PALEOCEANOGRAPHIC CONTROLS ON THE DISTRIBUTION OF CONODONTS IN A PENNSYLVANIAN EPICONTINENTAL BLACK SHALE

A depth-stratification model is generally invoked to explain the spatial and temporal distribution of different conodont taxa in North American cyclothems of Pennsylvanian age. According to this model, Idiognathodus is indicative of warm, open-marine surface waters, while Idioprionodus occupied deeper, slightly cooler water masses. Gondolellid taxa occupied even deeper and cooler waters beneath the thermocline. Here, we present geochemical proxies for paleoceanographic conditions within an extensive epicontinental black shale (the early Missourian Stage Hushpuckney Shale) that challenges aspects of this model.

Neodymium isotopes of conodont apatite and sedimentary geochemistry (nitrogen isotopes and molybdenum and uranium enrichment factors) suggest that an interplay of open-ocean upwelling and continental runoff led to distinct spatial and secular variations in watermass properties within the shallow Late Pennsylvanian Midcontinent Sea of North America. In particular, the intensity of terrestrial runoff influenced the flux of bulk organic matter to the sediment. Concurrent changes in watermass redox conditions not only drove authigenic enrichment of redox-sensitive trace elements across the basin, but also had a strong effect on the spatial distribution of various conodont taxa. The strongest influence on the distribution of conodont taxa appears not to have been water depth per se but, rather, watermass chemistry in the form of nutrients, turbidity, and/or dissolved oxygen levels.