Ancient Rocks in Japan Unveil Secrets of Ocean Extinction Catastrophes

By examining prehistoric rocks and fossils from Mount Ashibetsu in Japan, a team of international researchers has made groundbreaking discoveries about the Ocean Anoxic Event 1a (OAE 1a), a catastrophic environmental upheaval that radically depleted oxygen in the Earth’s oceans, leading to significant extinctions, particularly among plankton species.

For decades, scientists have postulated that enormous underwater volcanic eruptions might have triggered increases in carbon dioxide (CO2), consequently heating the planet and causing anoxic conditions in the ocean during the Mesozoic Era. This new research, which draws on collective insights from Northwestern University and other institutions, pinpoints the exact timing of these volcanic events, revealing that OAE1a commenced approximately 119.5 million years ago. These findings further substantiate the theory that volcanic CO2 emissions were primary instigators of the anoxic crisis.

The team determined that the duration of OAE 1a was just over 1.1 million years. This critical information can enhance our understanding of Earth's climatic responses, making it increasingly relevant as modern society confronts the challenges of present-day global warming.

Published in the esteemed journal *Science Advances*, this research represents the most precise and comprehensive dating effort concerning ocean anoxic events conducted to date.

Brad Sageman, a senior author on the paper and a professor at Northwestern’s Weinberg College of Arts and Sciences, emphasized the importance of these historical insights: “Ocean anoxic events partially result from climatic warming in a greenhouse world. To predict what may lie ahead due to human-induced warming, leveraging historical data becomes invaluable. The past is indeed our best teacher for the future.”

During the Cretaceous Period, two main ocean anoxic events occurred, alongside several minor ones, with OAE 1a identified as one of the largest. These events appear to correlate with rapid volcanic activities that injected immense quantities of CO2 into both the ocean and the atmosphere. Unlike ordinary volcanoes, these were large igneous provinces capable of erupting about a million cubic kilometers of basalt over millions of years. This dramatic increase in CO2 led to acidification of seawater, which harmed marine life by dissolving their calcium carbonate shells.

Research into ocean anoxic events began in earnest during the 1970s, following the discoveries made by geologist Seymour Schlanger from Northwestern and Oxford professor Hugh Jenkyns, who found organic carbon-rich shales that suggested widespread anoxia across vast regions of the ocean.

In this latest research stemming from Mount Ashibetsu, the team focused on stratified volcanic tuffs that were tangible indicators of historical eruptions. These tuffs, formed from volcanic ash that settled and solidified over eons, were elevated due to tectonic activities, accessible for study in Hokkaido's temperate rainforest.

Sageman elaborates on the process: "As magma cools, it forms crystals. By the time this tuff hardens, it seals atoms within, some of which, such as uranium, decay over time. This decay provides a mechanism for dating the eruptions, thus allowing us to date specific layers in sedimentary rock formations." Collaborators across various universities brought in complementary expertise, applying a suite of isotopic analyses to track changes in the carbon cycle and volcanic activity during the OAE1a interval.

These isotopic systems serve as markers that facilitate global correlation of geological events from Hokkaido to southern France and beyond, providing a precious glimpse into the interconnected narrative of Earth's climatic and ecological history.

As humanity faces environmental change at an unprecedented scale, insights such as these could illuminate how our planet might react in the years to come, reminding us that history, indeed, has a way of repeating itself.