Carbonate cementation in the surface layer of reefs and beachrock eliminates porosity and partly replaces detrital quartz grains. The uptake and release of CO₂ by photosynthesis and respiration in reef communities cause a shift in the carbonate buffer system of seawater. Field studies and experimentation show minimum values of CO₂ (1.9 mmol/l) and HCO^-₃ (2.4 Meg/l) in association with maximum values of pH (10) and O₂ (> 100% saturation) in waters covering corals and alga prior to sunset. The converse is true for these variables prior to sunrise, when minimum values of pH (7.6) and O₂ (< 66% saturation) occur with maximum values of CO₂ (2.7 mmol/l) and HCO^-₃ (2.8 Meg/l) Experimental tanks containing plain seawater showed almost no diurnal variability in pH (a constant 8.0) or O₂ (~ 58% saturation) measurements. Seawater adjacent to reef biomass with elevated pH and supersaturated with calcium carbonate is pumped into the underlying reef rock and beach rock where carbonate precipitation and quartz dissolution occurs. As the system equilibrates, pH values progressively decrease.

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