Paper No. 8
Presentation Time: 10:10 AM
CONSTRAINING THE REGIONAL PALEOENVIRONMENTAL VARIABILITY OF PENNSYLVANIAN BLACK SHALES, EAST-CENTRAL USA
Our work over the past several years has focused on spatial geochemical variability within the laterally continuous cyclic shales of the Midcontinent. Although light and uniform sulfur isotope ratios argue for pervasive euxinic deposition, regional gradients are present in the extent of Mo scavenging and in rates of siliciclastic sedimentation expressed in varying Fe/Al ratios. In proximal shales in Iowa, Mo/Al ratios in excess of 100 (ppm/wt.%) contrast values reaching only 20 in the more-distal deposits of Oklahoma, despite strongly sulfidic conditions at both localities. These trends--likely tracking relative inputs of terrestrial organic matter--are opposite of those observed for Fe. In Oklahoma, Fe/Al ratios are a factor of five greater than the continental ratio of 0.5 observed in the over- and underlying oxic shales. Enrichments in Fe resulted from scavenging in the euxinic water column during syngenetic pyrite formation, while the syngenetic flux in Iowa was swamped by siliciclastic input--yielding Fe/Al ratios only slightly above continental (riverine) contributions. Rare-earth element (REE) patterns in conodont apatite, like coexisting authigenic phosphate, show middle REE enrichments that are independent of taxonomy, paleoredox and paleogeography. These secondary (diagenetic) signals compromise the paleoenvironmental utility of REE data in the cyclothemic deposits.
To the east, 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, the C/S proxy in these settings is also vulnerable to weathering during sea-level lowstands and must be used with caution. In contrast to oxidative loss of sulfur, pyrite-S enrichments in oxic facies of the Midcontinent mark downward diffusion of hydrogen sulfide following the onset of euxinic deposition. These overprints, with diagnostic S isotope and concentration signatures, are analogous to those observed on glacial-interglacial timescales in Quaternary sediments of the Cariaco Basin and also highlight the care needed for effective use of C/S paleoenvironmental methodologies.