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Caldera chronicles: Aging Yellowstone's geysers no easy task

Caldera chronicles: Aging Yellowstone's geysers no easy task

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Excelsior Geyser and Grand Prismatic Spring

This aerial view shows Excelsior Geyser (in the foreground) and Grand Prismatic Spring in Yellowstone’s Midway Geyser Basin. The colors around the thermal features are locations of different thermophile communities. These thermophiles fix carbon from the atmosphere and from the hot water. 

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dakota Churchill, physical scientist with the U.S. Geological Survey and the University of California, Berkeley; Michael Manga, professor at the University of California, Berkeley; Shaul Hurwitz, research hydrologist with the USGS; Joe Licciardi, professor at the University of New Hampshire; and Jim Paces, research geologist at the USGS.

Many visitors to Yellowstone wonder, "how long have these geysers been erupting?” The answer lies in the deposits left by geyser eruptions.

We know that prior to about 15,000 years ago, Yellowstone was covered by a thick ice cap that likely scoured away any older geyser deposits. Thus, we can assume that the physical edifices of Yellowstone’s geysers formed since the glaciers retreated.

We also have the drawings and photos of geysers by scientists from the early expeditions to Yellowstone. If we compare current geysers images with those taken almost 150 years ago, we can infer that it takes millennia, rather than centuries, to construct large geyser cones. Yet we hope to know more about how geysers evolve: When did Yellowstone’s geysers begin forming? How long does it take to form large geysers? Are large geysers constructed mainly during specific climate episodes that are favorable to their formation?

When faced with questions regarding the ages of rocks or time scales of geologic processes, geologists often turn to radiometric dating. Different radiometric dating methods are available, and the use of any given method depends on the rate of decay of each naturally occurring radioactive isotope as well as the type of material available for analysis. For carbon-bearing deposits younger than about 50,000 years, radiocarbon dating is the primary method.

Radiocarbon (14C) dating depends on production of 14C in the upper atmosphere, which quickly mixes with air circulating around the globe. Atmospheric CO2 consists of two stable carbon isotopes, carbon-12 (98.9% of the total carbon) and carbon-13 (1.1%), along with a tiny amount of carbon-14 (one part per trillion carbon atoms) that decays to nitrogen-14 with a half-life of 5,730 years. This means that after 5,730 years half of the original number of 14C atoms will have transformed to nitrogen-14.

All three carbon isotopes are assimilated into plants as they take in CO2 for photosynthesis, or into animals when they eat the plants. Once the organism dies, it stops incorporating “modern” carbon, and the 14C already in the organic material decays without being replenished. The concentration of 14C in the organic material decreases at a predictable rate over time, forming a natural geologic clock. By measuring the concentration of 14C remaining in a sample of organic material, like charcoal from burned trees, its age can be calculated. For developing the radiocarbon dating method in the 1940s, Willard Libby received the 1960 Nobel Prize in Chemistry.

In a recent study, scientists attempted to determine the age of geysers using radiocarbon dating. Sinter samples were collected from Castle and Giant Geysers in the Upper Geyser Basin, near Old Faithful. The sinter was then dissolved in the laboratory, and the organic material residue was extracted for dating.

Concentrations of the three carbon isotopes were measured using an Accelerator Mass Spectrometer, which can measure extremely low concentrations of isotopes very precisely. However, lab results were surprising and unexpected. The 14C ages calculated for the samples did not coincide with their stratigraphic position in the geyser structure. Some of the oldest 14C ages were found at the top of the cone where sinter formed most recently, and young 14C ages were found in layers at the base of the geyser, where deposits are likely to be oldest.

What might cause this unexpected result?

Organic material that is trapped in the sinter comes either from material blown in by the wind, such as pollen or needles from nearby trees, or from microbial mats (thermophiles) that grow on the sinter. These microbes incorporate carbon not only from the atmosphere, but also from water erupted from the geysers.

The carbon dissolved in the water can be derived from multiple sources, including from older 14C-free rocks in the subsurface, or from deep magmatic gases that are also 14C-free. As a result, the 14C-derived ages do not have a strictly atmospheric origin and thus do not accurately reflect how long ago the microbes died.

Attempting to obtain more reliable results the study turned to alternative radiometric dating methods, such as the uranium-thorium disequilibrium method. This method is based on the isotopic compositions of radioactive uranium and thorium incorporated in the opal that makes up the sinter.

Unfortunately, the sinter contains very low uranium concentrations and elevated thorium concentrations that yielded post-glacial ages (younger than 10,000 years), but with large uncertainties that limit the utility of the resulting dates.

The study also applied a method called “surface exposure dating,” which uses radioactive isotopes to measure the length of time that a rock has been exposed to cosmic rays at, or near, the Earth's surface. In this study, concentrations of the radioactive isotope beryllium-10 measured in the sinter indicated that contamination from past rainwater strongly influenced concentrations in the opal, rendering the resulting date unreliable.

For now, we can only conclude that dates obtained from this study: 1) confirm that Yellowstone’s large geysers indeed formed after glaciers receded from the caldera about 15,000 years ago; and that 2) it takes thousands of years to construct the large geyser cones.

Ongoing research is exploring other geochemical approaches to deciphering geyser evolution. The story certainly doesn’t end here. Stay tuned for more information as geologists make new attempts to answer the question of how old an individual Yellowstone geyser might be.

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