Fe/Al ratios of suspended particulate matter from intermediate water in the Okhotsk Sea: Implications for long-distance lateral transport of particulate Fe

Abstract

We have found that the ratio of Fe to Al in suspended particulate matter (SPM) in the intermediate layer of the Okhotsk Sea increases to the open ocean from the dense shelf water on the northwestern continental shelf, which is the source of southward-flowing Okhotsk Sea Intermediate Water (OSIW). The SPM concentration and the Al content of the SPM decrease along the course of the OSIW, whereas the ratios of Ba and Mn to Al increase. The SPM samples on the continental shelf were collected in waters characterized by low N⁎ (the deviation from the stoichiometric relationship between nitrogenous nutrients and phosphate). These low N* values imply that sedimentary denitrification occurs on the continental shelf, and both Fe and Mn are concomitantly reduced in the sediments and diffuse from the sediments to the overlying waters, where they are subsequently oxidized and precipitate. We assume that the SPM in the intermediate layer is influenced mainly by horizontal transport from the continental shelf to the ocean interior and that Ba can be used as a proxy for organic matter. Based on these assumptions, the results could reflect the following processes: 1) a gradual loss of denser lithogenic matter, evidenced by the decreases of the SPM concentration and of the Al content of the SPM, and 2) a relative increase of the Fe and Mn fractions associated with the organic matter in the SPM, which is derived from the continental shelf. The organic matter-related Fe and Mn could also be affected by the precipitation of both Fe and Mn just above the reduced sediments of the continental shelf. Our results show that the particulate Fe associated with organic matter is preferentially transported from the northwestern continental shelf to the open ocean.