Prehistoric rock in Japan reveals clues to ancient ocean anoxic event

Rocks from Mount Ashibetsu in Japan have revealed important information about an ancient global environmental crisis, which choked the oxygen from Earth’s oceans.

The Ocean Anoxic Event 1a (OAE 1a) was one of the largest disruptions of the global carbon cycle and climate system in the past 200 million years.

New research published in Science Advances has determined that it was triggered 119.5 million years ago and lasted for 1.1 million years, during the Early Cretaceous Period.

The new information helps scientists better understand how the Earth’s climate and ocean system operates and responds to stress, especially as it relates to current warming.

“Ocean anoxic events occur in part as a consequence of climatic warming in a greenhouse world,” says Brad Sageman, senior author of the study from Northwestern University in the US.

“If we want to make accurate predictions about what we will see in the decades ahead with human-caused warming, this information is invaluable. The best way to understand the future is to look at data from the past.”

Researchers have long thought that the most likely cause of OAE 1a was undersea volcanic eruptions, which resulted in a rapid injection of massive amounts of carbon dioxide (CO₂₎ into the ocean and atmosphere.

This study analysed ancient strata on Japan’s northern island, Hokkaido. The rocks, or tuffs, formed from volcanic ash that settled and solidified over time. Volcanic activity then lifted these layers above sea level during the formation of Japan’s islands.

“Magma comes out of a volcano in liquid form and then begins to cool,” Sageman says.

“During this process, crystals start to form. By the time the tuff solidifies, the crystals become a tiny closed system. They lock in atoms, and some of those atoms, like uranium, start to decay, meaning they convert from one isotope to another.”

“These isotope systems provide tools for correlating the OAE1a interval between sites in Hokkaido, southern France and other sites all around the globe,” he explains.

“They give us markers for instants in geologic time.” 

Sageman and collaborators found an abrupt shift in carbon isotope ratios, caused by the spike in volcanic CO₂ and later by the burial of excess organic matter from dead plants and animals. At the same time, there was a change in osmium isotope ratios, reflecting a massive influx of volcanic material into ocean waters.

The timing of these events corresponds to eruption of the Ontong Java Nui complex, a large igneous province in the southwestern Pacific Ocean.

Now that researchers know it took the oceans 1.1 million years to recover from the sharp increase in CO₂ from this eruption, they have more insight into how long the effects of CO₂-driven warming events might last and what the associated effects, such as ocean anoxia, may be.

“We’re already seeing zones with low oxygen levels in the Gulf of Mexico,” Sageman says.

“The main difference is that past events unfolded over tens of thousands to millions of years. We’re driving roughly similar levels of warming (or more) but doing so in less than 200 years.”