A strange set of wrinkles in ancient Moroccan seafloor rocks has led scientists to a surprising idea: deep-sea microbes may have left them behind 180 million years ago, far below any sunlight.

The rocks were found in Morocco’s Dadès Valley by researchers studying ancient reef ecosystems, including Dr Rowan Martindale, a paleoecologist and geobiologist at The University of Texas at Austin, and Stéphane Bodin of Aarhus University.

To reach the reefs, the team crossed thick layers of turbidites, sediments left behind by underwater avalanches of mud, sand and debris. Ripple marks are common in those deposits, but Martindale said she spotted something unusual on top of them.

“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!'”

Wrinkle structures are small ridges and depressions that can form when microbial communities grow into mats across sandy sediment. They are usually linked to algae and bacteria in shallow coastal environments where sunlight supports photosynthesis.

That is why the Moroccan rocks posed a problem. The turbidites formed in deep water, at least 180 metres below the ocean surface, where sunlight could not reach.

Martindale said the team set out to test the idea carefully.

“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.”

The researchers first confirmed that the rock layers were deep-water turbidites. They then found elevated concentrations of carbon in sediment directly beneath the wrinkles, which the Geological Society of America said is often associated with biological activity.

The team also looked at video footage from remotely operated submersibles, which showed that microbial mats can form well below the sunlit upper ocean. In those dark settings, chemosynthetic bacteria, organisms that get energy from chemical reactions rather than sunlight, can build microbial communities.

Based on the geological evidence, chemical data and modern seafloor observations, the researchers concluded that chemosynthetic microbes formed the wrinkle structures.

Their proposed explanation is that turbidite flows delivered nutrients and organic matter to the deep seafloor. As that material decomposed, oxygen in the sediment dropped, creating conditions that suited chemosynthetic microbes. During quiet periods between debris flows, bacterial mats could spread over the sediment surface and form the wrinkles. In some cases, later sediment buried and preserved them.

The find is unusual for another reason. Wrinkle structures are often rare in rocks younger than about 540 million years old because animals burrowing through seafloor sediments tend to destroy them before they can be preserved. These Moroccan sediments formed about 180 million years ago, when seafloor animals were already abundant.

Martindale said the finding could change where scientists look for signs of ancient microbial life.

“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.”

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