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

Mississippian ramp slope muds, Virginia were not significantly reset isotopically by meteoric aquifer systems, because of their location in a rapidly subsiding foreland basin. The carbon and oxygen isotopes tend to covary with negative excursions toward 4th-order maximum floods and positive excursions toward correlative conformities beneath shallow water tongues in the slope carbonates; superimposed on this is a smaller magnitude, higher frequency fluctuation. There is an approximately 1 per mil increase in δ¹³C near 4th-order correlative conformities of high frequency sequences. This is opposite to what would be expected from meteoric diagenesis, and could be due to increased productivity during glacial stages due to increased ocean circulation, or to shallowing, which brought the ramp top into more positive δ¹³C waters.

Many of the high frequency sequences show a positive excursion of 1 to 2 per mil in δ¹⁸O near correlative conformities. This is opposite to that expected for meteoric diagenesis, but resetting of the oxygen isotopes to lighter values by burial fluids cannot be discounted, especially near grainy units above correlative conformities. Some of the preserved signal could reflect ice volume changes, although the presumed sea-level changes based on the stratigraphy would only account for 0.5 to 1 per mil change. Cooling/warming of ramp top waters and changes of a few parts per thousand in salinity, along with the ice volume effect, could account for much of the oxygen isotope excursions.