Temporal Change of Dissolved Inorganic Carbon in the Subsurface Water at Station KNOT (44°N, 155°E) in the Western North Pacific Subpolar Region

Abstract

(Received 3 December 2003; in revised form 11 May 2004; accepted 16 May 2004) The dissolved inorganic carbon (DIC) and related chemical species have been mea- sured from 1992 to 2001 at Station KNOT (44°N, 155°E) in the western North Pacific subpolar region. DIC (1.3~2.3 µmol/kg/yr) and apparent oxygen utilization (AOU, 0.7~1.8 µmol/kg/yr) have increased while total alkalinity remained constant in the intermediate water (26.9~27.3σ θ ). The increases of DIC in the upper intermediate water (26.9~27.1σ σ σ σ σ θ ) were higher than those in the lower one (27.2~27.3 σ θ ). The tem- poral change of DIC would be controlled by the increase of anthropogenic CO 2 , the decomposition of organic matter and the non-anthropogenic CO 2 absorbed at the region of intermediate water formation. We estimated the increase of anthropogenic CO 2 to be only 0.5~0.7 µ µ µ µmol/kg/yr under equilibrium with the atmospheric CO 2 con- tent. The effect of decomposition was estimated to be 0.8 ± 0.7 µ µ µ µmol/kg/yr from AOU increase. The remainder of non-anthropogenic CO 2 had increased by 0.6 ± 1.1 µmol/kg/yr. We suggest that the non-anthropogenic CO 2 increase is controlled by the accumulation of CO 2 liberated back to atmosphere at the region of intermediate wa- ter formation due to the decrease of difference between DIC in the winter mixed layer and DIC under equilibrium with the atmospheric CO 2 content, and the reduction of diapycnal vertical water exchange between mixed layer and pycnocline waters. In future, more accurate and longer time series data will be required to confirm our results.