DECIPHERING BULK d13CORG AND d15NORG IN DEVONIAN-MISSISSIPPIAN BLACK SHALES
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.