Dissolution rates of biogenic carbonates in natural seawater at different pCO2 conditions: A laboratory study

The bulk dissolution rates of six biogenic carbonates (goose barnacle, benthic foraminifera, bryozoan, sea urchin, and two types of coralline algae) and a sample of mixed sediments from the Bermuda carbonate platform were measured in natural seawater at pCO2 values ranging from approximately 3,000 to 5,500 muatm. This range of pCO2 values encompassed values regularly observed in pore waters at a depth of a few cm in carbonate sediments at shallow water depths (<15 m) on the Bermuda carbonate platform. The biogenic carbonates included calcites of varying Mg-content (2-17 mol%) and a range of specific surface areas (0.01-2.7 m2 g-1) as determined by BET gas adsorption. Measured rates of dissolution increased with increasing pCO2 treatment for all substrates and ranged from 2.5-18 mumol g-1 hr-1. The highest rates of dissolution were observed for the bryozoans and the lowest rates for the goose barnacles. The relative ranking in dissolution rates between different substrates was consistent at all pCO2 levels indicating that substrates dissolve sequentially and that some substrates will be more vulnerable than others to rising CO2 and ocean acidification. Furthermore, dissolution rates were found to increase with increasing Mg-content, though the relative dissolution rates were observed to be a function of both Mg-content and microstructure (surface area).