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Paper No. 6
Presentation Time: 2:50 PM

SULFUR ISOTOPIC CURVES FOR CARBONATE-ASSOCIATED SULFATE REFINE STRATIGRAPHIC INTERPRETATION OF THE ST LOUIS LIMESTONE IN THE ILLINOIS BASIN, SOUTH-CENTRAL INDIANA, USA


GRAY III, Walter E.1, KEITH, Brian D.2, MARKS, Jade A.3 and PRATT, Lisa M.3, (1)Indiana Geological Survey, Indiana University, 611 N Walnut Grove Avenue, Bloomington, IN 47405, (2)Indiana Geological Survey, Indiana Univ, 611 North Walnut Grove, Bloomington, IN 47405, (3)Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, grayw@indiana.edu

Carbonate-associated sulfate (CAS) has been used recently to generate high-resolution, stratigraphic curves for changes in the isotopic composition of seawater sulfate through the Phanerozoic, permitting comparison with temporal records for carbon, oxygen, and strontium isotopes. Here we present sulfur isotopic data (δ34S) for 117 samples of CAS and 104 samples of pyrite (pyr) from 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 (12.7‰) in lower St. Louis carbonates is about 2 per mil lower (enriched in 32S) than the global average for units of this age. The upper St. Louis is inferred to be an openly circulating seaway connected to the south with the ocean. The mean value of δ34SCAS (14.0‰) matches global seawater sulfate during the Middle Mississippian. Values of δ34Spyr. generally show an upward increase through the St Louis with strong variation and distinct positive shifts present in association with the interval containing beds of rugose coral. The difference between the sulfur isotopic composition of CAS and pyr (DCAS-pyr ) ranges from 13 to 50 per mil with higher DCAS-pyr values mostly in the lower St. Louis Limestone. Large differences in DCAS-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 paleoceanographic conditions and are valuable chemostratigraphic tools in carbonates lacking biostratigraphic markers.

Session No. 191

T4. Geochemical Proxies for Ancient Ocean Chemistry: Implications for Links Between Ocean Chemistry, Plate Tectonics, Sea Level, and Climate throughout the Late Precambrian and Phanerozoic
Tuesday, 6 November 2012: 1:30 PM-5:30 PM
202AB (Charlotte Convention Center)
Geological Society of America Abstracts with Programs. Vol. 44, No. 7, p.459
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