CONODONT GEOCHEMICAL RECORDS OF LATE PALEOZOIC PALEOENVIRONMENTAL VARIABILITY IN MIDCONTINENT NORTH AMERICA

Over the past several years we have studied the geochemistry of biogenic (conodont) apatite--including our current investigation of oxygen isotope relationships--collected across the full range of repetitive facies exposed within Pennsylvanian outcrops in Indiana, Iowa, Missouri, and Kansas. Rare-earth element (REE) patterns in these Desmoinesian and Missourian conodonts, like those of coexisting authigenic phosphate, show middle REE enrichments that are independent of taxonomy, paleoredox, and paleogeography. We have also observed middle REE depletions in the host shales, suggesting secondary (diagenetic) repartitioning of REE between the fine-grained, siliciclastic host sediments and both biogenic and authigenic apatite. Such secondary signals compromise the paleoenvironmental utility of the REE data.

Despite the loss of original trace element records, previous workers have demonstrated the potential for retention of primary oxygen isotope data in conodont apatite. The results of a 2001 study of Pennsylvanian midcontinent conodonts by M. Horacek and others, although cursory in scope, suggest oxygen isotope relationships analogous to the well-established model for isotopic variability in Quaternary foraminifera. Specifically, trends in the conodont data were attributed to the waxing and waning of Gondwanan ice storage, with only minor temperature effects. Glacial-interglacial temperature change would have been dampened in the low-latitude depositional setting of the midcontinent deposits. Horacek et al. thus presented their preliminary isotope data as confirmation of glacio-eustatic interpretations of the Pennsylvanian cycles. In our present study, we are taking a more thorough look at oxygen isotope variability within and among conodont genera spanning both paleogeographic and stratigraphic gradients within Pennsylvanian strata of the Midcontinent. Our approach is designed to evaluate potential vital effects, the integrity of preserved primary signals, local and regional geographic variability, and possibly ancient water-column structure--all with an aim of resolving longstanding debates centered on global versus regional controls on the cyclic patterns of late Paleozoic deposition.