It should also be noted that the Mediterranean Sea is one of the most polluted water bodies with microplastics in the world: in 2020, scientists reported finding 2 million particles in a single square meter of sediment that was only 5 centimeters thick. If aragonite crystals form around microplastics floating in the water column, Bialik does not know. “They could probably form around any nucleation center,” says Bialik. “I suspect microplastics may also be possible. But as scientists like to say, more research is needed.
What Bialik and his colleagues box say, however, is that when these crystals form, they release CO2. So much so, Bialik calculates, that they represent perhaps 15% of the gas that the Mediterranean Sea emits into the atmosphere.
As the sea heats up and loses its CO2both water spitting it out and crystals proliferating, its acidity actually goes down. This is the opposite process to that which causes widespread ocean acidification: as humans spit out more CO2 in the atmosphere, the oceans absorb more and the ensuing chemical reaction increases acidity. Acidification makes it harder for organisms like corals and snails (known collectively as calcifiers) to build shells or exoskeletons from calcium carbonate. But as the Mediterranean warms and releases its absorbed carbon into the atmosphere, it becomes more basic, reversing this acidification.
Should be great for calcifiers, right? Not necessarily. “Many of them have specific temperature ranges in which they can build their shells – not too hot, not too cold,” says Bialik. So even though the sea becomes less acidic as it warms, that heat stresses these organisms in a different way. (Not to mention the stress of being constantly exposed to extreme levels microplastics.)
It is not clear if aragonite crystals are forming in more places around the world. Scientists are already familiar with “whiting events,” in which calcium carbonate precipitates in a much more obvious way, turning the waters around the Bahamas and the Persian Gulf a milky color. In the eastern Mediterranean, there were no obvious occurrences of index whiting in Bialik et al. Instead, they came across the crystals in their sediment traps.
“This is a somewhat unique area with a variety of conditions that need to occur for it to work,” says marine chemist Andrew Dickson of the Scripps Institution of Oceanography, who was not involved in the research. “The question then is how special or how common is this environment around the oceans? And I don’t have a clear picture of that in my mind.
Eastern Mediterranean conditions may not be replicated in many other places, so Dickson leans towards the idea that this may not be particularly common. But Bialik points out that wherever it happens, it could cause a climate problem: the formation of aragonite crystals can impair the water’s ability to absorb atmospheric CO.2thus interfering with how the ocean reduces levels of planet-warming gases.
“I won’t say that we fully understand that at the moment and we fully understand what governs it – when it turns on and when it turns off,” Bialik says. “We didn’t even think this process occurs on this scale in open waters, under normal marine conditions. And so we still have a lot of things that we need to understand about that.