Detecting the progression of ocean acidification from the saturation state of CaCO3 in the subtropical South Pacific

Progression of ocean acidification in the subtropical South Pacific was investigated by using high-quality data from trans-Pacific zonal section at 17°S (World Ocean Circulation Experiment section P21) collected in 1994 and 2009. During this 15 year period, the CaCO3 saturation state of seawater with respect to calcite (Ωcal) and aragonite (Ωarg) in the upper water column (<400 dbar) decreased at rates of 0.037 a−1 and 0.025 a−1, respectively, east of 145°W longitude; these rates are among the fastest in the world's oceans. In contrast, at longitudes 170°E–145°W, Ωcal and Ωarg decreased relatively slowly, at 0.008 a−1 and 0.005 a−1, respectively. The Ωarg saturation horizon occurred at a depth of about 1200 dbar at the westernmost end of the section and shoaled eastward to about 20 dbar. From 1994 to 2009, it migrated upward at a rate of 5.2 dbar a−1 west of 145°W. Decomposition of the temporal changes of Ω (ΔΩ) showed that the accumulation of anthropogenic CO2 in the ocean accounted for more than half of ΔΩ. The more rapid rate of decline of Ω in the eastern section was attributable to a relatively large contribution of organic matter remineralization, whereas the slower rate in the central section was attributed to a decrease of anthropogenic CO2 uptake caused by rising water temperatures. An important finding of this study was that acidification of the upper water column was enhanced by processes related to the oxygen minimum zone in the eastern subtropical South Pacific Ocean.