Background 2 : Facies Model of of Hot Springs

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(1)BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. The Hot Springs Facies Model - Background The changes in the morphology (shape), water chemistry, and microbial life along the flow path of a hot spring differentiate the hot spring system into five distinct parts, called facies. The changes in water chemistry, water pH and flow rates influence the morphology (shape) along the flow path of a hot spring, which in turn differentiates the hot spring system into five distinct facies. Fouke et al., (2000), defines The Hot Springs Facies Model as the packets (or distinct structures) of limestone found along the primary flow path in a hot springs system. The facies represent the physical, chemical, and biological processes active in the environment at the time of deposition. They can be identified by the physical evidence left in sedimentary rock deposits. The type of sedimentary rock in hot springs facies is most often limestone, which is composed of the minerals calcite and aragonite. Both of these minerals are made up of calcium carbonate (CaCO3). Limestone forms in many different types of environments, but limestone that precipitates from hot springs is called travertine. The physical processes that create the travertine structures include the morphology (shape) of the underlying terrestrial surface and temperature of the water. Other physical influences include chemical. Figure 1a. characteristic of the spring water, such as pH, and biological communities. These biological communities include all of the plant, animal, and microbial populations that inhabit the environment. The water temperature of a travertine hot spring source is affected by the mixing of heated subsurface and meteoric (derived from precipitation) water. The mixing affects the amount of calcium carbonate (CaCO3) that dissolves out of the rock into the water and the rate at which the carbon dioxide degasses (or leaves the water). Recall that pH is the measure of the range of acidic to basic qualities of the water. Microbial organisms, or microbes, are responsible for the dominant biological activity in the hot springs facies. The microbial communities that inhabit many hot springs are easily identified by their various colors ranging from creamy white to dark green to brown.. Figure 1b.

(2) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. The Formation of Travertine UnderneathYellowstone there is considerable geothermal activity because theYellowstone region is volcanically active. Water from rain and snow filters through the ground over time and is heated to high temperatures through its contact with volcanic rock. This hot water moves upward through fissures (cracks) in the underground rocks and dissolves minerals in the rocks incorporating them into the water (Brock, 1994). This can take from 2000 to 11,000 years (Rye and Truesdell, 2007). Mammoth Hot Springs overlies limestone that was deposited during the Paleozoic era from 570 million to 245 million years ago when the region was covered by an inland sea (Smith and Siegel, 2000). Limestone (CaCO3) may be broken down by acidic volcanic waters into calcium (Ca +2) and carbonate (CO3-2) ions. Through further chemical reactions the carbonate is changed to carbon dioxide (CO2). As carbon dioxide levels increase, water becomes more acidic, resulting in a lower pH. The water is super-saturated with calcium ions (Ca +2) and carbon dioxide (CO2) by the time is arrives at the surface. Upon exposure to air at the surface, some of the carbon dioxide leaves the water through a process called degassing. An everyday example of degassing happens when a chilled can of soda pop is opened and allowed to sit on a counter for a few hours. Initially, there is a lot of carbon dioxide dissolved in the soda. However, once the pressure is released and the temperature lowers to match the air temperature, much of the carbon dioxide gas will escape into the air. Because of this, the soda becomes less acidic and its pH value increases. Degassing in a hot springs system occurs due to a variety of factors such as changes in temperature, pressure, turbulence, the length and angle of the flow path, and photosynthesis by microorganisms. When there is less carbon dioxide compared to calcium ions dissolved in the hot springs water, precipitation occurs. This is due to the reversal of the chemical reaction that caused the water to become super-saturated; calcium carbonate (CaCO3) precipitates out of the water. When a mineral precipitates out of the water it forms a solid. In this case, the mineral – calcium carbonate – creates travertine deposits. The Five Facies of Hot Springs Systems1 The hot springs facies model has five facies that can be found along the primary spring water flow path. They are: (a) vent, (b) apron channel, (c) pond, (d) proximal slope, and (e) distal slope. The vent (Figures 1a & 1b) is the location where the spring water first emerges at the surface. The vent may look like a bowl shaped depression or a narrow fissure. The water is hottest in this part of the facies because it is most affected by the underground heat source. The water temperature at the vent can be as high as 71-74 °C (159-165 °F). Vent water also contains very high amounts of dissolved carbon dioxide (CO2) and calcium (Ca+2) and carbonate (CO3 -2) ions. The dissolved carbon dioxide makes the water slightly acidic. Pure water is neutral and has a pH of 7. Water that emerges from the vent has a pH of approximately 6.1. All of the information for the facies model is based on Mammoth Hot Springs in Yellowstone National Park.. 1. Other hot springs can have lower or higher temperatures..

(3) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. The next facies along the flow path is called the apron channel facies (Figure 2a and 2b). It can be shaped like a gently sloped stream or have the characteristics of a billowing apron. The spring water moves quickly over this facies. As it flows, it degasses and precipitates calcium carbonate. At Mammoth, the water temperature at this facies ranges from around 69-74°C (156-165 °F). The pH increases a little to 7.0, due to the loss of carbon dioxide. A family of bacteria called aquificales can sometimes be found inhabiting the apron channel. These bacteria look like long, white or cream colored streamers and can be seen clearly in. Figure 2a Apron Channel. Figures 2a, 2b, and 3a. After the apron channel, the next facies is called the pond facies (Figure 3a and 3b). Fouke (2011) describes the ponds as “multiple semicircular depressions with a rim on one side” (page 92). Depending on the shape of the precipitated travertine, there may be many ponds in succession along the flow path, or a few very large ponds. The series of ponds form the characteristic set of step-like terraces found in the Upper Terraces at Mammoth Hot Springs. Ponds closer to the vent will have higher temperatures and lower pH (higher acidity) than ponds further from the vent. Figure 2b Apron Channel. There is also variability within the ponds themselves. If the primary flow path runs through the middle of the pond, it will have a higher temperature and lower pH than the water located in the more stagnant peripheral edges of the pond. The range in pond temperatures has been recorded from between 35-69°C (95-156 °F) and the pH varies from 7 to 7.9. Microbial life in the ponds can be very colorful (Figures 4a and 4b). It is not unusual to see aqua, peach, brown, green, or gold bacterial and algal mats. Some types of microbes are able to capture sunlight and undergo photosynthesis. Different types of microorganisms have been associated with different photosynthetic colors. The types of microbial communities that inhabit a hot spring will change due to fluctuations in environmental factors such as temperature, pH, sunlight, and flow rate (Fouke et al., 2000). The final two facies have names with the word “slope” in them. The first of these is called the proximal slope (Figures 5a & 5b) because it is located closer to the vent. This facies is usually found at an increase in the downward.

(4) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. Figure 3a Ponds. Figure 3b Ponds. Figure 4a Microbial Life - colors. Figure 4b Microbial Life - colors. Figure 5a Proximal slope. Figure 5b Proximal slope.

(5) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. slope. The “steps” on the proximal slope terraces tend to be more spread out than those of the pond facies and the corresponding pond-like features are much shallower. The temperature along the proximal slope flow path ranges from 28-54 °C (82-129 °F), and the pH increases to around 7.4. Remember that increasing pH values mean decreasing acidity. Microbial life along the proximal slope is not usually as colorful as it is in the ponds. This could be due to several factors including changes in acidity and temperature, as well as residence time of the microbes. The final facies is called the distal slope, (Figures 6a and 6b) so named because it is furthest from the vent. The distal slope is not as steep as the proximal slope. In fact, it can be nearly flat. At this point, the shape of the land. Figure 6a Distal Slope. Figure 6b Distal Slope. controls the flow path. The water in the distal slope facies has the lowest temperatures and highest pH values. This is because the water has traveled the greatest distance from the vent and much of the carbon dioxide has degassed, giving the constituents in the water time to reach equilibrium with the environment. The temperatures range from as low as ambient (environmental) temperature to as warm as 44 °C (111°F). The pH may be as high as 8.0. To have or not to have all five facies… Not every hot spring has all five facies. For example, Orange Spring Mound (in 2009) does not have all five facies. There are no ponds, and it is missing these characteristic terraced shapes (Figure 7a). Hymen Spring, near Liberty Cap had an Apron Channel that flowed right into a creek in 2006. This resulted in only two of the five facies (Figure 7b). The proximal slope may also be hard to identify; due to the road in 2008 (Figure 8), there is a limited area in which the spring water can flow naturally without taking over the roadbed. At other hot springs, the facies might be quite spread out along a long flow path, or there may be multiple vents, causing the facies to overlap among the flow paths. (See Figure 9). Sometimes, the vent is centered in a large basin, and there is no apron channel. The vent above Canary looked like this in 2008 (Figure 10). Additionally, the rate of flow, which can fluctuate wildly from day to day or even hour-to-hour, can cause the facies to prograde (move forward away from the vent) or retrograde (move backwards toward the vent). This may cause the water to be cooler or warmer and one facies may, for a time, have the physical characteristics of another facies. Finally, cooler carbonate springs, which have significantly different source water temperature will not develop the same five facies, and are not comparable. The mineral deposits used in these facies definitions can be formed only at carbonate hot springs whose source water is sufficiently similar (Veysey et al, 2008)..

(6) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. Figure 7a Orange Spring Mound. Figure 7b Hymen Spring. Figure 8 Orange Spring Mound. Figure 9 New Trail Spring panoramic All photos by A. Houseal taken at Mammoth Hot Springs between 2006-2010.. Figure 10 A basin-type vent at Canary Hot Spring.

(7) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. Summary of Broad Changes along the Flow Path In hot springs, water temperature decreases as it moves from the vent to the distal slope facies. This occurs because water heated underground emerges from the vent at temperatures up to 65-74 °C (149-165 °F). As the water moves further from the heat source, it becomes cooler, reaching temperatures of 44 °C (111°F) or less. The pH of the water also changes as water travels along the primary flow path. The initial pH is low because of the high amounts of carbon dioxide dissolved in the water. As the water travels down the flow path, it becomes less acidic. This transition occurs because carbon dioxide is degassed from the water due to changes in the temperature, pressure, turbulence, and interactions with microbial life. As the carbon dioxide degasses, pH increases and calcium carbonate precipitates and accumulates as travertine. Travertine accumulation rates depend on proximity to the vent. Accumulation rates may be as high as 30 cm/year in vent facies and decrease to 5 cm/year for distal slope facies (Fouke et al, 2000). In addition, the flow rate decreases as water moves away from the vent, flowing faster near the vent and much slower as it reaches the distal slope. However, there is not necessarily a linear phenomenon here as flow rates are also affected by slope and the result of flow on deposition of travertine. Water moves so slowly at the distal slope that it can sometimes be seen seeping into the ground. Higher temperatures are usually found within the primary flow path because the fast moving water has less time to cool than the slower moving water outside the flow path. Sometimes, the primary flow path can be identified by the lighter colored microbial communities (namely ones that flourish in these warmer temperatures), which may be flanked by darker colored microbial communities on either side An example of this can be seen in Figure 11a and 11b.. Figure 11a Microbial colors: lighter center, darker outside of flow path. Figure 11b Microbial colors: lighter center, darker outside of flow path. Finally, there are differences in the types of microbial life observed in the different facies. The apron channel is known for white streamer-like microorganisms from the family called aquificales. The ponds have brightly colored microbial mats and the proximal slope shows darker microbial mats. There is a correlation between the temperature of the water along the flow path and the types of microbial life. Lighter colors of microbes are associated with higher temperatures. Conversely, darker color microbes are associated with lower temperatures..

(8) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. Figure 12 (below) summarizes the hot springs facies model in a visual and table form. (Figure by B. W. Fouke).

(9) BACKGROUND Facies Model of Hot Springs. EXPEDITION: YELLOWSTONE! STaRRS. Extra Note for Teachers The Hot Springs Facies Model background information is intended for teacher use. It is not meant to be converted into a lecture and presented in this manner to students. In the curriculum, the student investigations piece includes the development of questions students may have about the hot springs. We have found that some of the more simple and profound questions that can be asked deal with the information presented above. Typically, students will hypothesize that the lighter colored microbial communities are cooler than the darker ones, which is actually opposite to what they can discover using simple tools (IR thermometers, meter sticks, photography, and observation). Inquiry-based teaching gives the students a chance to discover this on their own, which would be thwarted if the content were presented in lecture format and students were expected to memorize the characteristics of the system. In addition, this could also remove some of the inherently “cool” qualities of the hot springs system best discovered by simple observations; these observations can begin with students looking at Photo Point pictures taken of various hot springs provided in a lesson later on in this curriculum. References: Brock, T. D. (1994). Life at high temperatures. Yellowstone Association for Natural Science, History, and Education, Inc. Fouke, B. W., Farmer, J. D., Des Marias, D. J., Pratt, L., Sturchio, N. C., Burns, P. C., & Discipulo, M. K. (2000). Depositional facies and aqueous-solid geochemistry of travertine depositing hot springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, U.S.A.). Journal of Sedimentary Research. 70(3): 565–585. Fouke, B.W. (2011). Hot-Spring Systems Geobiology: Abiotic and Biotic Influences onTravertine Formation at Mammoth Hot Springs, Yellowstone National Park, USA. Sedimentology Decade Review. Rye, R.O., & Truesdell, A.H. (2007). The question of recharge to the deep thermal reservoir underlying the geysers and hot springs of Yellowstone National Park. In Integrated Geoscience Studies in the Greater Yellowstone Area: Volcanic, Tectonic, and Hydrothermal Processes in the Yellowstone Ecosystem. L.A. Morgan, ed., 235–270. U.S. Geological Survey. Smith, R. B., & Siegel, L. J. (2000). Windows into the earth: The geologic story of Yellowstone and Grand Teton national parks. Oxford University Press. Veysey, J., Fouke, B.W., Kandianis, M.T., Schickel, T.J., Johnson, R.W., & Goldenfeld, N. (2008). Reconstruction of water temperature, pH, and flux of ancient hot springs from travertine depositional facies. Journal of Sedimentary Research 78: 69–76..

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