USING DENSITY GRADIENT CENTRIFUGATION TO ASSESS STABLE ISOTOPE VARIABILITY IN DEVONIAN-MISSISSIPPIAN MARINE BLACK SHALES

Carbon isotopic composition of bulk kerogen from Devonian-Mississippian shales of the central Appalachian Basin may reflect, among other things, variations in organic matter (OM) source or variations in organic productivity levels. To interpret isotopic trends, it is important to fully understand the controls on carbon isotopic composition, particularly variations in the types and relative amounts of OM present and the isotopic composition of these individual components.

These shales are extremely high in organic carbon (often as much as 10-20%). However, the kerogen is so intimately associated with the inorganic matrix of the shale that the isolation of representative and unaltered organic matter presents several problems. Density-gradient centrifugation (DGC) is a biological separation process that has been applied successfully to the separation of coal and shale macerals. DGC was performed on a suite of samples from a single core from central Kentucky (D6) in order to assess carbon isotope variability in kerogen concentrates. Intervals analyzed included the Huron and Cleveland Members of the New Albany Shale (Upper Devonian), and the Sunbury Shale (Lower Mississippian).

OM was concentrated using an HCl-HF-HCl protocol, after which it was micronized in an inert atmosphere (N₂), and separated using a CsCl density gradient. Previous studies have isolated bituminite in the density range of 1.18-1.23 g/mL and alginite in the range of 1.00-1.11 g/mL. Examination of DGC profiles for our samples over those density ranges, followed by petrographic analysis, indicated that we were able to separate out hundreds of milligrams of very pure bituminite. The less dense alginite from the same bulk kerogens occurred in only small amounts, but by combining several adjacent density fractions, we were able to obtain 25 mg or so of relatively pure (>90%) alginite. Subsequent analysis of whole-rock samples and kerogen separates included organic petrography, C-H-N, Rock-Eval pyrolysis, and carbon isotopes. Our results demonstrate the influence of varying amounts and types of OM on overall carbon isotope composition, and may provide insights for studies that utilize carbon isotopes in paleoenvironmental reconstructions.