Ancient carbon ‘burps’ drained ocean oxygen, repeating now

Ancient carbon ‘burps’ caused massive ocean oxygen depletion millions of years ago, triggering widespread marine extinctions and reshaping life on Earth. Today, human-driven carbon emissions are repeating this dangerous trend, warming oceans and reducing oxygen levels, putting marine ecosystems at serious risk. Scientists warn that if emissions continue unchecked, we could witness a similar crisis, threatening ocean health, fisheries, and biodiversity for generations to come. Understanding this past event is vital for acting now.

Fresh studies by researchers from the University of California, Davis, the Chinese Academy of Sciences, and Texas A&M University reveal that massive carbon dioxide bursts from natural sources caused sharp drops in ocean oxygen levels roughly 300 million years ago.

Through analyzing deep-sea sediments and employing sophisticated climate modeling, the team identified five instances when ocean oxygen decreased by four to twelve percent, coinciding with sharp increases in atmospheric CO₂. Such low-oxygen events, called anoxia, are associated with significant harm to marine life and biodiversity.

Although rooted in deep time, these findings have serious implications for the present and future. Similar shifts today would heavily affect critical coastal areas and the fisheries and ecosystems that depend on them.

“This is the only record we have of huge shifts in CO₂ comparable to today’s levels,” explained senior author Isabel P. Montañez, a Distinguished Professor at UC Davis. “The difference is that in the past, volcanic activity caused these changes, whereas today human activity is responsible, and at a far faster rate, roughly hundreds to a thousand times quicker.”

The researchers analyzed sediment cores from the Naqing succession in South China, focusing on uranium isotopes within carbonate layers. “These ‘burps’ appear in both CO₂ levels and ocean uranium isotope records, confirming their link, and the scale of the uranium spikes gives an indication of how serious the anoxic conditions were,” said Montañez.

The team used this data to feed advanced climate models, which simulate ancient climate scenarios. “It’s a sophisticated mathematical framework that incorporates proxy information and processes it across hundreds of thousands of simulations to pinpoint the best match for reality, given all available data and uncertainty,” added Montañez.

These models revealed five distinct periods, between 290 and 310 million years ago, when ocean oxygen fell by four to twelve percent. Each lasted roughly 100,000–200,000 years. Though these events didn’t align with any known mass extinction, fossil evidence shows dips in biodiversity coinciding with these periods. “Each of these ‘burps’ impacted life, especially in coastal areas,” said Montañez.

Although ancient Earth had atmospheric oxygen levels roughly 40–50% higher than today, the magnitude of these CO₂ spikes mirrors modern increases, offering a sobering warning. “It’s remarkable that these events caused such disruption despite higher oxygen levels,” said Montañez. “The takeaway is clear, we can’t assume it won’t happen again with human-driven emissions.”

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Source: sciencedaily‍