Crystal Geyser: An Unusual Cold Spring System, Grand County

Crystal Geyser is a cold carbon dioxide (CO2) geyser, part of a natural spring system along the Little Grand Wash fault south of Green River, Utah (figure 1). The spring system hosts a series of CO2-driven geysers and springs with active and fossil microbial mats and tufa deposits composed of carbonate and iron oxide and iron oxyhydroxide minerals (Potter-McIntyre and others, 2017; Knuth and Potter-McIntyre, 2018) (figure 2). Additionally, progressively older carbonate spring deposits crop out on some of the topographic highs in the area because these relatively erosion-resistant deposits armor the paleo-land surface and slow down erosion (Shipton and others, 2004; Burnside, 2010). Recent radiometric U-Th dating of carbonate terraces and embedded veins reveal that CO2-charged fluid has constantly leaked to the surface for over 400 thousand years during the Pleistocene (Burnside, 2010). Crystal Geyser is a popular place for tourists, and it is not uncommon to see children playing in the spring.


Presidents Message
I have had the pleasure of working with many diff erent geologists from all around the world. As I have traveled around Utah for work and pleasure, many times I have observed vehicles parked alongside the road with many people climbing around an outcrop or walking up a trail in a canyon. Whether these people are from Utah or from another state or country, they all are quick to mention to me how wonderful our geology is here in Utah.
Utah is at the junction of several diff erent geological provinces. We have the Basin and Range to the west and the Central Utah Hingeline and Th rust Belt down the middle. Th e Uinta Mountains have outcrops of some of the oldest sedimentary rock in Utah. Utah also has its share of young cinder cones and basaltic lava fl ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. Th e general public comes to Utah to experience our wonderful scenic geology throughout our state and national parks. Driving between our national and state parks is a breathtaking experience.
Th e "Utah Geosites" has been a great undertaking by many people. I wanted to involve as many people as we could in preparing this guidebook. We have had great response from authors that visit or work here in the state. Several authors have more than one site that they consider unique and want to share with the rest of us. I wanted to make the guidebook usable by geologists wanting to see outcrops and to the informed general public. Th e articles are well written and the editorial work on this guidebook has been top quality.
I would like to personally thank Mark Milligan, Bob Biek, and Paul Inkenbrandt for their editorial work on this guidebook. Th is guidebook could not have happened without their support. I would like to thank Jenny Erickson for doing the great desktop publishing and the many authors and reviewers that helped prepare the articles. Your work has been outstanding and will certainly showcase the many great places and geology of Utah. Last, but not least, Th ank you to the American Association of Petroleum Geologists, Rocky Mountain Section Foundation for their fi nancial support for this publication.
Guidebook 48 will hopefully be a dynamic document with the potential to add additional "geosites" in the future. I hope more authors will volunteer articles on their favorite sites. I would like to fi ll the map with locations so that a person or family looking at the map or articles will see a great location to read about and visit. Enjoy Guidebook 48 and enjoy the geology of Utah.

INTRODUCTION
Crystal Geyser is a cold carbon dioxide (CO 2 ) geyser, part of a natural spring system along the Little Grand Wash fault south of Green River, Utah (figure 1). The spring system hosts a series of CO 2 -driven geysers and springs with active and fossil microbial mats and tufa deposits composed of carbonate and iron oxide and iron oxyhydroxide minerals (Potter-McIntyre and others, 2017;Knuth and Potter-McIntyre, 2018) (figure 2). Additionally, progressively older carbonate spring deposits crop out on some of the topographic highs in the area because these relatively erosion-resistant deposits armor the paleo-land surface and slow down erosion (Shipton and others, 2004;Burnside, 2010) (figures 1 and 2). Recent radiometric U-Th dating of carbonate terraces and embedded veins reveal that CO 2 -charged fluid has constantly leaked to the surface for over 400 thousand years during the Pleistocene (Burnside, 2010). Crystal Geyser is a popular place for tourists, and it is not uncommon to see children playing in the spring.
The Crystal Geyser conduit is actually an abandoned petroleum exploration well through which water emanates. Surrounding the pipe are terraces of primarily carbonate mineral deposits (Shipton and others, 2004; Potter-McIntyre and others, 2017; figure 2), dull to brilliant orange in color owing to minor iron precipitated from the spring water (figure 2). These terraces cascade down to the river, and include orange and green pools depending on the microbes within them-the green color indicates photosynthesizing microbes. Larger terraces are composed of multiple small terracettes that are thought to be microbially-induced structures (Fouke and others, 2000). Also present around the drill pipe are collections of spheroidal mineral masses called pisoids. These are formed from agitation of minerals when the geyser erupts, causing spheres of precipitate to roll around and accrete new layers of carbonate minerals.

DIRECTIONS
From I-70 head south off the east Green River exit 164 and then turn east. Take a right at the sign for Crystal Geyser and follow the road. The road is a graded dirt road that is generally in good condition. If it has been raining a lot, the road may be more difficult to navigate. About halfway between the hairpin turn and Crystal Geyser, an oil seep is just off the north side of the road.

Where Does the Water Come From?
The artesian spring water emanates from deep subsurface reservoirs along geologic faults that bound Salt Wash and Ten Mile graben (Jung and others, 2014; figure 3). The source reservoirs are Jurassic and Permian units that recharge at the San Rafael Swell to the west (Baer and Rigby, 1978;McPherson and Heath, 2009;Dubacq and others, 2011;Kampman and others, 2014). The spring water is CO 2 -and methane-charged, saline, and of neutral pH (6.2-7; Shipton and others, 2004;Potter-McIntyre and others, 2017). The source of CO 2 is likely from decarbonation of Paleozoic carbonate rocks (Leadville Limestone) deep below the reservoir   (Shipton and others, 2004;Heath and others, 2009;Kampman and others, 2009;Probst and others, 2017). Th e tufa deposits are spatially dispersed and of variable volumes, suggesting that the location of CO 2 leakage has varied over geologic time depending on the ability of the faults to transmit fl uid (permeabilty). Th e subsurface strata and faults exhibit strongly heterogeneous permeability owing to seismic activity, regional erosion, both mineral dissolution and precipitation, and changing fl uid fl ow volumes owing to variability in climate over time (Burnside, 2010;Burnside and others, 2013;Kampman and others, 2014).

How Does Crystal Geyser Work?
As mentioned in the introduction, Crystal Geyser formed via a human-drilled wellbore. An oil seep along the Little Grand Wash fault motivated drilling of the oil exploration well in 1935 (Baer and Rigby, 1978). Th e well never produced oil; however, CO 2 dissolved and pressurized in the artesian aquifer at depth now discharges through the wellbore. Th is open conduit allows rapid depressurization and discharge as episodic geyser eruptions. Following each eruption, the wellbore again fi lls with water from the bottom up, with pressure building up during fi lling. Th e artesian pressure ultimately exceeds the rate of refi ll and causes another eruption (Watson and others, 2014). Th e cycle repeats, sometimes aft er a few hours and sometimes as long as a day or more between eruptions. Crystal Geyser's eruption intervals, durations, and intensities were at one time regular and consistent, but timing now is quite erratic, possibly related to vandalism. Tourists have dropped rocks and even reportedly dynamite into the geyser (Shipton and others, 2004). Other possible factors for variable eruption rates include seismic activity (Han and others, 2013) and/or interactions between recharge rates and CO 2 migration rates within the artesian aquifers (Kampman and others, 2014).

SITE OF ACTIVE SCIENTIFIC RESEARCH
Crystal Geyser and its related spring system are subjects of active scientifi c research on topics ranging from global warming to the search for extraterrestrial life. Th is section discusses topics of recent research, including analysis of the geyser's source aquifer as an analog to engineered carbon capture and sequestration, followed by analysis of microbial life, interactions between microbes and mineral precipitation and how these processes off er insight regarding the search for life on Mars and beyond.

CO 2 Sequestration
Global warming of our planet is attributable to the greenhouse eff ect, specifi cally increasing concentration of anthropogenic CO 2 in the atmosphere that traps heat from solar radiation aft er it is refl ected from the earth's surface (e.g., Scheff er and others, 2006; Eby and others, 2009;Notz and Stroeve, 2016;Specht and others, 2016). Many ideas have been proposed to reduce CO 2 emissions and the greenhouse eff ect, one of which is carbon capture and sequestration (e.g., Yang and others, 2008;Dai and others, 2013;Rahman and others, 2017;Rackley, 2017). Carbon capture and sequestration (CCS) includes capture of CO 2 at point sources such as cement plants and power plants, pressurizing and condensing it to a fl uid and then injecting that fl uid into subsurface reservoirs. Th e fl uid that emanates at Crystal Geyser comes from a natural subsurface CO 2 reservoir, but it leaks via migration upward along faults. Th e spring water degasses its CO 2 at the springs and geysers and eff ectively emits CO 2 into the atmosphere, similar to industrial sources (but much smaller in volume). Understanding how this gas moves upward, how the emissions vary from site to site along faults, and what impedes or promotes fl ow are all very important parameters to know before CCS becomes a viable mitigation strategy for anthropogenic CO 2 emissions (e.g., Shipton and others, 2005;Gouveia and Friedmann, 2006;Burnside and others, 2013;Watson and others, 2014).

Astrobiology
Biosignatures are preserved fi ngerprints of past microbial life, which is the type of life scientists are searching for on Mars and icy moons within our solar system. Th ree types of biosignatures   north-northwest (upriver). To the right, the exposed rocks are the middle Jurassic section. As one drives along the Little Grand Wash Fault (see fi gure 1), the grey rocks to the south of the road are the Cretaceous Mancos Shale. Th ese rocks are younger than the exposed rocks to the north of the fault and were downthrown relative to the Jurassic rocks. Th e well is in green and it extends 2627 feet below the surface. It is not cased, so the CO 2 -charged water fl ows into the pipe in both the Entrada Sandstone and the Navajo Sandstone reservoirs (Watson and others, 2014). However, this fault serves as a conduit for fl uid to fl ow upward to the surface and come out at Crystal Geyser, and for the oil seep you passed on the way in. Jurassic rocks are in yellow and the Cretaceous rocks in green.

Icy Moons
Enceladus and Europa are high priority targets for future exploration because of their subsurface oceans, which make them potentially habitable environments (Hendrix and others, 2019). Th ese moons exhibit plumes (geysers) of subsurface water that erupts to the surface. Th ese plumes would make excellent targets for understanding the habitability of Enceladus and Europa because of their relative ease of accessibility. Studies of the microbial life deep within the Crystal Geyser waters have found a diverse population with adaptations to reside in CO 2 -rich, saline environments (Santillan and others, 2015;Emerson and others, 2016;Probst and others, 2017;Knuth and Potter-McIntyre, 2019; fi gure 4). Ongoing studies seek to fi nd ways to determine habitability from the geyser plumes to help design future missions.

SUMMARY
Crystal Geyser is a fascinating example of a rare cold spring and geyser system. It is a treasure trove of scientifi c information, as well as just a fun and scenic place to visit. Spend some time hiking around and looking at the fault and the older tufa deposits, and think about how these formed throughout the millennia-and think about similar features on Mars and other celestial bodies in our solar system! Burnside, N.M., Shipton, Z.K., Dockrill, B., and Ellam, R.M., 2013, Man-

Mars
On Earth, microorganisms commonly enhance mineral precipitation and mediate mineralogical and chemical compositions of resulting deposits (e.g., Reid and others, 2000;Dupraz and others, 2009;Petryshyn and others, 2012;Corkeron and others, 2012). Many of the features at Crystal Geyser are thought to be created by microbes, such as the terracettes and the green color of some of the pools and even the orange color (Emerson and others, 2016;Potter-McIntyre and others, 2017; fi gure 2). Even though some research seems to suggest abiotic precipitation plays a large part in carbonate formation at springs due to degassing of CO 2 (e.g., Fouke and others, 2000;Takashima and others, 2011;Knuth and Potter-McIntyre, 2019), those studies acknowledge that microbial metabolisms do aff ect precipitation, particularly in minerals forming away from the vents (Fouke and others, 2000;Takashima, 2011