Vigorous Anaerobic Methane Oxidation in the Upper Devonian Succession, Western New York; Possible Evidence for Devonian Gas Hydrates
Lash, Gary; Blood, Randy
Fine-grained organic-rich rocks that have accumulated under anoxic conditions can begin generating methane soon after deposition, sometimes at burial depths of several meters or so, as a consequence of microbially mediated degradation of organic matter. Biogenic methane migrates upward through the zone of methanogenesis to the sulfate-methane transition zone (SMTZ) where anaerobic methane oxidation (AMO) depletes interstitial water of seawater sulfate and methane to produce 13C-depleted biocarbonate and 34S-enriched sulfide. Methane flux is a principal control on the depth below the seafloor of the SMTZ. Specifically, high methane flux rates, such as those described from modern methane hydrates, result in the establishment of shallow (< 1 m) SMTZs. The Upper Devonian Rhinestreet-Angola-Pipe Creek-Hanover shale succession of western New York provides a record of non-steady state burial and related AMO. Evidence of the latter includes thin (10-30 cm) intervals moderately enriched in Fe and S, 13C-depleted authigenic concretionary carbonate (δ13C = -11 to -14‰ PDV), 34S-enriched pyrite (δ34S = 24 - 25% CDV) associated with the carbonate, and an interval of 34S-enriched (δ34S = 43% CDV) barite nodules. Differential compaction of host shale around concretions as well as the preservation of depositional clay grain microfabrics within concretions suggests that the latter formed as a consequence of passive carbonate precipitation at subseafloor depths of ~ 1 m. The organic-rich Rhinestreet Shale, then, appears to have been an especially active bioreactor that maintained a shallow SMTZ throughout the burial history of these deposits. Moreover, multiple concretion intervals from the lower Rhinestreet Shale through the overlying organic-lean Angola Shale reflect episodes of AMO induced by episodic reductions in sedimentation rate. Textural aspects of this succession, isotope data, and seemingly shallow SMTZ depths are consistent with a robust methane source, perhaps even a gas hydrate. Indeed, recent modeling suggests that gas hydrates could have formed beneath a warm, relatively shallow Middle to Late Devonian sea at burial depths of less than 300 m.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013