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Wednesday, July 17, 2024 | Back issues
Courthouse News Service Courthouse News Service

Ocean ‘Dead Zones’ Caused by Warming

WASHINGTON (CN) - The results of a National Science Foundation-funded research project point to a clear link between large-scale ocean warming events and the rapid development of marine dead zones, researchers said Wednesday.

Researchers collected sediment cores from the North Pacific to measure biomarkers produced by plankton in conjunction with geochemical data to provide a high-resolution record of environmental change over the past 60,000 years, according to the study's abstract.

The record reveals that two large-scale warming events at the end of the last ice age about 14,700 years ago and again 11,500 years ago caused an increased growth of plankton, which sank to the ocean floor and led to hypoxia - low-oxygen conditions, the foundation said.

"This work tackles a long-standing debate about what causes expansion of oxygen minimum zones, also known as dead zones, in the oceans," Candace Major, program director for the foundation's Division of Ocean Sciences, said. "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."

Researchers noted that warming water alone is insufficient to cause the hypoxia, but they indicated that it can start the oxygen level decline because warmer seawater has less dissolved oxygen in it. The plankton, specifically microscopic diatoms, also needs nutrients and iron to produce the massive 'blooms' that result in the dead zones.

"The large shells of the diatoms act as anchors that help to sink them more quickly (relative to smaller types of phytoplankton) to the bottom of the seafloor. This increases the amount of organic matter that sinks to the deeper ocean, where it consumes deep ocean oxygen through the process of respiration," lead author Summer Praetorius of the Carnegie Institution for Science said in an email to Courthouse News.

"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," Alan Mix of Oregon State University, a co-author of the paper, said. "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."

A dead zone study published last November by the Smithsonian Tropical Research Institute indicated that the process of anthropogenic eutrophication, or human-caused enrichment of water by dissolved nutrient like the phosphates and nitrates commonly found in agricultural run-off, can trigger hypoxic events. Maps of the more than 400 known dead zones worldwide show them mainly clustered along the coasts of developed countries.

However, that study's conclusions matched the foundation's research regarding rising ocean temperatures.

"Temperature is perhaps the climate-related factor that most broadly affects dead zones," Andrew Altieri and Keryn Gedan, the study's authors, said in the Smithsonian's article about their research.

Of immediate concern is a "record-breaking algae bloom dominated by a certain species of diatom" noted this year across the North Pacific in conjunction with unusually warm water temperatures, because it is "of a scale similar to events documented in the geologic record," the foundation researchers 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," Mix said.

The time scale indicated in the geologic core samples showed that large effects could happen within decades, or possibly centuries. "While we emphasize the past transitions to hypoxia occurred rapidly, this is based on geological records, which are capturing decade to century time scales, so we really can't pin down whether it occurred within a 10-year period or a 50-year period, for example," Praetorius said.

"While it's too soon to know how this event ties into the long-term climate patterns that will emerge in the future, current conditions seem eerily reminiscent of past conditions that gave way to extended periods of 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."

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