In 1963, the Glen Canyon Dam, in Hite Utah was completed, creating the Lake Powell reservoir along the Colorado River. The water levels of Lake Powell peaked in 1983 and have declined since, releasing over-pressure on the underlying sediment. This release in over-pressure created mud volcanoes, structures along the shoreline made of cavities that allow fluid and gases to rise to the surface and escape. Green house gases including methane are released from these structures, and to better understand how development of natural wetlands can result in unintended increased levels of greenhouse gas emissions, we asked 1) how much of each gas is generated or and whether the amount of each gas is changing through time and 2) how are these gases forming in the subsurface? We first measured the amounts of carbon dioxide (CO2), methane (CH4), and air (N) in volcano gas samples collected in 2014, 2015, and 2016. We found that from 2014 through 2016, methane levels from these volcanoes fluctuated significantly. In 2016, we looked at the amounts of carbon and hydrogen isotopes in the methane, which told us the gas is generated from microorganisms feeding on organic matter and is released during water-level fluctuations. We looked at mud volcanoes only located along the Lake Powell marina delta in Hite, Utah. The data spans geological structures restricted to one marina delta.
Credit
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Gas Chromatography : Gas Chromatography: Gas samples were collected by filling a 20-mL headspace vial (Restek) with liquid from within a mud volcano or the Colorado River (depending on the sampling site) then inverted. A small plastic funnel was immersed in the sampling site with the stem inserted into the mouth of the headspace vial. Gas bubbles were directed into the headspace vial using the funnel until the vial was approximately 4/5 full of gas (1/5 lake water). The vial was capped under water with a PTFE/Silicone septum lined cap (Restek), sealed with Parafilm and maintained in an upside-down position until analysis with the 1/5 water content preventing gas escape. At least twenty volcanoes were sampled once on a single day while three volcanoes were sampled over a two-day period, and six volcanoes were sampled over a three-day period. Gas mixtures were prepared in 20-mL headspace vials over water to mimic the conditions of the samples. Specifically, methane, carbon dioxide, nitrogen, and propane were collected in graduated cylinders over water at atmospheric pressure and room temperature. A gas-tight syringe was used to transfer aliquots of gas to water-filled, inverted 20-mL headspace vials leaving 4 mL of water remaining in the vials. Vials were capped underwater and remained upside-down until analysis.
Ion Ratio Mass Spectrometry: Isotope ratio mass spectrometry was used to determine the quantity of 13C and 2H, or deuterium in samples containing the greatest amount of methane. Isotopic measurements as reported in parts per million (‰), are described by the following: 𝛿=(R_sample/R_standard -1)* 10^3 Where R is the 13C/12C or the 2H/1H ratios relative to the Pee Dee Belemnite (PDB) and the Standard Mean Ocean Water standards, respectively. Via plotting 𝛿13C vs 𝛿D of the gaseous samples and comparing these signatures to those of gas samples with known origins, the sources of CH4 can be inferred (Figure 6)[Whiticar, 1999]. It is worth mentioning the following: carbon from both bacterial and thermogenic gases has or is involved in biological processes of near-surface carbon cycles [Whiticar, 1999]. Because IRMS was conducted on only methane in the eleven samples, analyses and conclusions regarding fractionation factors (carbon dioxide-methane, in this case) could not be made. For the isotopic ratio mass spectrometry, samples were separated and analyzed using an Agilent GC combustion unit (either HP 6890 or HP 6890/7890 gas chromatography) joined with a mass spectrometer (either to ThermoFinnigan Delta Plus Advantage or Thermo Scientific Delta V Plus) in continuous flow mode. Peak detection and quantification was completed in Finnigan's Isodat software. Liquid nitrogen was used to remove air and enrich concentrations of hydrocarbons. While gas is channeled through a pyrolysis furnace, methane was permuted to H2 gas and carbon, upon which the hydrogen and entered the mass spectrometer. Each sequence began with reference gases, and 10% of the analyses are check standards.
