Vertical fluxes of nutrients and carbon through the halocline in the western subarctic Gyre calculated by mass balance

Using data collected in the R/V Mirai (Japan Marine Science and Technology Center) cruises (November–December 1997, November–December 1998, May 1999, January–February 2000, and May–June 2000) we have computed the vertical fluxes of nutrients and carbon through the halocline, located at the base of the surface layer (∼100 m) in the Western Subarctic Gyre (WSG). The vertical fluxes were estimated by mass balance using a one-dimensional model and taking the fresh-water flux to the surface of the WSG to be 0.40 m yr−1 at two sites: in the central part of the WSG (50°N, 165–170°E) and close to its southwestern edge (44.0–44.5°N, 155°E). Based on our calculations, the annually averaged vertical export fluxes of the silica, carbonate, total organic nitrogen, phosphorus, and carbon at the study sites were 1.0±0.2, 0.34±0.06, 0.40±0.10, 0.024±0.006, and 3.2±0.6 mol m−2, respectively. Comparison with sediment-trap data collected at 1000 and 3000 m at the study sites between 1997 and 2000 (Fluxes and chemical compositions of particulate matter in the northwestern North Pacific: results from sediment-trap experiments (1997–2000), this volume) show that ∼93% of particulate organic carbon and nitrogen exported from the surface layer of the WSG had undergone remineralization from 100 to 1000 m. The computed annual uptake of excess CO2 and the change in δ13C due to the Suess effect in the study area were 0.74±0.1 mol m−2 and −7.4±1.0 per mil m, respectively. The depth-integrated δ13C change rate due to the Suess effect in the WSG was slightly lower than a global (Pacific, Indian, and Atlantic oceans) depth-integrated δ13C change rate (–9.7±2.4 per mil m yr−1 (Global Biochemical Cycles 13 (1999) 857)).