Long-distance transport of particulate iron from the Amur River to the western subarctic Pacific reinforced by the combination of Fe and Nd isotopes

Iron is an essential nutrient and limits primary productivity in High Nutrient Low Chlorophyll (HNLC) regions. The western subarctic Pacific (WSP) is one of HNLC regions, and the most important source of iron in the WSP has been thought to be the dust from East Asia such as Gobi dessert. In recent years, however, some studies suggest that the northwestern continental shelf region of the Sea of Okhotsk (OS) where the Amur River discharges large amounts of dissolved iron is one of the most important source area of iron in the WSP (e.g. Nishioka et al., 2007). The Amur has high concentration of dissolved iron, and more than 90% of the dissolved iron precipitates in the estuary mixing zone by coagulation. In the Amur estuary on the northwestern continental shelf of the OS, the strong tidal mixing causes the iron precipitates to be resuspended. The suspended particulate matter (SPM) is carried out to the Okhotsk Sea Intermediate Water (OSIW) by the Dense Shelf Water (DSW) (Nakatsuka et al., 2004). The OSIW flows out to the WSP through the Bussol strait, and adds to the North Pacific Intermediate Water (NPIW). The SPM is possibly transported to the WSP in the above processes, and re-dissolved iron from the SPM (Sugie et al., 2013) contributes to the biological productivity in the WSP. In this study, we analyzed stable isotopes of iron (Fe) and radiogenic isotopes of neodymium (Nd) in particulate iron chemically extracted from the continental shelf sediments and SPM of the OS and those in dissolved Fe and Nd of the Amur River water in order to clarify the transport process of the particulate iron from the Amur River in the OS. Iron isotopes have been recently applied to trace origin of marine iron because δ56Fe value varies depending on source, such as river, eolian dust and hydrothermal input. In addition, we used neodymium isotopes to draw information where the particulate iron in the OSIW precipitates because the particulate iron takes up large amounts of REEs from surrounding water in the precipitation process. The analytical results show that the dissolved iron in the Amur River has higher δ56Fe value than those in the other rivers in the world. The higher iron isotope ratio probably attributes to Fe-organic complexes which are enriched in heavy iron isotopes than the other iron species of dissolved iron. δ56Fe values of particulate iron extracted from the continental shelf sediments and SPM in the wide region of the OS are fairly similar to that of the Amur dissolved iron. The iron isotopes of the SPM around the Bussol strait also show the Amur River signal. Regional distribution of neodymium isotopes in the particulate iron support that the particulate iron derived from the Amur is long-transported to the southern part of the OS and probably to the WSP. The Fe and Nd isotope variations are consistent with the transport process of iron based on the previous studies on Fe concentration (Nishioka et al., 2007; 2011), and strongly suggest that particulate iron of the Amur origin is the main source of iron in the WSP.