Conditions of sedimentation have affected isotopes in coarse-grained sulfides from Pennsylvanian shales. For example, the Mecca Shale Member (Desmoinesian Series) contains highly variable values for δ34S (≈;-10‰ ; σ, standard deviation, ≈9) near the ancient shoreline of the Illinois basin, whereas equivalent strata in Missouri and Kansas, farther from the ancient shoreline, have more negative and less variable values (-14.5‰; σ ≈ 5). Sulfides from younger beds in the Missourian Series contain still lighter and less variable sulfur (δ34S ≈ -27‰; σ ≈ 3.1). Because low δ34S values are typical of sediments deposited in anoxic deep waters, these results suggest that Missourian shales formed slowly in deep waters. The Desmoinesian shales may have been deposited more rapidly in shallower seas. Alternatively, secular shifts for δ34S in sea-water SO4 may have been stronger than previously realized.

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