SULFUR AND TRACE METAL GEOCHEMICAL RECORDS OF LATE PALEOZOIC PALEOENVIRONMENTAL VARIABILITY IN NORTH AMERICAN BLACK SHALES

Our work is focused on spatial geochemical variability within the laterally continuous cyclic Pennsylvanian shales of the Midcontinent and Appalachian Basin. Black shales in the Appalachian Basin generally show high ratios of organic carbon to pyrite sulfur, as would be expected in lacustrine settings. These ratios correlate well with faunal records of low-salinity, nonmarine deposition. While encouraging, exceptions exists, and high sulfur samples were observed. Our current model favors downward diffusive overprints by marine sulfate following a depositional transition from nonmarine to marine conditions. Such overprints have diagnostic sulfur isotope character, which can be distinguished readily from pyrite formation under primary marine conditions or secondary overprints associated with transitions from oxic marine to euxinic marine conditions, as we observe in midcontinent transgressive gray shales. These two models for sulfur overprinting are directly analogous to the contrasting S isotope patterns recorded across the most recent glacial-interglacial transition in the Black Sea and Cariaco Basin. In the Black Sea, sulfate reduction in nonmarine layers are driven by anaerobic oxidation of methane diffusing from below and sulfate diffusing from above. This model will be tested for the overprinted Pennsylvanian lacustrine sediments through carbon isotope analysis of dispersed calcium carbonate.

In contrast to the anomalously heavy pyrite sulfur isotope values in the shales in the Appalachian Basin, with an average value of -3 per mil (n=26), Midcontinent (Missourian) black shales show light and uniform sulfur isotope ratios that suggest pervasive euxinic marine deposition. Metal distributions in the midcontinent shales, particularly Fe and Mo, are readily explained in light of models for euxinic deposition across possible spatial gradients in siliciclastic sedimentation, organic input and hydrothermal contributions, as well as temporal gradients in paleoredox. The contrasting environments of the Appalachian Basin are allowing us to evaluate mechanisms of metal mineralization, enrichment and remobilization in nonmarine settings. More importantly, the marine overprints should illuminate pathways of metal behavior within the context of a purely terrestrial organic reservoir.