Cretaceous shales from the western interior of North America: Sulfur/carbon ratios and sulfur-isotope composition

Carbon and sulfur abundance and δ³⁴S of pyrite sulfur were studied in cores of selected Cretaceous marine shales from the western interior of North America. Sulfur/carbon ratios average 0.67, a value greater than that observed in recent marine sediments and much higher than global values calculated for the Cretaceous. Increased S/C ratios probably result from generally low levels of bioturbation and enhanced efficiency of sulfate reduction due to low oxygen levels in the Cretaceous seaway. Isotopic compositions of pyrite sulfur vary systematically with the level of oxygenation of the depositional environment and therefore with organic carbon abundance and type of organic matter. Samples with organic carbon in excess of 4 wt% contain disseminated pyrite that is extremely depleted in ³⁴S (mean δ³⁴S = −31‰); these samples are laminated clay shales that contain hydrogen-rich (type II) organic matter. Samples containing less than 1.5% organic carbon display relatively “heavy” but wide ranging δ³⁴S values (δ³⁴S = −34.6‰ to +16.8‰; mean δ³⁴S = −12.4‰); these samples are highly bioturbated and contain only type III, hydrogen-poor organic matter. Samples containing intermediate amounts of organic carbon contain pyrite with δ³⁴S values averaging −25.9‰ and contain mixed type II and type III organic matter. The higher organic carbon content and the preservation of hydrogen-rich organic matter generally correlate with slow sedimentation. Samples rich in organic carbon and containing isotopically “light” sulfide sulfur accumulated beneath anoxic and perhaps sulfidic bottom waters. Samples with intermediate organic matter content and intermediate sulfur isotopic compositions accumulated under mainly dysaerobic bottom waters. Samples with relatively low amounts of organic carbon and wide-ranging but less negative sulfur isotopic values were deposited beneath oxygenated bottom waters. Sulfur-isotope data are apparently a sensitive indicator of diagenetic or depositional facies of fine-grained Cretaceous rocks in the western interior.