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"Geohydrobiology" sounds like a made-up word—as if the prefixes from multiple scientific fields were strung together to score big points in a game of Scrabble. But it makes more sense when you break it down: geo, meaning Earth; hydro, meaning water; and bio, meaning life.

It is easy to understand why studying geohydrobiology is an important endeavor at Yellowstone, a place where Earth, water, and life intersect in unusual and stunning ways.

Yellowstone Volcanic Plateau hosts the world's largest, most profound, and visually stunning example of an active continental hydrothermal system, with over 10,000 springs, geysers, fumaroles and mudpots. Equally stunning is the diversity of Yellowstone's hydrothermal fluids, which range in temperature from ambient to 93°C (the temperature of boiling water at Yellowstone's elevation), span pH levels from 1.5 (acidic) to 10 (basic), and have highly variable chemical compositions. Where does this diversity come from?

Doublet Pool, fringed by geyserite, in the Upper Geyser Basin of Ye...
Doublet Pool, fringed by geyserite, in the Upper Geyser Basin of Yellowstone National Park. National Park Service photograph by Jacob Frank.

The basic picture of Yellowstone's hydrothermal plumbing system is that a hydrothermal reservoir exists deep beneath the surface and is recharged by rain and meltwater from high elevations. These meteoric fluids descend into the Earth, are heated, and pick up new chemicals, like chloride (Cl) and sulfate (SO4), from the volcanic system. The chemically enriched hot waters then rise back up to surface to form Yellowstone's iconic thermal features.

During ascent to the surface, the water boils as pressure decreases. This boiling results in what is known as "phase separation," where the vapor and liquid separate and migrate to the surface along different paths. The fluids of the liquid phase tend to be neutral to basic in pH and retain most of the Cl. In contrast, the vapor-phase contains most of the SO4 and is acidic. These separated water and vapor phrases also mix with colder ground and surface water as they ascend, further influencing their compositions.

While this basic model can explain much of the chemical variation seen in Yellowstone's hydrothermal fluids, our understanding of this important process is severely limited by our lack of knowledge of the shallow plumbing of Yellowstone's hydrothermal features, and the time scales over which the phase separation takes place. In essence, we have a two-­?dimensional understanding of a four-dimensional problem. We know surface geometry, water chemistry, and heat flow for many of the thermal features, but we don't understand the ultimate composition of the deep hydrothermal reservoir, nor the timing and pathways for ascent of the fluids.

Silica sinter (an amorphous form of silicon dioxide) forms from the...
Silica sinter (an amorphous form of silicon dioxide) forms from the waters that flow from hot springs near Shoshone Lake, Yellowstone National Park. USGS photograph by Jake Lowenstern.

The problem of subsurface boiling and phase separation is important for understanding not only the chemistry of the water and gas discharged at the surface, but also the microbial life that inhabit Yellowstone's hot springs. To address this gap in knowledge, scientists from the University of Wyoming and Montana State University are collaborating to research the four-dimensional process of phase separation and its impact on the biological communities that thrive in thermal waters. The research has several goals:

  1. use geophysical tools, like electromagnetics, ground penetrating radar, and seismic refraction, to obtain an image of the subsurface plumbing of hydrothermal features
  2. measure the chemistry of the fluids, especially with regard to radioactive elements that change in composition over time, to understand the nature of the deep source reservoir, the timing of interactions between water and rock, and the ages of the fluids
  3. measure the fluid chemistry and DNA of microbial life in hot springs

This multi-faceted investigation will address all aspects of Yellowstone's hydrothermal system—geo (rock), hydro (water), and bio (life). Research into the geohydrobiology of Yellowstone will provide a better understanding of not just phase separation and the chemistry of the resulting fluids, but also how those fluids impact the diverse organisms that lend Yellowstone's thermal features their spectacular forms and colors.

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