While hiking in Morocco’s Dades Valley, Dr. Rowan Martindale found 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 terrain with colleagues including Stefan Bodin of Aarhus University. Their goal was to investigate the ancient coral reef ecosystem that once existed beneath the ocean that covered the region millions of years ago.
To reach these ancient coral reefs, the team had to cross vast rock formations known as turbidites. These deposits form when underwater mud, sand, and debris avalanche down the ocean floor and eventually settle into a thick layer of sediment. Ripples are common in such deposits, but Martindale noticed something unusual above the ripples.
“As I was walking through these turbidites, I looked around and saw this beautifully undulating bedding plane,” Martindale says. “I said, ‘Stefan, you need to get back here. These are wrinkle structures!’
What is the structure of wrinkles?
Wrinkle structures are small ridges and depressions that form as microbial communities grow into mats throughout sandy sediments. These mats are composed of microorganisms such as algae and bacteria that bind the sediment together and leave a unique surface texture.
These are important to scientists because they can provide evidence of ancient microbial life. However, the wrinkle structure is usually fragile. When animals began actively burrowing into 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 rare in rocks older than about 540 million years, which coincides with a significant expansion in animal diversity. Currently, they are most commonly 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 great mystery.
The turbidites, which contain wrinkled structures, formed at depths of at least 180 meters (590 feet) below the ocean surface. At that depth, photosynthetic algae cannot survive because sunlight does not reach them.
That immediately caused problems. If sunlight-dependent microbes couldn’t create that structure, what could?
Previous reports on the wrinkled structure of ancient deep-sea turbidites were controversial and widely debated. The age of the rocks made the mystery even more surprising. These deposits were formed about 180 million years ago, when seafloor animals were abundant and constantly disturbed the sediments. Such activities typically destroy the delicate tissues of microorganisms before they can be preserved.
Everything about the findings suggested that the wrinkle structure should not be present in that environment.
Martindale knew that extraordinary claims required strong evidence.
“Let’s look at all the evidence we can find to be confident that these are turbidite wrinkle structures,” Martindale says. That’s because wrinkled structures are usually of photosynthetic origin and “shouldn’t be in this deep-sea environment.”
Searching for evidence of ancient microbes
The research team carefully examined the rocks to determine both the environment in which the deposits formed and the biological origins of the unusual textures.
First, they verified that the layer was indeed turbidite deposited in the deep ocean. Next, they looked for chemical signatures that could reveal whether organisms played a role in forming the structures.
Their analysis showed increased carbon concentrations in the sediment layer directly beneath the wrinkles. Carbon enrichment is often associated with biological activity, providing an important clue that microorganisms are involved.
So researchers turned to the modern ocean for answers.
Video footage collected by remotely operated submersibles reveals that microbial mats can also form in parts of the ocean far below the photic zone, the sunlit upper layer where photosynthesis takes place. Instead of relying on sunlight, these communities are built by chemosynthetic bacteria.
Chemosynthetic organisms generate energy from chemical reactions rather than sunlight. Some organisms use compounds such as hydrogen sulfide and methane as fuel, allowing them to survive in dark environments where photosynthetic life cannot survive.
Deep sea bacteria may have caused the wrinkles
After a combined review of geological evidence, chemical data, and modern ocean floor observations, the researchers concluded that they had identified a chemosynthetic wrinkle structure preserved in the rock record.
Their proposed explanation is that turbidite flows transported nutrients and organic matter to the deep ocean floor. As that material decomposed, oxygen levels in the sediment decreased, creating favorable conditions for chemosynthetic microorganisms.
During quiet intervals in an underwater debris flow, bacterial mats can spread across the surface of the sediment. Over time, those mats developed distinctive wrinkles that remained in the rock.
In most cases, subsequent debris flows would have destroyed the microbial mats. However, in some cases, conditions allowed the mat and its wrinkled texture to remain buried and preserved for millions of years.
Expanding the search for early life
Martindale hopes future laboratory experiments will help scientists understand exactly how these structures form in deep-sea environments.
The discovery also has the potential to expand scientific thinking about the structure of wrinkles. Traditionally, researchers have associated them almost exclusively with photosynthetic microbial mats that live in shallow waters. New findings suggest that chemosynthetic communities may also produce similar features.
If so, geologists may need to revisit environments that have been previously dismissed as unlikely to preserve evidence of ancient microbial ecosystems.
“Wrinkle structure is very important evidence for the early evolution of life,” Martindale says. By ignoring their possible presence in turbidites, “we may be missing an important part of the history of microbial life.”
This discovery raises interesting possibilities. Some clues about Earth’s early microbial past may be hidden in places scientists never thought to look.
