Ocean warming blamed for North Pacific dead zones

Abrupt ocean warming at the end of the last ice age was linked to the sudden onset of low-oxygen, hypoxic, conditions that led to vast marine dead zones in the North Pacific, new research has found.

"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," said Candace Major from US National Science Foundation (NSF) that funded the research.

Large-scale warming events about 14,700 and 11,500 years ago occurred rapidly and triggered loss of oxygen in the North Pacific, raising concern that low-oxygen areas will expand again as the oceans warm in the future.

Although many scientists believe that a series of low-oxygen "dead zones" in the Pacific Ocean off Oregon and Washington during the last decade might have been caused by ocean warming, evidence confirming that link has been sparse.

The new study, however, found a clear connection between two historic intervals of abrupt ocean warming that ended the last ice age with an increase in the flux of marine plankton sinking to the seafloor, ultimately leading to a sudden onset of low-oxygen conditions or hypoxia.

"During each warming event, the transition to hypoxia occurred abruptly and persisted for about 1,000 years suggesting a feedback that sustained or amplified hypoxia," lead author Summer Praetorius Carnegie Institution for Science in Washington, DC, said.

The new discovery was the result of a decades-long effort by numerous researchers at Oregon State University to collect marine sediment cores from the North Pacific, creating comprehensive, high-resolution records of climate change in the region.

The temperature records came from traces of organic molecules, called biomarkers, produced by plankton.

The study was published in the journal Nature.