Researchers have discovered that massive bursts of carbon dioxide from natural Earth systems caused significant drops in ocean oxygen levels around 300 million years ago. The study, led by scientists from the University of California, Davis, the Chinese Academy of Sciences, and Texas A&M University, found five periods when ocean oxygen concentrations decreased by 4% to 12%, corresponding to significant increases in atmospheric carbon dioxide levels. These oxygen-less events had negative impacts on marine life and biodiversity.
The team analyzed sediment cores from South China to study the geochemical composition, with a focus on carbonate uranium isotopes, which provide a record of Earth’s environmental conditions from approximately 310 to 290 million years ago. Through that analysis, we see these ‘burps’ not just in carbon dioxide but also in the ocean’s uranium isotope signature. They’re totally aligned, and the size of those uranium spikes tells us about the magnitude of the ocean anoxia,” said Isabel P.
Montañez is a Distinguished Professor at UC Davis and the senior author of the study.
Ancient CO2 linked to marine anoxia
The researchers used this data to inform climate models, which were run hundreds of thousands of times on a supercomputer to estimate ancient climates based on the available information.
They identified five instances of decreased ocean oxygen, each lasting between 100,000 to 200,000 years. While these periods did not directly coincide with mass extinctions, they did align with pauses in biodiversity observed in the geological record. The Earth’s atmosphere 300 million years ago had 40% to 50% higher oxygen levels.
However, the increases in carbon dioxide levels are similar, potentially serving as a warning for the present day. This is a significant discovery because it reveals substantial ocean anoxia under conditions with significantly more atmospheric oxygen than today. The message is clear: don’t be so sure that it can’t happen again with our current human-driven release of carbon dioxide,” emphasized Montañez.
The study’s findings were published in the Proceedings of the National Academy of Sciences.
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