Moroccan deep-sea rocks point to ancient microbes living without sunlight

Researchers studying ancient seafloor deposits in Morocco say they have found wrinkled rock textures that may preserve traces of microbes living far below sunlight. The finding matters because these structures are usually linked to shallow, sunlit environments, while the Moroccan rocks formed in deep water.

The Geological Society of America said the features were identified in the Dadès Valley by a team including Rowan Martindale, a paleoecologist and geobiologist at the University of Texas at Austin, and Stéphane Bodin of Aarhus University. The researchers were examining ancient reef systems that once lay beneath an ocean covering the region.

To reach the reef deposits, the team crossed layers known as turbidites, according to the society. Turbidites form when sediment, mud and other material move downslope in underwater flows and settle on the seafloor.

Wrinkles in the wrong setting

The unusual textures appeared on rippled bedding surfaces within those turbidites. In geology, wrinkle structures are small ridges and hollows often made when microbial mats grow across sandy sediment and bind grains together.

Such features are useful evidence for ancient microbial life, the Geological Society of America said, but they are fragile. They also became less common in rocks younger than about 540 million years, after burrowing animals became more active and often disturbed seafloor sediment before delicate textures could be preserved.

The Moroccan rocks posed two problems for the standard explanation. The team determined that the sediments were deposited at least 180 meters, or 590 feet, below the ocean surface, beyond the reach of sunlight needed by photosynthetic algae. The rocks also date to about 180 million years ago, a time when sediment-churning seafloor animals were already common.

Previous claims of wrinkle structures in ancient deep-water turbidites have been disputed, according to the Geological Society of America. Martindale and colleagues therefore checked both the depositional setting and possible biological signals before proposing an explanation.

Chemistry and modern seafloor clues

The researchers report in the journal Geology that carbon levels were elevated in sediment directly below the wrinkled surfaces. The Geological Society of America said that carbon enrichment can be associated with biological activity and supported the interpretation that microbes helped create the textures.

The team also compared the Moroccan rocks with observations from modern oceans. Video from remotely operated submersibles has shown microbial mats living below the photic zone, where sunlight-driven photosynthesis is unavailable, according to the society.

Those communities can rely on chemosynthetic bacteria, which draw energy from chemical reactions rather than sunlight. Some chemosynthetic organisms use compounds such as methane or hydrogen sulfide as fuel, allowing them to live in dark marine settings.

Martindale and her co-authors propose that turbidite flows carried nutrients and organic matter to the deep seafloor. As the material broke down, oxygen levels in the sediment fell, creating conditions that could support chemosynthetic microbial mats during quieter periods between sediment flows.

Later flows would often have destroyed those mats, the researchers said. In some cases, however, burial may have protected the wrinkled surfaces long enough for them to survive in the rock record.

The study suggests geologists may need to look more closely at deep-water deposits when searching for evidence of ancient microbial ecosystems. Martindale said wrinkle structures are significant evidence in studies of early life, and that overlooking turbidites could leave out part of microbial history.

This story draws on original reporting from ScienceDaily.