3D maps can help researchers track and predict the ocean’s response to climate change
The oceans are often perceived as places full of life: organisms proliferate even in the most extreme circumstances, thousands of kilometers deep, where light barely reaches. However, there are certain places where oxygen naturally plummets, and the waters become uninhabitable for most aerobic organisms. These desolate areas are known as ‘ oxygen-deficient zones ‘ or ODZs. And although they make up less than 1% of the ocean’s total volume, they are a major source of nitrous oxide, a potent greenhouse gas. Its boundaries can also limit the extent of fishing and marine ecosystems.
Now MIT scientists have generated the most comprehensive three-dimensional ‘atlas’ of the world’s largest ODZs.
The new analysis, published in ‘ Global Biogeochemical Cycles ‘, provides high-resolution maps of the two main oxygen-deprived water bodies in the tropical Pacific, revealing the volume, extent, and different depths of each ODZ, along with characteristics of very precise scales, like streams of hydrogen peroxide flowing into otherwise impoverished areas.
El equipo utilizó un nuevo método para procesar más de 40 años de datos oceánicos, que comprende casi 15 millones de mediciones tomadas por muchos cruceros de investigación y robots autónomos desplegados en el Pacífico tropical. Los investigadores recopilaron y analizaron esta ingente cantidad de información para generar mapas de zonas deficientes en oxígeno a varias profundidades, similares a los muchos cortes de un escaneo tridimensional.
From these maps, the researchers estimated the total volume of the two major ODZs in the tropical Pacific more accurately than in previous efforts. The first zone, which extends from the coast of South America, measures about 600,000 cubic kilometers, approximately the volume of water that would fill 240,000 million Olympic swimming pools. The second zone, off the coast of Central America, is about three times larger.
The atlas serves as a reference for where the ODZs are today. The team hopes that scientists can update this document with ongoing measurements, to better track changes in these zones and predict how they may change as the climate warms.
“Overall, the oceans are expected to lose oxygen as the climate gets warmer. But the situation is more complicated in the tropics, where there are large areas that are deficient in oxygen,” explains Jarek Kwiecinski, who developed the atlas together with Andrew Babbin, the Cecil, and Ida Green Career Development Professor in the Department of Earth, Atmosphere, and Sciences. planetary. “It is important to create a detailed map of these areas so that we have a point of comparison for future changes.”
Phytoplankton-eating microbes
ODZs are large, persistent regions of the ocean that occur naturally, as a result of marine microbes devouring sinking phytoplankton along with all the available oxygen in the vicinity. These zones are in regions that bypass ocean currents, which would normally replenish the regions with hydrogen peroxide. As a result, these zones are locations of relatively permanent, oxygen-depleted water, and can exist in ocean depths of approximately 35 to 1,000 meters below the surface. To give some perspective, the oceans have an average depth of about 4,000 meters.
During the last 40 years, different expeditions have been used as a method to measure oxygen to throw bottles that are filled with water that is later analyzed. However, the authors point out that there are many artifacts that come from the measurement in this process, and it could overstate the true value of oxygen. So instead of relying on measurements from the bottle samples, the team analyzed data from sensors attached to the outside of the bottles or integrated with robotic platforms that can change their buoyancy to measure water at different depths. These sensors measure a variety of signals, including changes in electrical currents or the intensity of light emitted by a photosensitive dye to estimate the amount of dissolved oxygen in the water.
Scientists have attempted to use these sensor data to estimate the true value of oxygen concentrations in the ODZs, but have found it incredibly difficult to convert these signals accurately, particularly at near-zero concentrations. “We take a very different approach, using measurements not to see their actual value, but rather how that value changes within the water column,” says Kwiecinski. That way we can identify anoxic waters, regardless of what a specific sensor says.”
Bottoming out
The team reasoned that if the sensors showed a constant, unvarying value of oxygen in a continuous vertical section of the ocean, regardless of the actual value, then it would likely be a sign that the oxygen had bottomed out and that the section was part of a zone. deficient in oxygen.
The researchers brought together nearly 15 million sensor measurements collected over 40 years by various expeditions and robotic floats and mapped the regions where oxygen did not change with depth. “Now we can see how the distribution of anoxic water in the Pacific changes in three dimensions,” says Babbin.
The team mapped the boundaries, volume, and shape of two main ODZs in the tropical Pacific, one in the northern hemisphere and the other in the southern hemisphere. They were also able to see fine details within each zone. For example, oxygen-depleted waters are “thicker” or more concentrated toward the center, and appear to thin out toward the edges of each zone. “We were also able to see lagoons, where it appears that large mouthfuls of anoxic waters were taken out at shallow depths,” says Babbin. “There is some mechanism that brings oxygen into this region, making it oxygenated compared to the surrounding water.”
Such observations of the oxygen-deficient zones of the tropical Pacific are more detailed than has been measured to date. “How the edges of these ODZs are formed, and how far they extend, could not be previously resolved,” says the researcher. We now have a better idea of how these two zones compare in terms of areal extent and depth.” For his part, Kwiecinski adds: “This gives you an outline of what could be going on. Much more can be done with this data collection to understand how the ocean’s oxygen supply is controlled.”
