Distributions of natural organic Fe-binding ligands in the eastern and western subarctic Pacific

Iron (Fe) is well known as a limiting factor to control primary productivity especially in high-nutrient and low chlorophyll area such as the subarctic Pacific. In seawater, most dissolved Fe in seawater is complexed with natural organic ligands. Since bioavailability of Fe in seawater depends on its species, the chemical speciation of Fe is needed to evaluate the nutrient status for phytoplankton. In this study, vertical distributions of organic Fe-binding ligands were investigated in the eastern and western subarctic Pacific to clarify its impact on Fe cycle. Filtered seawater samples (<0.2 µm) were obtained through the whole water column during GEOTRACES-Japan cruise KH-17-03 using trace metal clean technique. The organic Fe-binding ligands’ concentration and their conditional stability constants (K’_Fe’L) were determined by competitive ligand equilibration-adsorptive cathodic stripping voltammetry method using salicylaldoxime (Abualhaija and van den Berg, 2014). The concentrations and log K’_Fe’L of organic ligands ranged from 0.7 nM to 5.9 nM and from 10.1 to 12.3, respectively. The concentrations of organic ligands exceeded those of dissolved Fe concentration more than 1 nM in most samples except for the surface (10-m) at the Oyashio-Kuroshio transition zone station; consequently >99% of dissolved Fe was estimated to be complexed with these natural organic ligands. Below 400-m depth, high excess ligands concentration (>4 nM) were often found at the western station; especially weak ligands (log K’_Fe’L <11) distributed in the lower part of North Pacific Intermediate Water (NPIW). While this trend was not observed at the eastern stations. In the subarctic Pacific, a concentration gradient of dissolved Fe has been demonstrated in the NPIW between western and eastern areas (Nishioka and Obata, 2017), a further research is necessary to shed light on the source of organic ligands in this region.