"The existence of life itself can change the physical and chemical attributes in an environment of deposition," said Bruce Fouke, a geologist at the University of Illinois. "By studying the effects of microbial metabolism on the chemistry of the water and on the way minerals are deposited in Earth environments, we can better interpret samples from other planets for signs of life."

For example, various carbonate features -- including tiny, rod-shaped calcite crystals -- found in the Martian meteorite ALH84001 could have been formed by either organic or inorganic means. To help interpret whether such shapes are indicative of life, Fouke has established a systematic model for the deposition of travertine by actively flowing hot springs at Angel Terrace at Mammoth Hot Springs.

"Travertine is a crystalline form of calcite that forms where subsurface waters erupt, cool, de-gas and precipitate calcium-carbonate minerals with a variety of crystal morphologies and chemical compositions," Fouke said. "In this setting, we are examining the environmental feedback mechanisms that exist between water, microbes and the precipitation of travertine."

Mammoth Hot Springs, near the northern boundary of Yellowstone National Park, is one of the world's largest sites of travertine accumulation. The travertine deposits at Mammoth Hot Springs are approximately 8,000 years old, 73 meters thick and cover more than 4 square kilometers.

"Yellowstone is an ideal laboratory because of the high precipitation rates and the abundance of microbes," Fouke said. "By documenting where we find certain calcite shapes in the spring system, we can link those shapes with a particular water flow, chemistry and microbe. With that environmental context, we can start to decipher the geological record and to reconstruct ancient environments."

Geochemical evaluation of the spring water and underlying travertine has suggested that inorganic processes such as carbon dioxide de-gassing, temperature decreases and possibly evaporation are the primary environmental controls on travertine mineralogy, Fouke said. "So the environmental context could be the key to determining whether or not a particular feature is an entombed microbe."

On Earth, microbes also can be found trapped in fluid inclusions in ancient calcite crystals. Fouke is working with UI microbiologist Abigail Salyers to develop techniques to liberate the microbes and isolate, extract, amplify and sequence their DNA.

"The genetic analysis will provide additional information about the microbes' metabolism," he said. "We will incorporate this information into our depositional model to help link the presence of ancient life with suspect, calcium-carbonate depositional features and chemical compositions."

Fouke published his findings in the May issue of the Journal of Sedimentary Research. Funding was provided by NASA, the National Research Council and the UI Critical Research Initiative.