Iron supply to the western subarctic Pacific: Importance of iron export from the Sea of Okhotsk

Abstract

Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface‐mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron‐rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.

The Iron Biogeochemical Cycle Past and Present

“…X-ray absorption spectroscopy showed the presence of Fe 2+ -bearing minerals from volcanogenic sediments which, along with redox-mobilised iron (oxyhydr)oxides, were also present on the shelf. Once again the peak concentrations of acid-leachable iron in the particulates occurred at a density surface just above the oxygen minimum, which was consistent with a supply from the Sea of Okhotsk (see also Nishioka et al, 2007). Subsequently Lam et al (2012) studied the mineralogical composition of the 1-51 µm fraction of suspended sediments collected over a 1000 km transect in the eastern tropical north Atlantic.…”

“…A sediment source of iron was clearly implicated, and resuspension of shelf sediments was the favoured explanation because diffusive fluxes of iron from reducing sediments (see above) were thought to be too low. Nishioka et al (2007) also proposed that the enrichment of iron in filtered (<0.22µm) and acidified seawater samples collected in the Western Subarctic Pacific were due to resuspension of shelf sediments from the Sea of Okhotsk.…”

Temporal and spatial variations in photosynthetic physiology of diatoms during the spring bloom in the western subarctic Pacific

“…The mixed layer depth (MLD) was estimated using the definition: depth at which the water density = the density + 0.1 σ θ of the sea-surface water. Water samples were collected using trace-metal-clean Niskin-X samplers on a Kevlar hydrowire (Nishioka et al 2007). Underwater downward irradiance of photosynthetically available radiation (PAR) was measured by a spectroradiometer (PRR-600/610, Biospherical Instruments).…”

“…In addition, since spring diatom blooms take up large amounts of nutrients (including dissolved inorganic carbon) in the Oyashio and Transition regions and significantly affect the biogeochemical cycles, nutrient and plankton dynamics during spring blooms have been investigated. Light, iron, and/or silicic acid were proposed as the important controlling factors of the ecosystem dynamics during spring blooms through field observations (Saito et al 2002, Saito & Tsuda 2003, Nishioka et al 2007, and (Yoshie et al 2003, Yamanaka et al 2004. However, the limited understanding of P-E parameters such as P B max and α B prevents further evaluation of the ecosystem model and determination of the factors controlling the ecosystem dynamics in these regions.…”

The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

“…Various processes are responsible for the exogenous inputs: deposition of iron-bearing dust (Duce and Tindale, 1991;Tegen and Fung, 1994;Mahowald et al, 1999;Jickells et al, 2005); reduction and resuspension of sedimentary iron (Johnson et al, 1999;Elrod et al, 2004;Moore et al, 2004;Lam et al, 2006;Nishioka et al, 2007;Lam and Bishop, 2008;Moore and Braucher, 2008;Tagliabue et al, 2009;Misumi et al, 2011); and fluvial and hydrothermal inputs (Wetz et al, 2006;Tagliabue et al, 2010;Klunder et al, 2011;Nishioka et al, 2013;Saito et al, 2013). Most sedimentary and hydrothermal iron is supplied from continental margins and the ocean abyss to open ocean surface waters by physical ocean processes.…”