Toxic oxygen-free and hydrogen sulfide-rich ocean conditions that existed during a major extinction event nearly 94 million years ago could repeat themselves in the future, according to new research being published online this week in the journal Proceedings of the National Academy of Sciences.

Research, led by biogeochemists at the University of California, Riverside, suggests that the previous estimates of these oxygen-free, hydrogen sulfide-rich conditions (also known as euxinia) extended across approximately five percent of the global ocean during a major climatic perturbation that occurred 93.9 million years ago.

As the study authors explain, oxygen in both the ocean and the atmosphere increased dramatically alongside the first proliferation of animal life some 600 million years ago. Since that time, numerous short-lived biotic events, marked by these climatic perturbations, have occurred whenever the ocean’s oxygen concentrations decreased.

By analyzing the chemistry of rocks deposited 93.9 million years ago, the UC-Riverside researchers found that the euxinic environment spread far more than the 0.1 percent found in the modern ocean, but less than experts had previously estimated during the time of this event.

However, even those past reports of oxygen-free and hydrogen sulfide-rich conditions were too high, they were still widespread enough to have had a “dramatic effect on the entire ocean’s chemistry and thus biological activity,” the university explained in a statement.

“These conditions must have impacted nutrient availability in the ocean and ultimately the spatial and temporal distribution of marine life,” explained investigator Jeremy D. Owens, a former graduate student at UC Riverside who is currently 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.”

According to UC Riverside biogeochemistry professor and principal investigator Timothy W. Lyons, the discovery is especially noteworthy because he and his colleagues were able to map out a landscape of the “bioessential elements” in ocean waters that were far more disturbed than they had anticipated. The effects on life, he said, were “big.”

A large, marine-based biological extinction has already been documented during the event 93.9 million years ago. However, other phenomena have also been linked to the occasion, including elevated levels of atmospheric carbon dioxide linked to increased temperature both in the air and the ocean waters. As a result, global rainfall and continental weathering likely increases, further altering the chemistry of the seas.

“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 explained. “What this says is that only portions of the ocean need to contain sulfide to greatly impact biota.”

As part of their research, Owens, Lyons and their associated collected rock samples (which had formerly been seafloor mud) from multiple locations in Italy and the UK. Those samples then underwent a chemical extraction process so that the study authors could analyze sulfur isotope compositions and estimate the ocean’s chemistry.

“The importance of their study is elevated by the large amount of previous work on the same interval and thus the extensive availability of supporting data and samples,” the university said. “Yet despite all this past research, the team was able to make a fundamental discovery about the global conditions in the ancient ocean and their impacts on life.”

“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,” added Lyons. “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.”

A grant from the National Science Foundation (NSF) supported the research, and researchers from Virginia Tech, the University of Oxford, Rutgers University, the Max Planck Institute for Marine Biology in Germany and British Petroleum (BP) also contributed to the study.