From the Israeli shore, you can see the tranquil, deep-blue waters of the Mediterranean Sea, which have provided food and shelter for mankind for thousands of years. But beneath the surface, an odd development is taking place: The sea’s ability to process carbon dioxide is being disrupted by a process known as stratification.
Consider this region of the Mediterranean as a kind of liquid cake. The top layer of water, which rests on cooler, deeper layers beneath, is heated by intense sunshine. Seas on Earth are able to absorb a quarter of the carbon emissions that humans put into the atmosphere because CO2 dissolves in saltwater out in the open ocean, where water temperatures are lower. However, throughout the summer, as the eastern Mediterranean Sea warms, it can no longer absorb that gas and instead begins to release it.
The same thing takes place when soda that has been carbonated with carbon dioxide is opened. Or Bialik, a geoscientist at the University of Münster in Germany, adds that in most cases, you maintain it cool so that the dissolved gasses will remain dissolved. “All the gasses will pop out at once if you try to open it after leaving it in your car for a while since as it warms, the fluid’s ability to hold CO2 decreases.” You’ve got a mess on your hands, Boom, Fizz.
This dynamic is more significant for the climate in the Eastern Mediterranean than a sticky car interior because the sea starts spitting up huge amounts of CO2 that the water can no longer contain. A second carbon issue is also rife in these warming, stratifying seas, according to Bialik and his associates, who recently found aragonite crystals in sediment traps. Snails and other aquatic animals use aragonite, a kind of calcium carbonate, to construct their shells. The aragonite is developing abiotically everywhere but the hotter Eastern Mediterranean. That’s a further indication that the water is becoming so heated that its carbon load is being released.
In contrast to deeper ocean areas, where upwelling brings up cooler water, the fluid on top of these hot, shallow, stable waters little interacts with the underlying colder layers. A recent publication presenting the discovery in the journal Scientific Reports was coauthored by Bialik. “The conditions are so harsh that we can absolutely manufacture calcium carbonate chemically from these liquids, which was somewhat of a shock for us,” says Bialik. (He conducted the research while attending the Universities of Malta and Haifa.) It’s basically like a beaker that sits there for a very long time, long enough to initiate these reactions and begin producing these crystals.
It reminds you of the sugar crystal experiments you might have performed as a child. You saturated the water with a lot of sugar. The sugar did not precipitate into fat clusters until a string was added, at which point they stuck to the thread. Similar to this, the Mediterranean becomes carbonate-rich when it stratifies and heats up. Bialik and his colleagues are unsure of how exactly the aragonite reactions begin, but they speculate that they may begin with dust-like nuclei that have been blown from adjacent land. From there, layers of aragonite develop into crystals, which are very little replicas of the thread in the sugar water.
It’s also important to note that the Mediterranean Sea is one of the water bodies with the highest level of microplastic pollution worldwide: In a square meter of sediment only 5 cm thick in 2020, researchers discovered 2 million particles. Bialik is unsure if aragonite crystals are growing around microplastics that are drifting in the water column. According to Bialik, “they could presumably form around any nucleation center.” “I think microplastics might possibly be a factor. However, as scientists are fond of saying, additional study is required.
However, Bialik and his associates can state that as these crystals form, CO2 is released. They contribute roughly 15% of the gas that the Mediterranean Sea emits into the atmosphere, according to Bialik’s estimation.
The acidity of the sea actually decreases as it heats and releases CO2, both from the water belching it up and from the growing crystals. In contrast to the process that is widely contributing to ocean acidification, this one: As more CO2 is released by people into the atmosphere, more of it is absorbed by the oceans, which causes a chemical process that increases acidity. Acidification makes it more difficult for creatures that form calcium carbonate exoskeletons or shells, such as corals and snails (which are collectively referred to as calcifiers). The Mediterranean is becoming less acidic as it warms and releases the carbon it has absorbed back into the atmosphere.
The calcifiers should benefit greatly from it, right? No, not always. According to Bialik, many of them require precise temperature ranges in order to construct their shells—neither too hot nor too cold. The heat thus strains these species in a different way, even though the sea is becoming less acidic as it warms. (Also stress from exposure to high levels of microplastics on a continual basis.)
It’s unclear whether aragonite crystals are growing in number globally. The waters near the Bahamas and in the Persian Gulf take on a milky color as a result of calcium carbonate precipitating in far more evident ways during “whiting episodes,” which are previously known to scientists. There was no visible whiting event in the Eastern Mediterranean to alert Bialik and his associates. Rather, they accidentally discovered the crystals in their sediment traps.
As a marine chemist at the Scripps Institution of Oceanography who wasn’t involved in the study, Andrew Dickson notes that “this is a relatively unique area with a variety of factors that have to happen to make this work.” “So, the question is: to what extent is that habitat truly unique, or is it typical of the oceanic region? And I don’t have a distinct mental image of that.
Dickson is leaning toward the assumption that this may not be extremely widespread; after all, the conditions in the eastern Mediterranean may not be repeated in many other areas. Wherever it may be occurring, however, Aragonite crystal formation may interfere with water’s capacity to absorb atmospheric CO2, impairing the ocean’s ability to reduce levels of the planet-heating gas, according to Bialik.
We still don’t fully understand this and what controls it—when it turns on and when it goes down, says Bialik—so I won’t state that we do. “We didn’t even believe that this process takes place on this magnitude in open waters under typical marine conditions. Therefore, there is still a lot about it that we need to grasp.