Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 3 3 2006 10.5194/bg-3-357-2006 http://www.biogeosciences.net/3/357/2006/ http://www.biogeosciences.net/3/357/2006/bg-3-357-2006.html http://www.biogeosciences.net/3/357/2006/bg-3-357-2006.pdf 357 369 2006-07-24 CO₃^2− concentration and pCO₂ thresholds for calcification and dissolution on the Molokai reef flat, Hawaii K. K. Yates kyates@usgs.gov R. B. Halley U.S. Geological Survey, Center for Coastal and Watershed Studies, St. Petersburg, FL 33701, USA The severity of the impact of elevated atmospheric pCO₂ to coral reef ecosystems depends, in part, on how seawater pCO₂ affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO₂ and CO₃^2− to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO₂ on calcification and dissolution processes. Rates of net calcification and dissolution, CO₃^2− concentrations, and pCO₂ were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcification ranged from 0.03 to 2.30 mmol CaCO₃ m^−2 h^−1 and dissolution ranged from –0.05 to –3.3 mmol CaCO₃ m^−2 h^−1. Calcification and dissolution varied diurnally with net calcification primarily occurring during the day and net dissolution occurring at night. These data were used to calculate threshold values for pCO₂ and CO₃^2− at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO₃^2− and pCO₂. Threshold pCO₂ and CO₃^2− values for individual substrate types showed considerable variation. The average pCO₂ threshold value for all substrate types was 654±195 μatm and ranged from 467 to 1003 μatm. The average CO₃^2− threshold value was 152±24 μmol kg^−1, ranging from 113 to 184 μmol kg^−1. 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