ASU Study on Rock Preservation of Life Signs Dedicated to Late Mentor

Jon Lima-Zaloumis, building on his PhD research at Arizona State University (ASU), is working on methods to find signs of life in extreme environments, such as Earth’s atmosphere and possibly other planets.

After completing his PhD in geoscience from ASU’s School of Earth and Space Exploration in late 2021, Lima-Zaloumis joined the Microbiology of the Deep Lab as a postdoctoral researcher. This lab is led by Elizabeth Trembath-Reichert, an assistant professor at the school.

Lima-Zaloumis is now the lead author of a new paper published in the Journal of Sedimentary Research. The paper explores how evidence of life is preserved in carbonate rocks, which could help in detecting signs of life on other planets. The study was co-authored by Trembath-Reichert and ASU Professor Jack Farmer, who passed away in February 2023. The paper honors Farmer, who was Lima-Zaloumis’s PhD advisor.

“This research highlights a key finding from my PhD, which Jack helped shape from the beginning of my graduate studies. Jack was an amazing field scientist, and I learned a lot from him that you can’t find in textbooks,” Lima-Zaloumis said. “With the help of my postdoctoral advisor Elizabeth Trembath-Reichert, we refined our interpretations over the years.”

Carbonate Rocks and Tracing Life’s History

Carbonate rocks on Earth can show signs of life that are visible to the naked eye, like animal shells and exoskeletons found in limestones in Arizona. These rocks are crucial for studying the evolution of life because they contain some of the oldest evidence of life on Earth, especially in the form of stromatolites—layered carbonates created by microorganisms.

However, carbonate rocks rarely preserve direct evidence of microorganisms, such as body fossils. Over time, the textures of these rocks are often destroyed, making it hard to determine their origins. Researchers usually blame this on “diagenesis,” which are processes that change the rocks over time. But there are few detailed examples of how and when these changes happen, especially in modern environments where carbonate is still forming.

“To understand ancient rocks and why microorganisms are often not preserved, we wanted to study rocks that are forming today,” Lima-Zaloumis said. “We wanted to see how microorganisms were preserved early on and if their signs were quickly lost.

“We applied a core idea from Geology 101 called the ‘Principle of Uniformitarianism’—the present is key to understanding the past.”

Study of Crystal Geyser, Utah

The team studied Crystal Geyser in Utah to understand how microorganisms are preserved.

Crystal Geyser is a partially human-made geyser on the shore of the Green River, about 10 miles south of Green River, Utah. It was created in 1936 when an oil exploration well hit a pressurized groundwater system with trapped carbon dioxide gas. This led to the formation of a cold-water spring where carbonate is actively forming today as travertine.

After taking samples back to the lab, the team found that the quickly forming carbonate captured evidence of life, such as tiny structures resembling body fossils of microorganisms and small, layered stromatolites like those in ancient carbonates.

However, they also found that these signs degrade within a few years—very quickly in geological terms. Using techniques like microscale Raman spectroscopy, the researchers showed that this degradation happens as the carbonate mineral aragonite quickly changes into the more stable mineral calcite.

This mineral transformation, along with the coarsening of carbonate crystals, rapidly erases the original biological information.

Implications for Preserving Biosignatures

The findings suggest that carbonate spring environments might not be as good for preserving evidence of life as previously thought, at least when looking for structural features like body fossils of microbes.

However, the researchers also found that despite the destruction of textures, organic material from the organisms is still retained in the altered rocks.

The research shows that to preserve small-scale structural biosignatures for long periods, a process like silica replacement must happen quickly before the carbonates degrade.

Broader Impact

This research has important implications for understanding how signs of life are preserved from early Earth and other planets, like Mars, where carbonates have been found. It emphasizes the need for rapid mineralization processes to preserve microscopic biosignatures and highlights the potential for organic material to survive changes in carbonate rocks.

“Rocks are more complex than we think, and if we don’t fully understand what’s happening on Earth, how can we understand what happened in the past or on other planets?” Trembath-Reichert said. “Research like this helps us make connections between what we see now and what might have happened billions of years ago or millions of miles away.”

Lima-Zaloumis added that Farmer’s work continues at ASU.

“When we think of searching for past life on other planets, we usually look for previously wet and habitable environments with lots of mineral precipitation where evidence of life can become fossilized. Our work provides important context and caution to this approach. It sets baseline expectations for the kinds of biosignatures we might find in such environments and why fossilized microbes are so rare in ancient carbonate rocks.

“This work continues Jack Farmer’s legacy, who spent his career exploring these questions in a field he founded called ‘exopaleontology.’ Jack’s legacy lives on through the work of his many students and colleagues interested in searching for signs of past life on other planets.”