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

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-H2S composition in the lower-middle halite series to a CH4-H-2 composition in the upper halite and potash series. Carbon isotope results for CH4 show a C-13 enrichment up-sequence from typical biogenic values of -45parts per thousand to -50parts per thousand to extremely unusual C-13-enriched values as high as +21parts per thousand. The deltaD values for these C-13-enriched CH4 gases range from -240parts per thousand to -377parts per thousand. A model is proposed for the formation of the CH4 gases whereby the dominant isotopic fractionation process controlling the system was evaporation of the brines. This generated a progressive C-13 enrichment in the carbon in the residual brines due to preferential loss of (CO2)-C-12 to the atmosphere. The resulting CH4 generated in the sediments, as evaporation and precipitation advanced, recorded this C-13 enrichment in the carbon reservoir. Therefore, the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration.