Wrinkled Rocks From Deep-Sea Morocco Preserve 180-Million-Year-Old Microbial Life

Rowan Martindale was hiking on a hillside in Morocco in 2016 when she stopped in front of a slab of sedimentary rock and felt certain something was wrong with the conventional explanation for what she was seeing.

The rock surface was covered in small, regular wrinkles - a texture reminiscent of elephant skin. To Martindale, an associate professor at the University of Texas at Austin's Jackson School of Geosciences, it was an unmistakable signature. She had seen the same pattern in dozens of samples during graduate school, all identified as microbial mat fossils - the preserved remnants of bacterial communities that coated ancient seafloors more than 180 million years ago during the Early Jurassic.

The problem was where the rock had formed. Geochemical analysis showed the sediment had originated in deep water, nearly 600 feet below the surface. The prevailing interpretation in geology held that wrinkle structures made by microbes were a shallow-water phenomenon - environments where sunlight was available for photosynthesis and where normal marine predators and scavengers were absent due to stress conditions following extinction events. At 600 feet, there should have been no sunlight, and no obvious reason for a microbial mat to form.

The standard explanation and why it did not fit

The usual account for wrinkle-like textures in deep-sea sediments involves physical forces. When an underwater landslide - technically a turbidite - pushes sediment across the seafloor, it can create ridges and furrows as the material piles up and stops. These physical structures can superficially resemble biological textures.

Martindale didn't accept that explanation for what she was seeing. The morphology was too specific, too consistent with what she knew biological wrinkle structures looked like from her graduate training. "It was one of those things, knowing what to look for and having that 'search image' of wrinkle structures in my head, that made me want to stop and dig into this," she said.

She spent years pursuing the question. The result is a paper published in Geology that proposes a new mechanism - one that unites physical and biological explanations by making the turbidite essential to the microbes' survival rather than an alternative explanation for the texture.

Landslide as food source

The hypothesis Martindale and her co-authors propose is that an underwater landslide delivered nutrients to the deep seafloor - organic material, chemicals, and reduced compounds that arrived with the turbidite deposit. A community of chemosynthetic bacteria, capable of making energy from chemical reactions rather than sunlight, colonized the fresh sediment surface and formed the mat that produced the wrinkle texture when preserved.

Chemosynthetic microbial communities exist in the deep ocean today. They colonize hydrothermal vents, cold seeps, and the carcasses of dead whales that drift to the seafloor - the so-called whale fall ecosystems, where bacterial mats coat organic-rich material and support diverse scavenger communities for years. These bacteria make energy from sulfur compounds and other chemicals rather than light.

In the Moroccan case, the landslide would have played the role of the whale carcass - delivering a concentrated pulse of reduced compounds to an otherwise nutrient-limited deep-sea floor. The microbial community that colonized the deposit may also have produced toxic sulfur compounds as metabolic byproducts, keeping marine predators and scavengers away and allowing the mat to spread undisturbed until it was eventually buried and fossilized.

"In the present, some of the largest microbial ecosystems on our planet are found in the dark ocean," said Jake Bailey, a professor at the University of Minnesota who studies environmental microbiology and was not involved in the study. "The research here shows that certain ancient sedimentary structures may record the presence of these chemolithotrophs rather than phototrophs."

Implications for the fossil record

If wrinkle structures in deep-sea sediments can form through chemosynthesis rather than only shallow-water photosynthesis, the implications for paleontology are significant. Wrinkle structures have been documented across the geological record for hundreds of millions of years and have typically been interpreted either as shallow-water biological features or as purely physical formations. Some of those physical interpretations may need revisiting.

The change also affects estimates of how prevalent chemosynthetic microbial life was in ancient oceans. If chemosynthetic communities left structural traces in deep-sea sediments that have been misclassified as physical features, the fossil record of these organisms is considerably larger than the current catalog suggests.

Martindale notes a practical obstacle to reexamination: the vocabulary for describing wrinkle textures in rock is imprecise. "Wrinkly can mean lots of things, so there's a lack of diagnostic language," she said. Before systematic reassessment of ambiguous wrinkle structures is possible, the field needs clearer criteria for distinguishing biological from physical origins.

The research was funded by the National Science Foundation.