May 25, 2020

The asteroid that killed dinosaurs acidified the sea thousands of years | Science

The history of the asteroid that ended the dinosaurs is missing some chapters or they are not quite well written. Now, a study with marine microorganisms so small that they could be confused with grains of sand shows how the impact of the carid acidified the surface of all the oceans. The rapid reduction in the pH of the water, recorded in its shells, ended a good part of marine life. Tens of thousands of years had to pass before the sea regained its balance.

Science is quite advanced what happened 66.04 million years ago. An asteroid about 10 or 12 kilometers in diameter impacted the Earth in what is now the Gulf of Mexico and the Yucatan Peninsula. The shock unleashed the apocalypse: released an energy equivalent to that of 10,000 million bombs like Hiroshima, raising a gigantic tsunami, volatilizing huge amounts of material and releasing billions of tons of CO into the atmosphere2 and sulfides.

The worst came later. The sky darkened, filtering solar radiation, which cooled the weather and, above all, hindered photosynthesis. The result was the disappearance of 75% of terrestrial biodiversity, starting with the non-winged dinosaurs. The cataclysm, which marked the end of the Cretaceous geological period and the beginning of a new one, the paleogen (what geoscientists call the K-Pg limit), was of such magnitude that it left a clear mark on the fossil record. One of the places where the scar is best appreciated is in the cave of Geulhemmerberg, in the southeast of the Netherlands, very far from the Mexican coast (see image above).

The foraminiferous shells show a marked decalcification after impact

"The cave is especially unique because it is considered to collect the first centuries, at most millennia, after the asteroid hit the Earth," says the researcher at the German Center for Geosciences Research GFZ and lead author of the study Michael Henehan. "It retains a wide layer of about ten centimeters thick that was deposited among a series of super storm events caused by climatic impact shocks," he adds.

In that layer, unicellular microorganisms appear that are not bacteria, neither plants nor animals. They are the foraminifera, protists who protected their only cell with a shell not very different from that of mussels and clams. "Because (the stratum) is very rich in clays, the foraminifera contained in these sediments have been very well preserved, as if they had been alive until yesterday," says Henehan, who started this work during his stay at the University of Yale (USA).

Thanks to the ratio of the different boron isotopes (atomic variations of the same chemical element) present in the shells of the foraminifera, Henehan and his colleagues have been able to see what happened at sea after the asteroid. The main ingredient of the shells is calcium carbonate and its formation has a lot to do with the alkalinity (or acidity) of the water, that is, its higher or lower pH, the concentration of hydrogen ions per liter.

"The isotopic composition of the shells tends to resemble that of the water in which they live. The ratio of oxygen isotopes, for example, can tell us if they lived in warm or cold waters, boron isotopes, water pH" , says the researcher at the University of Zaragoza and co-author of the study, Laia Alegret, a great expert in these organisms.

Foraminifera of the species' Heterohelix globulosa

enlarge photo
Foraminifera of the species' Heterohelix globulosa

The foraminifera of the Geulhemmerberg cave trapped in the K-Pg boundary show a marked decalcification of their shells. "At lower pH, lower availability of material to make them," Alegret recalls. The study, published in the magazine PNAS, shows that after 100,000 years of a stable pH, this fell by up to 0.3. The current one is around 8.3. "It's not that it was sulfuric acid, but it greatly prevented calcification," he adds.

The phenomenon was widespread. The authors of the study compiled more than 7,000 foraminifera from the Dutch cave, bathed by the disappeared sea of ​​Tethys, but also from two other locations today emerging in the United States and three more in the Pacific and Atlantic. All fossils show a marked decalcification in the layers after the impact of the asteroid. "Unlike many others, the foraminifera did not become extinct after the event, but did suffer significant changes," says the Spanish researcher.

"The acidification of the ocean that we observed could easily have been the trigger for mass extinction in the marine environment," Pincelli Hull, a professor of geology and geophysics at Yale, notes in a note. Although it did not reach the bottom, the deposition of sulfuric and nitric acids generated after the impact altered the acidity of the surface layers of all oceans. Net primary carbon production, an indicator of biodiversity, was reduced to at least half. The study estimates that a large part of ocean life, beginning with mosasaurs, the great marine reptiles, became extinct.

For Alegret, what happened 66 million years ago should be a lesson for the current climate change process. "After 100,000 years of stability, the pH dropped in a few hundred years and did not recover as fast as it fell: it began to do so after 40,000 years and did not reach pre-asteroid levels until after 80,000 years."

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