While hiking in the Dades Valley of Morocco, Dr. Rowan Martindale saw something so unusual that it immediately caught his attention.
Martindale, a paleontologist and geobiologist at the University of Texas at Austin, was exploring the rugged landscape with fellow researchers including Stefan Bodin of Aarhus University. Their goal was to investigate the ancient reef ecosystems that existed beneath the ocean that covered the region millions of years ago.
To reach those ancient rocks, the team had to cross extensive layers of rock known as turbidites. These deposits form when avalanches of underwater mud, sand, and debris move down from the ocean floor and eventually settle into thick sediment layers. Ripple marks are common in such deposits, but Martindale noticed something unusual sitting on top of the waves.
“As we were walking on these turbidites, I was looking around and this beautifully undulating bedding plane caught my eye,” says Martindale. “I said, ‘Stephen, you need to come back here. These are wrinkled structures!’
What are wrinkle structures?
Wrinkled structures are small ridges and depressions that can form when microbial communities grow into mats in sandy sediments. These mats are composed of microscopic organisms such as algae and bacteria that bind the sediment together and leave behind distinctive surface textures.
They are important to scientists because they may provide evidence of ancient microbial life. However, wrinkled structures are usually delicate. Once 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, wrinkled structures are uncommon in rocks less than about 540 million years old, a period that coincides with a large expansion in animal diversity. Today, they are often found in shallow coastal environments where sunlight supports photosynthetic algae.
A discovery that shouldn’t have been there
The rocks Martindale was examining turned out to be a big puzzle.
Turbidites with wrinkled structures were formed in deep water, at least 180 meters (590 ft) below the ocean surface. At those depths, sunlight cannot penetrate, making it impossible for photosynthetic algae to survive.
This immediately created a problem. If microorganisms that depend on sunlight couldn’t create structures, what did they create?
Previous reports of wrinkled 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 were formed about 180 million years ago, at a time when animals were abundant on the sea floor and were constantly disturbing the sediments. Such activity usually destroys the delicate microbial structure before it can be preserved.
Everything about the discovery suggests that wrinkled structures should not exist in that setting.
Martindale knew that extraordinary claims required strong evidence.
“Let’s look at every single piece of evidence we can to make sure that these are wrinkled structures in turbidites,” says Martindale, “because wrinkled structures, typically photosynthetic in origin, “should not be in this deep-water setting.”
Evidence of ancient microbial life discovered
The research team carefully examined the rocks to confirm both the environment where the sediments formed and the biological origin of the unusual texture.
First, they verified that the layers were indeed turbidites deposited in deep water. Next, they looked for chemical signals that could reveal whether living organisms played a role in the formation of the structures.
Their analysis showed increased concentrations of carbon in the sediment layers just below the wrinkles. Carbon enrichment is often associated with biological activity and provides an important clue that microorganisms were involved.
Researchers then turned to modern oceans for answers.
Video footage collected by remotely operated submersibles has shown that microbial mats can form even in parts of the ocean that are far below the photosphere, the sunlit upper layer where photosynthesis occurs. Instead of relying on sunlight, these communities are formed by chemosynthetic bacteria.
Chemosynthetic organisms produce energy from chemical reactions rather than from 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 caused wrinkles
When geological evidence, chemical data, and modern seafloor observations were considered together, the researchers concluded that 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 sea floor. As that material decomposed, oxygen levels in the sediment decreased, creating favorable conditions for chemosynthetic microbes.
During calm intervals between underwater debris flows, bacterial mats can spread on the sediment surface. Over time, those mats developed distinctive wrinkles preserved in the rocks.
Most of the time, subsequent debris flows will have wiped out the microbial mat. However, sometimes circumstances allow the mats and their wrinkled texture to be buried and preserved for millions of years.
Expanding the search for early life
Martindale hopes that future laboratory experiments will help scientists better understand how these structures form in deep-water environments.
The discovery could also broaden scientific thinking about wrinkled structures. Traditionally, researchers have associated them almost exclusively with photosynthetic microbial mats living in shallow waters. The new findings suggest that chemosynthetic communities can also produce similar features.
If this is the case, geologists may need to re-consider environments that were previously dismissed as unlikely places to preserve evidence of ancient microbial ecosystems.
“Wrinkle structures are really important evidence of the early evolution of life,” says Martindale. By ignoring their possible presence in turbidites, “we are missing an important part of the history of microbial life.”
The discovery raises an interesting possibility: Some clues to Earth’s early microbial past may be hidden in places scientists never thought about.