| Oxygen isotopic analysis of Late Carboniferous age conodonts were conducted in order to assess the potential preservation of isotopic trends that may speak to the ongoing debate over depositional controls of the repetitive sedimentary records seen throughout the midcontinent, United States and in similar deposits world-wide. Within three midcontinent cycles, the Upper Ft. Scott (Desmoinesian), Swope, and Dennis (Missourian), isotopic excursions of 1.7‰, 0.6‰, and 0.8‰ were seen within the black shale member of each cycle, respectively. These black shales have been interpreted as both indicators of deep water, anoxic conditions associated with glacial minima and shallow, near shore deposition. As the observed shifts are of a similar magnitude recorded by Cenozoic marine microfossils on a glacial/interglacial timescale and because they occur in phase with lithologic change attributed to waxing and waning of Gondwanan ice sheets, they suggest that variability in Late Carboniferous ice volume is responsible for the isotopic trends. Within a fourth cycle, the Pawnee (Desmoinesian), no isotopic excursion is observed, which is expected given the local, nearshore depositional controls of this section.{09}Due to the condensed nature of the black shale facies with respect to geologic time the isotopic records support models of rapid ice sheet collapse followed by slow glacial growth. Also, similarities between the isotopic shifts linked to Quaternary and Pennsylvanian glaciation suggest analogous styles and magnitudes of paleoclimatic control. |
