Carbonate saturation state of surface waters in the Ross Sea and Southern Ocean: controls and implications for the onset of aragonite undersaturation

Abstract. Predicting when surface waters of the Ross Sea and Southern Ocean will become undersaturated with respect to biogenic carbonate minerals is challenging in part due to the lack of baseline high-resolution carbon system data. Here we present ~ 1700 surface total alkalinity measurements from the Ross Sea and along a transect between the Ross Sea and southern Chile from the austral autumn (February–March 2013). We calculate the saturation state of aragonite (Ω_Ar) and calcite (Ω _Ca) using measured total alkalinity and pCO₂. In the Ross Sea and south of the Polar Front, variability in carbonate saturation state (Ω) is mainly driven by algal photosynthesis. Freshwater dilution and calcification have minimal influence on Ω variability. We estimate an early spring surface water Ω_Ar value of ~ 1.2 for the Ross Sea using a total alkalinity–salinity relationship and historical pCO₂ measurements. Our results suggest that the Ross Sea is not likely to become undersaturated with respect to aragonite until the year 2070.