Large-scale carbon isotope fractionation in evaporites and the generation of extremely 13C-enriched methane

Abstract

Petrographic, fluid-inclusion, geochemical, and gas stable isotope data are reported here for a Permian Zechstein evaporite sequence. This deposit is a geochemically unaltered sequence. Bromine concentrations show a continuous evaporation profile with little postdepositional alteration in halite chemistry. Bacterial fermentation gases, identified in primary inclusions, change from an N 2 -H 2 S composition in the lower-middle halite series to a CH 4 -H 2 composition in the upper halite and potash series. Carbon isotope results for CH 4 show a 1 3 C enrichment up-sequence from typical biogenic values of -45‰ to -50‰ to extremely unusual 1 3 C-enriched values as high as +21‰. The δD values for these 1 3 C-enriched CH 4 gases range from -240‰ to -377‰. A model is proposed for the formation of the CH 4 gases whereby the dominant isotopic fractionation process controlling the system was evaporation of the brines. This generated a progressive 1 3 C enrichment in the carbon in the residual brines due to preferential loss of 1 2 CO 2 to the atmosphere. The resulting CH 4 generated in the sediments, as evaporation and precipitation advanced, recorded this 1 3 C enrichment in the carbon reservoir. Therefore, the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration.