Researchers have conducted an analysis of radioisotopes found in layers of fossilized volcanic ash. The decay process of uranium into lead within small crystals allowed scientists to accurately date significant historical events. It was established that one such event happened 119.5 million years ago, aligning with evidence of extensive volcanic eruptions that lasted for 1.1 million years. The findings of this study enhance scientists’ understanding of the connections between atmospheric CO2 levels, climate changes, and oceanic conditions.
By examining ancient rocks and fossils exposed on the slopes of Mount Ashibetsu in Japan, researchers have accurately clarified the timing and duration of Ocean Anoxic Event 1a (OAE 1a), a significant environmental disturbance that depleted oxygen in Earth’s oceans, leading to widespread extinction, particularly among plankton.
There has been a longstanding hypothesis that large undersea volcanic eruptions led to increases in carbon dioxide (CO2), global warming, and a reduction of oxygen (known as anoxia) in the oceans during the Mesozoic Era. Now, an international team, including Earth scientists from Northwestern University, has successfully pinpointed the exact timing of the volcanic eruption and OAE 1a, which initiated 119.5 million years ago. This research contributes to the mounting evidence that volcanic emissions of CO2 were directly responsible for triggering this anoxic event.
Furthermore, the team found that OAE 1a lasted for just over 1.1 million years. This important data assists scientists in comprehending the functioning of the Earth’s climate and ocean systems, particularly in relation to current warming situations.
The research was published recently in the journal Science Advances, marking the most precise dating of an ocean anoxic event achieved to date.
“Ocean anoxic events partly arise as a result of climatic warming in a greenhouse environment,” stated Brad Sageman, a senior author of the study from Northwestern. “For us to accurately forecast what the future will hold in terms of human-induced warming, this information is crucial. The best method to understand what’s to come is to analyze historical data.”
Sageman, whose expertise is in ancient climates, is a professor within Northwestern’s Earth, Environmental and Planetary Sciences department and serves as a co-director of the Paula M. Trienens Institute for Sustainability and Energy.
A Northwestern connection
The Cretaceous Period saw two significant and several minor ocean anoxic events, with OAE 1a being among the two largest. The probable cause was volcanic eruptions that rapidly released substantial amounts of CO2 into the ocean and the atmosphere. These were no ordinary volcanic eruptions; they originated from large igneous provinces that expelled up to a million cubic kilometers of basalt over millions of years. When CO2 interacts with seawater, it produces a weak carbonic acid that literally dissolves the shells of marine creatures. This acid, combined with low oxygen levels, has serious repercussions for marine ecosystems.
Interest in ocean anoxic events began in the mid-1970s, following a discovery by Northwestern geologist Seymour Schlanger and Oxford professor Hugh Jenkyns. While analyzing sediment samples from the Pacific Ocean floor, they identified black, organic carbon-rich shales that were similar in composition and age to samples from the Atlantic Ocean and rock formations in Italy.
The most likely explanation for these deposits was the widespread lack of oxygen. Anoxia halts the decomposition of organic matter from deceased plants and animals, resulting in a global pattern of organic enrichment. Instead of breaking down, the sinking plankton and other remains accumulated to create organic carbon-rich layers found throughout the globe.
“What caused black shales to form simultaneously in deep oceans and terrestrial areas?” Sageman wondered. “Schlanger and Jenkyns recognized there must have been a massive global event resulting in a decrease of oxygen from the ocean surface down to the seafloor.”
History solidified in stone
In their new study, researchers turned their attention from the ocean’s depths to ancient strata on the northwest edge of a mountain in Hokkaido, Japan. The tuff rocks, which originated from volcanic ash that settled and hardened over time, were exposed due to tectonic activity elevating these layers above sea level during the formation of the Japanese archipelago.
By gathering and scrutinizing these tuffs, Sageman, along with his Ph.D. student, Luca Podrecca, and their team, were finally able to glean insights into geological history.
“When magma erupts from a volcano, it emerges as a liquid and starts to cool,” explained Sageman. “During this cooling, crystals begin to form. Once the tuff solidifies, these crystals create a tiny closed system that captures specific atoms, including uranium, which begins to decay. This decay process allows us to date the eruption and thereby determine the age of a particular layer within a sedimentary rock formation. While our collaborators from Tohoku University in Japan, Durham University in the U.K., and Northwestern focus on the characterization and global correlation of the layers, our partners at the University of Wisconsin-Madison and Boise State University are experts in geochronological analyses.”
Additionally, researchers employed other isotopes like carbon, which indicates simultaneous changes in the carbon cycle, and osmium, which tracks volcanic activity and variations in ocean chemistry.
“These isotopic systems are invaluable for correlating the OAE 1a interval across various locations including Hokkaido, southern France, and other places around the world,” Sageman explained. “They serve as geologic time markers.”
Pinpointing the exact timeline
The evidence suggests a sudden shift in carbon isotope ratios—initially triggered by the increase in volcanic CO2 introduced into the carbon cycle and later due to the excessive burial of organic matter—occurred at the start of OAE 1a in the early Cretaceous. A concurrent shift in osmium isotopic ratios indicates a significant influx of volcanic materials into the oceans. The timing of these events aligns with the eruption of the Ontong Java Nui complex, an extensive igneous province comparable in size to Alaska found in the southwestern Pacific Ocean.
With the understanding that it took the oceans 1.1 million years to recuperate from the rapid rise in CO2, researchers now have a clearer picture of how long the consequences of CO2-driven warming events may last, along with potential effects like ocean anoxia.
“We are already observing areas with low oxygen in the Gulf of Mexico,” noted Sageman. “The key difference is that prior events unfolded over tens of thousands to millions of years, while we are inducing similar or greater warming levels in under 200 years.”
