The asteroid that wiped out the dinosaurs acidified the oceans

The asteroid that wiped out the dinosaurs acidified the oceans

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An international study, with the participation of the University of Zaragoza together with scientists from the US, Great Britain and Germany, shows, for the first time, that the impact of an asteroid on the Mexican Yucatan Peninsula 66 million years ago caused the acidification of the oceans, contributing to the last great mass extinction.

These results confirm that the marine extinction was the gases emitted by said impact and not the cessation of photosynthesis due to the darkness generated by the resulting cloud of dust, as previously believed.

Precisely, this work confirms the research hypothesis in which Laia Alegret, a member of the University Institute for Research in Environmental Sciences (IUCA), one of the signatories of the study, has been working for a decade.

The oceans absorb a third of carbon dioxide (CO2) emissions into the atmosphere, which helps regulate the climate by capturing excess heat. However, this process also has unwanted effects, such as a decrease in the pH (acidity level) of the waters, which alters the fixation of calcium carbonate (CaCO3) in the skeletons of numerous species, which could even accelerate the climate change.

The impact of an asteroid in the Mexican Yucatan Peninsula 66 million years ago caused the last great mass extinction, and affected almost 70% of the species of our planet, ending the dominance of the great dinosaurs in terrestrial environments.

In the oceans, large reptiles such as Mosasaurs disappeared, like much of the calcareous plankton that lived in surface waters.

«For years, it was suggested that there would have been a decrease in the pH of the ocean because the impact of the meteorite hit rocks rich in sulfur and caused a shower of sulfuric acidBut until now no one had direct evidence that this had happened ”, explains Michael Henehan, co-author of the study, formerly a scientist at Yale University and now at the Geosciences Research Center in Potsdam (Germany).

Traditional hypotheses suggested that after the impact of the asteroid to late Cretaceous, the darkness generated by the resulting dust cloud prevented photosynthesis and ceased primary productivity in the oceans, causing successive extinctions along the food chain.

"The acidification of the ocean that we observe could have been the trigger for the mass extinction in the marine realm," adds Pincelli Hull, assistant professor of geology and geophysics at Yale University, another of the co-authors of the work.

However, in 2012 Alegret led a publication in the PNAS magazine showing that extinctions in the oceans were not related to the cessation of photosynthesis, proposing a quick event of ocean acidification, much faster than the current and resulting from the gases emitted by the impact, as the main cause of selective extinctions in marine environments.

Eight years later, the same journal publishes a study that demonstrates this hypothesis. Alegret also participated in the 2017 international expedition to the new continent, Zealand, which remains almost entirely submerged.

Microscopic fossils in a mine in the Netherlands

Analyzes of microscopic marine fossils (foraminifera) found in the Geulhemmerberg mine in the Netherlands have allowed the first measurement of the pH of marine waters to be obtained after the impact of the late Cretaceous, demonstrating that this was the key mechanism in ecological collapse of the oceans.

Geochemical analyzes of carbon and boron in foraminifera shells, which have required the study of up to 7,000 microfossils per sample, indicate a decrease in the pH of the waters of 0.3 units and a large increase in atmospheric CO2 (700 parts per million). This is the first empirical measure of the mechanisms that triggered the extinctions.

Samples from various locations in the US and from ocean soundings in the Atlantic and Pacific have also been analyzed.

The study also includes the modeling of global changes in the geochemistry of the oceans, and allows ruling out that the impact caused an increase in volcanic activity. It shows that the recovery of ocean chemistry and marine ecosystems was slowly reestablished following global disturbances, even though marine plankton and primary productivity evolved rapidly after extinctions.

The latter has recently been confirmed by another international study, in which Alegret also participates together with researchers from the American universities of Yale, Boulder Colorado and MITMA, published in the journal Paleoceanography and Paleoclimatology.

The publication is an excellent example that geologically rapid events such as a meteoric impact or ocean acidification can have profound consequences on long-term life, and has implications for studies of current climate change.

Bibliographic reference:

Henehan, M., Ridgwell, A., Thomas, E., Zhang, S., Alegret, L., Schmidt, DN, Rae, JWB, Witts, JD, Landman, NH, Greene, S., Huber, BT, Super, J., Planavsky, NJ, Hull, PM 2019. «Rapid ocean acidification and phased biogeochemical recovery following the end-Cretaceous Chicxulub impact«. Proceedings of the National Academy of Sciences (PNAS).

Video: Demystifying ocean acidification and biodiversity impacts