CARBON ISOTOPE VARIABILITY IN KEROGEN FROM DEVONIAN-MISSISSIPPIAN MARINE BLACK SHALES

Understanding the carbon isotopic composition of Devonian- and Mississippian-age organic constituents is key to understanding cyclicity in organic-rich shales such as those of central Kentucky. Carbon isotopic compositions are commonly attributed to global changes in CO₂, to changing redox conditions, or to variations in organic productivity levels. However, it is important to fully understand the controlling parameters of carbon isotopic composition. Bulk chemical analyses typically do not permit definitive identification of amounts and types of marine and terrestrial constituents of organic-rich rocks, and this inability to distinguish the types and relative amounts of organic matter presents inherent problems when it comes to interpretation of isotopic data.

Preliminary data from black shales in Kentucky indicate that not only does the mix of terrestrial versus marine components affect the carbon isotopic composition, but the type of marine organic components may as well. For example, whole-rock analysis of samples that contain abundant well-preserved algal matter have d¹³C values of -27 to –29 per mil; whereas, samples with an abundance of bacterially degraded bituminite have d¹³C values of –29 to –30 per mil. Terrestrial organic material in these sediments has a d¹³C value of around -27 per mil. Thus, variations in carbon isotopic composition appear to be related to alginite/bituminite ratios in addition to terrestrial/marine ratios.

Organic petrology and density gradient centrifugation were used to assess carbon isotope variability in kerogen from the Sunbury, Cleveland, and Huron shales. Analysis of whole kerogen samples and kerogen separates include petrography, C-H-N, Rock Eval pyrolysis, and carbon isotopes. These analyses show the effects of variations in organic components on overall carbon isotope composition, and may provide insights for studies that utilize carbon isotopes in paleoenvironmental reconstructions.