MOLECULAR EVIDENCE OF PHOTIC-ZONE EUXINIC CONDITIONS DURING DEPOSITION OF DEVONIAN BLACK SHALES OF THE APPALACHIAN BASIN

The high organic carbon content of the Devonian black shales of the Appalachian Basin have been attributed traditionally to enhanced preservation of organic matter in a primarily anoxic paleoenvironment. However, recent work has provided evidence for intermittently anoxic to dysoxic conditions. In this study, we investigate the molecular nature of the organic matter to further characterize the depositional environment of the Devonian black shales of the Appalachian Basin and to infer a mechanism of preservation of organic carbon. Both the soluble and insoluble organic matter (kerogen) fractions were analyzed using gas chromatography/mass spectrometry (GC/MS) and pyrolysis-GC/MS (using a platinum coil at 615°C for 20 seconds), respectively. The black shales analyzed included Givetian (Portwood Member/Harg facies) and Famennian (Huron Member, Three Lick Bed) horizons of the New Albany Shale collected from a drill core (D6) located in east-central Kentucky. The black shales have organic carbon contents ranging from 6.3% to 17.6% by weight and vitrinite reflectances around 0.5% placing the level of organic matter maturation within the oil window. The organic matter is predominantly marine (alginite and bituminite) with variable terrestrial (vitrinite and inertinite) content. The aromatic fraction of the soluble organic matter is rich in aryl-isoprenoids and contains isorenieratane. These organic compounds and isorenieratane in particular are diagenetic products of isorenieratene, a diaromatic carotenoid derived from photosynthetic sulfur bacteria. These bacteria are strict anaerobes requiring both light and sulfide-rich (euxinic) water columns. The kerogen pyrolyzates contain abundant 1,2,3,4-tetramethylbenzene, probably derived from bound diaromatic carotenoids. The abundance of organic sulfur in the pyrolyzates is very low, suggesting that sulfurization was not the primary mechanism leading to kerogen-bound organic compounds. These results indicate that during periods of anoxia in the Devonian Appalachian Basin, sulfide accumulated in the water column and reached the photic zone.