Revenge of the seabed diggers

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New Haven, Connecticut – Ancient seabed burrows have been the subject of love at first sight for years.

These prehistoric churners – a large assortment of worms, trilobites, and other animals that lived in Earth’s oceans hundreds of millions of years ago – are believed to have played a key role in creating the conditions for the flourishing marine life. Their activities altered the chemical makeup of the sea itself and the amount of oxygen in the oceans, in a process called bioturbation.

But did this bioturbation help or prevent the expansion of complex animal life? A new study from Yale, published in the journal Earth and Planetary Science Letters, found that the burrowers of the seabed were really very useful.

“Bioturbator animals are one of our prime examples of ‘ecosystem engineers,’ said lead author Lidya Tarhan, assistant professor of earth and planetary sciences at Yale.“ They play a major role. in the formation of the chemical composition of the oceans, and even, at geological time scales, of the atmosphere. “

Bioturbator animals that live and burrow in the sediments of the seabed became widespread and active for the first time in the early Cambrian Period, around 541 million years ago. They were part of the so-called “Cambrian Explosion,” when most groups of animals with sophisticated body plans and behaviors began to appear in rapid succession, according to the fossil record.

But there is a lot of debate among Earth scientists about the impact of these burrowers on their environment.

For example, there is the relationship between bioturbation and the availability of phosphorus – an essential nutrient that is necessary for all life. The availability of phosphorus determines the size of the global biosphere and the complexity of life it can support. Phosphorus reaches the seabed primarily in the form of plankton, the carcasses of which sink to the ocean floor after death, and ocean waters that flow upward along the edges of continents.

A lot of recent research has suggested that the early diggers took phosphorus and buried it, effectively choking off the supply of this life-creating nutrient. The theory also suggests that bioturbation has changed the way carbon is buried beneath the ocean floor, leading to widespread reduction of oxygen in the water.

A separate body of bioturbation research – grounded in evolutionary theory and fossil record observations – offers a much different premise. This theory argues that burying the seabed would have led to greater biological sophistication, not less, in terms of the size and behavior of the animals.

“We have long had these two major think tanks, fundamentally at odds with each other, regarding the role of early animals in shaping the chemistry, habitability and ecology of the oceans,” said said Tarhan.

The new work of the Yale team aims to solve the problem.

For the study, Tarhan and his colleagues created new models of phosphorus cycling and bioturbation that more accurately describe the two processes. For example, she said, previous models did not take into account the large amount of phosphorus-rich minerals that formed in the sediment at the bottom of the ocean. Likewise, the previous modeling assumed that bioturbation was an all-or-nothing activity that functioned almost like an on-off switch, rather than a behavior that gradually accelerated.

“Our work has, for the first time, reconciled the two main frameworks regarding the role of early animals in driving changes in the evolutionary and biogeochemical landscapes of Earth’s early oceans,” said Tarhan. “The first burrowing animals indeed favored the emergence of increasingly productive and complex ecosystems and helped to continue the Cambrian explosion, rather than stifling or delaying its impact.”

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The co-authors of the study are Noah Planavsky, associate professor of earth and planetary sciences at Yale, and Mingyu Zhao, former postdoctoral researcher at Yale who is now at the University of Leeds.

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