The locations of the lowest oxygen concentrations and deep teal indicate regions without enough dissolved oxygen in the eastern Pacific Ocean. Andrew and Jarek Kwiecinski are credited.
Life is abundant in the oceans, except in pockets where oxygen naturally plummets and waters become unlivable for most aerobic organisms. The pools are called ODZs. They make up less than 1 percent of the ocean's total volume, but they are a significant source of nitrous oxide, a potent greenhouse gas. Their boundaries can limit the amount of fish that can be caught.
The most detailed, three-dimensional "atlas" of the largest ODZs in the world have been created by MIT scientists. The two major oxygen-starved bodies of water in the tropical Pacific are mapped in the new atlas. The maps show the volume, extent, and varying depths of each ODZ, along with fine-scale features such as ribbons of oxygenated water that intrude into otherwise depleted zones.
The team used a new method to process over 40 years' worth of ocean data, which included over 15 million measurements taken by many research cruises. The researchers used the data to create maps of oxygen- deficient zones at various depths, similar to the many slices of a three-dimensional Scan.
The researchers estimated the total volume of the two major ODZs more precisely than before. The volume of water that would fill a billion Olympic-sized pools is roughly 600,000 cubic kilometers. The second zone is three times larger.
ODZs can be found in the Atlas as a reference. The team hopes that scientists can add to the atlas with more data to better track changes in these zones as the climate warms.
As the climate gets warmer, it's expected that the oceans will lose oxygen. In the tropics, there are large oxygen-deficient zones, which is why the situation is more complicated. We have a point of comparison for future change if we have a detailed map of these zones.
The journal Global Biogeochemical Cycles contains the team's study.
Airing out artifacts.
Oxygen- deficient zones are large, persistent regions of the ocean that occur naturally, as a consequence of marine microbes gobbling up sinking phytoplankton along with all the available oxygen in the surroundings. These zones lie in regions that miss ocean currents, which replenish regions with oxygenated water. ODZs are places of relatively permanent, oxygen-depleted waters and can be found at depths between 35 to 1,000 meters below the surface. The oceans run about 4,000 meters deep.
Over the last 40 years, research cruises have explored these regions by dropping bottles down to various depths and hauling up seawater that scientists measure for oxygen.
There are a lot of artifacts that come from a bottle measurement. Oxygen can get into the sample from the plastic that we deploy at depth. Artificial oxygen inflates the ocean's true value when all is said and done.
The team looked at the data from the sensors attached to the bottles or the robotic platforms that could change their buoyancy to measure water at different depths. The amount of oxygen dissolved in water can be estimated by the intensity of light emitted by a photosensitive dye. The sensors record signals continuously as they descend through the water column, unlike the seawater samples that represent a single depth.
Scientists have tried to use the sensor data to estimate the true value of oxygen concentrations in ODZs, but have found it difficult to convert the signals accurately.
"We took a very different approach, using the measurement to see how the value changes within the water column," Kwiecinski says. "That way we can identify anoxic waters, regardless of what a specific sensor says."
It was bottoming out.
The team believed that if the sensors showed a constant, unchanging value of oxygen in the ocean, it would be a sign that the section was part of an oxygen- deficient zone.
The researchers mapped the regions where oxygen did not change with depth after collecting over 15 million sensor measurements.
The distribution of anoxic water in the Pacific can now be seen in three dimensions.
The ODZs in the tropical Pacific and the Northern Hemisphere were mapped by the team. They were able to see the fine details in each zone. Oxygen-depleted waters are more concentrated towards the middle and appear to thin out towards the edges of each zone.
Big bites were taken out of the anoxic waters at shallow depths. "There's a mechanism that brings oxygen into this region, making it oxygenated compared to the water around it."
Observations of the tropical Pacific's oxygen- deficient zones are more detailed than what has been measured to date.
"How the borders of these ODZs are shaped, and how far they extend, could not be previously resolved," says Babbin. We now have a better idea of how the two zones compare.
"This gives you an idea of what could be happening," Kwiecinski says. There's a lot more that can be done with this data to understand how the ocean's oxygen supply is controlled.
The story is being re-posted by MIT News, a popular site that covers news about MIT research, innovation and teaching.
The new atlas of ocean's oxygen-starved waters was retrieved 27 December 2021.
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