Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

: Methane (CH 4 ) is an economic resource and a greenhouse gas, but its migration through rocks is not immediately associated with speleogenesis. Sulfuric-acid speleogenesis is a cave-forming mechanism that has produced a variety of economically important oil fields and aquifers, and is theorized to be related to the oxidation of CH 4 and hydrocarbons. Despite hypotheses that the oxidation of CH 4 may provide a basis for the generation of sulfides during sulfuric-acid speleogenesis, evidence from active systems has not yet been obtained. In this study, we address how CH 4 influences the development of sulfidic cave systems by sampling the CH 4 , H 2 S, and CO 2 concentrations, as well as d 13 C CH 4 , d 2 H CH 4 , and d 13 C CO 2 values, in a cave currently forming by sulfuric-acid speleogenesis, Cueva de Villa Luz. CH 4 , H 2 S, and CO 2 concentrations were highest directly above springs in the cave, showing that all three gases enter by means of the spring water. The d 13 C CH 4 and d 2 H CH 4 in the air of CVL ranged from (cid:2) 47.92 6 0.15 to (cid:2) 35.47 6 0.12 % (VPDB) and (cid:2) 117 to (cid:2) 83 % (VSMOW), respectively. Keeling plots suggest that CH 4 with d 13 C CH 4 ¼(cid:2) 24 6 3 % and d 2 H CH 4 ¼(cid:2) 40 6 50 % was outgassing from spring water. This stable-isotope signature does not fall within traditional published d 13 C CH 4 versus d 2 H CH 4 fields. Our data suggest that the CH 4 entering Cueva de Villa Luz is the remnant of a larger thermogenic CH 4

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