FOURTH ORDER EUSTATIC SIGNAL FROM CARBON-OXYGEN ISOTOPES, MISSISSIPPIAN SLOPE CARBONATES, VIRGINIA

Basinal muddy carbonates from the Appalachian Basin ramp slope, Virginia, were sampled at 1.5 meter (5 feet) intervals in order to construct C-O isotope curves for the Chesterian Appalachian Basin carbonate ramp. The downdip deeper water lime mudstones were not significantly reset isotopically by meteoric aquifer systems, which did not extend into the more rapidly subsiding proximal foreland. The δ¹⁸O and δ¹³C of the Appalachian deep water section do not show the same strong 3rd order trends common to those of the mid-continental United States and western Europe. The deeper water lime muds appear to preserve a eustatic or climatic signal. The dominant isotopic signal in the Appalachian slope section are at the 4th order sequence scale. Six out of nine of the sequences show a positive excursion of 1 to 2 per mil in δ¹⁸O near 4th order correlative conformities. A portion of this signal could reflect ice volume changes, although the presumed sea level changes of less than 50 m in the late Meramecian-early Chesterian would only account for 0.5 per mil, whereas the larger late Chesterian events might have generated 1 per mil change (Smith and Read 2000). In addition, cooling of shallow platform waters (from which the lime muds were derived) by 5°C or so could have generated 1 to 1.5 per mil oxygen isotope shifts, that along with the ice volume effect and increased salinity, could account for the magnitude of the excursions. The 4th order carbon excursions contrast to the long-term positive carbon isotope excursion common in the geological record, which are due to long-term sequestering of isotopically light organic carbon. The approximately 1 per mil increase in δ¹³C near 4th order correlative conformities could be due to increased productivity during glacial stages due to increased ocean circulation or to effects of shallowing, which would bring the sea floor into more positive δ¹³C waters.