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

Our work over the past several years has focused on spatial geochemical variability within the laterally continuous cyclic Pennsylvanian shales of the Midcontinent and Appalachian Basin. Although light and uniform sulfur isotope ratios suggest pervasive euxinic deposition of our Midcontinent (Missourian) shales, regional gradients can be inferred for the efficiency of Mo scavenging and for rates of siliciclastic sedimentation expressed in spatially varying Fe/Al ratios. Black shales in Iowa show Mo enrichment roughly five times greater than that observed in coeval shales in Oklahoma. Trends for Fe are opposite those observed for Mo. In Oklahoma, Fe/Al ratios in black shales are up to five times larger than the continental ratio of 0.5 observed in the over- and underlying oxic shales and in the coeval black shales in Iowa. Enrichments in Fe often result from scavenging in a euxinic water column during syngenetic pyrite formation. Despite complications linked to possible hydrothermal contributions and epigenetic mineralization, observed Fe and Mo trends are reasonably interpreted in terms of early mineralization controlled by independently predicted regional patterns in (1) the organic reservoir, including relative inputs of terrestrial versus marine organic matter, and (2) rates of siliciclastic input. We have also explored possible variations in the intensity and persistence of water-column euxinicity.

To the east, Pennsylvanian black shales in the Appalachian Basin show strong correspondence between faunal records of low-salinity deposition and predicted high ratios of organic carbon to pyrite sulfur. While encouraging, exceptions exist. Our current model favors 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 formed under primary marine conditions, including 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.