Biogeochemical interactions among carbon, sulfur, iron, selenium, and trace metals in upper cretaceous chalks and shales: Inferences regarding paleoenvironmental conditions and implications for modern environmental chemistry

The Upper Cretaceous Smoky Hill Member of the Niobrara Formation and the Sharon Springs Member of the Pierre Shale in South Dakota and Wyoming are organic-rich chalks and shales characterized by interbedded bentonitic ash and enrichment of many trace elements. Correlations among selected major elements (Fe, Al, Si, Ti, Mn, Ca, C, S) and trace elements (Se, As, Mo, V) are interpreted in the context of depositional and early diagenetic redox conditions. Concentration and isotopic signature of four sedimentary sulfur species (sulfate, acid-volatile, pyrite, and organic) were determined for each lithofacies. Results suggest that early diagenetic reduction of sulfate and other oxidized species proceeded via microbial reduction under reducing conditions in sediment pore water. Reactive marine organic matter was abundant in both sediments, but siliciclastic muds contained more organic debris derived from terrestrial vegetation than calcareous mud. Trace metals are associated with the organic fraction in shales, while sulfide minerals represent a significant phase for their accumulation in chalk. Variations in trace metal partitioning appear to be related to differences in organic matter reactivity, sedimentation rate, or mineralogy.; The Niobrara Formation and the Pierre Shale are notorious for Se contamination in the Great Plains. Soils developed from Pierre and Niobrara bedrock often support seleniferous vegetation that is toxic to herbivorous animals. The solubility and toxicity of Se are controlled by partitioning among various organic and inorganic species and by valence state. A sequential extraction protocol was devised and utilized in conjunction with analysis by Hydride Generation-Atomic Absorption Spectroscopy to quantify the forms and oxidation states of Se in these units. Kerogen-associated selenide is the predominant form of Se in these rocks. Chalks contain larger concentrations of selenide associated with pyrite than shales, in which base-soluble humic forms are prevalent. Comparison between core and outcrop samples suggest that Se oxidized by weathering is more readily retained by adsorption in shales and bentonites than in chalks. Differences in mineralogy and the nature of organic matter that characterize these lithofacies are significant to the natural release of Se to the environment, and to the remediation of Se released during agricultural irrigation.