Researchers have analyzed prehistoric rocks from Mount Ashibetsu in Japan, refining the timeline of Ocean Anoxic Event 1a (OAE 1a), a significant environmental disruption that led to ocean oxygen depletion and extinction, particularly among plankton. The event is believed to have been triggered by massive volcanic eruptions undersea during the Mesozoic Period, which increased carbon dioxide levels and global warming.
The study, involving an international team including Northwestern University Earth scientists, determined that OAE 1a began 119.5 million years ago and lasted for over 1.1 million years. This research adds evidence that volcanic CO2 emissions were directly responsible for this anoxic event.
Published in Science Advances, the study provides detailed dating of an ocean anoxic event. Brad Sageman from Northwestern University stated, "Ocean anoxic events occur in part as a consequence of climatic warming in a greenhouse world." He emphasized the importance of historical data for predicting future climate scenarios.
The Cretaceous Period experienced several ocean anoxic events, with OAE 1a being one of the largest due to volcanic eruptions injecting large amounts of CO2 into the atmosphere and oceans. These eruptions created weak carbonic acid in seawater, affecting sea life significantly.
Seymour Schlanger and Hugh Jenkyns first explored these events in the mid-1970s after discovering black shales rich in organic carbon on the Pacific Ocean floor. This finding suggested widespread ocean oxygen depletion during that period.
In their recent study, researchers examined ancient strata on Hokkaido Island's northwest edge. These rocks formed from volcanic ash lifted above sea level by tectonic activity. By analyzing these tuffs, Sageman and his team gained insights into geologic history using isotopes like uranium for dating purposes.
Other isotopes such as carbon and osmium were used to track changes in the carbon cycle and volcanic activity respectively. "These isotope systems provide tools for correlating the OAE1a interval between sites," said Sageman.
Evidence indicates a shift in carbon isotope ratios at OAE 1a's onset due to increased volcanic CO2 followed by excess burial of organic matter. Osmium isotopic shifts reflect massive volcanic material input into oceans coinciding with Ontong Java Nui complex eruptions.
This research helps understand how long effects from CO2-driven warming may last based on past recovery timescales like those seen post-OAE 1a when it took oceans about 1.1 million years to recover fully from heightened CO2 levels.
"We're already seeing zones with low oxygen levels," noted Sageman regarding current Gulf of Mexico conditions compared against historical patterns unfolding over much longer periods than today's rapid changes within just two centuries.
Supported by various foundations including National Science Foundation (NSF), U.K.'s Natural Environment Research Council (NERC) & Japan Science Foundation; this work offers valuable insights into understanding Earth's climate system responses amid ongoing human-induced warming trends globally today through its comprehensive analysis titled “Radioisotopic chronology: Framework analysis driving mechanisms.”