“LIVE” SAND DISSOLUTION IN RESPONSE TO ELEVATED CARBON DIOXIDE: FLORIDA KEYS

Increasing anthropogenic CO₂ is predicted to have adverse effects on many ecosystems and may be particularly devastating to coral reefs.  This paper reports the results of experiments during which CO₂ is artificially increased over ″live″ reef sand.  These aragonitic sands begin to dissolve at night in seawater with pH 8.2, pC0₂ of 325 matm, and aragonite saturation index of 3.9.  Additions of CO₂ initially increase pCO₂, and decrease pH and carbonate saturation.  However, within 48 hours small amounts of sand dissolution decrease pCO₂, increase pH, and elevate the carbonate saturation index.  Dissolution, in seawater supersaturated with carbonate minerals, may result from the presence of highly soluble magnesian calcite, extremely small particle sizes, biologically mediated dissolution, and/or recrystallization processes.  The experiments suggest that carbonate reef sediments should not be considered as metastable, largely inert sediments immersed in supersaturated seawater.  Rather, they are in dynamic dissolution/precipitation disequilibrium, continually responding to subtle changes in seawater chemistry.  In ″live″ carbonate systems, elevated CO₂ affects both dissolution and calcification rates through processes that are not yet well characterized.