Refining the Timeline of Ocean Anoxic Event 1a and Its Impacts

• Dic 30, 2024 - 15:13

Researchers have used ancient rocks and fossils from Mount Ashibetsu in Japan to precisely determine the timing and duration of Ocean Anoxic Event 1a (OAE 1a), a significant environmental disturbance that depleted oxygen from Earth's oceans, leading to widespread extinction, particularly among plankton. This study, which sheds new light on the causes and effects of this catastrophic event, reveals that volcanic eruptions likely triggered a rise in carbon dioxide (CO2) levels, resulting in global warming and oceanic anoxia during the Mesozoic Era.

An international team, including Earth scientists from Northwestern University, has now established that OAE 1a began 119.5 million years ago and lasted just over 1.1 million years. This refined timeline helps scientists better understand the Earth's climate and ocean systems, offering valuable insights into the responses of these systems to stress, particularly in the context of current climate change.

The study, published in Science Advances, represents the most detailed and precise dating of an ocean anoxic event to date. Brad Sageman, a senior author and professor of Earth, Environmental, and Planetary Sciences at Northwestern University, emphasizes the importance of this research in understanding future climate change. "The best way to understand the future is to look at data from the past," Sageman noted, highlighting the relevance of this study for predicting the effects of human-induced warming.

The Cretaceous Period, which saw two major and several minor ocean anoxic events, experienced OAE 1a as one of the largest. Volcanic eruptions, particularly large igneous provinces that released up to a million cubic kilometers of basalt over millions of years, are believed to have rapidly increased CO2 levels in the atmosphere and oceans. This excess CO2 reacted with seawater, forming carbonic acid, which dissolved the shells of marine creatures, while simultaneously depleting oxygen in the ocean—a combination that had catastrophic effects on marine life.

The concept of ocean anoxic events was first suggested in the 1970s when researchers discovered organic carbon-rich shales in ocean floor sediments, which were found to be similar in composition and age to rocks from both the Atlantic Ocean and Italy. These shales suggested widespread oxygen depletion in the oceans, leading to the global accumulation of organic material that would otherwise decompose.

To further investigate, the researchers turned to volcanic ash deposits, or tuffs, found on the northwest edge of Mount Ashibetsu on Japan's Hokkaido Island. These layers, formed from volcanic ash that solidified over time, were exposed through tectonic activity and made accessible for study. By analyzing these ancient tuffs, Sageman, Ph.D. student Luca Podrecca, and their collaborators gained critical insights into the geological history of OAE 1a.

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