Isotopic Studies of Late Paleozoic Cyclical Sedimentary Deposits

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The goal of this project is to establish a high resolution isotope stratigraphy for the late Carboniferous using marine deposits from the south-central portion of the North American continent. Earlier work (Grossman et al., 1985; Adlis et al., 1987) on Virgilian (late Pennsylvanian) cyclic deposits in north- central Texas has shown that original isotopic composition is preserved in many calcitic shells of marine organisms. The availability of such material provides a unique opportunity to develop a near-continuous isotopic record for the latest Carboniferous and earliest Permian (a 15-20 million year time interval). Such a record, with proper paleontologic and sedimentologic control, can be used 1) to document secular variation in the 18O of seawater, 2) top determine changes in paleotemperature and paleodepth of the sea floor at the time of deposition, and 3) using paleodepth data, to determine the magnitude of late Carboniferous eustatic changes in sea level. Distinguishing the 18O record paleotemperature trends from changes in the 18O of seawater has always hampered isotopic studies. Detailed paleontologic and lithologic data to distinguish isotopic trends caused by local salinity change, or temperature change associated with changes in paleodepth, from isotopic trends derived from changes in seawater 18O associated with glaciation will be used. As a further check on depth effects we will determine a separate isotopic curve based on thick-shelled brachiopods from the zero depth horizon. This methodology will permit quantification of the eustatic sea level changes that produce Carboniferous cyclic deposits, but more importantly, will produce an isotope stratigraphy for testing and extending the emerging cycle stratigraphy of the Carboniferous and Permian, eventually leading to a global isotopic event stratigraphy.