Authigenic barite nodules and carbonate concretions in the Upper Devonian shale succession of western New York – a record of variable biogenic methane
Authigenic barite nodules associated with 13C-depleted calcium carbonate concretions and 34S-enriched pyrite within a 2.8-m-thick interval at the bottom of the Upper Devonian Hanover Shale of western New York provide evidence of sulfate reduction fueled by upward-diffusing biogenic methane, perhaps sourced within the Middle Devonian Marcellus Shale. Strong 34S enrichment and high δ34S/δ18O values of the barite nodules reflect (1) a high rate of kinetic fractionation induced by microbial sulfate reduction in a semi-enclosed system and (2) a slow seepage rate of the upward-diffusing Ba- and methane-bearing fluid. However, the association of authigenic calcium carbonate and barite in the same stratigraphic interval, especially the presence of barite overgrowths on carbonate concretions, is not entirely consistent with what is known of mineralization induced by anaerobic oxidation of methane focused at the sulfate-methane transition (SMT). The observed relations may reflect a reduced CH4/Ba2+ ratio of pore fluids ascending the sediment column induced by a diminished rate of methanogenesis. The tempered methane flux caused the SMT to descend the sediment column enabling barite to form within the same interval 13C-depleted calcium carbonate had most recently precipitated. Diminished methane flux may have been caused by burial-related passage of the organic-rich Marcellus below the biogenic methane window and its replacement in that depth interval by overlying organic-lean deposits of the Hamilton Group. Subsidence of the SMT would have increased the preservation potential of labile authigenic barite by enabling the compacting host shale to buffer the nodules. Thus, the barite would be protected from sulfate-deficient interstitial fluids produced when burial eventually carried the organic-rich Upper Devonian Rhinestreet Shale into the biogenic methane window inducing an upward shift of the SMT through the sediment column.
AAPG Datapages/Search and Discovery Article #90195 © 2014 Eastern Section Meeting, London, Ontario, Canada, September 27-30, 2014