Good news for marine life? Study finds ocean oxygen levels remained high even as seas seared in the ancient past

From ocean warming and acidification to habitat destruction due to coral bleaching, there is no shortage of threats to marine life in the current climate change scenario. However, a new study shines a rare strand of hope.

Despite the fact that we're caught up in a constant downhill battle trying to keep global warming to a minimum, the Earth is no stranger to such calamities, having faced extended periods of similar warmth in the past.

Most notably, the mid-Miocene was one such era where global temperatures reflected what has been forecast for the next few centuries under current climate change conditions. While life might've looked very different in this period from 16 million years ago, one particular factor appears very promising.

Analysis of ocean sediments from the mid-Miocene era revealed that ocean oxygen levels in a key area were much higher, despite the fact that oceans were much hotter back then.

One of the most significant stresses on current aquatic life comes from global warming-exacerbated oxygen loss from the oceans. The seas already host a plethora of 'dead zones' — places where oxygen falls dangerous below what's required to support aquatic life. As the Earth warms up, experts are concerned that these dead zones could expand over time.

Estimates show that the oceans have already lost 1-2% of their oxygen since the middle of the 20th century, and further heating means we could be facing a decline of up to 7% in the next 100 years. And yet, it seems as though the seas adapted somehow in the past.

"Our study shows that the eastern equatorial Pacific, which today is home to the largest oxygen-deficient zone in the oceans, was well oxygenated during the Miocene warm period, despite the fact that global temperatures at that time were higher than at present," remarks Anya Hess, lead author of the study. "This suggests that current oxygen loss may ultimately reverse."

The testing process was thus: researchers found fossilised remains of microorganisms from the mid-Miocene period, called foraminifera. Testing the composition of these teeny tiny creatures would reveal the biological processes during their ancient reign, which is ultimately linked to their environment, the ancient oceans of the past.

This, and a variety of other methods studying deep sea sediments, helped the researchers make informed predictions into oxygen levels from the period. They found that even at the height of the Miocene warming, the area remained well oxygenated.

"These results were unexpected and suggest that the solubility-driven loss of oxygen that has occurred in recent decades is not the end of the story for oxygen's response to climate change," rejoices Yair Rosenthal, a co-author of the study.

The findings of this study have been published in Nature and can be accessed here.

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