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Thursday, 24 October 2019

Cretaceous-Paleogene Extinction: The impact of an asteroid has been the main cause of extinction


66 million years ago, dinosaurs and many other species disappeared during mass extinction known as Cretaceous-Paleogene (K-Pg) extinction. The main hypothesis used to explain this event is the combination of intense volcanism and the impact of an asteroid. However, so far, there is no empirical evidence as to which of the two had contributed most to this extinction. But recently, a team of geologists has shown that the impact of the asteroid has caused a brutal acidification of the oceans with massive disruption of the carbon cycle, making this impact the main cause of K-Pg extinction.

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The fossil remains of tiny calcareous algae not only provide information about the end of the dinosaurs, but also show how the oceans have recovered from the impact of the asteroid. Experts agree that a collision with an asteroid has caused mass extinction on our planet, but some hypotheses have been made that ecosystems were already under pressure from increasing volcanism.

Our data shows a gradual deterioration of environmental conditions 66 million years ago, " says Michael Henehan of GFZ's German Geoscience Research Center. With colleagues from Yale University, he published in PNAS a study describing ocean acidification during this period.


Massive acidification of the oceans due to the impact of the asteroid
Henehan has studied boron isotopes in the calcareous shells of plankton (foraminifera). According to the findings, there was a sudden impact that led to massive acidification of the oceans. The oceans have taken millions of years to recover from this acidification. " Before the event, we could not detect any increasing acidification of the oceans, " explains Henehan.

The impact of a celestial body left traces: the Chicxulub crater in the Gulf of Mexico and tiny amounts of iridium in the sediments. Up to 75% of all animal species disappeared at the time. The impact marks the boundary of two geological epochs - the Cretaceous and the Paleogene (formerly known as the Cretaceous-Tertiary boundary).

The study of fossilized oceanic foraminifera (in color on the graph) revealed a rapid acidification of the oceans during the extinction K-Pg (red vertical line), caused by the impact of an asteroid. Credits: Michael J. Henehan et al. 2019

Henehan and his team at Yale University reconstructed environmental conditions in the oceans, using fossils from deep-sea drill cores and rocks formed at that time.

After the impact, the oceans became so acidic that the organisms that made their calcium carbonate shell could no longer survive. As a result, as life forms in the upper oceans have disappeared, carbon uptake by photosynthesis in the oceans has been reduced by half.

The collapse of the carbon cycle and the slow recovery of ecosystems

This state lasted several tens of thousands of years before the spread of calcareous algae. However, it took millions of years for the flora and fauna to recover and for the carbon cycle to reach a new equilibrium. The researchers found decisive data on this subject during an excursion to the Netherlands, where a particularly thick layer of rock from the Cretaceous-Paleogene boundary is conserved in a cave.

The Cretaceous-Paleogene geological boundary is clearly visible in this cave of Geulhemmerberg (Netherlands), where the samples of the study were taken. Credits: Michael Henehan

In this cave, a particularly thick layer of clay accumulated immediately after the impact, which is really quite rare, " says Henehan. In most cases, sediments accumulate so slowly that such a rapid event, such as an asteroid impact, is difficult to identify during rock analysis. " Because so much sediment was deposited at a time, we were able to extract enough fossils to analyze, which allowed us to isolate the transition.

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