You'll eventually notice darkened rocks in deserts anywhere on the planet. This is especially true where the sun shines brightest and water trickles down. If you're fortunate, you might find ancient art, called petroglyphs, in some places. Researchers have been studying the petroglyphs for years and more than the dark stain (also known as rock varnish) in which they were drawn.Science has not yet come to an agreement about the origin of rock varnish, which is extraordinarily rich in manganese.Scientists at the California Institute of Technology and the Department of Energy's SLAC National Accelerator Laboratory think they have the answer. A paper published in Proceedings of the National Academy of Sciences recently found that rock varnish is created by microbial communities using manganese to protect themselves from the harsh desert sun.According to Usha Lingappa (Caltech graduate student and study's author), rock varnish has been a mystery for a long time. She said that Charles Darwin wrote about it, Alexander von Humboldt also wrote about it. There is still a debate over whether the rock varnish has a biological origin or not.Lingappa explained that she and her coworkers didn't set out to discover where rock varnish is made. They were more interested in how rock varnish interacts with desert microbial ecosystems. They used as many techniques as possible: DNA sequencing, mineralogical analysis, electron microscopy and, with the help of Samuel Webb, advanced X-ray spectroscopy techniques that could map different types of manganese within rock varnish samples.Lingappa stated that by combining different perspectives, we might be able to draw a picture and understand the ecosystem in new ways. "That's where it all started and then we just stumbled upon this hypothesis" about rock varnish formation.One of the key findings by the team was that while desert dust usually contains manganese in particles, it was deposited in continuous layers in varnish. This was revealed by Xray spectroscopy techniques at SSRL, which can not only determine the chemical compounds in a sample, but also the distribution, on a microscopic level, throughout the sample.The same analysis revealed that varnish's manganese content was not due to being exposed to the sun for many millennia. Lingappa's team was led to the conclusion that rock varnish had been left behind by the Chroococcidiopsis bacteria.Webb, for his part, said that he enjoys a project with manganese -- "I have been a mangaphile since a while now." This project came at the perfect moment, considering advances in Xray spectroscopy at SSRL. Webb said that the researchers were able to see finer details of rock varnish thanks to X-ray beam sizes and other improvements. They could also get a better look at the samples without risk. We're always tweaking things and it was the right time for this project, which would have been impossible five or ten years ago.
