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December 27, 2021

by Jennifer Chu, Massachusetts Institute of Technology

The oceans are teeming with life almost everywhere, except in certain pockets, where oxygen naturally drops and the water becomes uninhabitable to most aerobic organisms. These desolate pools are “oxygen starvation zones” or ODZs. And although they make up less than 1 percent of the total volume of the ocean, they are a significant source of nitrous oxide, a powerful greenhouse gas. Their limits can also limit the extent of fisheries and marine ecosystems.

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Now MIT scientists have created the most detailed three-dimensional “atlas” of the largest ODZs in the world. The new atlas offers high-resolution maps of the two large, oxygen-poor bodies of water in the tropical Pacific. These maps show the volume, extent, and varying depths of each ODZ along with fine-scale features such as ribbons of oxygen-rich water that invade otherwise depleted zones.

The team used a new method to process ocean data spanning over 40 years , spanning nearly 15 million measurements made by many research cruises and autonomous robots in the tropical Pacific. The researchers collected and analyzed this huge and fine-grained data to create maps of oxygen starvation zones at various depths, similar to the many layers of a three-dimensional scan.

From these maps, the researchers estimated the total volume of the two large ODZs in the tropical Pacific more accurately than previous attempts. The first zone, which extends off the coast of South America, measures approximately 600,000 cubic kilometers – roughly the amount of water that would fill 240 billion Olympic pools. The second zone off the coast of Central America is about three times as large.

The atlas serves as a reference for where ODZs are today. The team hopes that scientists can add ongoing measurements to this atlas to better track changes in these zones and predict how they might change as the climate warms.

“The oceans are widely expected to be oxygen lose when the climate gets warmer. But the situation is more complicated in the tropics, where there are large areas of oxygen deficiency, “says Jarek Kwiecinski ’21, who developed the atlas with Andrew Babbin. Cecil and Ida Green Career Development Professor at MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It is important to make a detailed map of these zones so that we have a point of reference for future changes.”

Zones of oxygen deficiency are large, persistent regions of the ocean that occur naturally as marine microbes are sinking phytoplankton along with all that is available Consume oxygen in the environment. These zones happen to be in regions with no ocean currents that would normally fill regions with oxygen-rich water. As a result, ODZs are locations of relatively persistent, deoxygenated water and can exist in mid-ocean depths between around 35 and 1,000 meters below the surface. From a certain perspective, the oceans run an average of around 4,000 meters deep.

For the past 40 years, research cruises have explored these regions by dropping bottles at various depths and pulling up seawater, which scientists then measure for oxygen.

“But there are a lot of artifacts that come from a bottle measurement when you’re trying to really measure zero oxygen,” says Babbin. “All the plastic that we use in depth is full of oxygen that can get into the sample. All in all, this artificial oxygen increases the real value of the ocean. ”

Instead of relying on measurements from bottle samples, the team examined data from sensors attached to the outside of the bottles or integrated into robotic platforms that monitor their buoyancy can change to measure water at different depths. These sensors measure a variety of signals, including changes in electrical current or the intensity of light emitted by a photosensitive dye, to estimate the amount of oxygen dissolved in water. Unlike seawater samples, which represent a single discrete depth, the sensors continuously record signals as they descend through the water column.

Scientists have tried to use this sensor data to estimate the true value of oxygen concentrations in ODZs however, it is incredibly difficult to convert these signals precisely, especially at concentrations close to zero.

“We took a completely different approach in that we used measurements to investigate not their true value but how this value varies within the The water column changes, ”says Kwiecinski. “That way, we can detect anoxic waters regardless of what a particular sensor says.”

The team argued that when sensors show a constant, unchanging level of oxygen in a continuous, vertical section of the ocean regardless of the true value , this is probably a sign that the oxygen has bottomed out and that the section is part of an oxygen depleted zone.

The researchers merged nearly 15 million sensor readings collected over 40 years from various research cruises and robotic swimmers, and mapped the regions where oxygen did not change with depth.

“We can now see the distribution of anoxic water in the Pacific changing in three dimensions,” says Babbin.

The team mapped the boundaries, volume and shape of two large ODZs in the tropical Pacific, one in the northern hemisphere and the other in the southern hemisphere. You could also see fine details within each zone. For example, deoxygenated waters are “thicker” or more concentrated towards the center and appear to thin out towards the edges of each zone.

“We could also see gaps where it looks like big bites from anoxic waters in lesser Depth, ”says Babbin. “There is a mechanism that brings oxygen into this region, which makes it more oxygenated than the surrounding water.”

Such observations of the deoxygenated zones of the tropical Pacific are more detailed than previous measurements.

” How the boundaries of these ODZs are shaped and how far they extend has not yet been clarified, “says Babbin. “Now we have a better idea of ​​how these two zones compare in terms of area and depth.”

“That gives you a sketch of what could happen,” says Kwiecinski. “With this collection of data, there is much more that can be done to understand how the ocean’s oxygen supply is controlled.”

This story was published courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that contains news about MIT research, innovation, and teaching.

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