Some microorganisms not only coat rocks with a thin black veneer, they do so much faster than earlier estimates indicate, according to geomicrobiologist Michael Spilde from the Institute of Meteoritics at the University of New Mexico, Albuquerque, N.M., and his collaborators. Ther results were presented last year during the Geological Society of America (GSA) annual meeting in Portland, Ore.

The microbes are actively making rock varnish in caves. Although no one claims to observe comparable biotic processes in desert settings, the varnish in caves appears the same as varnish found on desert rocks, suggesting it, too, has biologic origins. The humidity in caves may help to account for the relative speed of microbial varnish production there, according to Spilde. Based on manganese-oxidizing microbes that are actively producing rock varnish in Fort Stanton Cave in New Mexico, growth greatly exceeds the 40 μm per 1,000 years rate that others in this field consider standard.

Puzzlement over rock varnish dates back at least to naturalist Alexander von Humboldt. Considered the first geographer, he puzzled over the dark coatings that he observed on rocks and boulders while exploring South America during the early 1800s.

Two centuries later, geomicrobiologists Tanzhuo Liu and Wallace S. Broecker from the Lamont-Doherty Earth Observatory of Columbia University at Palisades, N.Y., did not say how rock varnish is made but concluded that it accumulates ever so slowly. Based on a set of 42 rock varnishes from western U.S. deserts, they reported in 2000 that that rock varnish grows at a rate of 1-40 μm per 1,000 years, making it the slowest known accumulating sedimentary deposit.

However, if Spilde and his collaborators are correct, cave varnish grows very quickly. For instance, in places where they removed samples a year ago, some regrowth has occurred. That comparative speed suggests that the availability of water or water vapor could accelerate growth of varnish in caves compared to rock varnish in desert settings.

"The walls are coated with black manganese oxide deposits that look so much like desert varnish that you would think you were on the desert surface instead of hundreds of meters underground," Spilde says, referring to Fort Stanton Cave. "Some of the cave varnish is smooth and shiny just like varnish in the Mojave Desert, and contains micro laminations and microstromatolite structures."

Those structures are consistent with microbial activity, Spilde continues. Several lines of evidence point to microbial origins for varnish, including: (i) laminated microstomatolite structure of manganese oxides, (ii) upward growth of structures toward the surface, (iii) manganese- oxidizing bacteria from the cave producing such materials in the lab, and (iv) DNA from the diverse suite of microorganisms from the cave varnish. "A few microorganisms are known manganese oxidizers, but a large percentage are novel," he says. "We still at this point have many more questions."

Geographer Ron Dorn of Arizona State University, an expert on rock varnish, is skeptical about comparing cave varnish with varnish on desert rocks, even though he suspects that the latter has biological origins. "I have no doubt that Spilde and his team have found rock varnish in the Fort Stanton Cave," he says. "I do not think that they have proved the biogenic origin of rock varnish." He also questions the legitimacy of "using a cave to ‘prove' the biogenic formation of rock varnish that grows in the middle of a desert." Adds another expert on rock varnish, Kim Kuhlman of the University of Wisconsin, "I personally don't believe the cave coatings will ever be able to prove anything about varnishes outside in the desert simply because the conditions are so very different."

Meanwhile, geologist David Krinsley from the University of Oregon examined several back-scattered electron microscope images from Spilde. "They look almost identical to the pictures I have taken of varnish in various places in the world," Krinsley says. "I'd have to say that they are varnish."

Barry E. DiGregorio
Barry E. DiGregorio is a freelance science writer in Middleport, N.Y.

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