Dolomitization of Holocene shallowmarine deposits mediated by sulfate reduction and methanogenesis in normal-salinity seawater

ABSTRACT: Dolomite constitutes an average of 12 % of the Holocene organic-rich sediments over a 15 km2 area of the Cangrejo Shoals mudbank in northern Belize. Although it defines a laterally persistent stratiform body that averages 3 m thick, it is present throughout the 7.6-m-thick sediment section. These transgressive sediments are less than;6400 years old and were deposited in shallow-marine environ-ments of normal salinity. The dolomite is dominantly cement, and av-erage crystal size is 7 m. There are no significant correlations among amount of dolomite vs. sediment texture, mineralogy, porosity, or mole % MgCO3 in associated particulate high-Mg calcite, depth, or location on the shoals. The dolomites are poorly ordered and calcic (39.5–44.5 mole % MgCO3), with low mean Mn (210 ppm) and relatively high mean Sr (1034 ppm) concentrations. There is no evidence of recrys-tallization or geochemical alteration of the dolomite. d18O values of the dolomites range from 0.5 to 2.8‰PDB, and the mean value (2.1‰) sug-gests that the dolomite precipitated from normal-salinity pore water. Dolomite d13C values range from 25.2 ‰ to 111.6‰PDB (mean sea-water d13C 5 0.5‰), which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water. Dolomitization attending these organodiagenetic re-actions apparently was reversible over time, and episodic rather than continual precipitation is indicated. Requisite Mg and Ca were provid-ed by seawater and by some dissolution of host sediments. The most rapid period of dolomitization may have been during early transgres-sion, when relatively high sedimentation rates sustained high levels of organodiagenesis and pore-water alkalinities