Unveiling the ancient secret behind the portrait-like rock carvings in the desert landscapes
In a groundbreaking discovery, a team of researchers, led by Usha F. Lingappa, a graduate student at Caltech, have found that rock varnish - the dark-stained rock commonly found in deserts - is produced by microbial communities. The study, published in the Proceedings of the National Academy of Sciences on June 22, 2021, sheds light on the unexpected role of these microorganisms in desert geology.
Lingappa and her team did not initially set out to understand the origin of rock varnish. Instead, they were studying microbial ecosystems in the desert and their interaction with rock varnish. However, their observations led them to a surprising conclusion.
Rock varnish, it turns out, is unusually rich in manganese. One key observation made by the team was that manganese in desert dust is usually in particle form, but it is deposited in more continuous layers in varnish. This led the researchers to believe that the varnish is left behind by these bacteria.
The team, which included scientists from Caltech, SLAC National Accelerator Laboratory, and elsewhere, used various techniques to study rock varnish. These included DNA sequencing, mineralogical analyses, electron microscopy, and advanced X-ray spectroscopy methods. Improvements in X-ray beam size provided a more detailed image of rock varnish, according to Webb, a researcher involved in the study.
Webb, who has a long-standing interest in manganese projects and refers to himself as a "mangaphile", also mentioned that the timing was right for this project. Recent advancements in X-ray spectroscopy at SSRL, a DOE Office of Science user facility, made it possible to tell not only what chemical compounds make up a sample but also how they are distributed, on a microscopic scale, throughout the sample.
Another significant finding was the prevalence of bacteria called Chroococcidiopsis in the samples of rock varnish. These bacteria use manganese to combat the oxidative effects of the harsh desert sun. The kinds of manganese compounds in varnish were found to be the result of ongoing chemical cycles, rather than being left out in the sun for millennia.
The research was supported by grants from the National Science Foundation, National Institutes of Health, and National Aeronautics and Space Administration. The DOI for the study is 10.1073/pnas.2025188118.
This discovery not only deepens our understanding of desert geology but also opens up new avenues for research in microbiology and astrobiology. The team's findings suggest that life may be more resilient and adaptable than previously thought, even in the harshest of environments.
