| Summary: | Models project that with current CO2 emission rates the Southern Ocean surface will be undersaturated with respect to aragonite and calcite by the end of the 21st century resulting in widespread impacts on biogeochemistry and the ocean ecosystem. However, accurate assessment of future acidification changes and impacts require a better understanding of present-day saturation state and depth of the saturation horizon in the Southern Ocean. We analyze present-day carbonate chemistry and assess the current vulnerability of the Southern Ocean with respect to ocean acidification using freely-available cruise data published in the Global Ocean Data Analysis Project Version 2 (GLODAPv2), along with an interpolated version of these data that gap fills when data have not been collected at a particular location or time. We find that the present-day saturation horizon for aragonite varies from 400-2000m depth. The aragonite saturation horizon is shallowest around 60 degrees S and deeper North and South of this latitude at 40 degrees S and 70 degrees S. The Calcite saturation horizon is shallowest in the South Pacific and along 60 degrees S. We find very low data density at the depth of the saturation horizon, which may bias the interpolated product's estimate of the horizon depth in many locations. We further assess whether predictions of the present-day horizon depth from Earth system models are consistent with these observations.
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