Sulfur and Carbon Isotopic Composition and Carbonate Associated Sulfate Concentration of Calcitic Concretions of the Upper Cretaceous Holz Shale, Silverado, California
The Holz Shale, an organic rich marine slope deposit, contains disseminated nodular calcite concretions and concretionary layers. The concretions range in size from ~2 to 100 cm in diameter and formed before significant compaction of the host shale as indicated by deflection of external laminae around the nodules. Carbonate d13C values range from –3 to –10‰ (PDB) with values decreasing systematically from center-to-edge in all concretions analyzed. This trend is consistent with outward concentric growth and an increase in carbonate sourced from organic carbon remineralization with progressive burial. Carbonate associated sulfate (CAS: trace sulfate incorporated into the carbonate crystal lattice) concentrations are reduced compared to marine-precipitated carbonates. CAS concentrations of ≤500 ppm are consistent with carbonate precipitated from modified pore waters depleted in sulfate compared to seawater. d34SCAS is low with values ranging from –10 to –20‰ (CDT). Such isotopically depleted sulfate can result from the oxidation of microbially produced H2S in pore waters or from pyrite oxidation during CAS extraction. While SEM analysis reveals that Holz Shale concretions do contain framboidal pyrite, preliminary step-wise leaching experiments suggest that pyrite was not significantly oxidized during the extraction process. The d34SCAS results are curious for several reasons. Production of significant H2S occurs within the sulfate reduction zone (which is anaerobic), whereas oxidation of H2S is thought to occur above or at the interface with the sulfate reduction zone, where oxygen or nitrate is present. Typical SO42- concentrations at this interface should be nearly entirely marine and overwhelm the isotopic signature of oxidized H2S. Perhaps another oxidant plays a role in sulfide oxidation deeper within the zone of sulfate reduction, there is an unappreciated microbial pathway for sulfide oxidation, or the flux of sulfide during diagenesis was great enough to overwhelm sea water isotopic signatures at the interface between oxic and sulfidic porewaters.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009