(ORDO NEWS) — The growing concentration of carbon dioxide in the atmosphere not only contributes to global warming, but also leads to ocean acidification.
This process has several consequences at once, including non-obvious ones.
For example, in addition to threatening marine life, ocean acidification itself can reduce atmospheric carbon dioxide. True, the effect will be short-lived, and it has its price.
To understand how global warming threatens us all, scientists are studying the past of the Earth – fortunately, in its rich history of large-scale cataclysms, you can find many illustrative examples.
For example, the Cenomanian-Turonian boundary biotic event ( OAE 2 ), during which more than a quarter of marine invertebrates died out due to a decrease in oxygen concentration in the water.
Together with them, of course, predators disappeared – pliosaurs and almost all ichthyosaurs.
Then, in the second half of the Cretaceous period, that is, about 94 million years ago, for almost 500 thousand consecutive years, the content of carbon dioxide in the atmosphere, according to various estimates, fluctuated between 320 and 900 parts per million.
For comparison: now this figure is 410-420 parts per million. Such a wide spread in estimates is explained by the fact that we have almost no direct data on the concentration of carbon dioxide in the ancient air.
And circumstantial evidence is highly dependent on the model against which it is being compared.
However, while we do not know the exact values of carbon dioxide in the atmosphere during and before OAE 2 , we do know something.
For example, that this event was preceded by half a million years of increasing underwater volcanism. In several places, mantle plumes melted the thin oceanic crust at once – in the area of what is now Madagascar, in the Arctic and in the eastern Pacific Ocean.
During OAE 2, foraminifera shells almost ceased to settle on the seabed, the deposits of this period are black – they contain mainly silicon and its oxides (the material of radiolarian shells).
Two new complementary scientific papers shed light on a number of important details of the Cenomanian-Turonian boundary biotic event. Both are based on the study of the chemical composition of the ancient seabed.
Samples of rocks formed from bottom sediments during the period of interest to scientists were collected at their outcrops in the commune of Gubbio (Italy) and the regions of the former Western Inland Sea (North America).
Cores obtained by deep sea drilling during the mission of the International Ocean Exploration Program ( IODP ) were also used.
The onset of OAE 2 in the samples is clearly visible even without radioisotope dating the rocks darken sharply. The remains of foraminifera disappear in them.
And before disappearing, these protists, or rather, their calcium carbonate shells, deform and shrink. This may be due to both ocean acidification and other factors, which required verification.
To do this, the researchers compared how modern foraminifera respond to conditions that presumably preceded the Cenomanian-Turonian boundary biotic event.
Microorganisms placed in an environment with high acidity not only change externally, but the chemical composition of their shells also differs. In particular, the balance between the isotopes of calcium-44 and -40 is shifting (there is less of the former).
Such a marker is much more reliable than the appearance of shells, which are poorly preserved in bottom sediments. And it has been observed for studied fossils.
That is, scientists have received solid evidence that the ancient foraminifera lived in conditions of insufficient biocalcification. This, in turn, proves that the ocean then became increasingly acidic.
The second study focused on the causes of acidification.
The chemical composition of cores from the OAE 2 period was distinguished not only by the loss of calcium carbonate. In it, the ratio of the isotopes of osmium-187 and -188 was also different (there was more heavy).
The only reason why such a picture could have developed is the release of this metal to the Earth’s surface along with mantle rocks. And not in the air, but directly in the water.
Thus, scientists strongly associated the beginning of the Cenomanian-Turonian boundary biotic event with underwater volcanism, and the latter with ocean acidification.
Moreover, the analysis of new data has made it possible to find the most probable explanation for the anomaly, which has remained a mystery for a long time since the discovery of OAE 2.
Right in the middle of nearly half a million years of hot temperatures caused by the greenhouse effect, almost 40,000 years of comparative cooling have been hidden.
As a result of the two works described above, it turns out that volcanism introduced an excess of carbon dioxide into the atmosphere and water, and this led to global warming and ocean acidification.
In sea water, it became more difficult for carbon ions to bind to calcium ions, the concentration of dissolved CO 2 increased, and oxygen decreased.
The trouble is that if foraminifers absorb less calcium, then the alkalinity of the environment increases. And with it, the ability of water to absorb carbon dioxide from the air directly, and this slows down global warming or even reverses it.
On the one hand, recent data show that promoting seawater alkalinity is a working strategy to combat global warming. On the other hand, you need to remember that this will not be in vain for the inhabitants of the ocean.
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