BIOGEOCHEMICAL CONTROLS ON CARBONATE DISSOLUTION IN A BAHAMIAN BLUE HOLE: POTENTIAL IMPACTS ON ATMOSPHERIC C FLUXES

Atmospheric fluxes of carbon are important for controlling climate. Earth’s largest C reservoir is carbonate minerals (primarily CaCO₃), which may contribute to atmospheric fluxes of C depending on what type of acid dissolves them. Dissolving with carbonic acid (H₂CO₃), which is generated by dissolution and hydration of atmospheric CO₂, results in no net gain or loss of atmospheric CO₂. In contrast, dissolution of carbonate minerals by sulfuric acid (H₂SO₄) generates a net gain of C to the atmosphere. To examine the relative roles of H₂CO₃ and H₂SO₄ in dissolution of carbonate platforms, we collected water samples and deployed sondes at 0.75 and 2 m depth, respectively through seven tidal cycles in Inkwell Blue Hole, San Salvador, Bahamas, which is stratified with an H₂S rich layer at the pycnocline. The sondes and samples provided data for temperature (T), pH, dissolved oxygen (DO%), major cation, anion, alkalinity, and hydrogen sulfide (H₂S) concentrations. DO% values increased only during the day suggesting a control by primary productivity. Conversely, pH decreased both at night and during the day. Nighttime decreases in pH reflect production of CO₂ by respiration and hydration to H₂CO₃, while daytime decreases in pH in part reflect oxidation of H₂S to form H₂SO₄. The nighttime decrease in pH ranged from 0.07 to 0.18 pH units, but during the day decreases ranged from 0.2 to 0.31 pH units. Mass balance models of these changes in pH indicate that CO₂ production ranges from 0.2 to 0.4 mmol/L at night and O₂ production ranges from 0.5 to 0.8 mmol/L during the day. The drop in pH due to formation of H₂SO₄ during the day is at least twice the drop in pH corresponding to H₂CO₃ production at night, and could be larger considering that primary production consumes CO₂ during the day, which would raise the ph. Photosynthetic production of oxygen in the presence of H₂S appears to be an important control on carbonate dissolution, and causes a net flux of C to the atmosphere. Since H₂S oxidation is enhanced by the photosynthetic production of oxygen, this process is most likely to occur as carbonate platforms flood with rising sea levels. The generation of CO₂ resulting from this process could represent a positive feedback in climate cycling.