New research from an international team of scientists, including Northwestern University, has pinpointed the timing and duration of Ocean Anoxic Event 1a (OAE 1a), a catastrophic episode 119.5 million years ago that depleted ocean oxygen and drove mass extinction. This discovery comes from studying ancient volcanic ash deposits on Japan's Mount Ashibetsu.
During the Cretaceous Period, volcanic eruptions from large igneous provinces injected immense amounts of carbon dioxide (CO2) into the oceans and atmosphere, causing global warming and ocean anoxia. This study reveals that OAE 1a persisted for 1.1 million years, offering insights into Earth's climate system and its response to extreme CO2 levels – a critical comparison to modern climate change.
"Ocean anoxic events occur in part as a consequence of climatic warming in a greenhouse world," said Brad Sageman, senior author and professor at Northwestern University. "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."
Historical discovery and new insights
Ocean anoxic events were first identified in the 1970s when researchers, including Northwestern's Seymour Schlanger, discovered organic-rich black shales in deep-sea sediments. These deposits suggested widespread oceanic oxygen loss that prevented organic matter from decomposing, forming strata rich in organic carbon.
In the new study, researchers analyzed tuff layers formed from volcanic ash on Hokkaido Island, Japan. These ancient rocks, exposed by tectonic uplift and erosion, provide a window into geologic history. Sageman's team, alongside collaborators from Japan, the U.K., and the U.S., used isotopic dating methods to determine the timing of volcanic eruptions and OAE 1a.
"Magma comes out of a volcano in liquid form and then begins to cool," explained Sageman. "During this process, crystals start to form, locking in atoms like uranium. As these atoms decay, they create a tool for precise dating, helping us understand the age of sedimentary layers."
Carbon and osmium isotopes revealed the sequence of events during OAE 1a, including a massive injection of volcanic CO2 followed by extensive burial of organic matter. These isotopes also allowed the researchers to correlate findings with other global sites, such as southern France.
Implications for today
The volcanic eruptions linked to OAE 1a originated from the Ontong Java Nui complex, a vast igneous province in the Pacific Ocean. This event caused significant disruptions to marine life, with acidic waters dissolving shells and low oxygen levels exacerbating ecosystem collapse.
The study sheds light on the long-lasting impacts of CO2-driven warming events. Although ancient events unfolded over millions of years, Sageman noted the alarming parallels to modern climate change: "We're already seeing zones with low oxygen levels in the Gulf of Mexico. 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 research, published in Science Advances, offers a framework to analyze how Earth's systems respond to rapid environmental stressors, providing a crucial tool for understanding modern climate challenges.
Research Report:Radioisotopic chronology of Ocean Anoxic Event 1a: Framework for analysis of driving mechanisms
