CARBON-OXYGEN ISOTOPE SIGNAL OF MISSISSIPPIAN SLOPE CARBONATES OF VIRGINIA: A COMPLEX RESPONSE TO CLIMATE-DRIVEN FOURTH-ORDER GLACIO-EUSTASY

Mississippian slope carbonates in Virginia, deposited on the margin of a rapidly subsiding foreland basin, were not significantly reset isotopically.  C and O isotopic ratios of these muds tend to covary at the scale of the 4^th-order sequences that dominate the platform succession.  The d¹³C values of the slope muds range from +0.5 to +4‰ PDB and lie in the range of least-altered contemporaneous brachiopods from North America, indicating little resetting of carbon isotopes by diagenesis.  There is ~1 to 2‰ decrease in d¹³C in the maximum flooding zone relative to the lowstand, followed by a positive shift upward in the highstand before becoming more negative toward 4^th-order correlative conformities.  The ¹³C-enriched values in d¹³C profiles could be due to increased productivity during glacial stages. The d¹⁸O values of muds range from about -2.2 to  -4.6‰ PDB, falling within and beyond the lighter part of the range of contemporaneous North American brachiopods. These values likely have been reset by burial diagenesis, and perhaps by some meteoric diagenesis just beneath correlative conformities.  The sequences show negative excursions of 1 to 2‰ in d¹⁸O in the maximum flooding zone relative to the lowstand, and then become more positive upward in the highstand, then decrease toward the correlative conformities.  Some of the preserved signal could reflect ice-volume changes, although this would account for only 0.5 to 1‰ change.  The remaining 0.5 to 1.5‰ of oxygen isotope excursions could be accounted for by cooling of ramp-top waters and small increases in salinity due to increased aridity in the Appalachian Basin, with the opposite effect occurring during high sea levels.  The d¹⁸O and d¹³C of the Appalachian ramp slope facies lack the strong 3^rd-order trends of the mid-continental United States and Europe.  The remnant 4^th-order C and O isotope signal is similar in structure to Cenozoic deep-sea isotope profiles that are due in large part to 4^th-order glacio-eustasy.  This would be compatible with onset of Late Paleozoic glaciation of Gondwana.