GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 7 2020 © 2020 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. METHANE EMISSIONS FROM MUDS DURING LOW WATER-LEVEL STAGES OF LAKE POWELL, SOUTHERN UTAH, USA Margariete Malenda, Thomas A. Betts, Wendy S. Simpson, Michael C. Wizevich, Edward Simpson, and Laura Sherrod GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Examples of the fluid-escape features related to lower water levels in the Lake Powell near Hite, Utah. The mud volcanoes range from being extensive (left, with 1.7-m-tall individual) to relatively small in size, such as the bubbling volcano on the right (frog is approximately 5 cm). i Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. 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Sprinkel Azteca Geosolutions 801.391.1977 GIW@utahgeology.org dsprinkel@gmail.com Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov John R. Foster Utah Field House of Natural History State Park Museum 435.789.3799 eutretauranosuchus@ gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 7 2020 121 ABSTRACT The Glen Canyon Dam, along the Colorado River in Page, Arizona, was completed in 1963, creating the Lake Powell reservoir which spans the Arizona-Utah border. The water levels of Lake Powell peaked in 1983 and have declined since, releasing overlying pressure on the underlying sediment. In general, water levels experience seasonal highs and lows, with punctuated periods of considerable and steady decreases (1987 to 1993, 1999 to 2005, and 2011 to 2014) and less dramatic recoveries (1993 to 1999 and 2005 to 2011). This release of overpressure coupled with increasing pore pressures due to biological methane pro- duction has created mud volcanoes, structures along the shoreline made of cavities that allow fluid and gas to rise to the surface and escape. Although these sedimentary structures have been assessed using geo- physical techniques and excavation to characterize their morphologies and fracture propagation, limited chemical data has been reported on the inputs and products of these gas- and fluid-escape features. This research investigates the relative proportions of methane (CH4), carbon dioxide (CO2), and air (unseparated nitrogen [N2] and oxygen [O2]) gas released, the variability of these proportions through time, and how these gases formed in the subsurface. The field site is along the Lake Powell near Hite, Utah. Three gas samples were collected from mud volcanoes along the delta in July 2014, whereas 21 samples were collected in July 2015 and were analyzed via gas chromatography (GC). The GC analyses from 2014 and 2015 have a mean CH4 concentration of 81.47 ± 9.29 percent of volume (% v/v) and 32.40 ± 15.31% v/v, respectively. In May 2016, 50 samples from 25 vents were collected and analyzed via GC for bulk composition, and 11 of which were analyzed by isotope ratio mass spectrometry (IRMS) for carbon and hydrogen isotope content of CH4. The 2016 GC analysis detected average relative concentrations for CH4, CO2, and air of 74.51 ± 14.08% v/v, 2.82 ± 3.76% v/v, and 22.67 ± 14.28% v/v, respectively. Gas composi- tions from individual vents varied over the three-day sampling timeframe in the summer of 2016 including CH4 decreases of up to 66% v/v and increases of up to 38% v/v. IRMS signatures of samples collected in 2016 indicate the gasses are in part generated during microbial respiration through hydrogenotrophic and acetoclastic methane production. Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Margariete Malenda1, Thomas A. Betts2, Wendy S. Simpson3, Michael C. Wizevich4, Edward Simpson2, and Laura Sherrod2 1Department of Geophysics, Stanford University, 397 Panama Mall, Mitchell Building, Stanford, CA 94305; malenda@stanford.edu 2Department of Physical Sciences, Kutztown University of Pennsylvania, 15200 Kutztown Road, Kutztown, PA 19530 3Parkland High School, 2700 North Cedar Crest Blvd., Allentown, PA 18104 4Department of Geological Sciences, Central Connecticut State University, Copernicus Hall, 1615 Stanley St., New Britain, CT 06050 Citation for this article. Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L., 2020, Methane emissions from muds during low water-level stages of Lake Powell, southern Utah, USA: Geology of the Intermountain West, v. 7, p. 121–136, 4 appendices, https://doi.org/10.31711/giw.v7.pp121-136. © 2020 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. 122 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 INTRODUCTION Methane Generation in Lacustrine Settings Mud volcanoes, pockmarks, water-filled craters (salses), and sedimentary deformation features in la- custrine settings can serve as natural gas seeps and can provide insight on mechanisms behind hydrocarbon and greenhouse gas generation and expulsion (Dlugo- kencky and others, 1995; Judd, 2005; Forster and others, 2007; Etiope and others, 2009; Bussmann and others, 2011). For example, up to 40 to 60 million tons (40–60 teragrams) of methane (CH4) are released annually via geological sources including mud volcanoes (Etiope, 2004), and 70 L/min of CH4 has been estimated to vent from the Dashgil mud volcano along the coast of Azer- baijan alone (Kopf and others, 2010). More specifically, terrestrial and marine mud volcanoes are recognized as significant sources of CH4 that may be of either bio- genic, thermogenic, or a combination of both origins (Hovland and others, 1997; Tinivella and Giustiniani, 2012). One primary source of CH4 is the respiration of microorganisms such as those prevalent in lacustrine settings, wetlands, rice paddies, and landfills among other natural and anthropogenic sources (Chanton and others, 2005). Large reservoir systems created by dams are also sites of significant biogenic CH4 (St. Louis and others, 2000; Joyce and Jewell, 2003; Maeck and others, 2014; Deemer and others, 2016; Harrison and others, 2017), and water level declines in such systems have been linked to increased rates in CH4 release (Maeck and others, 2014; Beaulieu and others, 2017; Harrison and others, 2017). Similarly, prior studies explain that the gas seeps of Utah’s Lake Powell reservoir delta (figure 1) are the result of increasing pore-water pressures from biogen- ic CH4 production and decreasing overlying pressure from the lowering water table (Netoff and others, 2010; Livingston and others, 2014, 2015; Sherrod and others, 2016; Miller and others, 2018). These studies identify algal blooms and the organic-rich clays as substrates to sustain methanogens (CH4-producing microorgan- isms). For example, the organic-rich clay layer described in Miller and others (2018) contained mm thick zones of fine, macerated plant materials and stem fragments decomposed, in part by microorganisms. This layer un- derlies heterolithic layers containing gas-filled cavities and ranges in depths from 0 to 6 m due to fluid migra- tion and subsequent sediment mobilization. These prior studies have discussed the origins and evolution of the Lake Powell gas seeps from primarily physical observations. For the first time, here we doc- ument the chemical signature of these gases through relative hydrocarbon compositions, then relate isotopic composition to the biogenic mechanisms for methano- genesis, and finally consider potential biological, hy- drological, and chemical controls on CH4 production in discussing future field measurements. Geologic Setting of the Lake Powell Delta Lake Powell is a reservoir located on the Utah-Ar- izona border and results from the 1963 completion of the Glen Canyon Dam near Page, Arizona (figure 1). Water levels experience seasonal highs and lows, with punctuated periods of considerable and steady decreas- es (1987 to 1993, 1999 to 2005, and 2011 to 2014) and less dramatic recoveries (1993 to 1999 and 2005 to 2011) (figure 2; U.S. Bureau of Reclamation, 2017). Such dras- tic water level fluctuations trigger undercutting erosion of canyon walls, subaqueous gravity flows, and up to one-hundred-fold increases in sediment accumula- tion (Pratson and others, 2008; Anderson and others, 2010). Over the three-year sampling period presented here (July 2014, July 2015, and May 2016), water levels were approximately 1100 m above sea level in 2014 and 2015, and 1097 m high during the 2016 period (figure 2). Overall, the reservoir has been gradually decreasing since the 1127 m full pool high in 1983. Lake Powell, in the study area, is underlain by the Cedar Mesa Sandstone, a confined aquifer sealed by Lake Powell muds. The uppermost 2 m of the delta con- sists of clays, silts, and fine sands (Willis, 2012). Nota- ble soft-sediment deformation, gas- and fluid-escape features including domes, pockmarks, craters, salses, sediment-filled craters, mud and sand volcanoes, dike systems, and sub-centimeter gas bubble cavities are rec- ognized at Hite (Netoff and others, 2010; Livingston and others, 2014, 2015; Sherrod and others, 2016; Mill- er and others, 2018). These soft-sediment deformation 123 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 structures (SSDS) are non-seismic in origin as the re- gion has only experienced two earthquakes since 1850, both under 3.5 magnitude, and are well below the 5.5 magnitude threshold for liquefaction and fluidization (Allen, 1986; Obermeier, 1996; Galli, 2000; Anderson and others, 2010; University of Utah Seismograph Sta- tions, 2020). Decreasing overlying water pressure and increasing underlying pore water and gas pressures are the primary triggers for the Lake Powell SSDS (Netoff and others, 2010; Livingston and others, 2014, 2015; Sherrod and others, 2016; Miller and others, 2018). These prior studies have attributed CH4 production to decaying organic matter in the subsurface and from al- gal blooms. In particular, macerated plant material in a mm-thick clay-rich, laterally extensive layer has been identified and is a likely source of organics on which methanogens can feed (Sherrod and others, 2016; Mill- er and others, 2018). Methanogenic Pathways Lacustrine greenhouse gases can be produced through various mechanisms. The following processes, Salt Lake City Lake Powell Field work location near Hite Marina. UTAH Escalante River G re en R iv er Colo ra do R ive r San Juan River Dirty Devil River Boat Ramp (270 m E to W) Volcanoes Sampled Colorado River N Figure 1. Sample locations along the Lake Powell delta, near Hite, Utah, during low lake level in May 2016. Sampled mud volcanoes are denoted with white X’s. Lake Powell reservoir is located in southeast Utah along the Colorado River and was created by the construction of Glen Canyon Dam near Page, Arizona (not shown here). Source of aerial photograph is from U.S. Department of Agriculure (USDA)-Farm Service Agency (FSA)-Aerial Photography Field Office (APFO)-National Ag- riculture Imagery Program (NAIP). 124 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 hydrogenotrophic production and acetoclastic produc- tion, are the focus of this study as they are most relevant to the Lake Powell delta system. These two mechanisms are mediated by the amount of substrates and hydrogen available to the methanogens as well as the amount of sulfate-reducing bacteria present. In this study, we use the ratios of carbon (12C versus 13C) and hydrogen (1H versus 2H—also known as deuterium, D) measured in mud volcano CH4 to interpret the methanogenic pro- cesses responsible for the gas ebullition. Further details regarding the isotopic characterization can be found in appendix A, but provided here are general descriptions of methanogenic and thermogenic processes sourced from Whiticar and others (1986) and Whiticar (1999). The first process described here, hydrogenotrophic production, is characterized by the following reaction: CO2 + 8 H+ + 8e- → CH4 + 2H2O It is the most predominant process of biogenic gas production and the main source of CH4 in marine/sa- line settings. However, it may occur in freshwater set- tings when acetate sources are exhausted, and meth- anogens require a new substrate. Hydrogenotrophic production is often the secondary means of microbial CH4 generation in freshwater environments, but will predominate when acetate pools, such as those neces- sary for certain acetoclastic formation, are exhausted. At this point, the microorganisms switch to reducing the bicarbonate with hydrogen. The second process described is acetoclastic pro- duction. Upon the breakdown of organic matter, acetate (CH3COOH) is generated and subsequently converted to CH4 in the respiration processes of microorganisms (Whiticar and others, 1986): CH3COOH → CH4 + CO2 This is the dominant means of biogenic gas produc- tion in freshwater environments (Whiticar and others, 1986). The carbon dioxide (CO2) produced through this fermentation can subsequently serve as a reactant in hydrogenotrophic production (above) when the ace- tate source is depleted (Whiticar, 1999). Sulfate-reduc- ing bacteria will (chemically) reduce the acetate other- wise used in fermentation with a greater energy yield, and therefore can limit the amount of fermentation and subsequently the amount of hydrogenotrophic produc- tion. In sulfate-rich marine settings, sulfate-reducing bacteria out-compete methanogens for acetate, mitigat- ing acetate fermentation (Chanton and others, 2005). Therefore, acetate fermentation tends to be more char- acteristic of the freshwater rhizosphere than marine. Finally, thermogenic CH4 occurs when organic matter is broken down by elevated temperatures (> 100°C) and pressures. Although Lake Powell is located within the Kaiparowits Basin, which has been assessed to contain gas (National Assessment of Oil and Gas, 2012), we will 2014 2015 2016 sampling 1100 1105 1095 1090 1085 Lake Powell Hydrograph: Daily Measurements Lake Powell Hydrograph: All Time Water Levels 2014-2016 1963 until Jan. 2017 1100 1090 1120 1110 1080 1070 1060 1050 1040 1130 E le va ti o n ( M e te rs A b o ve S e a L e ve l) E le va tio n ( M e te rs A b o ve S e a L e ve l) Month Measured Date Measured Oct. Nov. Dec. Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. 19 65 19 70 19 75 19 80 19 85 19 90 19 95 20 00 20 05 20 10 20 15 Figure 2. Lake Powell historical water levels from 1963 (in- ception) to 2017 along with daily water level data during the three years of sampling. Sampling events are noted with cir- cular symbols (U.S. Bureau of Reclamation, 2017). 125 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 discuss why this is an unlikely mechanism for gas gen- eration along the delta. MATERIALS AND METHODS Gas Collection Samples were collected during the field seasons of July 2014 and 2015, and in May 2016. The 2014 and 2015 samples were collected during lake level highs for each season (between 1100 and 1102 m above sea level) whereas the 2016 samples were collected during lake level lows (about 1095 m above sea level, figure 2). The majority of samples reported here were collected in 2016 from 25 vents (figure 3). Fifty of the samples collected in 2016 were analyzed via gas chromatography (GC) for bulk composition, and 11 samples with elevat- ed percent volumes of CH4 were selected for isotope ra- tio mass spectrometry (IRMS) of carbon and hydrogen isotope content of the CH4. Gas samples were collected by filling a 20-mL head- space vial (Restek) with liquid from within a mud vol- cano or the Colorado River (depending on the sampling site) then inverted for sampling. A small plastic funnel was immersed in the sampling site with the stem insert- ed 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 Para- film and maintained in an upside-down position until analysis with the 1/5 water content preventing gas es- cape. At least 20 volcanoes were sampled once on a sin- gle day, whereas three volcanoes were sampled over a two-day period, and six volcanoes were sampled over a three-day period. Samples for the 2016 season were re- frigerated until September, when chromatography tests were completed. Remaining gas from the 2016 season continued to be refrigerated until April of 2017 when isotopic analyses were completed. The authors did not conduct tests to determine whether biological activity altered relative gas compositions during storage. Gas Chromatography Gas standards and samples collected in 2014 and 2015 were analyzed using a Hewlett-Packard 5890 gas chromatograph equipped with a packed, 2.4-m-long by 0.3 cm (8-ft-long by 1/8 in) O.D., 80/100 mesh Po- ropak Q column and a thermal conductivity detector. Gas mixtures were prepared in 20-mL headspace vials over water to mimic the conditions of the samples. 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. Gas standards and samples from 2016 were an- alyzed using an Agilent 7890 gas chromatograph equipped with G1888 automated headspace sampler (Agilent Technologies). A 200°C injector operating at a 20:1 split ratio was connected to a 30 mm by 0.53 mm Carboxen 1006 PLOT column (Supelco) to separate the gas components. Thermal conductivity and flame ion- ization detectors were used in series to quantify and confirm chromatographic peak identities (see appendix B for headspace sampler and chromatograph operating conditions). Isotope Ratio Mass Spectrometry Isotopic compositions of carbon and hydrogen in CH4 can be used to identify the processes behind the CH4 generation (Whiticar and others, 1986). Details on the theory behind carbon to hydrogen isotopic ratios can be found in appendix A. Eleven samples from 2016 were favorably selected for IRMS analyses if they had relatively higher quantities of CH4 (as detected with the gas chromatography) and if the samples were in a set of multiples taken from the same volcano (appendix C). For IRMS, 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 con- tinuous flow mode. Peak detection and quantification were completed in Finnigan’s Isodat software. Hydro- carbon components were isolated in the GC unit and were introduced into a combustion furnace to produce CO2 which was sent to the mass spectrometer for isoto- pic analyses of 13C. Liquid nitrogen was used to remove air and enrich concentrations of hydrocarbons. For the 126 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 deuterium analysis, gas was channeled through a pyrol- ysis furnace to permute CH4 to H2 and carbon, upon which the H2 entered the mass spectrometer. Each se- quence began with reference gases, and 10% of the anal- yses were check standards. ANALYTICAL RESULTS Gas Chromatography All GC values are noted in percent of volume of specific gas to total sample volume when denoted with “v/v” unless otherwise specified. The average percent volume of CH4 for each of the three years, 2014, 2015, and 2016, is 81.47% v/v with a ± 9.29% v/v standard deviation, 32.40 ± 15.31% v/v, and 74.51 ± 14.08% v/v, respectively (figure 4, tables C1 to C3 in appendix C). The average CH4 composition of the 50 gas samples collected in 2016 is approximately 7% v/v lower than the average concentration of CH4 in gas samples pre- viously collected from the marina during the 2014 field season (Livingston and others, 2014) and is about 42% a b dc Figure 3. Field photographs during May 2016. (a) Salse near the edge of Lake Powell with high effusive rates disrupting the water surface. Individual is 1.7 m tall. (b) Mud volcano with gas effusion. The individual is 1.7 m tall. (c) Salse with slow effusing gas emission. The card (bottom left) is 15 cm long. (d) Field scientist (1.75 m tall in prone position) sampling a salse. Note the shore of the Lake Powell in the background. 127 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 v/v greater than the average CH4 collected in 2015. Although sample sizes (n) vary from year to year, the following data sets convey the variation in gas con- tent between samples between and from the same field seasons. For example, although at 81.47% v/v, the 2014 sampling year had the greatest CH4 on average, the 2016 sampling year includes CH4 compositions ranging from 25.31% v/v to 93.34% v/v. Overall, the 2015 sam- pling year had the least amount of CH4 on average, and also yielded the lowest minimum and maximum con- centrations of CH4 (0.00% v/v minimum and 45.50% v/v maximum). For each sampling year, of the three gases tested, CO2 yields the lowest percent content on average, and for each year, when compared to CH4 and air (unseparated nitrogen [N2] and oxygen [O2]), CO2 also yields the lowest minimum and maximum percent content (tables C2 and C3 in appendix C; figure 4). Of the 2016 data, chromatographic analyses yielded an average concentration of 74.51 ± 14.08% v/v CH4, 2.82 ± 3.76% v/v CO2, and 22.67 ± 14.28% v/v air (ta- bles C1 and C3 in appendix C). The CH4, CO2, and air concentrations ranged from 25.31 to 93.34% v/v, 0.00 to 23.58% v/v, and 5.28 to 74.69% v/v, respectively. Four volcanoes (16-1, 16-3, 16-5, 16-10) were sam- pled over the course of two days, whereas six volcanoes (16-7, 16-12, 16-17, 16-20, 16-25, 16-27) were sampled over a three-day timespan (figure 5). For the following fluctuation calculations, duplicates taken from the same volcano in the same day were averaged to a single mea- surement for that day (tables C3 and C4 in appendix C; figure 5). For example, all four CH4 measurements collected at volcano 16-20 on the third day (samples 16- 20C1 to 16-20C3, and 16-20C5) were averaged to rep- resent a single measurement at volcano 16-20 on day three. Daily fluctuations were calculated using the aver- aged measurements. In 2016, there was a mean increase of 6.08% v/v in CH4 from the first to second day and a 24.68% v/v decrease from the second to third day while the CO2 content showed a 0.89% v/v increase between the first two days of sampling and a 0.5 %v/v decrease from the second to third day. The average variation from day to day was a 5.46% v/v decrease in CH4 and a 0.37% v/v increase in CO2 (table C4 in appendix C). Geographically, there is a cluster of eight volcanoes, a single volcano to the north (16-1), and a single volca- no to the south (16-10). Changes in relative CH4 con- centration range from extremely low variation of only 2% v/v increases (volcano 16-17), to great variations of up to 38% v/v increases (volcano 16-5) and 66% v/v decreases (volcano 16-7) across multiple days. Several volcanoes within the southern end of the cluster (volca- noes 16-3, 16-5, 16-7, 16-20, 16-27) exhibited increases 2014 n=3 2015 n=21 2016 n=50 23% 3% 75% 67%<1% 32% 16% 3% CH4 CO 2 Air (N + O )2 2 82% All values are in % v/v Figure 4. Average relative gas concentrations and sample sizes during the 2014–2016 sampling seasons. In gener- al, the relative CH4 content is high in July 2014 at 82% v/v and in May 2016 at 75% v/v and is lower at 32% v/v in July 2015. Relative carbon diox- ide content remains minimal ranging between less than 1% v/v to 3% v/v between the three years, and the resulting relative concentration of air ranges from 16% v/v in July 2014 to 67% v/v in July 2015. 128 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 in CH4 from day one to day two while 16-12 and 16-7 had steady CH4 fluxes, and only 16-25 observed a de- crease. Two volcanoes, 16-7 and 16-12, are proximal to one another and both exhibited decreases in CH4 from 80% v/v or more down to less than 40% v/v between the second and third day. Volcano 16-20 also shows a decrease from day two to three, but not as severe (about a 35% v/v fall). Vent 16-10 to the south exhibits a slight increase from day one to day two while vent 16-1 in the north shows the opposite trend. Isotope Ratio Mass Spectrometry All IRMS results are detailed in table D1 in ap- pendix D. When plotted on a carbon-deuterium (CD) CH4 source discrimination diagram (figure 6; Whiticar, 1999), δ13C and δD CH4 signatures of all but two sam- ples lie within the ranges characteristic of bacterially produced CH4 (regions 1 and 2 in figure 6) (>-100 to about -45 δ13C‰ PDB, and about -400 to -150 δ13D‰ SMOW). This excludes the possibilities of geothermal or hydrothermal (region 5), abiogenic or mantle (region 6), or artificial (region 7), production of these gases. DISCUSSION Gas Chromatography Compositions Flux measurements are commonly used to assess CH4 and CO2 emissions with production mechanisms. Although gas fluxes were not collected from the Lake Powell gas seeps, relative magnitudes of CH4, CO2, and air should be considered during future flux measure- ments from vents along the delta near Hite. For this study, temporal changes in relative lacustrine CH4 con- tent are discussed across a short-term (within a three- day time frame) and long-term period (beyond a three- day time frame) below. Sampling of the following would be useful in understanding controls on temporal cor- relations with methanogenesis in the Lake Powell delta: (1) seasonal flux sampling throughout the year and over a longer period of time, (2) soil and water temperatures, (3) soil and root redox potentials, and (4) identification 1166--2255 1166--2277 1166--2200 1166--1177 1166--1122 1166--77 1166--551166--33 1166--1100 400 m 16-1 CH4 CO 2 Air (N + O )2 2sampling days in white (1,2,3) 16-1 16-25 16-271166--220016-171166--11221166--771166--551166--33 1166--1100 1 1 1 1 11 1 1 1 12 2 2 2 3 3 3 3 3 322 2 2 2 2 vent sampled c o m p o s iti o n ( % v /v ) composition variation during 2–3 day sampling period, 2016 Colorado River Figure 5. (Left) Graph of relative gas concentrations of mud volcanoes sampled over multiple days in May 2016. Concentra- tions are in percent volume gas to volume of sample (% v/v), assuming 100% v/v of the gas was detected during GC analyses. (Right) Locations of the vent sources sampled in May 2016. Aerial photograph was taken June 2014. Source of the aerial photograph is from the USDA-FSA-AFPO-NAIP. 129 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 -450 -400 -350 -300 -250 -200 -150 -100 -50 -100 -80 -60 -40 -20 0 δD methane (‰, SMOW) 1 3 δ C m e th a n e ( ‰ , P D B ) Predicted methane oxidation shift as applied to IRMS samples Samples reported in this study Shift due to substrate depletion Shifts in isotopic variation due to secondary effects 1 2 3 4 5 Atmospheric 6 7 1) Bacterial: Hydrogenotrophic (Carbonate Reduction) 6) Abiogenic or Mantle 7) Artificial 3) Bacterial: Mixed 4) Thermogenic 5) Geothermal or Hydrothermal Crystalline 2) Bacterial: Acetoclastic (Methyl Type Fermentation) Figure 6. δ13C versus δD diagram used for classification of natural gas (adapted from Whiticar, 1999). Higher values of δ13C and δD will plot closer to zero than lower values. The isotopic signatures follow more closely the oxidation trend as empha- sized with the single-headed arrow and the region outlined with a dashed border. 130 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 and quantification of vegetation and organic sources among the sampled vents. Short-Term Changes in Relative Methane Content Short-term, specifically diurnal fluxes in CH4 ebul- lition have been attributed to acute water depth fluctu- ations and water circulation, soil redox potentials, and vegetation. These variables are explored in context of the ten mud volcanoes sampled for relative CH4 content over multiple days in 2016. In the instance of water depth fluctuation, Sebacher and others (1986) found an increase in methanogen- ic CH4 fluxes of about 300 mg m-2 d-1 with increasing water depth of about 45 cm in Alaskan bogs, fens, and marshes. Among other variables including water tem- perature and permafrost depth, water level held the strongest correlation to fluxes in their study. Moore and Roulet (1993) synthesized aerobic and anaerobic peat columns in the laboratory using bog, fen, and swamp soils. They observed that CH4 fluxes increased with ini- tially falling water levels (0 to 20 cm depth) and then a decrease in fluxes upon further falling water levels (20 to 50 cm depth). The authors concluded that there is a strong but complex relationship between water table fluctuations and CH4 outgassing. Similarly to Moore and Roulet (1993), if there is a relationship between the Lake Powell delta water table level and relative CH4 content variations from day to day, then it may be complex and vary to some degree from volcano to volcano. During the most recent sam- pling period (2016), water levels increased daily between 17 and 19 cm (U.S. Bureau of Reclamation, 2017). Yet upon steady water level increase, relative CH4 content experienced an average 6.08% v/v increase from sam- pling day one to day two, and on average a 24.68% v/v decrease from day two to three (table C4 in appendix C). Variations in water levels could impact certain gas fluxes in volcanoes more than others. Changes in water levels will impact a greater volume of pores in spatially larger volcanoes and impart a more extensive impact on gas fluxes. Additionally, volcanoes that experience more extreme or frequent episodes of wetting and drying may exhibit different patterns of gas production. Therefore in order to better understand how water levels are af- fecting day-to-day CH4 release from volcanoes along Lake Powell, it would be beneficial to measure not only the CH4 flux, but also overall water levels and water lev- els local to each gas seep. Water circulation has also been shown to impact short-term CH4 production in settings similar to the Lake Powell delta. Effects of water circulation were ob- served when Podgrajsek and others (2014) found that the greatest CH4 generation of a freshwater lake (38 km2 area and 1.3 average depth) occurred in the morning (average of about 10 nmol m-2s-1 and outliers of over 100 nmol m-2s-1) and lowest in the evenings (average of about 5 nmol m-2s-1 and outliers of less than 30 nmol m-2s-1). They consider that water-side convection is one mechanism of transporting CH4 from various depths in the water column to the surface. In measuring convec- tion, they installed a 6-m-high monitoring tower, which may not be feasible along the clay and mud-rich Lake Powell delta. To investigate the impact of water circula- tion on short-term changes in relative CH4 concentra- tions, an alternative approach to measuring water circu- lation measurements might be necessary. Duan and others (2005) found pronounced varia- tions in CH4 ebullition fluxes correlated with the pre- dominant vegetation (Phragmites australis and Pota- mogeton pectinatus) but no direct correlation with mean water level (about 46 cm depth versus about 83 cm depth) in Wuliangsu Lake of Inner Mongolia. However, they noted that water level will determine the type of vegetation which thrives and indirectly CH4 generation as well. Their study also showed that increased diurnal CH4 fluxes were correlated with increasing photosyn- thetically active radiation (PAR) in reeds and more so than in pondweeds. Coupling PAR measurements with surrounding vegetation and daily flux measurements of the Lake Powell volcanoes would be useful in under- standing the effects of low versus high photosynthetic activity along the delta on CH4 generation. Prior studies show that redox potentials of plant roots and soils have also been correlated with daily changes in CH4 production. Chen and others (2010) found CH4 fluxes in a ponded system of up to 15 mg CH4 m-2 hr-1 with flux increases of about 10 mg CH4 m-2 hr-1 in a six-hour period. These diurnal variations were significantly correlated with paralleled soil redox 131 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 potentials taken at 5 and 10 cm depth. Fluxes did not correlate well with changes in water and soil tempera- tures at these depths however, and similar trends (strong correlation with redox potential but not soil tempera- tures) were also identified in Bansal and others (2018). Flux and redox potential measurements at each volcano could also be useful in determining to what extent these influence short term CH4 generation. Long-Term Fluctuations in Methane Content from 2014 to 2016 The influencing factors on long-term changes in methanogenic production discussed here are tempera- ture and precipitation as previous studies show these factors vary between seasons and impact methanogene- sis in similar freshwater systems. For example, Pugh and others (2018) showed that average monthly CH4 fluxes were strongly correlated with air temperatures. In their study, CH4 fluxes increased from near-zero fluxes in winter and up to 39 mg C m-2 day-1 in summer months. Schulz and Conrad (1996) demonstrated that after in- cluding a chloroform inhibitor, a temperature increase of 16°C resulted in ten times more methanogenesis over a six-day-long incubation period. With a 25 to 32 day incubation and a sampling time frame of less than two weeks, Schulz and others (1997) showed that CH4 pro- duction rates peaked around 35°C (in a comparison to temperatures ranging from 0 to 50°C). Chanton and others (2005) showed that elevated temperatures in the summer result in sulfate depletion and more availability of acetate to methanogens. Alternatively, Rask and oth- ers (2002) demonstrated with 300 days’ worth of data, that temperature had variable influence on CH4 fluctu- ations depending on the location (flark, string, deep bay or shallow bay) in their freshwater system. Although we did not sample for temperature or pre- cipitation variables, we relate relative CH4 concentra- tions to regional temperature and precipitation and dis- cuss below how these would impact gas ebullition along the Lake Powell delta. The average temperatures of July 2014 and 2015 were similar (23.27°C and 21.69°C, re- spectively), while the average temperature of May 2016 was lower at 12.5°C (U.S. Climate Data, 2020). If tem- perature is a predominant predictor of CH4 ebullition along the Lake Powell delta, one might suspect the July 2014 and 2015 CH4 relative concentrations to be more similar and deviate from the 2016 data; however, this trend is not observed in the data. Future work mea- suring gas fluxes, air temperature, and temperature of water pooled in each volcano throughout longer term sampling could allow us to constrain the correlation be- tween heat energy and CH4 production in this system. Precipitation and wetting may be one explanation for the variation in CH4 between sampling seasons. Es- top-Aragones and others (2016) conducted a year-long study on 15 bogs in Europe focusing on an initial wet period, a prolonged dry period (when the water table de- creased over 35 cm), and a rewetting episode (when the water table increased 30 cm). They observed a peak in CH4 fluxes during the early dry period (520.7 mg C-CH4 m-2d-1) yet continued drying decreased fluxes through time (down to 14.5 mg C-CH4 m-2d-1). Upon rewetting, CH4 fluxes increased once more, but not to the point of early dry period fluxes. Similar trends (sharp peaks at the beginning of drought conditions and slow recovery of CH4 emissions) were found in gully-mires over a six- year period (Hughes and others, 1999) and a 10-week period (Dowrick and others, 2006). Drought conditions (lasting two weeks) in the Auchencorth Moss peatland of Scotland also resulted in delayed increase in CH4 flux and subsequent dramatic decrease without rebounding to original fluxes (Dinsmore and others, 2009). The Lake Powell delta experienced slightly less precipitation (on average 3.48 and 2.16 cm precipitation, respectively) and elevated CH4 concentrations (greater than 70% v/v) in both the July 2014 and May 2016 sampling months while the month of July 2015 experienced slightly greater pre- cipitation (5.23 cm) and experienced the lowest average CH4 concentrations (less than 35% v/v). Lake Powell temperature and precipitation data were sourced from usclimate.data.com. Isotope Ratio Mass Spectrometry Compositions In this study, we use the ratios of carbon (12C versus 13C) and hydrogen (1H versus 2H—also known as deu- terium, D) measured in mud volcano CH4 to interpret the methanogenic processes responsible for the gas eb- ullition. Upon plotting ‰ δ13C and ‰ δD against one 132 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 another, nine of the eleven CH4 samples lie within the “mixed” methanogenic region (region 3 of figure 6). Re- gion 1 is denoted with signatures of bacterial hydrog- enotrophic production (carbonate reduction) and re- gion 2 is denoted with bacterial acetoclastic production (methyl-type fermentation) signatures (Whiticar and others, 1986; Whiticar, 1999). The isotopic signatures of this study fall relatively well within the freshwater sedi- ment range (-65 to -50‰ δ13C and -400 to -250‰ δD) described in Whiticar and others [1986]). The Δ δD/Δ δ13C value (the slope of the line when plotting δD versus δ13C) of our samples is 4.81, R = 0.8344, which lies within the range (2.5 to 13.5 Δ δD/Δ δ13C) associated with methanotrophic activity in Chan- ton and others (2005). In addition to the physical water datum discussed above, water chemistry such as sul- fate, acetate, and organic content measurements would offer additional insight into gas generation within the Lake Powell delta mud volcanoes. The impacts of these chemical variables are discussed in more detail below in context of specific methanogenic pathways. Isotopic signatures within the “mixed” region of the δ13C and δD plot may result from one or both of two scenarios. Scenario one is that separate pools of gas (some derived from acetoclastic production and others from hydrogenotrophic production) are initially gener- ated in isolation and are later transported and converge during migration. Scenario two is that the CH4 source itself is shifting either spatially or temporally. For example, CH4 of isolated sources may have been mixed due to migration or diffusion of gas (Whiticar, 1999). Decreasing surface water levels of Lake Powell contribute to depressurizing of pore fluids and gases, allowing trapped CH4 from separate subsurface sources (with varying initial substrates and isotopic signatures) to migrate upwards through the sediment as gas bub- bles. This proposed mechanism would parallel pre- viously reported evidence of gas release and upward migration by pressurization (Netoff and others, 2010; Livingston and others, 2014, 2015; Sherrod and others, 2016; Miller and others, 2018). In this instance, the isotopic composition of the original substrate may shift the signatures of the pro- duced CH4. For example, if a precursor substrate had both 13CH3OOH and 12CH3OOH acetate, the substrate will more easily diffuse the lighter compound with less- er mass, than the former acetic acid molecule. This will result in a greater amount of bacterial 12CH4 generat- ed and very little bacterial 13CH4 created. As 13C is left behind in the original substrate, the next generation of bacterial CH4 will be more enriched in 13C than the first, and so on (Whiticar, 1999; Chanton and others, 2005). This transition would result in differences in the amount of δ13C generated (Whiticar, 1999). Additionally, a change in substrate composition and methanogenesis is often observed with increasing depth. For example, Hornibrook and others (1997) showed that acetate fermentation processes predominated in shallow organic-rich soils, while in deeper (greater than 45 cm depth), older, less reactive peat, CO2 re- duction predominated. Similar observations have been previously reported in the literature (Hornibrook and others, 1997, 2000a, 2000b; Chasar and others, 2000a, 2000b). This transition will result in decreasing δ13C (of CH4) signatures with depth similar to the decrease in the 11 samples presented here. Quality of the substrate throughout the delta could impact the scarcity of nutri- ents and mechanism of methanogenesis. However, should these be the only processes taking place, the carbon isotopic signatures alone would exhib- it a wide range and result in a linear shift (figure 6: “shift due to substrate depletion”) (Whiticar, 1999). These two processes alone do not account for the wider range in δD. Increasing δ13C and δD of isotopic signatures, which results in a “sympathetic” trend (linear and trend- ing from lesser to greater signatures of both isotopes), is often the result of oxidizing methanotroph bacteria (Chanton and others, 2005). This aerobic oxidation of- ten occurs during bacterial CH4 consumption in fresh- water settings at the anoxic/oxic interface (Whiticar, 1999) which lies in the top several centimeters of lake sediments. Similar oxidation and subsequent δD/δ13C trends have been observed in landfill soils, wetlands, and laboratories incubation studies (Chanton and oth- ers, 2005). Thermogenic sources tend to have greater δ13C sig- natures than biogenic sources, and will have further in- creased δ13C signatures with greater maturity (Whiticar, 1999), supporting the conclusion that the Lake Powell delta gas seeps are not related to thermogenic activity. 133 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 CONCLUSIONS • This is the first reporting of CH4, CO2, and air (N2+ O2) compositions as well as CH4 isotope signatures of samples collected over a multiple day and mul- tiple year period at Lake Powell, Hite, Utah. Aver- age relative concentrations of CH4, CO2, and air released from the Lake Powell gas volcanoes in the spring of 2016 ranged from 74.51 ± 14.08% v/v, 2.82 ± 3.76% v/v, and 22.67 ± 14.28% v/v, and ranged from 25.36 to 93.34% v/v, 0.00 to 23.58% v/v, and 5.28 to 74.69% v/v. • Gas chromatography data show changes in rela- tive CH4 concentrations ranging from extremely low variations of only 2% v/v increases, to greater variations of up to 38% v/v increases and 66% v/v decreases across multiple days. Acute water depth fluctuations and water circulation, soil redox poten- tials, and vegetation have been shown to result in short-term changes in CH4 ebullition rates in the literature, but more work must be done to under- stand the extent to which these factors influence CH4 production along the Lake Powell delta. • Carbon and hydrogen isotopes from 11 samples with the greatest concentrations of CH4 were ana- lyzed using IRMS. The δ13C and δD signatures of these samples support that the CH4 ebullition along the Lake Powell delta is biogenic and may source from a mixture of acetoclastic production and hy- drogenotrophic production. This mixture may be the result of two possible mixing mechanisms. First, there may exist isolated CH4 generation (of either predominantly acetoclastic production or predom- inantly hydrogenotrophic production), subsequent upward migration during pore depressurization, and finally mixing. Alternatively, original CH4 sources are shifting from fermentation to reduc- tion, resulting in a mixed isotopic signature. Isoto- pic signatures may potentially result from original substrates becoming depleted in lighter isotopes (12C ), and heavier isotopes (13C) are subsequently used in bacterial respiration as well as CH4 oxida- tion. Additionally, if methanogenesis occurs in an organic-rich zone and shifts to a depleted zone, the process may shift from fermentation to reduction. • Future field work should include measuring CH4 concentrations through time to calculate and relate fluxes to literature values. Additional biological, hy- drological, and chemical measurements within and around gas-producing volcanoes are necessary to further constrain impacts on biological CH4 pro- duction along the delta. Additional data must be gleaned to constrain long-term (seasonal and an- nual) changes in CH4 ebullition and potential in- fluences. Two possible controls on CH4 production are temperature and wetting of the delta subsurface. Measurements of soil and water temperature, obser- vations of water table fluctuations, and monitoring of wetting within and surrounding the mud volca- noes are necessary to better understand controls on CH4 production. ACKNOWLEDGMENTS The authors greatly appreciate the aid of John Spence, “Chief Scientist” of the National Park Service at the Glen Canyon and the encouragement and guidance of Utah State Paleontologist, Jim Kirkland. We would also like to thank William Parry for his helpful review and Doug Sprinkel for his thorough and timely editing. This work was supported by the Kutztown University Undergraduate Research Committee and the Pennsylvania State System of Higher Education Professional Development Committee. All field work was completed by Kutztown University affil- iates. There exist no conflicts of interest among any author related to this research, and all reported data can be found in the appendices and has been submitted to the Mende- ley Data repository (https://data.mendeley.com/datasets/ yv9s92r5rh/1). REFERENCES Allen, J.R.L., 1986, Earthquake magnitude-frequency, epicentral distance and soft-sediment deformation in sedimentary ba- sins: Sedimentary Geology, v. 46, p. 67–75, doi:10.1016/0037- 0738(86)90006-0. Anderson, P.B., Chidsey, T.C., Jr., Sprinkel, D.A., and Willis, G.C., 2010, Geology of Glen Canyon National Recreation Area, Utah-Arizona, in Sprinkel, D.A., Chidsey, T.C., Jr., and An- derson, P.B., editors, Geology of Utah’s parks and monuments: 134 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 Utah Geological Association Publication 28 (third edition), p. 301–335. Bansal, S., Tangen, B., and Finocchiaro, R., 2018, Diurnal patterns of methane flux from a seasonal wetland—mechanisms and methodology: Wetlands, p. 933–943, doi:10.1007/s13157-018- 1042-5. Beaulieu, J.J., Balz, D.A., Birchfield, M.K., Harrison, J.A., Nietch, C.T., Platz, M.C., Squier, W.C., Waldo, S., Walker, J.T., White, K.M., and Young, J.L., 2017, Effects of an experimental wa- ter-level drawdown on methane emissions from a eutrophic reservoir: Ecosystems, v. 21, no. 4, p. 657–674, doi:10.1007/ s10021-017-0176-2. Bussmann, I., Schlömer, S., Schlüter, M., and Wessels, M., 2011, Ac- tive pockmarks in a large lake (Lake Constance, Germany)— effects on methane distribution and turnover in the sediment: Limnology and Oceanography, v. 56, p. 379–393, doi:10.4319/ lo.2011.56.1.0379. Chanton, J., Chaser, L., Glasser, P., and Siegel, D., 2005, Carbon and hydrogen isotopic effects in microbial, methane from terres- trial environments, in Flanagan, L.B., Ehleringer, J.R., and Pa- taki, D.E., editors, Stable isotopes and biosphere—atmosphere interactions—processes and biological controls: Physiological Ecology, San Diego, California, Elsevier Academic Press, p. 55–105., doi:10.1016/B978-012088447-6/50006-4. Chasar, L.S., Chanton, J.P., Glaser, P.H., and Siegel, D.I., 2000a, Methane concentration and stable isotope distribution as evi- dence of rhizospheric processes—comparison of a fen and bog in the glacial Lake Agassiz peatland complex: Annals of Bota- ny, v. 86, p. 655–663, doi:10.1006/anbo.2000.1172. Chasar, L.S., Chanton, J.P., Glaser, P.H., Siegel, D.I., and Rivers, J.S., 2000b, Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minne- sota peatland: Global Biogeochemical Cycles, v. 14, no. 4, p. 1095–1108. Chen, H., Wu, N., Yao, S., Gao, Y., Wang, Y., Tian, J., and Yuan, X., 2010, Diurnal variation of methane emissions from an alpine wetland on the diurnal variation of methane emissions from an alpine wetland on the eastern edge of Qinghai-Tibetan Pla- teau: Environmental Monitoring and Assessment, v. 164, p. 21–28, doi:10.1007/s10661-009-0871-3. Deemer, B.R., Harrison, J.A., Li, S., Beaulieu, J.J., Delsontro, T., Barros, N., Bezerra-Neto, J.F., Powers, S.M., Dos Santos, M.A., and Vonk, J.A., 2016, Greenhouse gas emissions from reser- voir water surfaces—a new global synthesis: BioScience, v. 66, no. 11, p. 949–964, doi:10.1093/biosci/biw117. Dinsmore, K.J., Skiba, U.M., Billett, M.F., and Rees, R.M., 2009, Effect of water table on greenhouse gas emissions from peat- land mesocosms: Plant Soil, v. 318, p. 229–242, doi:10.1007/ s11104-008-9832-9. Dlugokencky, E.J., Steele, L.P., Lang, P.M., and Masarie, K.A., 1995, Atmospheric methane at Mauna Loa and Barrow observa- tories—presentation and analysis of in situ measurements: Journal of Geophysical Research, v. 100, p. 23,103–23,113, doi:10.1029/95JD02460. Dowrick, D.J., Freeman, C., Lock, M.A., and Reynolds, B., 2006, Sulphate reduction and the suppression of peatland methane emissions following summer drought: Geoderma, v. 132, p. 384–390, doi:10.1016/j.geoderma.2005.06.003. Duan, X., Wang, X., Mu, Y., and Ouyang, Z., 2005, Seasonal and diurnal variations in methane emissions from Wuliangsu Lake in arid regions of China: Atmospheric Environment, v. 39, p. 4479–4487, doi:10.1016/j.atmosenv.2005.03.045. Estop-Aragones, C., Zajac, K., and Blodau, C., 2016, Effects of ex- treme experimental drought and rewetting on CO2 and CH4 exchange in mesocosms of 14 European peatlands with dif- ferent nitrogen and sulfur deposition: Global Climate Change Biology, no. 22, p. 2285–2300, doi:10.1111/gcb.13228. Etiope, G., 2004, New directions—GEM—geologic emissions of methane, the missing source in the atmospheric meth- ane budget: Atmospheric Environment, v. 38, p. 3099–3100, doi:10.1016/j.atmosenv.2004.04.002. Etiope, G., Feyzullayev, A., and Baciu, C.L., 2009, Terrestrial meth- ane seeps and mud volcanoes—a global perspective of gas origin: Marine and Petroleum Geology, v. 26, p. 333–344, doi:10.1016/j.marpetgeo.2008.03.001. Forster, P., Ranaswamy, V., Artaxo, P., Bernsten, T., Betts, R., Fahey, D.W., Haywood, J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., and others, 2007, Changes in atmospheric constituents and in radiative forcing, in Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., and Miller, H.L., editors, Climate change 2007— the physical science basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmen- tal Panel on Climate Change, p. 129–234, doi:10.1103/Phys- RevB.77.220407. Galli, P., 2000, New empirical relationships between magnitude and distance for liquefaction: Tectonophysics, v. 324, p. 169– 187, doi:10.1016/S0040-1951(00)00118-9. Harrison, J.A., Deemer, B.R., Birchfield, M.K., and O’Malley, M.T., 2017, Reservoir water-level drawdowns accelerate and amplify methane emission: Environmental Science and Technology, v. 51, no. 3, p. 1267–1277, doi:10.1021/acs.est.6b03185. Hornibrook, E.R.C., Longstaffe, F.J., and Fyfe, W.S., 1997, Spatial distribution wetland of microbial methane production path- ways in temperate soils—stable carbon and hydrogen isotope evidence zone: Geochimica et Cosmochimica Acta, v. 61, no. 4, p. 745–753, doi: 0.1016/S0016-7037(96)00368-7. Hornibrook, E.R.C., Longstaffe, F.J., and Fyfe, W.S., 2000a, Factors 135 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 influencing stable isotope ratios in CH4 and CO2 within suben- vironments of freshwater wetlands—implications for δ-signa- tures of emissions: Isotopes in Environmental and Health Stud- ies, v. 36, no. 2., p. 151–176, doi:10.1080/10256010008032940. Hornibrook, E.R.C., Longstaffe, J.J., and Fyfe, W.S., 2000b, Evolu- tion of stable carbon isotope compositions for methane and carbon dioxide in freshwater wetlands and other anaerobic en- vironments: Geochimica et Cosmochimica Acta, v. 64, no. 6, p. 1013–1027, doi: 10.1016/S0016-7037(99)00321-X. Hovland, M., Hill, A., and Stokes, D., 1997, The structure and geo- morphology of the Dashgil mud volcano, Azerbaijan: Geomor- phology, v. 21, p. 1–15, doi:10.1016/S0169-555X(97)00034-2. Hughes, S., Dowrick, D.J., Freeman, C., Hudson, J.A., and Reyn- olds, B., 1999, Methane emissions from a gully mire in Mid- Wales, U.K. under consecutive summer water table drawdown: Environmental Science & Technology, v. 33, no. 2, p. 362–365, doi:10.1021/es980563z. Joyce, J., and Jewell, P.W., 2003, Physical controls on methane ebul- lition from reservoirs and lakes: Environmental and Engineer- ing Geoscience, v. 9, no. 2, p. 167–178, doi:10.2113/9.2.167. Judd, A., 2005, Gas emissions from mud volcanoes, in Martinelli, G., and Panahi, B., editors, Mud volcanoes, geodynamics and seismicity: NATO Science Series (Series IV: Earth and Envi- ronmental Series: Dordrecht, The Netherlands, Springer, v. 51, p. 147–157, doi:10.1360/zd-2013-43-6-1064. Kopf, A., Delisle, G., Faber, E., Panahi, B., Aliyev, C.S., and Guliyev, I., 2010, Long-term in situ monitoring at Dashgil mud volca- no, Azerbaijan—a link between seismicity, pore-pressure tran- sients and methane emission: International Journal of Earth Sciences, v. 99 (Supplemental 1), p. 227–240, doi:10.1007/ s00531-009-0487-4. Livingston, K., Bogner, E., Ireland, S., Simpson, E., Betts, T., and Laub, E., 2014, The geomorphic evolution of shallow-sourced methane produced mud volcanoes—Lake Powell, Hite Utah [abs.]: Geological Society of America Abstracts with Pro- grams, v. 46, no. 6, p. 766–767. Livingston, K., Bogner, E., Simpson, E.L., Malenda, M., Sherrod, L.A., Betts, T.A., and Laub, E., 2015, The proposed evolution of shallow-sourced methane mud volcano geomorphology Lake Powell, Hite Utah [abs.]: Geological Society of America Ab- stracts with Programs, v. 47, no. 7, p. 588. Maeck, A., Hofmann, H., and Lorke, A., 2014, Pumping methane out of aquatic sediments—ebullition forcing mechanisms in an impounded river: Biogeosciences, v. 11, no. 11, p. 2925– 2938, doi:10.5194/bg-11-2925-2014. Miller, K., Simpson, E.L., Sherrod, L., Wizevich, M.C., Malenda, M., Morgano, K., Richardson, A., Livingston, K., and Bogner, E., 2018, Gas bubble cavities in deltaic muds, Lake Powell del- ta, Glen Canyon National Recreation Area, Hite, Utah: Marine and Petroleum Geology, v. 92, no. 4, p. 904–912, doi:10.1016/j. marpetgeo.2018.03.032. Moore, T.R., and Roulet, N.T., 1993, Methane flux—water table re- lations in northern wetlands: Geophysical Research Letters, v. 20, no. 7, p. 587–590, https://doi.org/10.1029/93GL00208. National Assessment of Oil and Gas, 2012, Assessment of undis- covered oil and gas resources in the Paradox Basin Province, Utah, Colorado, New Mexico, and Arizona, 2011: U.S. Geolog- ical Survey Fact Sheet 2012–3031, 1–4 p. Netoff, D., Baldwin, C.T., and Dohrenwend, J., 2010, Non-seismo- genic origin of fluid/gas escape structures and lateral spreads on the recently exposed Hite delta, Lake Powell, Utah, in Car- ney, S.M., Tabet, D.E., and Johnson, C.L., editors, Geology of south-central Utah: Utah Geological Association Publication 39, p. 61–92. Obermeier, S.F., 1996, Use of liquefaction-induced features for pa- leoseismic analysis—an overview of how seismic liquefaction features can be distinguished from other features and how their regional distribution and properties of source sediment can be used to infer the location and strength of Holocene pa- leo-earthquakes: Engineering Geology, v. 44, no. 1–4, p. 1–76, doi:10.1016/S0013-7952(96)00040-3. Podgrajsek, E., Sahlee, E., and Tugersson, A., 2014, Diurnal cycle of lake methane flux: Journal of Geophysical Research Biogeo- sciences, v. 119, no. 3, p. 236–248, doi:10.1002/2013JG002327. Pratson, L., Hughes-Clarke, J., Anderson, M., Gerber, T., Twichell, D., Ferrari, R., Nittrouer, C., Beaudoin, J., Granet, J., and Crockett, J., 2008, Timing and patterns of basin infilling as documented in Lake Powell during a drought: Geology, v. 36, no. 11, p. 843–846, doi:10.1130/G24733A.1. Pugh, C.A., Reed, D.E., Desai, A.R., and Sulman, B.N., 2018, Wet- land flux controls—how does interacting water table levels and temperature influence carbon dioxide and methane fluxes in northern Wisconsin?: Biogeochemistry, v. 137, no. 1, p. 15–25, doi:10.1007/s10533-017-0414-x. Rask, H., Schoenau, J., and Anderson, D., 2002, Factors influencing methane flux from a boreal forest wetland in Saskatchewan, Canada: Soil Biology and Biochemistry, v. 34, no. 4, p. 435– 443, https://doi.org/10.1016/S0038-0717(01)00197-3. Schulz, S., and Conrad, R., 1996, Influence of temperature on path- ways to methane production in the permanently cold profun- dal sediment of Lake Constance: FEMS Microbiology Ecology, v. 20, no. 1, p. 1–14, https://doi.org/10.1111/j.1574-6941.1996. tb00299.x. Schulz, S., Matsuyama, H., and Conrad, R., 1997, Temperature de- pendence of methane production from different precursors in a profundal sediment (Lake Constance): FEMS Microbi- ology Ecology, v. 22, no. 3, p. 207–213, doi:10.1016/S0168- 6496(96)00091-8. 136 Methane Emissions from Muds During Low Water-Level Stages of Lake Powell, Southern Utah, USA Malenda, M., Betts, T.A., Simpson, W.S., Wizevich, M.C., Simpson, E., and Sherrod, L. Geology of the Intermountain West 2020 Volume 7 Sebacher, D.I., Harriss, R.C., Bartlett, K.B., Sebacher, S.M., and Grice, S.S., 1986, Atmospheric methane sources—Alaskan tun- dra bogs, and alpine fen, and a subarctic boreal marsh: Chem- ical and Physical Meteorology, v. 38, p. 1–10, doi:10.3402/tel- lusb.v38i1.15059. Sherrod, L., Simpson, E.L., Higgins, R., Miller, K., Morgano, K., Snyder, E., and Vales, D., 2016, Subsurface structure of wa- ter–gas escape features revealed by ground-penetrating radar and electrical resistivity tomography, Glen Canyon National Recreation Area, Lake Powell delta, Utah, USA: Sedimentary Geology, v. 344, p. 160–174, doi:10.1016/j.sedgeo.2016.02.005. St. Louis, V., Kelly, C., Duchemin, É., Rudd, J., and Rosenberg, D., 2000, Reservoir surfaces as sources of greenhouse gases to the atmosphere—a global estimate: BioScience, v. 50, no. 9, p. 766–775, doi:10.1641/0006-3568(2000)050[0766:rsasog]2.0. co;2. Tinivella, U., and Giustiniani, M., 2012, An overview of mud vol- canoes associated to gas hydrate system, in Nemeth, K., edi- tor, Updates in volcanology—new advances in understanding volcanic systems: Rijeka, Croatia, Intech Open, p. 225–267, doi:10.5772/3390. U.S. Climate Data, 2020: https://www.usclimatedata.com/climate/ lake-powell/utah/united-states/usut0284. U.S. Bureau of Reclamation, 2017, Lake Powell—daily data: https:// www.usbr.gov/rsvrWater/HistoricalApp.html. University of Utah Seismograph Stations, 2020, Utah’s earthquake threat: https://quake.utah.edu/outreach-education/utahs- earthquake-threat. Whiticar, M.J., 1999, Carbon and hydrogen isotope systematics of bacterial formation of methane: Chemical Geology, v. 161, p. 291–314. Whiticar, M.J., Faber, E., and Schoell, M., 1986, Biogenic meth- ane formation in marine and freshwater environments— CO2 reduction vs. acetate fermentation—isotopic evidence: Geochimica et Cosmochimica Acta, v. 50, no. 5, p. 693–709, doi:10.1016/0016-7037(86)90346-7. Willis, G.C., 2012, Geologic map of the Hite Crossing–lower Dirty Devil River area, Glen Canyon National Recreation Area, Gar- field and San Juan Counties, Utah: Utah Geological Survey Map 254DM, 12 p., 1 plate, scale 1:62,500, GIS data. A-1 APPENDIX A Carbon to Hydrogen Isotope Theory Isotope ratio mass spectrometry (IRMS) was used to determine the quantity of 13C and 2H, or deuterium (D), in samples containing the greatest amount of CH4. Isotopic measurements as reported in parts per million (‰), are described by the following: 𝛿 = ( 𝑅𝑠𝑎𝑚𝑝𝑙𝑒 𝑅𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 − 1) ∗ 103 Where R is the 13C/12C or the 2H/1H ratios relative to the Pee Dee Belemnite (PDB) and the Standard Mean Ocean Water (SMOW) 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 CH4 in the 11 samples, analyses and conclusions regarding fractionation factors (carbon dioxide-methane [CO2-CH4], in this case) could not be made. B-1 APPENDIX B Gas Chromatography Sample Conditions Table B. The sampling and operating conditions used in the Agilent-based gas chromatography are listed below. Agilent G1888 Headspace Sample Conditions Headspace oven temperature: 42oC Loop temperature: 110oC Transfer line temperature: 110oC GC cycle time: 35 min Vial equilibration time: 0 min Pressurization time: 0.2 min Loop fill time: 0.2 min Loop equilibration time: 0.2 min Injection time: 0.1 min Agilent 7890 Operating Conditions Inlet temperature: 200oC Split ratio: 20:1 He carrier gas flow: 3 mL/min Column: Carboxen PLOT 30 m x 0.53 mm Oven temperature: 35oC for 5 min, 20oC/min to 200oC, hold for 10 min TCD temperature: 230oC TCD ref gas flow: 20 mL/min TCD makeup gas flow: 2 mL/min FID temperature: 230oC FID H2 flow: 30 mL/min FID air flow: 400 mL/min FID makeup gas flow: 3 mL/min TCD = Thermal Conductivity Detector FID = Flame Ionization Detector C-1 APPENDIX C Gas Chromatography Results Average gas compositions from all three years; individual compositions of samples collected during each year and compositions from volcanoes sampled over a multiple day period in 2016 are provided. Table C1. Summary of averaged gas chromatography results from all three years of sampling. All gas contents are reported in percent by volume (% v/v). Sampling Period Sample Size Known Sample Locations CH4 Standard Deviation CO2 Standard Deviation Air (N2 + O2) Standard Deviation July 2014 3 No 81.47 9.29 2.67 0.47 15.60 9.70 July 2015 21 No 32.40 15.31 0.19 0.21 67.37 15.29 May 2016 50 Yes 74.51 14.08 2.82 3.76 22.67 14.28 Tables C2A and C2B. Chromatography results from 2014 and 2015, respectively. All data are reported in percent by volume (% v/v). Table C2A: 2014 Gas Chromatography Data (% v/v) Table C2B: 2015 Gas Chromatography Data (% v/v) Sample # CH4 CO2 Air (N2 + O2) Sample # CH4 CO2 Air (N2 + O2) 1 91.40 3.20 5.40 1 0.00 0.13 99.90 2 80.00 2.50 16.70 2 13.20 0.65 86.10 3 73.00 2.30 24.70 4 37.80 0.21 62.00 Average 81.47 2.67 15.60 5 43.50 0.14 56.40 Standard Deviation 9.29 0.47 9.70 6 33.10 0.21 66.70 Maximum 91.40 3.20 24.70 7 23.10 0.11 76.80 Minimum 73.00 2.30 5.40 8 43.50 0.14 56.40 9 25.30 0.98 73.70 C-2 Sample # CH4 CO2 Air (N2 + O2) 10 37.10 0.30 62.60 11 44.60 0.18 55.20 12 45.50 0.12 54.30 13 41.10 0.18 58.70 14 45.10 0.13 54.70 15 45.20 0.20 54.60 16 3.00 0.07 97.00 17 41.40 0.07 58.50 18 32.70 0.18 67.10 19 2.00 0.05 97.90 20 44.20 0.24 55.50 21 39.10 0.10 60.80 22 40.00 0.23 59.80 Average 32.40 0.19 67.37 Standard Deviation 15.31 0.21 15.29 Maximum 45.50 0.98 99.90 Minimum 0.00 0.05 54.30 C-3 Table C3. Gas chromatography data from 2016 field season. The “X” in “16-X” corresponds to the volcano of interest, “A to C” indicates which day of the sampling period the gas was collected (with A being day 1, B is day 2, and C is day 3). Subsequent values following these letters distinguish duplicate samples from one another. For example, 16-20B1 and 16-20B2 were both taken from vent 16-20 on the second sampling day, while 16-7 was taken from a volcano on the first sampling day, 16-7B was taken from that same volcano on the second day, and 16-7C was taken on the third day. 2016 Gas Chromatography Data (% v/v) Volcano Sample Name CH4 CO2 Air (N2 + O2) 16-1A 89.68 2.09 8.23 16-1B 73.01 1.63 25.36 16-1 75.76 0.28 23.96 16-3A 74.60 1.50 23.89 16-3B 84.11 1.28 14.60 16-L3 80.45 0.77 18.78 16-4 76.08 1.04 22.88 16-5 54.30 1.99 43.71 16-5B 91.58 2.86 5.56 16-6 74.94 0.72 24.34 16-7 80.46 3.04 16.50 16-7B 90.52 4.20 5.28 16-7C 25.31 0.00 74.69 16-8B 78.98 1.42 19.59 16-9 77.61 0.70 21.70 16-10 84.25 0.00 15.75 16-10B 93.34 0.64 6.02 C-4 Volcano Sample Name CH4 CO2 Air (N2 + O2) 16-11 78.09 1.17 20.74 16-12 79.54 0.55 19.91 16-12B 79.05 2.16 18.78 16-12C 38.85 0.00 61.15 16-14 79.25 0.88 19.86 16-15 86.06 3.04 10.90 16-16 71.77 0.40 27.83 16-17 82.16 1.52 16.32 16-17B 80.00 2.55 17.44 16-17C 79.69 2.08 18.23 16-18 63.77 1.86 34.38 16-20 73.74 2.04 24.22 16-20B1 83.03 5.27 11.70 16-20B2 90.34 4.04 5.62 16-20B3 89.66 5.04 5.29 16-20C1 42.84 4.07 53.10 16-20C2 65.04 5.17 29.79 16-20C3 46.86 3.95 49.19 16-20C5 58.97 5.40 35.63 16-21 75.02 4.80 20.18 16-22 89.01 5.41 5.58 16-23 66.25 5.17 28.58 C-5 Volcano Sample Name CH4 CO2 Air (N2 + O2) 16-24 63.55 0.54 35.91 16-25A 91.09 2.37 6.54 16-25B 83.49 3.57 12.93 16-25B-1 75.40 2.71 21.89 16-25C 63.60 13.10 23.30 16-25C1 76.73 2.27 21.00 16-26 76.06 1.31 22.63 16-27 67.23 0.00 32.77 16-27B 79.08 0.77 20.15 16-27C 80.25 0.17 19.58 16-28 64.86 23.58 11.56 Average 74.51 2.82 22.67 Standard Deviation 14.08 3.76 14.28 Maximum 93.34 23.58 74.69 Minimum 25.31 0.00 5.28 C-6 Table C4. Temporal fluctuations over the three-day sampling period. Duplicate samples were averaged in this calculation. For example, 16-20B1 through 16-20B3 were averaged and considered as one sample of gas from vent 16-20 on day 2. CH4 CO2 CH4 CO2 CH4 CO2 Day 1 2 3 Max % (v/v) 91.09 3.04 93.34 4.79 80.25 7.68 Min % (v/v) 67.23 0.00 73.01 0.64 25.31 0.00 Average % (v/v) 77.71 1.51 76.17 2.40 57.95 2.43 Average Change from Prior Day % (v/v) 6.08 0.89 -24.68 -0.50 Average Day to day Percent Change % (v/v) -5.46 0.37 D-2 APPENDIX D Table D1. Isotope ratio mass spectrometry (IRMS) results of eleven 2016 field-season samples with elevated methane. Gas chromatography compositions presented below were analyzed in spring 2017 prior to IRMS characterization. Gas Chromatography Compositions (% v/v) IRMS Volcano Sample Name CH4 CO2 Air (N2 + O2) 𝛿13C‰ 𝛿D‰ 16-7B 74 6 20 -71.7 -306.2 16-10 61 4 35 -71.7 -298.8 16-17 60 2 38 -65.5 -281.5 16-12 59 1 40 -66.0 -284.7 16-10B 59 1 40 -68.5 -302.4 16-20B2 57 8 35 -60.7 -254.1 16-22 55 4 40 -67.0 -270.2 16-17B 54 3 43 -64.7 -277.9 16-20B1 53 9 39 -59.3 -247.0 16-3B 53 2 46 -63.1 -261.1 16-7 44 4 52 -63.7 -251.2 Average 57.18 4.00 38.91 -65.63 -275.92 Standard Deviation 7.32 2.68 7.94 3.99 21.05