the oxygen minimum zone (OMZ). Again, most of the pFe was within the labile fraction, but the δ 56 Fe labile associated with the OMZ was lighter than in the hydrothermal plume (− 0.68 ± 0.32‰, n  = 41), reflecting its origin within reducing continental margin sediments. The open ocean away from these features was characterized by labile and total pFe concentrations of 56 Fe labile values, with 80% in the range of − 0.5‰ to + 0.2‰ ( n  = 169). Below the hydrothermal and OMZ plumes, benthic nepheloid layer labile pFe had similar δ 56 Fe labile to particles in the overlying water column features, suggesting that Fe isotopes in surface sediments may be a tracer for the location of active hydrothermal venting and oxygen minimum zones in the past ocean.