Preservation of Depositional Marine Sedimentary Sulfur Isotope Records Under Thermal Alteration

Session: New Voices in Geobiology

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Abstract:

The sedimentary sulfur record is critical for interpreting how our marine environments have evolved throughout Earth’s history because its burial of reduced species (i.e pyrite or kerogen) impacts global climate through drawdown of atmospheric CO₂ and gradual buildup of  O₂ levels. Sedimentary sulfur isotopic records (i.e. δ³⁴S) are used to reconstruct changes in ocean chemistry, microbial activity, and redox conditions in deep geologic time. Therefore, it is important to deconstruct whether the δ³⁴S records preserved in the geologic record reflect the values preserved during sedimentation or rather a diagenetically altered signal. Sulfur, present as sulfate, is ubiquitous in marine environments and is largely preserved in the marine rock record as three to four end members – carbonate-associated sulfate, pyrite, gypsum or organic sulfur (minor sink). Through microbial sulfate reduction, sulfate gets reduced to produce sulfide, which can then be scavenged rapidly by iron or react with organic matter to form pyrite and organic sulfur, respectively. This process occurs in low-oxygen environments when other respiration processes, such as oxic respiration or nitrogen fixation, are unavailable to organisms. Thus, the deposition of sulfur as pyrite, or organic sulfur, in the sedimentary record can indicate periods of anoxia in marine environments through time. However, it is unclear whether original δ³⁴S signals are retained after later stage metamorphism, and if the preservation of δ³⁴S  differs between organic and pyrite sulfur. This is important to distinguish as all sediments have undergone heating and compaction during the lithification process. We seek to assess how well δ³⁴S records are preserved under metamorphic conditions through the analysis of organic and pyrite δ³⁴S in variably thermally altered sediments, paired with in-situ sulfur isotope analyses of δ³⁴S variation within individual pyrite grains. Samples were collected within a single stratigraphic horizon along a transect perpendicular to an igneous intrusion into the Pierre Shale in the Raton Basin, NM, where heating decreases with distance from the dike. We collected SIMS data on individual pyrite grains, external heating constraints (e.g. Rock Eval pyrolysis and vitrinite reflectance), and bulk δ³⁴S records for pyrite and organic sulfur. This work supports interpretations of measured δ³⁴S values of thermally altered sediments, future studies on wider degrees of thermal alteration, and the development of correction models for reconstructing original δ³⁴S.