Carbonate system distribution, anthropogenic carbon and acidification in the western tropical South Pacific (OUTPACE 2015 transect)

Abstract. The western tropical South Pacific was sampled along a longitudinal 4000 km transect (OUTPACE cruise, 18 February, 3 April 2015) for the measurement of carbonate parameters (total alkalinity and total inorganic carbon) between the Melanesian Archipelago (MA) and the western part of the South Pacific gyre (WGY). This paper reports this new dataset and derived properties: pH on the total scale (pH T ) and the CaCO 3 saturation state with respect to aragonite ( Ω ara ). We also estimate anthropogenic carbon ( C ANT ) distribution in the water column using the TrOCA method (Tracer combining Oxygen, inorganic Carbon and total Alkalinity). Along the OUTPACE transect a deeper penetration of C ANT in the intermediate waters was observed in the MA, whereas highest C ANT concentrations were detected in the subsurface waters of the WGY. By combining our OUTPACE dataset with data available in GLODAPv2 (1974–2009), temporal changes in oceanic inorganic carbon were evaluated. An increase of 1.3 to 1.6  µ mol kg −1  a −1 for total inorganic carbon in the upper thermocline waters is estimated, whereas C ANT increases by 1.1 to 1.2  µ mol kg −1  a −1 . In the MA intermediate waters (27 kg m −3 σ θ 27.2  kg m −3 ) an increase of 0.4  µ mol kg −1  a −1 C ANT is detected. Our results suggest a clear progression of ocean acidification in the western tropical South Pacific with a decrease in the oceanic pH T of up to − 0.0027 a −1 and a shoaling of the saturation depth for aragonite of up to 200 m since the pre-industrial period.