Seeking the origin of Yellowstone’s Travertine Terrace Formation: are the bugs involved?

The essential tool. As a microbial ecologist, Dr. Alison Murray spends a lot of time at a microscope when she’s not sampling hot springs for microbes. This epifluorescence microscope is in the Microscopy Laboratory of DRI’s Division of Earth and Ecosystem Sciences.(Photo by John Doherty)

About three million people each year visit Mammoth Hot Springs on the northern edge of Yellowstone National Park to see the extraordinary, quickly changing geological formations produced by the springs’ mineralized waters. As the geothermal fluids descend toward exotic algae-colored ponds, the dissolved limestone settles out rapidly to create elaborate travertine terraces, shelves and dams that constantly rearrange the flow patterns as well as the landscape’s architecture.

Among the visitors to the springs this June was a scientist from the Desert Research Institute who has shifted her analytical gaze from such settings as deep ocean thermal vents and freezing oceans along Antarctica’s ice sheets. A microbial ecologist, Dr. Alison Murray is fascinated by the nature of microscopic life in extreme environments, and in this project she’s curious to determine whether microbes play a role in the park’s popular terraces.

Murray’s work is funded by a $1.1 million grant from the National Science Foundation. The principal investigator on the project is Dr. Bruce Fouke, a geologist from the University of Illinois Urbana-Champaign. The interdisciplinary team also includes University of Illinois physicist Dr. Nigel Goldenfeld, who will develop models to predict the travertine formation process. The multidisciplinary nature of the team reflects the complexity of the challenge.

Winter terraces at Yellowstone. Looking like frozen waterfalls, these are actually limestone deposits connecting travertine terraces formed from geothermal springs.(Photo by Dr. Bruce Fouke)

The geothermal water temperatures steadily decline from a high of 160°F as the fluids run from pool to pond, Murray says. Preliminary visits to the hot springs by the Fouke research group have determined that the terraces are home to a vastly diverse assemblage of microbes. Many of the microbes are limited to a specific temperature regime, perhaps to a single pool, while others are found throughout the system.

Colorful microbial mats coat the travertine terraces throughout the thermal spring system. Murray notes that recent work suggests that there’s a good possibility that a more complex biogeochemical process is at work that involves the microscopic “bugs.”

“The conventional explanation for these formations has always assumed that they form by purely physical processes,” Murray says. “This basically involves the dissolved limestone or calcium carbonate precipitating out as the water cools on rocks and previous layers of the terrace.

Yellowstone’s intricate terrace architecture. This close up of one of Mammoth Hot Springs’ travertine terrace shows the elaborate natural layering resulting from the rapid deposition of calcium carbonate—dissolved limestone—as the geothermal fluids flow downhill. The layers are about a half inch thick.(Photo by Dr. Bruce Fouke)

“The basic question is: are biodiversity and activity of specific living microbes and/or microbial communities required to create the terraced architecture? These features are observed in high-temperature and low-temperature carbonate spring deposits all over the world,” Murray points out, “but the possible role of a microbial ecology being involved has never been fully examined.”

The terraces can rise up to five millimeters, or about a fifth of an inch a day, a rate of deposition that makes it possible for the research team to conduct nearly real-time experiments. These will involve comparisons between natural deposition and experimental conditions where the microbial communities are removed with filtration or rendered inactive with ultraviolet rays. A detailed “census” of microbial communities and assessment of the range of various microbial populations will also be conducted to help describe the ecologies in the system.

“What we find here will provide fundamental knowledge of microbe-water-mineral interactions during carbonate precipitation. This is knowledge needed to reconstruct more accurately the history of microbial life not only on Earth, but also to anticipate what might be encountered on other planets.”

From Murray’s point of view, if the bugs are involved in the springs’ amazing architecture, it’s time they got credit for their work.

–John Doherty