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

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.