Calculations of Sulfate Diffusivity for Lithofacies of the Miocene Monterey Formation

Abstract

Integration of stable sulfur isotopic data, total sulfur content, uncompacted porosity, and estimated sediment density of the Miocene Monterey Formation have been used to calculate the extent of sulfate diffusion in the upper tens of centimeters of sediment during early diagenesis. Extent of sulfate diffusion, expressed as the number of pore volumes of water filtered through a volume of sediment, varies from three to infinity with the majority of values in the range of three to seven. Diffusion of sulfate into sediments is strongly influenced by bioturbation. Interbedding of laminated and massive Monterey strata within the Santa Maria basin indicates sporadic presence of bioturbating organisms during deposition. In addition to a negative correlation between bioturbation and calculated sulfate diffusivity, a positive correlation exists between calculated sulfate diffusivity and clastic input. Clastic indices are highest for the upper siliceous facies, intermediate in the lower calcareous facies, and lowest for the phosphatic facies. Calculation of sulfate diffusivity may be used to infer the extent of biomineralization within sediments. High sulfate diffusivity indicates extensive recycling of organic matter by aerobes; low values indicate minor recycling by aerobes and/or major decomposition of organic matter by anaerobes provides sulfate concentration in the sediment was highmore » enough to sustain biogenic sulfate reduction.« less