Low-Oxygen 'Dead Zones' Linked to Climate Change

Scientists may have found a link between climate change and dead zones. They've discovered a connection between abrupt ocean warming at the end of the last ice age and the sudden onset of low-oxygen conditions.

"This work tackles a long-standing debate about what causes expansion of Oxygen Minimum Zones, also known as dead zones, in the oceans," said Candace Major, one of the researchers, in a news release. "The results demonstrate a link between warming surface temperatures and dead zones at great depths. The findings also show that the response time between warming and dead zone expansion is quite fast."

Warmer water, by itself, is not enough to cause diatom blooms or hypoxia. Just as warming soda loses its fizzy gas, warmer seawater contains less dissolved oxygen, and this can start oxygen decline. It wasn't until accelerated blooming of microscopic diatoms that de-oxygenation is increased.

"The high-latitude North Pacific is rich in common nutrients such as nitrate and phosphate, but it is poor in iron and that seems to be the key," said Mix, one of the researchers, in a news release. "A partial loss of oxygen causes a chemical reaction that releases iron previously trapped in continental margin sediments. That iron then fuels diatoms, which bloom, die and sink to the seafloor, consuming oxygen along the way."

There are competing effects and the final result generally depends on which effect "wins." Warming may actually decrease for a time when there's mixing from below. But if the major nutrients are present, then warming favors plankton growth.

The findings may reveal a bit more about what to expect in the future. While it's too soon to know how warm ocean temperatures will affect long-term climate patterns, current conditions seem eerily reminiscent of past conditions that gave way to extended periods of hypoxia.

The findings are published in the journal Nature.

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