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The murky world at the bottom of the oceans is now a little clearer, thanks to a new study that tracks the evolution of marine sediment layers across hundreds of millions of years.

It is a story of world-building on a grand, yet granular, scale, accomplished by a succession of marine animals that burrowed and tunneled their way through heat and cold, species expansions and mass die-offs. Scientists call the process bioturbation—the excavation and mixing of sediments and soils by burrowing animals, particularly for shelter and sustenance.

"Bioturbation is one of the most important forms of ecosystem engineering today, both in the oceans and on land," said Lidya Tarhan, assistant professor of Earth and planetary sciences in Yale's Faculty of Arts and Sciences, and lead author of the study in the journal Science Advances.

"In the oceans, bioturbation plays a critical role in shaping the habitability and ecology of the seafloor, as well as in regulating nutrient cycling in overlying ocean waters," Tarhan said. "However, how bioturbation has varied through Earth's past, and the evolutionary timing of when bioturbators became the enormously impactful 'engineers' they are today, has long been poorly understood."

In addition to their own data—which includes observations from geologic field work in the U.S., Canada, Spain, and Australia and sediment drill cores collected from the modern oceans—Tarhan and her collaborators surveyed more than 1,000 previous scientific studies. They looked specifically for information about how intensively seafloor sediments were churned, as well as six types of fossilized burrows that have typically been among the deepest burrows in the seafloor. Ultimately, they amassed a database covering 540 million years of Earth's history—nearly the full evolutionary history of animal life.

The team gleaned several new insights from their research.

First, they found that the two main types of bioturbation—burrowing and sediment mixing by the animals—developed separately. Deep burrowing began early in the evolution of animals; sediment mixing took hundreds of millions of years to develop.

"Burrowing animals such as worms, and later, clams and crustaceans were abundant and widespread, at least in the shallow oceans," said Tarhan, who is also an assistant curator at the Yale Peabody Museum. "It took longer for them to venture to the deep oceans. But sediment mixing lagged behind. We hypothesize that ocean oxygen stress, particularly in intervals of warm, 'greenhouse' climates, may have been a major driver."

Ocean oxygen levels were likely very low when seafloor animal communities were first establishing themselves, she said. Under warmer water temperatures, animals' metabolic rates increase and so does their need for oxygen. That likely meant that sediment mixing, which requires a great expenditure of energy, took a backseat to less-intensive burrowing.

The researchers were also able to begin documenting how bioturbation has been affected by major environmental changes and mass extinction events throughout history. For example, during the End-Permian mass extinction 252 million years ago, when potentially more than 90% of animal species were wiped out, bioturbation ceased for a time. Then small, horizontal burrows slowly began to reappear.

Further research will explore what role this greatly reduced bioturbation had in the reestablishment of nutrients in the ocean and the eventual regrowth of ecosystems.

"Without a clear picture of how bioturbators responded to environmental stressors and how quickly they were able to rebound following extinctions, our understanding of the mechanics of the ecological cascades that drive extinction and dictate recovery is decidedly murky," Tarhan said. "This certainly compounds the challenges we face in attempting to predict the ecological impacts of our current extinction crisis."

Kate Pippenger, a graduate student in Tarhan's lab at Yale, is co-author of the study. Additional co-authors are Alison Cribb of the University of Southampton, Michelle Zill and David Bottjer of the University of Southern California, William Phelps of Riverside Community College, Mary Droser of the University of California-Riverside, and Matthew Clapham of the University of California-Santa Cruz.