MICROBIAL-GENERATED METHANE OUTGASSING FROM LAKE POWELL SEDIMENTS, GLEN CANYON NATIONAL RECREATION AREA, UTAH

Sedimentary structures linked to microbial generation and release of methane (CH₄) in terrestrial settings is an important emerging topic as CH₄ has a compounding impact on local and global climate change. A wide spectrum of non-seismically, gas-generated landforms and associated sedimentary structures have been recognized on the Lake Powell delta at Hite, Utah. The effort to understand the origin and development of these features is in its early stages. This work explores the chemical composition of effusive gases associated with these features.

Gas samples from 25 vents, or “mud volcanoes” were collected over spans of one to three days during the Lake Powell high water in July 2015 and low-water level in May 2017. Fifty of these samples were analyzed via gas chromatography (GC) for bulk composition, eleven of which underwent isotope ratio mass spectroscopy (IRMS) for carbon and hydrogen isotope content of methane. The GC analysis revealed three major components, air (unseparated nitrogen and oxygen), methane, and carbon dioxide that vary in abundance. The average concentration of methane in the effusive gases of was 74.06 ± 13.57% with a range of 25-93%. Average air concentration was 23.35 ± 13.76% with a range of 5-75%. Average carbon dioxide concentration was 2.99 ± 3.66% with a range of 0 to 24%. Gas composition from individual vents varied over the three-day sampling timeframe in 2016 including methane decreases of up to 66% and increases of up to 13%.

A plot of the 𝛿¹³C methane (-59 to -72 per mil) vs 𝛿D methane (-247 to -307 per mil) values indicates that the methane was produced by a combination of carbonate reducing and methyl-type fermenting methanogenic pathways by microorganisms. The presence of the two separate pathways may be due to mixing of two different methane pools during vertical gas migration caused by depressurizing of pore gasses after lake level lowering. Bacterial methane oxidation may have been the cause in a linear trending of the data on the 𝛿¹³C methane vs 𝛿D methane plot. Chemical data indicate a shallow source for the methane that was integral to the formation of the gas-generated features on the Lake Powell delta.