Unraveling an ancient enigma surrounding the painterly seascape of the desert's stones

In a groundbreaking discovery, a team of scientists from the California Institute of Technology (Caltech), the SLAC National Accelerator Laboratory, and other institutions have determined the origin of rock varnish, the dark-stained rock commonly found in deserts. The research, supported by the National Science Foundation, the National Institutes of Health, and the National Aeronautics and Space Administration, was published in the Journal of Proceedings of the National Academy of Sciences.

The project arrived at a suitable time due to recent advancements in X-ray spectroscopy at the DOE Office of Science user facility, SSRL. This technique allowed the researchers to not only identify the chemical compounds in a sample but also understand their distribution on a microscopic scale.

Usha F. Lingappa, a graduate student at Caltech and the study's lead author, stated that the team didn't initially set out to understand where rock varnish comes from, but rather to study microbial ecosystems in the desert and their interaction with rock varnish. One key observation was that manganese in desert dust is usually in particle form, but it is deposited in more continuous layers in varnish.

The team found that the kinds of manganese compounds in varnish were the result of ongoing chemical cycles, rather than being left out in the sun for millennia. This discovery suggests that rock varnish is left behind by microbial communities, specifically bacteria like Chroococcidiopsis, which use manganese to combat the oxidative effects of the harsh desert sun.

Improvements in X-ray beam size allowed the researchers to obtain a more detailed image of rock varnish. Other improvements in X-ray spectroscopy techniques ensured that the samples could be studied without risk of damage.

The research paper, titled "Microbial origin of desert rock varnish" (DOI: 10.1073/pnas.2025188118), was published in the Journal of Proceedings of the National Academy of Sciences. The paper proposes that microorganisms play a key role in rock varnish formation in deserts by catalyzing the oxidation and accumulation of manganese, which is essential in varnish development.

Webb, a researcher involved in the study, has a personal interest in manganese studies, stating he has been a "mangaphile" for some time. The team used various techniques such as DNA sequencing, mineralogical analyses, electron microscopy, and X-ray spectroscopy methods to study rock varnish.

The discovery of the microbial origin of rock varnish not only deepens our understanding of desert geology but also opens up new avenues for research into the role of microorganisms in shaping our planet.