NO EVIDENCE FOR LARGE-SCALE THERMOGENIC METHANE RELEASE BY THE KAROO-FERRAR LARGE IGNEOUS PROVINCE

The onset of the Early Jurassic Toarcian oceanic anoxic event is associated with increased oceanic temperatures, high atmospheric CO₂ levels, and negative δ¹³C excursions in the ocean-atmosphere reservoirs; the latter having been attributed to either the release of CH₄ gas hydrates or thermogenic CH₄ gas associated with the intrusion of the Karoo-Ferrar large igneous province (LIP) into coals and organic-rich shales. ¹²C-enriched thermogenic CH₄ production associated with the Karoo-Ferrar LIP would result in the residual material being ¹²C-depleted near the intrusions. In this study, geochemical analyses (stable carbon isotopes, proximate and ultimate analyses, and vitrinite reflectance, R_o) are reported for two coal transects that are intruded by dykes in the Highveld Coalfield, Karoo Basin.

In the transects studied, volatile matter (VM) decreases from >35% to around 15% in one transect, with a slight increase (23%) directly adjacent to the dike; the second transect shows a less pronounced decrease (from >25% to around 16%). Accompanying the decrease in VM content is an increase in R_o from background levels of around 0.7% to over 4% adjacent to the dyke; converted to paleotemperature, R_o values indicate background temperatures of ~100°C increasing to > 300°C close to the contact (within ~0.5 m). Despite the change in both VM and R_o, there are no significant changes in δ¹³C, certainly not of the magnitude that would be expected associated with large-scale thermogenic CH₄ generation. These and other Gondwanan coals have low vitrinite and liptinite contents (components more prone to CH₄ generation), in part explaining the modest decreases in VM adjacent to the dikes. This, combined with the relatively narrow metamorphic aureole surrounding the intrusions and the likelihood that at least some of the volatiles generated by the intrusion were trapped in the coal as coal-bed CH₄ or condensed as pyrolytic carbon, suggests only limited CH₄ release. The absence of a significant shift towards heavier δ¹³C within the contact aureoles questions whether the major negative δ¹³C shift recorded in the Toarcian can be attributed to the release of isotopically light thermogenic CH₄ associated with the dolerite intrusions of the Karoo-Ferrar LIP.