Circulation patterns in the Late Pennsylvanian North American Midcontinent Sea inferred from spatial gradients in sediment chemistry and mineralogy

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© 2018 Elsevier B.V. The North American Midcontinent Sea (NAMS) reached its maximum extent during glacio-eustatic highstands of the Late Paleozoic Ice Age, during which organic-rich “core shales” accumulated under a stratified water column with anoxic deep waters. The core shales of cyclothems are composed of a lower organic-rich laminated black shale and an upper bioturbated gray shale. The present study investigates circulation patterns within the NAMS by examining secular variations and spatial gradients in the concentrations and ratios of major elements, trace elements, and organic carbon, as well as carbon and nitrogen isotopes, within one cyclothemic core shale: the Heebner Shale. The Heebner Shale represents the core shale member of the Upper Pennsylvanian (lowermost Gzhelian; Virgilian) Oread Limestone Formation in Kansas and Iowa, and it is correlative with the Teutopolis Shale of the Mattoon Formation in Illinois. The Heebner was sampled at high stratigraphic resolution at five sites on the northern Midcontinent Shelf and two sites in the Illinois Basin. Clay and silt fraction proxies show influences from multiple distinct terrestrial sources. The paleosalinity proxy Sr/Ba in combination with spatial patterns of clay and silt proxies implies large-scale counterclockwise (cyclonic) rotation of the NAMS watermass, which was possibly driven by the hydrodynamics of upwelling on the southern Midcontinent Shelf margin and/or riverine discharge into the eastern NAMS. Spatial patterns of δ13C and δ15N are consistent with regionally variable productivity and redox conditions. Differences in the magnitude of a positive δ15N excursion among the study sites record spatially variable water-column denitrification rates. Enrichment of trace metals on the northern Midcontinent Shelf was a product of intensified water-column recycling within an estuarine saltwater wedge. Trace-metal enrichments and Fe/Al ratios are consistent with moderate watermass restriction throughout the NAMS. The results of the present study permit major refinement of the existing superestuarine circulation model for the NAMS [Algeo, T.J., et al., 2008, Modern and ancient epeiric seas and the super-estuarine circulation model of marine anoxia. In: B. Pratt and C. Holmden (eds.), Dynamics of Epeiric Seas, Geol. Assoc. Canada Spec. Publ. 48, pp. 7–38].

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Palaeogeography, Palaeoclimatology, Palaeoecology

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