First Critters to Kick Up Dust on the Seafloor Immediately Affected Ocean Chemistry

UMD geologists find evidence for massive shifts in ocean chemistry directly after the first animals began piercing microbial mats

For more than 3 billion years, life on Earth consisted of single-celled organisms, most of which grew in thick mats at the bottom of the ocean floor. Then, near the dawn of the Cambrian Period more than 500 million years ago, the first true animals evolved—along with the ability to pierce, churn and burrow into these microbial mats.

As a result of this widespread poking and prodding, the microbial mats released large amounts of trapped gases and waste products—drastically and permanently changing the chemistry of the world’s oceans. While geologists broadly agree that these changes happened, they have debated how quickly ocean chemistry began to change on a global scale.

New research by geologists at the University of Maryland and Queen’s University in Ontario, Canada suggests that these chemical changes were almost immediate, affecting global ocean chemistry within a few thousand years or so. The findings call into question a competing theory that suggests sediment mixing by Earth’s earliest animals took as long as 100 million years to significantly affect the oceans. The researchers published their findings in the November 2018 issue of the Canadian Journal of Earth Sciences.

“When animals first became large and strong enough to begin ventilating the mats, they released gases such as methane and hydrogen sulfide that could then be oxidized,” said Alan Kaufman, a professor of geology at UMD and a co-author of the research paper. “We came to the conclusion that you only have to poke a hole in a balloon for the air to come out. Our findings suggest that these gases were oxidized to form carbon dioxide, sulfate and soluble bicarbonate right away, resulting in nearly immediate changes to global ocean chemistry.”

Kaufman and his colleagues, including lead author Kalev Hantsoo (B.S. ’14, geology), currently a graduate student at Johns Hopkins University, collected rock samples from a formation on the wind-swept coast of Newfoundland, Canada. The area contains deposits that have been recognized by the international research community as a gold standard for the transition between the Precambrian and the Cambrian Period.

According to Kaufman, many of the sediment layers in the formation lacked enough organic carbon and pyrite sulfur for the researchers to test their hypothesis. Kaufman and his team addressed this with a brute-force approach, exhaustively collecting and analyzing large quantities of rock deposited before, during and after the Precambrian-Cambrian boundary.

“The first thing we did was embark on several seasons of intense collecting along the sea coast. Our team was out there climbing cliffsides and taking boats into small inlets,” Kaufman said. “For the first couple of seasons, we collected from the Cambrian boundary upward and saw the first appearance of a lot of very interesting looking fossils. But we also wanted to know what ocean chemistry was like before the boundary. These rocks contained no fossils, but we were able to investigate them with geochemical tools.”

Back in the lab at UMD, Hantsoo took the lead on crushing and dissolving pounds of rock and performing the geochemical analyses. The results showed a spike in specific carbon isotopes and a decline in sulfur isotopes, indicating an increase in oxidized gases and ions dissolved in seawater. These observations allowed the researchers to conclude that the piercing and burrowing of early animals had a near-immediate effect on global seawater chemistry.

In another interesting twist, the team found that seawater began to chemically stabilize several million years later, right around the time that shell-forming animals began to evolve. Kaufman suggests that the widespread alteration of seawater chemistry could be directly related to the Cambrian explosion, a 10-million-year-long event during the early Cambrian period, when the ancestors of most major forms of life first appeared in the fossil record.

“The end of the carbon and sulfur anomalies we discovered coincide with the first evidence of organisms with shells,” Kaufman explained. “It seems as though seawater chemistry changed enough that organisms might have been forced to build shells to deal with an excess of dissolved bicarbonate in seawater.”

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In addition to Kaufman and Hantsoo, UMD-affiliated co-authors of the research paper include Huan Cui (Ph.D. ’15, geology), currently a postdoctoral researcher at the Free University of Brussels, Belgium and former faculty research associate Rebecca Plummer.

The research paper, “Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada,” Kalev Hantsoo, Alan Kaufman, Huan Cui, Rebecca Plummer, and Guy Narbonne, was published in the Canadian Journal of Earth Sciences on September 6, 2018.

This work was supported by NASA (Award No. NNX12AR91G), the National Science Foundation (Award No. EAR0844270), the Natural Sciences and Engineering Research Council of Canada (NSERC), Queen’s University, the Society of Economic Geologists, and the Explorers Club. The content of this article does not necessarily reflect the views of these organizations.

Media Relations Contacts: Matthew Wright, 301-405-9267, mewright@umd.edu

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