A research team documented tiny bubbles in 830-million-year-old salt crystals of central Australia that contain traces of cells and algae.
This discovery shows that microorganisms from saline depositional environments can remain well preserved in salt over hundreds of millions of years and can be detected in situ with optical methods alone.
This study, published in the journal Geology, has implications for the search for life in both terrestrial and extraterrestrial chemical sedimentary rocks.
As salt crystals grow in an evaporating body of water, they can trap tiny bubbles of water in the crystalline structures, called by geologists primary inclusions. In addition to trapping water, the inclusions can trap any solids that were in the water near/on the crystal face. These solids include tiny crystals of evaporite minerals or organic remains. Previous studies of modern to 250-million-year-old Permian salt deposits have documented the presence of microorganisms and organic compounds.
The researchers Sara Schreder-Gomes, Kathleen Benison, and Jeremiah Bernau had access to core samples from the Neoproterozoic Browne Formation thanks to the Geological Survey of Western Australia. The Browne Formation is a succession of silt- and mudstone layers deposited 800 million years ago in a shallow lagoon or a dry lake in a very hot environment. As the water evaporated, crystals of salt formed and became buried in the mudstone.
The researchers used transmitted light petrography and UV-visible light petrography to identify primary fluid inclusions and their contents preserved in salt crystals. The team found that solids trapped in fluid inclusions were consistent with prokaryotic and eukaryotic cells, and with organic compounds, based on their size, shape, and fluorescent response to UV-visible light.
The discovery that microorganisms can be preserved in salt crystals for millions of years also has implications for the search for extraterrestrial life.
In September 2021, NASA’s Perseverance Rover discovered salt deposits in rocks collected at its landing site inside Jezero Crater. These salts may have formed when groundwater flowed through and altered the original minerals in the rock, or more likely when liquid water evaporated, leaving the salts. The salt minerals in these first two rock cores may also have trapped tiny bubbles of ancient Martian water. If present, they could serve as microscopic time capsules, offering clues about the ancient climate and habitability of Mars, and maybe even preserve the remains of ancient Martian microbes.