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 N2-H2S composition in the lower-middle halite series to a CH4-H2 composition in the upper halite and potash series. Carbon isotope results for CH4 show a 13C enrichment up-sequence from typical biogenic values of -450/00 to -500/00 to extremely unusual 13C-enriched values as high as +210/00. The deltaD values for these 13C-enriched CH4 gases range from -2400/00 to -3770/00. 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 13C enrichment in the carbon in the residual brines due to preferential loss of 12CO2 to the atmosphere. The resulting CH4 generated in the sediments, as evaporation and precipitation advanced, recorded this 13C enrichment in the carbon reservoir. Therefore, the isotopic profile observed in this sequence today represents a primary feature with little evidence for postdepositional migration.