Calcic dolomite cements compose an average of 5% of the upper 4.3 m of subtidal deposits <5600 yr old in the Cangrejo shoals in Belize. Mean δ18O (+2‰) compositions of the high-Sr dolomites (mean October 1900 ppm), together with near-normal salinity and inherently normal Mg/Ca ratio of pore fluids, suggest precipitation from near-normal seawater. Tidal and wind-driven circulation of seawater through the sediments supplies most of the Mg for dolomitization, which appears to be promoted by elevated pore-water alkalinity resulting from bacterially mediated oxidation of organic matter and, locally, early stages of methanogenesis. Rapid dolomitization here supports the idea that significant quantities of dolomite can form syndepositionally, from normal seawater, in shallow subtidal deposits.

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