DECIPHERING BULK d13CORG AND d15NORG IN DEVONIAN-MISSISSIPPIAN BLACK SHALES

Devonian-Mississippian black shales from the central Appalachian Basin contain an intricate mix of terrestrial and marine components, including vitrinite and inertinite (both terrestrial) and alginite and bituminite (both marine). Previous work indicated that d¹³C ratios for Devonian terrestrial organic matter (OM) are approximately -26‰ whereas marine OM has d¹³C ratios of around -30‰. Thus, bulk OM stable isotopic composition is a function of varying amounts of components that have significantly different isotopic signatures. Petrographic data for bulk kerogens isolated from these shales show that varying amounts of different marine OM components also influence carbon isotope composition: samples with low alginite/(alginite + bituminite) ratios have d¹³C values approaching -30.0‰ whereas samples with high alginite/(alginite + bituminite) ratios range between -29 and -28.5‰; abundant terrestrial OM can result in values around -27‰.

Relatively pure (> 95%) alginite and bituminite fractions were separated from the Cleveland Shale (Upper Devonian, Fammenian) and the Sunbury Shale (Lower Mississippian, Tournasian) using density gradient centrifugation (DGC). Pure alginite separated out in density fractions of < 1.1 g/ml, pure bituminite (the bulk of the OM in these samples) at 1.18-1.22 g/ml. Maceral composition was confirmed by optical microscopy. C/N ratios were relatively consistent and lower for the bituminite fractions (35 to 40); C/N ratios for the alginite fractions were higher and more variable than those for bituminite from the same sample (42 to 64). A reproducible pattern was observed for variations in both d¹³C and d¹⁵N across each density gradient with alginites having consistently heavier isotope ratios than bituminites. For example, alginite from the Sunbury typically had d¹³C values between -28 and -29‰, while bituminite had a d¹³C value of -30.5‰. A similar, though less pronounced pattern was observed for the d¹⁵N data. Values of d¹³C increased to -27 to -28.5‰ towards higher density fractions (1.3 to 1.4 g/ml) that contain a mixture of marine and terrestrial OM. These results further demonstrate isotopic differences between terrestrial versus marine macerals, but also clearly show the isotopic and geochemical heterogeneity of OM components believed to be exclusively of marine origin.