Contact Metamorphosed Coal and Global Warming: No Evidence for the Large-Scale Release of 13C-Depleted Methane
L. E. Yoksoulian1, S. M. Rimmer1, H. D. Rowe2, and D. R. Gröcke3
1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506
2Department of Geology, University of Texas at Arlington, Arlington, TX 76019
3Department of Earth Sciences, University of Durham, Durham, DH1 3LE, UK
Growing concern regarding the accelerated rate of carbon input to the atmosphere has prompted the search for comparable events throughout Earth's history. There has been considerable effort to determine the cause of such events, represented as δ13C excursions in the sedimentary record, and their subsequent effect on the Earth. One hypothesis put forth to account for these events is the large-scale release of 13C-depleted thermogenic methane by the intrusion of igneous rocks into organic-rich rocks (Svenson et al., 2004; McElwain et al., 2005; Svensen et al., 2007). Additionally, the study of McElwain et al., (2005) implies that the volume of thermogenic gas released during an intrusion event depends primarily on the amount of coal present and the volume of igneous material affecting that coal. Methane derived from coal is 25‰ lighter than the coal itself (Sackett, 1978) and our study asserts that if these models are to be applied to the release of 13C-depleted thermogenic methane from intruded coals, 13C-enriched coal should be observed adjacent to the intrusions.
Using data from various intruded coals, we present evidence to challenge the hypothesis of McElwain et al. (2005) and rather emphasize the importance other variables, such as maceral content, in-seam retention of volatiles, and rank at time of intrusion. We find that an intruded Permian lignite to sub-bituminous rank coal in the Karoo Basin, South Africa, referred to in the McElwain et al. (2005) study, shows little, if any, 13C enrichment as the intrusion is approached, a total of 0.4‰. The Karoo Basin coals also show a distinct correlation between inertinite composition and δ13C values, indicating a relationship between maceral composition and thermogenic gas release. This lack of enrichment is also seen in an intruded Pennsylvanian bituminous rank coal in southern Illinois, with a shift of only 0.5‰, hardly what might be expected if the intrusion had resulted in a significant release of 13C-depleted gases.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas