Warm climates linked to low-oxygen dead zones in the North Pacific

A new study from UC Santa Cruz has revealed that over the past 1.2 million years, low-oxygen dead zones have repeatedly emerged in the subarctic North Pacific Ocean during warm climates. The findings provide critical information about the causes of low oxygen – or hypoxia – in the North Pacific, and will help to predict these conditions in the future.

“It is essential to understand whether climate change is pushing the oceans toward a ‘tipping point’ for abrupt and severe hypoxia that would destroy ecosystems, food sources, and economies,” said study first author Karla Knudson.

The investigation was focused on the analysis of deep sediment cores from the Bering Sea. When organisms living in the seafloor sediments have been killed due to hypoxia, an orderly pattern of layers is preserved. Thus provides experts with a record of past low-oxygen events. 

At the end of the last ice age, a massive influx of fresh water into the ocean caused widespread hypoxia. The new study has produced the first records of earlier low-oxygen events.

“It doesn’t take a huge perturbation like melting ice sheets for this to happen,” said study co-author Professor Ana Christina Ravelo. “These abrupt hypoxic events are actually common in the geologic record, and they are not typically associated with deglaciation. They almost always happen during the warm interglacial periods, like the one we’re in now.”

While it remains unclear what triggers a hypoxic event, there is evidence that ocean warming, rising sea levels, and iron availability play a role. Hypoxia occurs after a surge in phytoplankton, which ultimately deplete the water of oxygen. 

“Our study shows that high sea levels, which occur during warm interglacial climates, contributed to these hypoxic events,” said Knudson. “During high sea levels, dissolved iron from the flooded continental shelves can be transferred to the open ocean and promote intense phytoplankton growth in the surface waters.”

Changes in ocean circulation – including intensified upwelling to bring more nutrients into the surface waters and stronger currents that could transfer iron from the continental shelf to the open ocean – likely play a critical role in triggering a hypoxic event, explained Knudson.

The sediment cores examined for the study were collected from a single site, which means the researchers do not know if the low-oxygen dead zones extended beyond the Bering Sea into other parts of the Pacific Ocean.

“We don’t know how extensive they were, but we do know they were very intense and lasted longer than the deglaciation event that has been so well studied,” said Professor Ravelo.

The findings raise new concerns about whether climate change and ocean warming will lead to a tipping point that will trigger widespread hypoxia in the North Pacific Ocean.

“The system is primed for this type of event happening,” said Professor Ravelo. “We need to know how extensive they were, and we need to rethink how these events are triggered, because we now know that it doesn’t take a huge perturbation. This study sets the stage for a lot of follow-up work.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer