CARBON ISOTOPES AS INDICATORS OF PALEO-PRODUCTIVITY CHANGES DURING THE MARCELLUS SHALE DEPOSITION

Measurement of the carbon isotopic composition of inorganic carbon (δ¹³C_carb) and organic carbon (δ¹³C_carb) in about 100 samples of the Marcellus Shale in Eastern Gas Shales Project cores WV6 and WV7 suggest that carbon isotopes can be used to understand changes in paleo-productivity during shale deposition. In both cores, we found higher total organic carbon (TOC) content and δ¹³C_carb values at the base of the Marcellus Shale. We hypothesize that these relatively higher δ¹³C_carb values are the result of rapid burial of large amounts of organic carbon from high algal paleo-productivity. The higher photosynthetic demand was expected to preferentially remove lighter ¹²C, leaving the dissolved inorganic carbon pool enriched in ¹³C and resulting in an increase in δ¹³C_carb in the precipitating carbonate. We found instead that δ¹³C_org values decrease (average = 1‰) through the basal organic-rich intervals in both WV6 and WV7. This has been interpreted as higher isotopic fractionation because of the tenfold higher atmospheric CO₂ concentration during the Devonian time period. We propose that higher atmospheric CO₂ could also have resulted in high paleo-productivity and larger carbon isotopic fractionation between dissolved inorganic carbon and produced organic carbon at the base of Marcellus Shale. Our preliminary results indicate the carbon isotopes can be used to better understand the paleo-productivity changes during the shale deposition.