In 2016, geologist Rowan Martindale was hiking a hillside in Morocco when something unusual caught his eye. The slabs of sedimentary rock were covered with a wrinkled texture that looked strikingly like elephant skin. The pattern was so unexpected that it immediately stood out.
“When I saw the wrinkles, I thought, ‘This can’t be in this rock. What’s going on here?'” said Martindale, an associate professor at the Jackson School of Geosciences at the University of Texas at Austin.
The texture of a rock reveals the process by which it was formed over millions of years. To Martindale, the wrinkled surface looked just like a fossilized microbial mat. These structures form as microbial communities grow throughout the sediment, leaving behind unique patterns. In this case, the texture appeared to preserve a dense layer of microorganisms that existed during the Early Jurassic Period, more than 180 million years ago.
Martindale recognized the pattern right away. During graduate school, she studied similar textures through photos and samples shared by colleagues in her lab, which focused on microbial fossils from the Early Triassic.
puzzle in the wrong place
There was one big problem. This setting did not match what the scientists expected.
The wrinkled rock formations originate from deep ocean waters approximately 600 feet below the earth’s surface. However, scientists have long believed that this type of microbial wrinkled structure forms only in shallow environments. In such environments, microbes rely on sunlight for energy and can avoid animals for energy, especially during stressful times or after mass extinctions.
In deeper waters, similar patterns are usually explained differently. Geologists often attribute it to underwater landslides that push sediment into ridges and trenches. But Martindale wasn’t convinced. The patterns she saw had clear signs of microbial activity.
“Knowing what to look for and having a ‘search image’ of the wrinkle structure in my head made me want to stop and dig into this,” she said.
A new explanation for the wrinkle structure of the deep sea
In a recent study published in geologyMartindale and her colleagues are proposing a new interpretation that links geological processes and biological activity. They suggest that while underwater landslides did occur, they did not directly create the wrinkles. Instead, they delivered nutrients to the ocean floor, allowing microorganisms to grow and structure to form.
According to the research team, these microbes were not dependent on sunlight. Instead, they likely relied on chemicals for energy, a process known as chemosynthesis. Nutrient inputs from landslides may have supported these communities, while the release of toxic sulfur compounds may have prevented other marine life from disturbing them.
Modern clues from deep sea ecosystems
Similar ecosystems exist in today’s oceans. Some microbial mats thrive in deep, dark environments, feeding on chemical energy rather than sunlight. An example of this can be seen in the carcass of a whale that has sunk to the ocean floor. These “whale drop” sites create ephemeral but rich ecosystems in which microbes quickly colonize and thrive.
Jake Bailey, a professor at the University of Minnesota who studies how microbes affect Earth systems, said the discovery challenges long-held assumptions about these rock structures.
“Some of the largest microbial ecosystems on Earth are now found in the dark ocean,” said Bailey, who was not involved in the study. “The research here shows that certain ancient sedimentary structures may record the presence of these chemotrophs rather than photosynthetic organisms (organisms that require sunlight to make energy).”
Rethinking fossils in the rock record
Martindale explained that this discovery could have far-reaching implications. If chemosynthetic microbial communities are more widespread than previously thought, their fossils may also become more common. However, scientists may have interpreted the wrinkled rock textures as purely physical formations and overlooked them.
Part of the challenge lies in the language used to describe these features. Without clear terminology, it can be difficult to distinguish between structures formed by physical forces and structures formed by living organisms.
“The terminology is pretty loose,” Martindale says. “There’s so much meanness about wrinkles that we lack diagnostic language.”
Follow an unexpected scientific path
Martindale typically studies ancient coral reefs and mass extinctions. She never expected that this observation would lead to studies of deep-sea microbial mats. But the mystery turned out to be too compelling to ignore.
“I’m really happy that it went in a direction that I didn’t expect at all,” she said. “We had no hypothesis that we would find these microbial mats here. We just had the right search image in the right place at the right time. And we were stubborn enough to not let it go.”
This research was funded by the National Science Foundation.
