Abrupt warming behind ocean ''dead zones''?

Washington

Abrupt ocean warming at the end of the last ice age may have caused the sudden onset of low-oxygen conditions that led to vast marine dead zones in the North Pacific, according to a new study.

"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, programme director in US National Science Foundation's Division of Ocean Sciences.

"The results demonstrate a link between warming surface temperatures and dead zones at great depths," said Major.

Large-scale warming events at about 14,700, and again 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.

Anomalous warmth that occurred recently in the Northeastern Pacific Ocean and the Bering Sea - dubbed "The Blob" - is of a scale similar to events documented in the geologic record. If such warming is sustained, oxygen loss becomes more likely.

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

The study 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," said lead author Summer Praetorius of the Carnegie Institution for Science in US.

Warmer water, by itself, is not sufficient to cause diatom blooms, nor hypoxia, the researchers noted.

Just as warming soda loses its fizzy gas, warmer seawater contains less dissolved oxygen.

However, it is not until accelerated blooming of microscopic diatoms - which have large shells and tend to sink more rapidly than other smaller types of plankton - that de-oxygenation is increased.

Diatoms are known to thrive in warm, stratified water, but they also require sources of nutrients and iron, according to Alan Mix of Oregon State University, a co-author of the study.

Warming may, for a time, decrease mixing from below, but if the major nutrients are there, as they are in the high North Pacific, then warming favours plankton growth.

"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," Mix said.

"Many people have assumed that climate change effects will be gradual and predictable, but this study shows that the ecological consequences of climate change can be massive and can occur pretty fast with little warning," he said.

The study was published in the journal Nature. — PTI