Decadal change of dissolved inorganic carbon in the subarctic western North Pacific Ocean

Dissolved inorganic carbon (DIC) was measured from 1992 to 2008 at two time-series sites in the subarctic western North Pacific; this region is a source of atmospheric CO₂ in winter due to vertical mixing of deep waters rich in DIC. To estimate the decadal DIC increase resulting from CO₂ uptake from the atmosphere, we corrected DIC for the contribution of biological activity below the temperature minimum (T_min) layer (~100 m), which is the remnant of the mixed layer from the preceding winter. Decadal DIC increases in the T_min layer and upper intermediate water (1.3–C1.5 μmol kg^-1 yr^-1; 100–200 m) were higher than those expected from oceanic equilibration with increasing atmospheric CO₂ and those previously reported in the open North Pacific. The increase in water column CO₂ was estimated to be 0.40 ± 0.08 mol m^-2 yr^-1. The decadal DIC change in the T_min layer affects winter xCO₂ emission. The increase of atmospheric xCO₂ in winter (2.1 ± 0.0 ppm yr^-1) is higher than that of oceanic xCO₂ (0.7 ± 0.5 ppm yr^-1) that calculated from DIC and total alkalinity in the T_min layer. This difference suggests reduction of CO₂ emission in winter is possibly controlled by the increase of total alkalinity.