Processes controlling precipitation of carbonate cement and dissolution of silica in reef and near-reef settings

Abstract

Carbonate cementation in the surface layer of reefs and beachrock eliminates porosity and partially replaces detrital quartz grains. The uptake and release of CO 2 by photosynthesis and respiration in reef communities cause a shift in the carbonate buffer system of seawater. Field studies and experimentation simulating the natural settings show minimum values of CO 2 (1.9 mmoles 1 −1) and HCO 3 − (2.4 meq 1 −1) in association with maximum values of pH (9.8) and O 2 (> 100% oxygen saturation) in waters covering corals and algae prior to sunset. The converse is true for these variables prior to sunrise, when minimum values of pH (7.6) and O 2 (<66% oxygen saturation) occur with maximum values of CO 2 (2.7 mmoles 1 −1) and HCO 3 − (2.7 meq 1 −1). Experimental tanks containing plain seawater showed almost no diurnal variability in pH (a constant 7.5–7.6) or O 2 (80–90% oxygen saturation) measurements. Seawater adjacent to reef biomass, with elevated pH and supersaturated with calcium carbonate, is periodically pumped into the underlying reef and beach sediments due to pressure-buildups between the reef framework or algal zones and open waters. Carbonate precipitation and quartz dissolution follow. As the system equilibrates, pH values progressively decrease.