Journal of Oceanography, Vol. 61 (No. 1), pp. 129-139, 2005
Masahide Wakita1,2*, Shuichi Watanabe1,2, Yutaka W. Watanabe1, Tsuneo Ono4, Nobuo Tsurushima3 and Shizuo Tsunogai1
1Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
2Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
3National Institute for Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan
4Hokkaido National Fisheries Research Institute, Kushiro 085-0802, Japan
(Received 3 December 2003; in revised form 11 May 2004; accepted 16 May 2004)
Abstract: The dissolved inorganic carbon (DIC) and related chemical species have been measured from 1992 to 2001 at Station KNOT (44°N, 155°E) in the western North Pacific subpolar region. DIC (1.3~2.3 mmol/kg/yr) and apparent oxygen utilization (AOU, 0.7~1.8 mmol/kg/yr) have increased while total alkalinity remained constant in the intermediate water (26.9~27.3sq). The increases of DIC in the upper intermediate water (26.9~27.1sq) were higher than those in the lower one (27.2~27.3sq). The temporal change of DIC would be controlled by the increase of anthropogenic CO2, the decomposition of organic matter and the non-anthropogenic CO2 absorbed at the region of intermediate water formation. We estimated the increase of anthropogenic CO2 to be only 0.5~0.7 mmol/kg/yr under equilibrium with the atmospheric CO2 content. The effect of decomposition was estimated to be 0.8 ± 0.7 mmol/kg/yr from AOU increase. The remainder of non-anthropogenic CO2 had increased by 0.6 ± 1.1 mmol/kg/yr. We suggest that the non-anthropogenic CO2 increase is controlled by the accumulation of CO2 liberated back to atmosphere at the region of intermediate water formation due to the decrease of difference between DIC in the winter mixed layer and DIC under equilibrium with the atmospheric CO2 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.