Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

Abstract. Rising atmospheric CO₂ contents have led to greater CO₂ uptake by the oceans, lowering both pH due to increasing hydrogen ions and CaCO₃ saturation states due to declining carbonate ion (CO₃²⁻). Here we used previously compiled data sets and new data collected in 2010 and 2011 to investigate ocean acidification of the North Pacific western subarctic gyre. In winter, the western subarctic gyre is a source of CO₂ to the atmosphere because of convective mixing of deep waters rich in dissolved inorganic carbon (DIC). We calculated pH in winter mixed layer from DIC and total alkalinity (TA), and found that it decreased at the rate of −0.0011 ± 0.0004 yr⁻¹ from 1997 to 2011. This decrease rate is slower than that expected under the condition of seawater/atmosphere equilibration, and it is also slower than the rate in the subtropical regions (−0.002 yr⁻¹). The slow rate is caused by a reduction of CO₂ emission in winter due to an increase in TA. Below the mixed layer, the calcite saturation horizon (~ 185 m depth) shoaled at the rate of 2.9 ± 0.9 m yr⁻¹ as the result of the declining CO₃²⁻ concentration (−0.03 ± 0.01 μmol kg⁻¹ yr⁻¹). Between 200 m and 300 m depth, pH decline during the study period (−0.0051 ± 0.0010 yr⁻¹) was larger than ever reported in the open North Pacific. This enhanced acidification rate below the calcite saturation horizon reflected not only the uptake of anthropogenic CO₂ but also the increase in the decomposition of organic matter evaluated from the increase in AOU, which suggests that the dissolution of CaCO₃ particles increased.