Ancient warming fueled massive marine dead zones in North Pacific

Over the past 1.2 million years, marine life repeatedly disappeared from large swaths of the North Pacific.

According to a new survey of Bering Sea sediment cores -- the results of which were published Wednesday in the journal Science Advances -- ancient periods of warming regularly produced dead zones in the northern half of the Pacific Ocean.

The analysis could inform today's efforts to forecast hypoxic conditions in Earth's oceans, where oxygen levels are already steadily declining.

"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," study first author Karla Knudson, who led the study as a graduate student in Earth sciences at the University of California, Santa Cruz, said in a press release.

In parts of the ocean where oxygen levels are sufficient to support marine life, sediment layers aren't neatly formed. They're mixed up and messy -- the evidence of biological activity.

In hypoxic zones, or dead zones, the layers are much more orderly.

When scientists examined lengthy sediment cores retrieved from the bottom of the Bering Sea, they found evidence of several early low-oxygen episodes beginning 1.2 million years ago.

Researchers have previously found evidence of a widespread hypoxia event at the end of the last ice age, when rapidly melting glaciers flushed massive amounts of freshwater into the planet's oceans.

But the latest findings suggest hypoxia events in the North Pacific occurred more frequently than previously estimated.

"It doesn't take a huge perturbation like melting ice sheets for this to happen," said corresponding author 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," said Ravelo, a professor of ocean sciences at UC Santa Cruz.

Rapid deoxygenization is typically triggered by massive phytoplankton blooms.

As masses of blue green marine algal cells die and sink to the bottom, the decomposition process eats up oxygen and releases large amounts of CO2.

Studies suggest phytoplankton blooms are encouraged by warming water temperatures, rising sea levels and an increase in the availability of iron.

"Our study shows that high sea levels, which occur during warm interglacial climates, contributed to these hypoxic events," Knudson said.

"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," Knudson said

Researchers estimate increases in upwelling and ocean mixing patterns could have also played a role in fueling hypoxia events.

When ocean layers are overturned, nutrient rich water from the deep gets brought to the surface, feeding the phytoplankton blooms that ultimately deplete local oxygen supplies.

Because the surveyed sediment cores were from a single site in the Bering Sea, however, the scientists said they can't say for certain how big the hypoxia events were.

"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 Ravelo, who served as co-chief scientist of Integrated Ocean Drilling Program Expedition 323, which retrieved the deep sea sediment cores in 2009.

The scientists said they hope future expeditions will help them get a better sense of how expansive these early hypoxia events were -- and what that might mean for future oxygen levels in Earth's warming oceans.

"The system is primed for this type of event happening," Ravelo said. "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," Ravelo said.

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