Research on how rocks preserve signs of life honors late ASU mentor
At Crystal Geyser, Utah, travertine rocks form in the presence of microbial communities. Small layered features called stromatolites can be seen growing, which likely form by the direct activity of microorganisms. These and other ‘biosignatures’, such as fossils of bacterial cells, can be seen preserved in the youngest upper layers of the travertine. Surprisingly, these signs of life are quickly erased due to crystal transformations and continued growth. These processes help explain why ancient carbonate rocks are often missing important signs of life, and helps us understand what kinds of biosignatures might be found in similar environments beyond Earth. Image courtesy of Jon Lima-Zaloumis/ASU
Building from his PhD work at Arizona State University, Jon Lima-Zaloumis is developing methods to look for signs of active life occurring in extreme environments, namely in the Earth’s atmosphere and potentially those of other planets.
After earning his PhD in geoscience from ASU's School of Earth and Space Exploration in late 2021, Lima-Zaloumis became a postdoc in the Microbiology of the Deep Lab led by Elizabeth Trembath-Reichert, an assistant professor in school.
Now, Lima-Zaloumis is the lead author of a recently published paper in the Journal of Sedimentary Research, which identifies the mechanisms involved in preserving evidence of life in carbonate rocks with implications for detecting biosignatures in planetary environments.
The work was co-authored by Trembath-Reichert and ASU Professor Jack Farmer, who died in February 2023, and honors the life and legacy of Farmer, who was Lima-Zaloumis's PhD advisor.
“This work reflects one of the important findings of my PhD, which Jack played a huge role in conceptualizing and refining from the start of my graduate program. Jack was an incredible field scientist, and here I was able to learn field methods and modes of thinking that are difficult to convey in textbooks," Lima-Zaloumis said. “Over the years, and with the help of my postdoctoral advisor Elizabeth Trembath-Reichert, we were able to hone our interpretations on what we were seeing.”
Carbonate rocks and tracing the history of life
On Earth, carbonate rocks preserve signs of life visible to the naked eye, such as animal shells and exoskeletons seen within limestones in Arizona. These kinds of rocks are especially important in the context of astrobiology and the study of the evolution of life because they represent some of the oldest evidence of life on Earth, particularly in the form of stromatolites — layered carbonates thought to be formed directly by the activity of microorganisms.
However, carbonates very rarely preserve evidence of microorganisms directly (such as body fossils). At small scales, the textures of these rocks are typically destroyed over time making it difficult or impossible to determine of their origins. Traditionally, researchers attribute the lack of preservation as likely due to “diagenesis,” or processes that alter the rocks over time. However, there are very few detailed examples of how, and when, these changes occur, especially in modern environments where carbonate is actively being formed.
“To help interpret ancient rocks and understand why we see this preservation bias against microorganisms, we wanted to look at rocks that were forming today," Lima-Zaloumis said. "We were interested in seeing how microorganisms were being preserved early on and whether signatures were quickly lost.
“In effect, we looked to apply one of the core ideas taught in Geology 101 called the ‘Principle of Uniformitarianism’ — how the present is key to understanding the past.”
Study of Crystal Geyser, Utah
The team investigated Crystal Geyser in Utah to explore processes related to the preservation of microorganisms.
Crystal Geyser is a partially human-made geyser located on the shore of the Green River, approximately 10 miles south of Green River, Utah. The geyser originated in 1936 when an oil exploration well tapped into a pressurized groundwater system containing trapped carbon dioxide gas. A cold-water spring developed, where carbonate actively forms today as travertineA beautiful rock commonly used as building material, even around the ASU campus. .
Having taken samples back to the lab, the team discovered that because carbonate here forms quickly, it effectively captures evidence of life such as tiny structures resembling body fossils of individual microorganisms, as well as small, layered stromatolites like those seen in ancient carbonates.
However, further investigation revealed that these signatures degrade within several years — extremely quickly with respect to geological time. Using microscale Raman spectroscopy and other techniques, the researchers showed that degradation occurs as the carbonate mineral aragonite quickly transforms into the more stable mineral calcite.
This mineral transformation, along with the coarsening of carbonate crystals, rapidly erases the original biological information.
Implications for preservation of biosignatures
The findings suggest that carbonate spring environments might not be as good of an environment to search for evidence of life as previously thought, at least when looking for "structural" features like body fossils of microbes.
However, the researchers also show that despite the textural destruction, organics (carbonaceous material) derived from the organisms are still retained throughout the disrupted/altered rocks.
The research highlights that to preserve small-scale structural biosignatures for long time scales, a process like silica replacement (i.e. permineralization) must happen very quickly before the carbonates degrade.
Broader impact
This research has significant implications for understanding the preservation of signs of life from early Earth and other planets, like Mars, where carbonates have been identified. It underscores the importance of rapid mineralization processes in preserving microscopic biosignatures and highlights the potential for organic material to survive diagenetic changes in carbonate rocks.
“Rocks are messier than we give them credit for, and if we don’t understand fully how it is happening in front of us on Earth, how will we understand what might have happened further back in time or on other planets?” Trembath-Reichert said. “Research like this is key to being able to make these connections to what we see now and what might have happened billions of years ago or millions of miles away.”
Lima-Zaloumis went on to share how Farmer’s work continues on here at ASU.
“Typically, when we think of searching for past life on other planets, our intuition tells us to go to a previously wet and habitable environment, preferably coinciding with lots of mineral precipitation where evidence of life can become readily fossilized. Our work provides some important context and caution to this approach. It gives some baseline expectations about the kinds of biosignatures we might expect to find in such an environment and why fossilized microbes are so rare in the ancient carbonate rock record.
"This work carries on the legacy of Jack Farmer, who spent a career exploring these questions in a field he founded called ‘exopaleontology.’ Jack’s legacy continues through the work of his many students and colleagues interested in searching for signs of past life on other planets.”
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