Eastern Mediterranean stratifies like a cake because of warming, scientists say
Is the eastern part of the Mediterranean really melting? That was the question Wired has been asked recently.
As reported, this process, called stratification due to the increase in temperature, also harms the way carbon dioxide in the sea is processed.
Under normal circumstances, the intense sunlight heats the top layer of water, which rests on cooler, deeper layers beneath. Seas on Earth can absorb a quarter of humans' carbon emissions into the atmosphere because CO2 dissolves in saltwater out in the open ocean, where water temperatures are lower. However, as the eastern Mediterranean Sea warms throughout the summer, it can no longer absorb that gas and instead begins to release it.
Or Bialik, a geoscientist at the University of Münster in Germany, gave an example to Wired to make the situation more understandable:
“You usually keep it cold, so the dissolved gasses will stay dissolved. If you leave it in your car for a while and try to open it, all the gasses are going to pop out at once because when it warms, the capacity of the fluid to hold CO2 goes down.”
Aragonite crystals have been caught
As the sea starts releasing vast amounts of CO2, the water can no longer contain in the Eastern Mediterranean. Bialik and his team have found that a second carbon issue is rife in these warming, stratifying seas.
Or Bialik and his colleagues have discovered that aragonite crystals could cause stratification.
Snails and other aquatic animals use aragonite, calcium carbonate, to construct their shells. The aragonite is developing abiotically everywhere but the hotter Eastern Mediterranean. That further indicates that the water is becoming so heated that its carbon load is being released.
“The conditions are so extreme that we can definitely generate calcium carbonate chemically from these waters, which was kind of a shock for us,” says Bialik, who co-authored the paper describing the discovery in Scientific Reports.
“It's basically like a beaker that sits there for a very long time, and it's long enough to get these reactions going and start generating these crystals.”
The Mediterranean Sea releases back the carbon
The acidity of the sea decreases as it warms and loses CO2, both from the water belching it up and from the proliferating crystals. This is the inverse of the process causing widespread ocean acidification: As humans emit more CO2 into the atmosphere, the oceans absorb more of it, causing a chemical reaction that increases acidity. However, as the Mediterranean warms and releases the carbon it has absorbed back into the atmosphere, it becomes more basic, reversing the acidification.
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 due to calcium carbonate precipitating in far more evident ways during "whiting episodes," previously known to scientists.
The oceans play a major role in the Earth’s climate by regulating atmospheric CO2. While oceanic primary productivity and organic carbon burial sequester CO2 from the atmosphere, precipitation of CaCO3 in the sea returns CO2 to the atmosphere. Abiotic CaCO3 precipitation as aragonite is potentially an important feedback mechanism for the global carbon cycle, but this process has not been fully quantified. In a sediment-trap study conducted in the southeastern Mediterranean Sea, one of the fastest warming and most oligotrophic regions in the ocean, we quantify for the first time the flux of inorganic aragonite in the water column. We show that this process is kinetically induced by the warming of surface water and prolonged stratification resulting in a high aragonite saturation state (ΩAr ≥ 4). Based on these relations, we estimate that abiotic aragonite calcification may account for 15 ± 3% of the previously reported CO2 efflux from the sea surface to the atmosphere in the southeastern Mediterranean. Modeled predictions of sea surface temperature and ΩAr suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO2.
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