Carbonate-associated sulfate (CAS) can be used to generate high-resolution, temporal curves reflecting changes in the isotopic composition of seawater sulfate through the Phanerozoic and permitting comparisons with the well-established records for carbon, oxygen, and strontium isotopes. Sulfur isotopic comparisons (δ34S) for 118 samples of CAS and 104 samples of pyrite (pyr) are reported from two cores of the St. Louis Limestone (Middle Mississippian) in south-central Indiana along the eastern margin of the Illinois Basin. The St. Louis Limestone is informally divided into upper and lower sections based on a mappable bed of colonial rugose coral (Acrocyathus proliferum ). The lower St. Louis Limestone is inferred to be a restricted and evaporitic depositional environment containing anhydrite and gypsum deposits. The mean value of δ34SCAS (13.2 /) in the lower St. Louis carbonates is about 1.5 / lower (enriched in 32S) than the global secular curve for units of this age. The upper St. Louis (14.2 / δ 34SCAS) is inferred to be an open marine environment with a circulating seaway connected with the ocean to the south. A mean value of δ 34SCAS (13.5 /) for the entire St. Louis Limestone matches the global seawater sulfate isotopic composition during the Middle Mississippian. Values of δ34Spyr generally show an overall upward increase in the St. Louis. The difference between the sulfur isotopic composition of CAS and pyr (Δ34SCAS-pyr) ranges from 13.1 to 50.1 per mil with higher Δ34SCAS-pyr values occurring mostly in the lower St. Louis Limestone. Large differences in Δ34SCAS-pyr are consistent with changing microbial pathways for cycling of sulfur in the sediment and water column as circulation in the seaway evolved from restricted to open. This study demonstrates that high-resolution curves for δ34SCAS are useful indicators of change in local paleooceanographic conditions and are valuable chemostratigraphic tools in carbonates units that lack biostratigraphic markers.
|Advisor:||Pratt, Lisa M.|
|Commitee:||Keith, Brian D., Ripley, Edward M.|
|School Location:||United States -- Indiana|
|Source:||MAI 51/04M(E), Masters Abstracts International|
|Keywords:||Carbonate-associated sulfates, Illinois Basin, Meramecian, St. Louis Limestone, Sulfide, Sulfur|
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