Science and Tech

They predict ‘dead zones’ in the North Atlantic due to warming

Distribution of the low-oxygen and 600-m oxygen-affiliated planktonic foraminifera G. hexagonus in modern and Pliocene oceans.


Distribution of the low-oxygen and 600-m oxygen-affiliated planktonic foraminifera G. hexagonus in modern and Pliocene oceans. -DAVIS ET AL. /NATURE COMMUNICATIONS

Jan. 5 () –

Researchers have created a map of the oceanic “dead zones” that existed during the Pliocene, when Earth’s climate was two to three degrees warmer than it is today.

The job, published in Nature Communications, could offer insight into the location and potential impacts of future low-oxygen zones in the oceans of a warmer Earth. In particular, the work predicts more dead zones in the North Atlantic.

Oxygen Minimum Zones, or MOZs, are areas of the ocean where midwater oxygen levels (100 to 1,000 meters below the surface) are too low to support most marine life. These dead zones play an important role in the overall health of the ocean.

“MOZs are very important to the geochemical cycling of the ocean,” says Catherine Davis, associate professor of marine, earth, and atmospheric sciences at North Carolina State University and corresponding author of the research. “They are produced in areas where sunlight and atmospheric oxygen do not reach. Their location determines the availability of carbon and nitrogen (an essential nutrient for life on Earth) in the ocean, so they are important drivers of nutrient cycling.”

Being able to predict the location of OMZs is important not only for understanding nutrient cycling, but also for its effects on marine life. Oceanic dead zones restrict the range of animals to the shallow surface of the ocean, where oxygen is most abundant.

Davis and his colleagues wanted to find out how a warmer climate might affect future OMZs. To do this, they went back to the Pliocene (between 5.3 and 2.6 million years ago), when CO2 levels in the Earth’s atmosphere were similar to today’s.

“The Pliocene is the last time we had a stable, warm climate globally, and the global mean temperature was 2-3C warmer than it is now, which is what scientists predict could be about 100 years from now. years,” Davis says.

To determine where the Pliocene OMZs were located, the researchers used a tiny fossilized plankton called foraminifera. Foraminifera are single-celled organisms about the size of a grain of sand. They form hard calcium carbonate shells that can remain in marine sediments.

One species in particular, Globorotaloides hexagonus, is only found in areas with little oxygen. After trawling Pliocene sediment databases to locate that species, the team was able to map the OMZs of the Pliocene. They superimposed their map on a computer model of Pliocene oxygen levels and found that the two matched.

ESPECIALLY IN THE NORTH ATLANTIC

The OMZ map showed that, during the Pliocene, oxygen-depleted waters were much more widespread in the Atlantic Ocean, especially in the North Atlantic. In contrast, in the North Pacific there were fewer areas with low oxygen content.

“This is the first global spatial reconstruction of past oxygen minimum zones,” says Davis. “And it matches what we’re already seeing in the Atlantic in terms of lower oxygen levels. Warmer water holds less oxygen.” This map of Pliocene dead zones could give us an idea of ​​what the Atlantic might look like 100 years from now on a warmer Earth.”

What would a future with much less oxygen in the Atlantic mean? According to Davis, it could have a huge impact on everything from carbon storage and nutrient cycling in the ocean to the management of fisheries and marine species.

OMZs act as a ‘floor’ for marine animals, which get squashed to the surface,” explains Davis. So anglers can suddenly see a lot of fish, but that doesn’t mean there are more than normal, it just means they are forced to occupy a smaller space. Fisheries will need to take into account the effects of OMZs when managing stocks.

“Subtle but far-reaching changes are also possible in the amount of nutrients available to life in those surface waters, as well as where CO2 captured by the ocean is stored.”

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