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δ13C and δ34S were determined for 47 coal samples from the Williams Fork Formation—31 samples from the Wadge coal bed and 16 samples from the Lennox coal bed. δ13C ranges from −23.4 to −27.2‰). Organic sulfur δ34S ranges from +5.3 to +13.5‰ for the Wadge bed and from +13.7 to +20.1‰ for the Lennox bed. The organic sulfur content of the coal samples ranges from 0.23 to 0.71 percent for the Wadge bed and from 0.65 to 2.72 percent for the Lennox coal bed. The ash content of both beds is low, averaging 8.5 percent for the Wadge bed and 6.4 percent for the Lennox bed. The carbon isotopic homogeneity of the Wadge and Lennox beds indicates that the plants in each mire were similar with respect to the carbon fixation processes and carbon source.

Previous sulfur isotopic studies of coral and the coal-forming processes have shown that δ34S is determined by the aquatic composition of sulfur in the peat-forming environment. In a freshwater mire, the δ34S of aquatic sulfate fluctuates about a mean of 5 ± 3‰, whereas in the marine environment, δ34S of aquatic sulfate clusters around +20‰. In peat-forming mire that is inundated by marine water, much of the sulfate is reduced by sulfate-reducing bacteria. As a result of this microbiologic activity, the active sulfur, which is assimilated into the decaying organic substrate, is depleted in 34S. However, if the sulfate-reducing bacteria are absent, the peat possess only sulfur with the isotopic composition of the growth environment. In a coastal mire, this sulfur could be similar to that of the marine water.

The low sulfur content and the isotopic composition in the lower part of the Wadge bed are consistent with sulfur assimilation in a freshwater growth environment. The increasing sulfur content and the increasing abundance of heavier isotopes toward the top of the bed suggest that, during the later stage of development of the coal mire, the peat-forming plants were increasingly influenced by a marine source and that marine sulfur was assimilated. The moderately high sulfur content and the 34S enrichment in the Lennox coal samples suggest that this mire was clearly influenced by a marine source.

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