Abstract
The mechanism by which nutrients in the deep ocean are uplifted to maintain nutrient-rich surface waters in the subarctic Pacific has not been properly described. The iron (Fe) supply processes that control biological production in the nutrient-rich waters are also still under debate. Here, we report the processes that determine the chemical properties of intermediate water and the uplift of Fe and nutrients to the main thermocline, which eventually maintains surface biological productivity. Extremely nutrient-rich water is pooled in intermediate water (26.8 to 27.6 σθ) in the western subarctic area, especially in the Bering Sea basin. Increases of two to four orders in the upward turbulent fluxes of nutrients were observed around the marginal sea island chains, indicating that nutrients are uplifted to the surface and are returned to the subarctic intermediate nutrient pool as sinking particles through the biological production and microbial degradation of organic substances. This nutrient circulation coupled with the dissolved Fe in upper-intermediate water (26.6 to 27.0 σθ) derived from the Okhotsk Sea evidently constructs an area that has one of the largest biological CO2 drawdowns in the world ocean. These results highlight the pivotal roles of the marginal seas and the formation of intermediate water at the end of the ocean conveyor belt.
Highlights
The mechanism by which nutrients in the deep ocean are uplifted to maintain nutrient-rich surface waters in the subarctic Pacific has not been properly described
An important factor for controlling biological production in the nutrient-rich region is the formation of chemical properties of intermediate water [6], including nutrients and the limiting micronutrient “Fe.” North Pacific Intermediate Water (NPIW) is formed under the strong influence of the marginal seas [13,14,15] and may play a major role in the connection of nutrients between the deep water and the surface water above it [6]
Previous studies imply that the nutrient return path to the surface exists in the northwest corner of the Pacific [6, 11], detailed mechanisms by which nutrients return to the surface layer and how high-nutrient low-chlorophyll (HNLC) water is formed in the North Pacific have not been described
Summary
We first constructed a diagram showing the three-dimensional (3D) distribution of dFe in the North Pacific, including its subpolar marginal seas (the Okhotsk Sea and the Bering Sea) (Fig. 1B). The eastern side of the subarctic Pacific has a continental shelf source of dFe along the Alaskan Stream (AS) (Fig. 1B and SI Appendix, Fig. S1F), as previously reported [22] This highdFe water of the AS is basically confined to the nearshore area, because the boundary current (AS) passes along the coast, eddy transports of the high-dFe water to offshore occasionally occur [30]. The horizontal distribution indicated by isopycnal analysis in this study clearly shows evidence that the high dFe source in the intermediate waters in the western subarctic Pacific is the marginal seas. The Okhotsk Sea to U-intermediate water and the EKC influence on L-intermediate water (SI Appendix, Fig. S2 A–F)
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