Except in special places where oxygen naturally drops and waters become unlivable for most aerobic species, the oceans are teeming with life. “Oxygen-deficient zones,” or ODZs, are these lonely lakes. They are also a large generator of nitrous oxide, a strong greenhouse gas, despite accounting for less than 1% of the entire volume of the ocean. Fisheries and marine habitats can be limited by their borders.
MIT researchers have created the world’s most detailed three-dimensional “atlas” of the world’s largest ODZs. The new atlas includes high-resolution maps of the tropical Pacific’s two major oxygen-depleted bodies of water. The volume, extent, and variable depths of each ODZ are depicted on these maps, as well as fine-scale phenomena like streamers of oxygenated water that intrude into otherwise impoverished zones.
Over 40 years of ocean data, including nearly 15 million measurements recorded by various research cruises and freed robots across the tropical Pacific, was processed using new technology. Like the many slices of a three-dimensional scan, the researchers collated and processed this huge, fine-grained data to develop maps of oxygen-deficient zones at various depths.
The researchers were able to more precisely estimate the overall volume of the two major ODZs in the tropical Pacific using these maps than earlier efforts. The first zone, which runs out from South America’s coast, is around 600,000 cubic kilometers in size, or enough water to fill 240 billion Olympic-sized swimming pools. The second zone is nearly three times larger, and it is located off the coast of Central America.
The atlas provides a guide to where ODZs are currently located. The researchers expect that by continuing to collect data, scientists will be able to follow changes in these zones better and forecast how they will alter as the climate warms.
“As the climate warms, it is widely believed that the oceans would lose oxygen. “However, in the tropics, where there are significant oxygen-deficient zones, the situation is more problematic,” says Jarek Kwiecinski. “It’s critical to generate a precise map of these zones so we can compare changes in the future.”
Artifacts Are Being Aired Out
Oxygen-deficient zones are huge, persistent areas of the ocean that develop naturally due to marine bacteria consuming sinking phytoplankton and all available oxygen. These zones are located in areas where passing ocean currents, which would ordinarily replenish these areas with oxygenated water, are absent. As a result, ODZs are areas of generally persistent, oxygen-depleted waters that can be found at depths ranging from 35 to 1,000 meters below the surface of the ocean. The oceans are around 4,000 meters deep on average, to put things in perspective.
Over the last four decades, research cruises have studied these areas by lowering bottles to varying depths and retrieving saltwater, which scientists then test for oxygen.
“However, when you’re attempting to detect genuinely zero oxygen, there are a lot of artifacts that occur from a bottle measurement,” Babbin explains. “All of the plastic we use at depth has oxygen, which can leak into the sample.” Artificial oxygen inflates the genuine value of the ocean.”
Rather than relying on data from bottle samples, researchers looked at data from sensors mounted outside the bottles. They were also integrated with robotic platforms to adjust their buoyancy to monitor water at various depths. These sensors analyze a range of signals, such as changes in electrical currents or the power of light radiated by a photosensitive dye to determine the quantity of oxygen dissolved in water. The sensors capture signals constantly as they descend through the water column, unlike seawater samples that indicate a single discrete depth.
Using sensor data, scientists have attempted to estimate the true value of oxygen concentrations in ODZs. Still, they have found it extremely difficult to convert these signals effectively, especially at values near zero.
“We used a totally different approach,” Kwiecinski explains, “looking at measurements not for their true value, but for how that value fluctuates within the water column.” “That way, regardless of what a given sensor says, we can recognize anoxic waters.”
Getting to the bottom of things
If sensors in a continuous, vertical piece of the water indicated a constant, unchanging value of oxygen, regardless of the true value, the researchers reasoned, it was likely a sign that oxygen had bottomed out and that the region was part of an oxygen-deficient zone.
The researchers plotted the locations where oxygen did not fluctuate with depth using over 15 million sensor measurements collected over 40 years by several research cruises and robotic floats.
“We can now visualize in three dimensions how the distribution of anoxic water in the Pacific varies,” Babbin explains.
The researchers studied the limits, volume, and structure of two large ODZs in the tropical Pacific, each in the Northern and Southern Hemisphere. They could see minute details within each zone as well. Oxygen-depleted waters, for example, appear to be “thicker” or denser in the middle, then thin out at the boundaries of each zone.
“We could also observe gaps,” Babbin says, “where it appears that large bites were taken out of anoxic waters at shallow depths.” “Some mechanism is transporting oxygen into this region, oxygenating it in comparison to the surrounding water.”
Such measurements of the oxygen-deficient zones of the tropical Pacific are more detailed than what has previously been measured.
“How these ODZs are shaped and how far they stretch has not been addressed earlier,” Babbin explains. “Now, we have a better grasp on how these two zones differ in terms of area and depth.”
“This provides you a rough idea of what might be going on,” Kwiecinski explains. “This data collection can be used to learn a lot more about how the ocean’s oxygen supply is managed.”
This article was based on a post from MIT. The Simons Foundation is funding some of this study.