Desert/Rock Varnish Ferromanganese Deposits

The Strange Things Microbes Can Do---Who Painted the Desert?

by Daniel Dehm

Why would anyone want to “paint” the vibrant orange, brown and tan rocks of the desert a blotchy black or a rusty red? Diana Northup, Michael Spilde, Penny Boston and other scientists at the University of New Mexico and New Mexico Tech (hotlinks) would like to know – they have been working hard to catch the “artist” responsible for what is commonly called desert varnish. Many scientists believe desert varnish is the result of mineral-rich water washing over the rocks, leaving behind things like iron and manganese to turn the rock various shades from red to black. But the SLIME Team (hotlink) took a different and surprising approach. They are trying to find evidence that microbes are responsible: millions of microscopic artists working together to change the color of the desert. What’s even stranger is that they think these microbes might “eat” the rock in order to produce the varnish.

Good example of desert varnish with lighter original surface rock below. Note hand for scale. Photo Credit: Penny Boston

Desert varnish is usually black, but can vary in hue from red to black. The visible color of a varnish surface is determined by a standard soils color chart seen above.
Photo Credit: Diana Northup.

Collecting samples from three sites in the southwest (Mars Desert Research Station near Hanksville UT, Carlsbad Caverns National Park, NM and the Luis Lopez mining district outside Socorro, NM), they set out to discover what lay behind the creation of desert varnish. Each of the sites chosen represented a different climate and a distinct rock type. Working like detectives, part of their project will involve eliminating the possibility that it’s the weather or the rocks themselves responsible for the varnish. But their first objective was getting desert varnish under the microscope, literally.
In the field, small flakes of varnish were hammered off the rock with sterile tools and placed into culture tubes. Other samples were placed into special tubes with a special buffer to be used for DNA analysis. Water samples, from precipitation runoff, were also collected to test the popular theory that only water was responsible. At one of the sites the team discovered a curious thin layer of green material lying underneath the varnish. They thought it might be bacteria! With samples in hand, the scientists headed back to the lab to get to the bottom of the mystery of the varnish.

Researchers chip away samples with a hammer and spike. Note the lighter material under the varnish.

Location of the three sample collection sites.

Back at the lab, they performed molecular analysis, and were able to isolate a particular group of microbes from some of the samples, called cyanobacteria. They were particularly interested in the types of cyanobacteria that might be able to oxidize iron and manganese. Oxidation happens when oxygen removes electrons from a material, especially a metal, leaving it “oxidized.” When oxygen encounters a metal in this way, we usually call it rust. But in the case of some bacteria, possibly including cyanobacteria, they metabolize (eat) metals like iron and manganese, leaving behind something very much like rust.
The scientists are currently growing bacteria in the labs cultured from the original field samples. If these bacteria are found to be iron and manganese eaters, part of the mystery about who painted the desert might have been solved. In addition to desert varnish aboveground, the SLIME Team is also investigating the link between these microbes and those thought to produce some of the striking features in two caves located in Carlsbad Caverns: Lechuguilla and Spider Caves (hotlink).
Another mystery about desert varnish has to do with a different kind of artist. Native Americans found that the varnish was relatively soft and easily carved into. Underneath is usually lighter rock, which provides an excellent natural contrast to use as a canvas. And so they carved. The symbolic drawings they left behind are called petroglyphs (literally “stone carvings”). Though anthropologists aren’t entirely sure why native people marked the rock, tens of thousands of such rock drawings have been found throughout the Southwest.

So the next time you see desert varnish, realize it might not be as simple as water washing over the rocks. Instead it might just be the work of millions of invisible artists – artists caught at work by curious scientists.

Petroglyph of a snake and dog?, carved into desert varnish, Rinconada Canyon, Petroglyph National Monument.

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