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Prehistoric rock in Japan reveals clues to major ocean anoxic event

New analysis pinpoints timing and duration of Ocean Anoxic Event 1a

• Researchers analyzed radioisotopes in layers of fossilized volcanic ash
• Decay of uranium to lead within tiny crystals enabled scientists to precisely pinpoint dates for certain events
• They determined this event occurred 119.5 million years ago, coincident with evidence for massive volcanic eruptions, and lasted for 1.1 million years
• Study results help scientists better understand links between atmospheric CO₂ levels, climate changes and conditions in the oceans

EVANSTON, Ill. --- By studying prehistoric rocks and fossils emerging from the side of Mount Ashibetsu in Japan, researchers have precisely refined the timing and duration of Ocean Anoxic Event 1a (OAE 1a), an extreme environmental disruption that choked oxygen from Earth's oceans to cause significant extinction, especially among plankton.

Researchers have long suspected that massive volcanic eruptions undersea caused carbon dioxide (CO₂) increases, global warming and depleted oxygen (called anoxia) in the ocean during the Mesozoic Period. Now, an international team of researchers, including Northwestern University Earth scientists, determined the precise timing of the volcanic eruption and OAE1a, which started 119.5 million years ago. The work adds to a growing volume of evidence that volcanic CO₂ emissions directly triggered the anoxic event.

The new study also determined that OAE 1a lasted for just over 1.1 million years. This 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.

The study was published late last month in the journal Science Advances. It marks the most detailed and highly resolved dating of an ocean anoxic event ever achieved.

"Ocean anoxic events occur in part as a consequence of climatic warming in a greenhouse world," said Northwestern's Brad Sageman, a senior author of the study. "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."

An expert on ancient climates, Sageman is a professor of Earth, Environmental and Planetary Sciences at Northwestern's Weinberg College of Arts and Sciences and a co-director of the Paula M. Trienens Institute for Sustainability and Energy.

A Northwestern connection

The Cretaceous Period experienced two major and several minor ocean anoxic events, with OAE 1a as one of the two largest. The most likely cause: volcanic eruptions rapidly injected massive amounts of CO₂ into the ocean and atmosphere. These aren't ordinary volcanoes but large igneous provinces that erupt up to a million cubic kilometers of basalt over several millions of years. When CO₂ reacts with seawater, it forms a weak carbonic acid, which literally dissolves sea creatures' shells. The acid, combined with low oxygen levels, has significant consequences for sea life.

Researchers first began pondering ocean anoxic events in the mid-1970s, after a discovery by Northwestern geologist Seymour Schlanger and Oxford professor Hugh Jenkyns. When examining sediment samples from the Pacific Ocean floor, Schlanger and Jenkyns discovered black, organic carbon-rich shales that matched samples - in composition and age - from both the Atlantic Ocean and rock formations in Italy.

Widespread lack of oxygen was the most likely explanation for these deposits. Anoxia prevents the breakdown of organic matter from dead plants and animals, leading to a global pattern of organic enrichment. Instead of decomposing, the settling plankton and other fossils accumulated to form organic carbon-rich strata scattered around the globe.

"How were black shales forming at the same time in the deep oceans and up on land?" Sageman asked. "Schlanger and Jenkyns realized there must have been a massive global event that caused oxygen to decrease from the ocean surface all the way down to the seafloor."

History solidified in stone

In the new study, researchers looked not to the depths of the oceans but to ancient strata along the northwest edge of a mountain on Japan's Hokkaido Island. The rocks, or tuffs, formed from volcanic ash that settled and solidified over time. Tectonic activity lifted these layers above sea level during formation of the Japanese islands, leaving them exposed and accessible where streams carve through the temperate rainforest of Hokkaido. By collecting and analyzing the tuffs, Sageman, his Ph.D. student, Luca Podrecca, and their collaborators gained a glimpse into geologic history.

"Magma comes out of a volcano in liquid form and then begins to cool," Sageman said. "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. That provides a tool to date the eruption, and, thus, date a specific layer within a stack of sedimentary rock. While the expertise of team members from Tohuku University in Japan, Durham University in the U.K. and Northwestern focuses on the characterization and global correlation of the strata, our collaborators at the University of Wisconsin-Madison and Boise State University are experts in the geochronological analyses."

The researchers also used other types of isotopes, such as carbon, which tracks synchronous changes in the carbon cycle, and osmium, which tracks volcanic activity and changes in ocean chemistry.

"These isotope systems provide tools for correlating the OAE1a interval between sites in Hokkaido, southern France and other sites all around the globe," Sageman said. "They give us markers for instants in geologic time."

Pinpointing the exact timeline

According to this evidence, an abrupt shift in carbon isotope ratios - caused first by the spike in volcanic CO₂ added to the carbon cycle (and later by the excess burial of organic matter) - occurred in the early Cretaceous at the beginning of OAE 1a. A concurrent shift in the isotopic ratios of osmium reflects a massive input of volcanic material into ocean waters. The timing of these events corresponds to eruption of the Ontong Java Nui complex, an enormous igneous province about the size of Alaska located 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, 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 said. "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."

The study, "Radioisotopic chronology of Ocean Anoxic Event 1a: Framework for analysis of driving mechanisms," was supported by the National Science Foundation, the U.K. Natural Environment Research Council and the Japan Science Foundation.

Photos from the field in Japan

Please credit all photos to Luca Podrecca/Northwestern University

Brad Sageman

Professor of Earth and Planetary Sciences

• [email protected]