SOLUTION OF SHALLOW-WATER CARBONATES: AN INSIGNIFICANT BUFFER AGAINST RISING ATMOSPHERIC CO2

Model predictions suggest that the saturation state of surface ocean waters with respect to carbonate minerals will decline during the twenty-first century owing to increased invasion of atmospheric CO₂. At this time sea surface temperatures could also be significantly higher. As a result calcareous organisms may have difficulty calcifying, leading to production of weaker skeletons and greater vulnerability to erosion. Alternatively, it has been suggested that there will be no significant impact on coral reef ecosystems because any changes in saturation state and pH will be restored by dissolution of metastable carbonate minerals. To resolve this controversy, we employ a physical-biogeochemical box model representative of the shallow-water ocean environment. Numerical simulations demonstrate that the carbonate saturation state of surface waters could significantly decrease and hamper the biogenic production of CaCO₃ during the twenty-first century. The effect and magnitude of increasing temperature are not clear. Our results suggest that an increase in this parameter could have either a positive or a negative impact on biogenic calcification. The average saturation state of marine pore waters could significantly decline, inducing dissolution of metastable carbonate phases within the pore-water–sediment system. Such dissolution could buffer the carbon chemistry of the pore waters, but overlying surface waters of reefs and other shallow-water carbonate environments will not accumulate sufficient alkalinity to buffer pH or carbonate saturation state changes owing to invasion of atmospheric CO₂. Sensitivity analyses indicate that dissolution of carbonate minerals is mainly driven by remineralization of organic matter. Consequently increases in the flux of organic matter to the sediment system via rivers or in situ production could have important implications for the carbonate geochemistry of the shallow-water ocean environment. Future decrease in pore water saturation state could affect the composition and rates of precipitation of carbonate cements in contemporary shallow-water marine sediments.