Dr. Rowan Martindale, a paleoecologist and geobiologist at the University of Texas at Austin, was hiking through the Dades Valley in Morocco’s Central High Atlas Mountains when something unusual caught his attention.
A research team including Martindale and Aarhus University’s Stefan Bodin were exploring the rugged valley to study the ecology of the ancient coral reef system that existed there when the area was once under the sea. Reaching these reefs required crossing several layers of turbidites, deposits formed by dense underwater debris flows. These deposits often exhibit ripple patterns. However, Martindale noticed that the small bumps and wrinkles superimposed on the ripples were unusual.
“As I was walking through these turbidites, I was looking around and I 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 bumps and depressions with diameters ranging from millimeters to centimeters. It occurs when communities of algae and microorganisms grow in mats across the sandy ocean floor. These delicate textures are rarely preserved in young rocks, as they are often disturbed and destroyed by animals. As a result, wrinkled structures are rare in rocks younger than about 540 million years, when animal life rapidly diversified and began actively stirring up marine sediments.
Currently, scientists typically find wrinkled structures in shallow tidal environments where sunlight supports photosynthetic algae.
Why shouldn’t there be these wrinkles?
The wrinkled structures Martindale discovered appeared in rocks that formed far below the ocean surface. The turbidites discovered were deposited at a depth of at least 180 meters, far too deep for sunlight to penetrate. This meant that the structure could not have formed from the same sunlight-dependent algae that create wrinkle patterns in today’s shallow environments.
Previous claims about the wrinkle structure of deep-sea turbidite deposits have also been challenged. Another issue is the age of the rocks. It was formed about 180 million years ago, when animals actively disturbed seafloors around the world, usually erasing delicate microbial textures. In other words, the wrinkle structure that Martindale saw should not have been preserved at all.
Realizing how rare this find was, she decided to see if her first impressions were correct.
“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.”
Evidence of chemosynthetic microbial life
The research team carefully examined the surrounding rock layers and confirmed that the deposits were indeed turbidites. They then investigated whether the unusual texture was actually formed by biological activity.
Chemical tests provided important clues. Sediments just below the wrinkles contain high carbon levels, which often indicate a biogenic origin. The research team also looked to modern marine environments for comparison. Images from remotely operated submersibles exploring the ocean floor far below the photic zone have revealed that microbial mats can also occur there, but they are produced by chemosynthetic bacteria. These microorganisms get their energy from chemical reactions instead of sunlight.
How deep-sea microorganisms created wrinkles
By combining geological observations, chemical evidence, and modern-day examples from the deep sea, scientists concluded that they had discovered chemosynthetic wrinkle structures preserved in the rock record.
Turbidite flow likely played an important role in creating suitable conditions. These debris flows transport nutrients and organic matter into the deep ocean, while simultaneously reducing oxygen levels in the surrounding sediments. Such conditions may support a community of chemosynthetic bacteria.
During quiet periods between debris flows, these bacteria can spread across the ocean floor and form mats on top of the sediment. As the mat grows, it develops the wrinkled surface pattern that Martindale observed in Moroccan rocks. In most cases, the next debris flow will destroy the mat, but in some cases the structure may be buried and preserved.
Expanding the search for ancient life
Martindale now hopes to conduct laboratory experiments to better understand how wrinkle structures develop within the turbidite environment. She also hopes the discovery will prompt scientists to reconsider the long-held assumption that wrinkled structures are created only by photosynthetic microbial mats.
If synthetic mats can produce these features, geologists may begin to explore environmental wrinkle structures that have been overlooked in the search for ancient life.
“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.”
