The relationship of diagenesis with a complex microbial ecosystem in the phosphatic interval of the Miocene Monterey Formation: evidence from stable isotopes and mineralogy

Abstract

Abstract A combination of evidence from mineralogy, sulfur and oxygen stable isotope ratios from early diagenetic carbonate associated sulfate (CAS) and structurally substituted sulfate in phosphate (both termed CAS here), and carbon and oxygen stable isotope ratios from carbonate, describe a complex redox system and complex microbial ecosystem within the phosphate-rich interval of the Monterey Formation. δ34S and δ18O of CAS are lower than Miocene seawater sulfate, as reconstructed from marine evaporite and barite minerals. Combined with mineralogical speciation modeling and mineralogical observations, isotopically low CAS isotope values suggest mixing from three isotopically characterized sulfate pools: two isotopically higher sulfate pools, one with the composition of Miocene seawater sulfate and a second isotopically evolved porewater modified by sulfate reducing microbes (SRM), and a third isotopically lower porewater sulfate pool resulting from the oxidation of H2S (produced by SRM) by sulfide oxidizing bacteria (SOB). High carbonate δ13C and diagenetic dolomite validates prior claims of methanogenesis. Differences in mineralogy are consistent with differences in isotope ratios (e.g. higher δ13C in dolomites), suggesting products from a vertical distribution of oxic to anoxic redox environments and associated ecosystems are preserved. These mineralogical and isotopic fluctuations repeat in various facies changes, suggesting that redox environments fluctuated over time. This interpretation is supported by prior sedimentologic analysis which interpreted that deposition of the phosphatic interval of the Monterey Formation was influenced by gravity-deposition processes promoted by tectonic activity. Gravity-deposition would serve to transport more oxygenated waters to the deep-water depositional environment and support microbially-mediated sulfide oxidation at depth, whereby H2S generated by SRM in the absence of sufficient iron could diffuse upward to a more oxygenated zone. Near the suboxic-oxic boundary, H2S oxidation would decrease porewater pH slightly and inhibit the preservation of carbonate minerals and promote precipitation of phosphate. Increasing depth in sediments generates successively more reducing conditions, promoting sulfate reduction, which may have been coupled with the anaerobic oxidation of methane and carbonate mineral precipitation. The most reducing conditions would support methanogenesis and dolomite production after consumption of the majority of sulfate.