Geotimes - September 2004

Geotimes

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Geophenomena

Vostok’s complicating ridge
Gauging The Geysers with Quakes

Vostok’s complicating ridge

Lake Vostok lies isolated beneath 4,000 meters of Antarctic ice. Since confirmation of its existence in 1996 and the discovery of microbes in the ice above, scientists have been looking to the lake to learn about life in a pristine and extreme environment. Now, the first-ever comprehensive map of the bottom of Lake Vostok has revealed two distinct basins, which may constitute separate ecosystems and could alter current efforts to sample the lake.

At Lake Vostok in Antarctica, scientists from Columbia University’s Lamont-Doherty Earth Observatory camp in tents and use the DeHavilland Twin Otter aircraft for geophysical surveys. The researchers recently published results showing that the isolated lake has two distinct basins, which may represent two different ecosystems. Copyright M. Studinger, 2001.

In 1998, a joint Russian, American and French team drilled through the ice to within 150 meters of the lake’s surface. Analysis of the 3,623-meter-long core revealed signs of microbial life in the lowermost part — believed to be frozen lakewater — providing tantalizing evidence that extreme forms of life may yet be found there. For this reason, the lake is somewhat analogous to Europa, a moon of Jupiter whose icy surface may cover a water ocean that astrobiologists believe could harbor extraterrestrial life.

“The water in Lake Vostok is like the ocean on Europa in that it is isolated by a thick ice cover — cut off from sunlight,” says Chris McKay, an astrobiologist with NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The key question with respect to life in Lake Vostok is: What is the fundamental energy source?”

McKay says that there are three possible sources of chemical energy: organic material in the ice carried over the lake; reduced gases, such as hydrogen sulfide from a geo-thermal or volcanic source; or organics and other reduced chemicals seeping from rocks on the lake bed or sides. “In the latter case, the circulation of the lake and the depth of the basins would matter,” McKay says.

Helping to elucidate these factors is a study in the June 19 Geophysical Research Letters by Michael Studinger, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory, and colleagues. The team found evidence of a ridge running east to west, 200 meters below the lake’s surface, separating a smaller, round northern basin from a larger, elongate southern basin. Additionally, ice-penetrating radar shows that the bottom of the ice sheet is melting into the lake’s northern basin, while lake water is freezing onto the ice sheet over the southern basin, Studinger and co-authors reported.

The two basins “may have implications for the chemistry of the water and the circulation of the water in the lake and these in turn may affect life,” McKay says. The key to how bathymetry could affect life is the pattern of melting and freezing at the bottom of the ice sheet, Studinger says. Meltwater enters only the northern basin where it sinks. But because the ridge limits water exchange between the two basins, “the chemical and biological compositions of these two ecosystems are likely to be different,” he says.

The team derived water depth and topography of the lake bottom from gravity measurements made from an airplane flying in a grid pattern above the East Antarctic ice sheet between December 2000 and January 2001. The strong density contrast between rock and water allowed the team to use gravity measurements to map the lake. “Because of the less dense water in the lake, the gravitational pull from the Earth is lower over the lake than outside,” Studinger says.

The researchers then subtracted the gravity signal from the ice sheet to derive the signal from the water and thus were able to calculate the depth. They estimate the lake volume is approximately 5,400 cubic kilometers, which is roughly the volume of Lake Michigan, but with a maximum depth of 800 meters, Lake Vostok reaches almost four times deeper.

Due to the bathymetry and the melting and freezing pattern, Studinger says, researchers should expect to find two different types of sediments on the bottoms of the basins. “Sediments released by basal melting are likely to accumulate in the northern basin,” Studinger says, “while pre-glacial sediments that recorded the onset of glaciation in Antarctica are more likely to be found in the southern, deeper basin.”

Sediment samples will not be retrieved, however, until an international consensus is reached on how to breach the pristine ecosystem, isolated for millions of years, without introducing contamination.

Sara Pratt
Geotimes contributing writer

Gauging The Geysers with Quakes

The Geysers, located on 30 square miles of ridges in the Mayacamas mountains 100 miles north of San Francisco, is the world’s largest geothermal power facility. It began commercial power production in 1960 and currently supplies 6 percent of Northern California’s electricity. In the late 1970s, scientists began noticing that regional seismic networks were detecting increasing numbers of small-magnitude earthquakes. Then, as steam pressure dropped in the 1980s, energy suppliers at the site began injecting water to augment steam production, a practice that continues today, further increasing seismic activity and triggering complaints from shaken residents. Researchers are taking advantage of the seismicity to better understand what’s happening below the surface and to estimate the longevity of the geothermal field.

Between 1985 and 1996, The Geysers averaged about 18 earthquakes per year with magnitudes of 3.0 or greater. In 1997, Calpine Corporation, which eight years earlier had begun takeover and consolidation of 19 of the 21 plants at The Geysers, began injecting treated wastewater and lakewater from Lake County. Calpine stepped up injections again in November 2003, with a 41-mile-long wastewater pipeline from Santa Rosa, raising the total amount of injected water to 36 million gallons a day. Since 1997, seismic activity in the magnitude-3.0 range has remained stable, with about 16 of that size per year, but earthquakes below magnitude 3.0 have numbered in the thousands per year.

“At lower magnitude ranges (greater than or equal to 1.5), we have seen an increase of approximately 30 percent in microearthquake activity,” says Mitch Stark, a seismologist with Calpine in Middletown, Calif. And the community has noticed too, with some residents seeking financial compensation in the form of a community fund to offset the cost of the damages that they attribute to the earthquakes, such as cracks in home walls.

While the link between well injections and increased seismicity is well-established, “not all Geysers seismicity is caused by injection,” Stark says. “A large proportion is due to pressure and temperature decline related to production.” He also points out that geothermal areas are often seismically active. “Presumably, there are natural tectonic events in the mix as well, since this part of California is definitely earthquake country.”

Regardless of the relative sources of the quakes, they are proving to be one of many useful tools for monitoring the geothermal reservoir, as well as the outcome of injections. “Most injection wells have a plume of microearthquakes that tell us, in three dimensions, where at least some of the water goes in the reservoir,” Stark says.

Other researchers are using the earthquakes to study the reservoir with a technique called 4-D seismic tomography. “The earthquake waves pass through Earth, and we can record them on the surface and produce a three-dimensional picture of what’s under the surface,” says Gillian Foulger, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif., and at the University of Durham in England, who studies geothermal fields.

Foulger and colleagues set out to measure the reservoir, and found that between 1991 and 1999, despite increasing injections, the reservoir was becoming increasingly depleted of fluid, as published in a 2003 Journal of Geophysical Research study. “It appears they were taking more out than they were putting in,” Foulger says.

By the late 1980s, close to 600 wells had been drilled, but as steam pressure in the reservoir decreased, wells were closed. Today, only about 350 wells are operational.

According to Stark, an engineering study of the reservoir that was conducted by Calpine in 2000 estimated that the total remaining steam reserves were approximately equal to all the steam produced since commercial production began 40 years ago.

Foulger says that closing some wells may extend the life of the field. “If it’s managed sensibly, it can make the difference between lasting 20 years and lasting a century,” she says.

In the meantime, residents continue to notice the impacts of four decades of development at The Geysers. Stark says that in response to community concern, Calpine has cut back injection into two wells that seem to generate “more than their share” of felt seismicity, either because they were closer to residential communities or produced larger quakes.

Sara Pratt
Geotimes contributing writer

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