Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lauren Harrison, a postdoctoral researcher with the U.S. Geological Survey.

Travertine is a form of limestone composed of the minerals calcite and aragonite, which are both made of calcium carbonate (CaCO₃). In contrast to limestones that are formed in the ocean from the shells of tiny plankton and other marine organisms, travertine precipitates from high-temperature hot springs when hot water (25–73 degrees Celsius or 77–163 Fahrenheit) is expelled from the subsurface. The decrease in pressure and temperature at the surface causes degassing of carbon dioxide dissolved in the water (similar to when you open a can of soda), which in turn causes calcium carbonate to precipitate.

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Rates of travertine deposition at Mammoth Hot Springs in Yellowstone National Park are very high (~3 millimeters per day) compared to other calcium carbonate systems. For example, fast-growing corals deposit travertine at an average rate of 1 millimeter per day, and the rate is ~0.2 millimeters per day for calcium carbonates deposited by cold freshwater (called tufas). Rates for calcium carbonate growth in the deep ocean or in terrestrial caves are even slower! One of the causes of the high deposition rate of travertine in Yellowstone is because thermophilic bacteria living in Yellowstone’s thermal waters encourage the precipitation of travertine.

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Travertine is a particularly interesting “geologic clock” because of the unique arrangement of calcium carbonate atoms in the aragonite and calcite. The spaces available between calcium, carbon, and oxygen atoms in the CaCO₃ mineral lattice allow for some trace elements that were dissolved in the water, like uranium, to be incorporated into the mineral structure. Other elements that are too big and don’t have the right ionic charge—like thorium, a product of the radioactive decay of uranium—are excluded. Scientists can therefore assume that any thorium measured within the crystal lattice of the mineral originated from the decay of the uranium that was locked into place when the aragonite or calcite formed. Because the rate of uranium radioactive decay is known, the age of the mineral can be calculated by measuring the amounts of parent uranium atoms and daughter thorium atoms in a sample of travertine. The concept is essentially the same as how rhyolite lava flows are dated at Yellowstone, although that procedure makes use of the decay of different radioactive elements.

The technique for determining the age of travertine was originally used by Neil Sturchio and his USGS collaborators on old travertines from Terrace Mountain and Mammoth Hot Springs in the north part of Yellowstone National Park. They used these ages to determine the timing of glacial advance and retreat during past ice ages in Yellowstone, finding that travertine-depositing hot springs migrated to higher elevations when glaciers occupied valleys or ceased activity entirely when glaciers covered the entire area.

The most well-known travertine-depositing hydrothermal area in Yellowstone is Mammoth Hot Springs. There, new travertine is continuously deposited, and old travertine outcrops extend for many miles to both the north and south. Travertine also occurs in small amounts at other locations within Yellowstone National Park, including in the Upper Geyser Basin, near Firehole Lake in Lower Geyser Basin, at Terrace Spring near Madison Junction, and along the Upper Snake River near the Southern boundary of Yellowstone National Park.

Travertine deposited outside of Yellowstone caldera, like at Mammoth Hot Springs, occurs in thick deposits because the subsurface geology outside the caldera includes shallow Paleozoic and Mesozoic sedimentary rocks that hot waters dissolve as they flow through, providing the large amounts of calcium and carbonate necessary for travertine formation. Conversely, the subsurface within Yellowstone caldera is underlain by thick rhyolite flows that are typically poor in calcium, so thermal waters have much lower calcium concentrations. The existence of travertine within Yellowstone caldera, like at Lower Geyser Basin, therefore requires conditions that are different from those at Mammoth Hot Springs, such as large influxes of cold meteoric water into the hydrothermal system to cool and dilute the amount of silica in caldera thermal waters. One potential source for this water is melting of glaciers, which during the most recent glaciation covered geyser basins within the caldera with up to a kilometer of ice until they receded about 15 thousand years ago. Another possibility is that travertine deposition in the geyser basin records periods of extreme precipitation that lasted hundreds to thousands of years and therefore may be an excellent indicator of Yellowstone’s past climate conditions.

Researchers at the U.S. Geological Survey are currently working on deciphering the record contained within travertine deposits from Yellowstone caldera. Stay tuned to hear the story Yellowstone’s travertines have to tell!

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