TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 55 (No. 6), pp. 717-729, 1999

Improved Method for Calculating Anthropogenic CO2 in the Upper Layer of the North Pacific Subtropical Gyre along 175°E

Yoshiyuki Takahashi1, Eiji Matsumoto1 and Yutaka W. Watanabe2

1Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
2National Institute of Resources and Environment, Onogawa 16-3, Tsukuba, Ibaraki 305-0053, Japan

(Received 10 August 1998; in revised form 12 August 1999; accepted 16 August 1999)

Abstract: Seawater samples were collected in the North Pacific along 175°E during a cruise of the Northwest Pacific Carbon Cycle Study (NOPACCS) program in 1994. Many properties related to the carbonate system were analyzed. By using well-known ratios to correct for chemical changes in seawater, the CO2 concentration at a given depth was back calculated to its initial concentration at the time when the water left the surface in winter. We estimated sea-surface CO2 and titration alkalinity (TA) in present-day winter, from which we evaluated the degree of air-sea CO2 disequilibrium in winter was. Using a correction factor for air-sea CO2 disequilibrium in winter, we reconstructed sea-surface CO2 in pre-industrial times. The difference between the back-calculated initial CO2 and sea-surface CO2 in pre-industrial times should correspond to anthropogenic CO2 input. Although the mixing of different water masses may cause systematic error in the calculation, we found that the nonlinear effect induced by the mixing of different water masses was negligible in the upper layer of the North Pacific subtropical gyre along 175°E. The results of our improved method of assessing the distribution of anthropogenic CO2 in that region show marked differences from those obtained using the previous back-calculation method.

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