Human industrial, transportation, and agricultural activities have caused global atmospheric concentrations of carbon dioxide (CO2) to increase from a pre-industrial average of 280 parts per million (ppm) to the current value of approximately 400 ppm (IPCC AR5 2013). Atmospheric CO2 concentrations are now higher than experienced on Earth for more than 800,000 years (Luthi et al. 2008), and the rate of CO2 release into the atmosphere is likely unprecedented in Earth history (Kump et al. 2009; Honisch et al. 2012). Ocean carbonate chemistry is a natural buffering system, but this buffering...