Lakeside Junior High
Mammoth Hot Springs, Yellowstone National Park.
Madison limestone formed about 370 million years ago at the bottom of a shallow ocean that covered much of what is now the western United States. This formation is hundreds of meters thick and was later covered by volcanic rocks from the Absaroka Volcanoes and then by the three eruptions of Yellowstone. These volcanic layers have eroded sufficiently to leave the limestone closer to the surface in this area.
Mammoth Hot Springs are located just outside and approximately 34 kilometers northwest of the present Yellowstone Caldera. Mammoth Hot Springs occur along the same fault system as the hot springs of Norris Basin. Norris is closer to the caldera, located less then 3 kilometers from the rim. This proximity translates into higher heat flow at Norris. Because Mammoth is further from the heat source, the water does not get as hot as the Norris Basin and we find temperatures ranging from 45-90 degrees Celsius - not boiling hot, but close!
Meteoric water (from rain and snow) seeps into the ground and is warmed deep within the limestone deposits by heat from the magma chamber below. Calcium carbonate (CaCO3) of the Madison limestone is dissolved into the heated and rising water. The Madison also contains a bit of the calcium sulfate mineral gypsum, CaSO4•2H2O, and it dissolves too. The sulfate from the gypsum reacts with hydrogen dissolved in the water to create hydrogen sulfide (H2S).
As the hot water rises through the limestone and volcanic rocks to the surface, the pressure decreases and hydrogen sulfide and carbon dioxide gases are released. These gases bubble out at the surface. The hydrogen sulfide is the source of the rotten egg smell at the springs and is utilized by chemotrophs, resulting in sulfuric acid (H2SO4) (chemotrophs are organisms that obtain energy by the oxidation of electron-donating molecules, in contrast to phototrophs, which utilize solar energy as their source of energy).
As carbon dioxide escapes, the acidity of the water decreases. The sulfuric acid is neutralized by the calcium carbonate (the “Tums effect”). These reactions result in a hot springs pH of 7.5 – 8 according to the data collected in the field. With the increase in pH, the dissolved calcium carbonate is comes out of solution to be deposited on the surface as travertine (CaCO3).The travertine deposits grow quickly, some as much as three meters in one year. Because these deposits grow so rapidly they cover the surface and anything on it (trees, stumps, the boardwalk etc.) in a very short time — often covering the source of spring water and causing new springs to open in other locations. The travertine forms terraces with raised edges that create a pool of water on the terrace. The terraces continue to grow as water flows over the edge and down the sides. Other factors that can affect the building of the formations are the water flow increasing and decreasing with the seasons, air temperature changes, and water temperature gradient changes resulting in altered biological activity. Earthquakes can also cause a source to close or open.
This harsh environment is a perfect place for extremophiles, organisms adapted to life in extreme conditions. There are three types present at Mammoth Hot Springs, all use different sources of energy and electrons to drive their metabolism. The chemotrophs are organisms that thrive in high temperatures and metabolize hydrogen sulfide coming from the source. These organisms used the hydrogen sulfide for both energy and electron sources. Because the concentration of hydrogen sulfide is greatest nearer the source, we find these organisms in the clear, colorless center of the pools. This location is also where the highest temperatures (75 – 90 degrees Celsius) are found and why no photosynthetic organisms are present. It’s too hot for those critters!
Anoxygenic phototrophs drive their metabolic processes by taking one photon of light in the red to deep red end of the visible light spectrum (and near-infrared light) for energy, and using sulfur for their electron source. There were no anoxygenic organisms observed at this location, though they can be observed in other formations of Mammoth.
The oxygenic phototrophs drive their metabolic processes by using two photons of visible light (red and blue), which more than triples the energy available. This abundant energy is needed to break the strong bond between the hydrogen and oxygen in water, their electron source. These photosynthesizers aggressively push out other organisms when temperatures and pH are ideal. They cannot tolerate temperatures above 72 or below 40 degrees Celsius. The pH levels in the alkaline range are optimal for these organisms.
Which organisms are where? Let’s begin in the center of the pool which is actually located a bit off to the right, but you
can see the bluish grey color surrounding the source of the spring (middle right hand side of the photo).
This area is where we would expect to find the chemotrophs “eating” the hydrogen sulfide as it bubbles up to the surface.
Again, this is the hottest part of the pool. Moving towards the edge of the pool, the temperatures decrease and the pH becomes more
alkaline allowing for the oxygenic photosynthesizers (the cyanobacteria Calothrix and Phormidium) to thrive.
These organisms create the orange and brown colors we see at the edge of the pool.
This image shows the terraces just below Canary Springs. The white sections are the travertine and the orange,
yellowish, and brown colors are due to the cyanobacteria living in the springs. The trees that you see have thick deposits
of travertine around them, basically burying them in a calcite tomb. There is a fallen tree on the bottom left side of the picture that
is being covered by the deposits. The trees, leaves, sticks, and other organisms that are covered in the travertine
could also be used to inform scientists about the life that was once there.
What is the connection between this formation in Mammoth and astrobiology? Because the organisms present at Mammoth represent the earliest forms of life on Earth, scientists can study them to learn how life has evolved on Earth and about the possibility for existing or extinct life forms on other planets. By understanding the conditions in which life can live, researchers can explore other planets for similar conditions – at present or in the past. Scientists can search other planets for "biomarkers," physical or chemical signatures indicating the presence of life or of past life on a planet. One method is to examine reflectance spectra from other planets to see if there are indications of photosynthesis, or mineral deposits associated with organisms or the resources that life needs. Scientists could look for calcite deposits like the ones in Mammoth as evidence of environments water is present and where organisms may live. They could look for the layered terrace patterns formed by the downstream flow of the water as the deposits are laid down. Perhaps there could be fossilized bacterial mats created by the cyanobacteria or bacteria filaments that were covered by calcite deposits.
Earth's Extremophiles home page