While hiking through Morocco’s Dadès Valley, Dr. Rowan Martindale spotted something so unusual that it immediately caught her attention.
Martindale, a paleoecologist and geobiologist at The University of Texas at Austin, was exploring the rugged landscape with fellow researchers, including Stéphane Bodin of Aarhus University. Their goal was to investigate ancient reef ecosystems that once existed beneath an ocean covering the region millions of years ago.
To reach those ancient reefs, the team had to cross extensive layers of rock known as turbidites. These deposits form when underwater avalanches of mud, sand, and debris rush down the seafloor and eventually settle into thick sediment layers. Ripple marks are common in such deposits, but Martindale noticed something unusual sitting on top of the ripples.
“As we’re walking up these turbidites, I’m looking around and this beautifully rippled bedding plane caught my eye,” says Martindale. “I said, ‘Stéphane, you need to get back here. These are wrinkle structures!'”
What Are Wrinkle Structures?
Wrinkle structures are small ridges and depressions that can form when microbial communities grow into mats across sandy sediment. These mats are made up of microscopic organisms such as algae and bacteria that bind sediment together and leave distinctive surface textures behind.
They are important to scientists because they can provide evidence of ancient microbial life. However, wrinkle structures are usually fragile. Once animals began actively burrowing through seafloor sediments hundreds of millions of years ago, these delicate features were often destroyed before they could be preserved.
As a result, wrinkle structures are uncommon in rocks younger than about 540 million years old, a period that coincides with a major expansion in animal diversity. Today, they are most often found in shallow coastal environments where sunlight supports photosynthetic algae.
A Discovery That Shouldn’t Have Been There
The rocks Martindale was examining presented a major puzzle.
The turbidites containing the wrinkle structures had formed in deep water, at least 180 meters (590 feet) below the ocean surface. At those depths, sunlight cannot penetrate, making it impossible for photosynthetic algae to survive.
That immediately raised a problem. If sunlight dependent microbes could not have created the structures, what did?
Previous reports of wrinkle structures in ancient deep water turbidites had been controversial and widely debated. The age of the rocks made the mystery even more surprising. These sediments formed roughly 180 million years ago, during a time when seafloor animals were abundant and constantly disturbing sediment. Such activity typically destroys delicate microbial textures before they can be preserved.
Everything about the discovery suggested the wrinkle structures should not exist in that setting.
Martindale knew extraordinary claims required strong evidence.
“Let’s go through every single piece of evidence that we can find to be sure that these are wrinkle structures in turbidites,” says Martindale, because wrinkle structures, usually photosynthetic in origin, “shouldn’t be in this deep-water setting.”
Searching for Evidence of Ancient Microbial Life
The research team carefully investigated the rocks to confirm both the environment where the sediments formed and the biological origin of the unusual textures.
First, they verified that the layers were indeed turbidites deposited in deep water. Next, they looked for chemical signatures that could reveal whether living organisms had played a role in creating the structures.
Their analysis showed elevated concentrations of carbon in the sediment layers directly beneath the wrinkles. Carbon enrichment is often associated with biological activity and provided an important clue that microbes were involved.
The researchers then turned to modern oceans for answers.
Video footage collected by remotely operated submersibles revealed that microbial mats can form even in parts of the ocean far below the photic zone, the sunlit upper layer where photosynthesis occurs. Instead of relying on sunlight, these communities are built by chemosynthetic bacteria.
Chemosynthetic organisms generate energy from chemical reactions rather than sunlight. Some use compounds such as hydrogen sulfide or methane as fuel, allowing them to thrive in dark environments where photosynthetic life cannot survive.
Deep Sea Bacteria May Have Created the Wrinkles
When the geological evidence, chemical data, and modern seafloor observations were considered together, the researchers concluded they had identified chemosynthetic wrinkle structures preserved in the rock record.
According to their proposed explanation, turbidite flows delivered nutrients and organic matter to the deep seafloor. As that material decomposed, oxygen levels in the sediment decreased, creating favorable conditions for chemosynthetic microbes.
During quiet intervals between underwater debris flows, bacterial mats could spread across the sediment surface. Over time, those mats developed the distinctive wrinkles preserved in the rocks.
Most of the time, later debris flows would have erased the microbial mats. Occasionally, however, conditions allowed the mats and their wrinkled textures to be buried and preserved for millions of years.
Expanding the Search for Early Life
Martindale hopes future laboratory experiments will help scientists better understand exactly how these structures form in deep water environments.
The discovery could also broaden scientific thinking about wrinkle structures. Traditionally, researchers have linked them almost exclusively to photosynthetic microbial mats living in shallow water. The new findings suggest that chemosynthetic communities may also produce similar features.
If that is the case, geologists may need to revisit environments that were previously dismissed as unlikely places to preserve evidence of ancient microbial ecosystems.
“Wrinkle structures are really important pieces of evidence in the early evolution of life,” says Martindale. By ignoring their possible presence in turbidites, “we might be missing out on a key piece of history of microbial life.”
The discovery raises an intriguing possibility: some clues to Earth’s early microbial past may be hiding in places scientists never thought to look.