Composition of Toxic Ancient Ocean Gives Clues for the Future

By analyzing seafloor mud samples, a research team from University of California, Riverside has quantified the toxic conditions of the oceans about 94 million years ago, correcting data yielded by previous research and providing a clearer picture of the chemical composition of the oceans.

Along with the first major proliferation of animal life on Earth about 600 million years ago, the levels of oxygen in the ocean and atmosphere rose dramatically. Since then, oxygen concentrations in the ocean have dipped episodically during other biotic events, which are typically marked by notable climate disruptions. The most well-understood of these events took place about 93.9 million years ago.

The team of biogeochemists examined the chemistry of rocks deposited in the seabed during that time period, finding that oxygen-free and hydrogen sulfide-rich -- or euxinic --  waters extended across about 5 percent of global ocean during the biotic event 94 million years ago. The figure is far more than the present conditions of the ocean, but much less than previous studies suggests for the same biotic event.

The researchers suggests that previous estimates of euxinia in the ancient ocean were too high, noting that levels as low as 5 percent would still be strong enough to have powerful effects on global ocean chemistry.

"These conditions must have impacted nutrient availability in the ocean and ultimately the spatial and temporal distribution of marine life," said team member Jeremy D. Owens, a former UC Riverside graduate student, who is now a postdoctoral scientist at the Woods Hole Oceanographic Institution. "Under low-oxygen environments, many biologically important metals and other nutrients are removed from seawater and deposited in the sediments on the seafloor, making them less available for life to flourish."

Biogeochemistry professor Timothy Lyons, who advised Owens at UC Riverside, said the study was particularly noteworthy because the researchers were able to map out a landscape of bioessential elements in the ocean and that the impacts were much larger than expected.

"Our work shows that even though only a small portion of the ocean contained toxic and metal-scavenging hydrogen sulfide, it was sufficiently large so that changes to the ocean's chemistry and biology were likely profound," Owens said. "What this says is that only portions of the ocean need to contain sulfide to greatly impact biota."

Lyons said that by better understanding the oceanic conditions of the past, we will be better equipped to understand our current situation.

"Today, we are facing rising carbon dioxide contents in the atmosphere through human activities, and the amount of oxygen in the ocean may drop correspondingly in the face of rising seawater temperatures," Lyons said. "Oxygen is less soluble in warmer water, and there are already suggestions of such decreases. In the face of these concerns, our findings from the warm, oxygen-poor ancient ocean may be a warning shot about yet another possible perturbation to marine ecology in the future."

The research is published in the Proceedings of the National Academy of Sciences.