Long-term trend of CO₂ and ocean acidification in the surface water of the Ulleung Basin, the East/Japan Sea inferred from the underway observational data

Abstract. Anthropogenic carbon is responsible for both global warming and ocean acidification. Efforts are underway to understand the role of ocean in a high CO₂ world on a global context. However, marginal seas received little attention despite their significant contribution to biogeochemical cycles. Here we report the CO₂ increase and ocean acidification in the surface waters of the Ulleung Basin (UB) of the East/Japan Sea, and possible causes are discussed. Fourteen observations of surface fCO₂ were made in the period from 1995 to 2009. The contribution of temperature variation to the seasonality of fCO₂ was almost equivalent to the non-thermal effect in the UB. However, the difference of relative contribution with the season makes two seasonal peaks of fCO₂ in the surface water of the UB. Non-thermal effect contributed to the surface fCO₂ drawdown in summer, whereas the surface fCO₂ elevation in winter. The decadal trend of fCO₂ increment was estimated by harmonic analysis. The estimated rates of increase of fCO₂ were 1.8 ± 0.4 μatm yr⁻¹ for the atmosphere and 2.7 ± 1.1 μatm yr⁻¹ for the surface water. The ocean acidification trend, calculated from total alkalinity and fCO₂, was estimated to be −0.03 ± 0.02 pH units decade⁻¹. These rates seem to be higher than observations at most other ocean time-series sites during the same period of time. Sustained observations are required to understand more accurate trend in this area.