Asteroid That Killed Dinosaurs Acidified the Oceans
The picture shows the Cretaceous-Palaeogene boundary at Geulhemmerberg in the Netherlands, where the boundary clay samples were taken. The event bed is clearly visible as a grey clay-rich layer, between the otherwise yellowish carbonate sediments. It was thought to have been laid down during calm periods between strong storm events.
Credit: Michael Henehan
The impact of a celestial body left traces: the “Chicxulub crater” in the Gulf of Mexico and tiny amounts of iridium in sediments. Up to 75 percent of all animal species went extinct at the time. The impact marks the boundary of two geological eras – the Cretaceous and the Palaeogene (formerly known as the Cretaceous-Tertiary boundary).
Henehan and his team at Yale University reconstructed the environmental conditions in the oceans using fossils from deep-sea drill cores and from rocks formed at that time. According to this, after the impact, the oceans became so acidic that organisms that made their shells from calcium carbonate could not survive. Because of this, as life forms in the upper layers of the oceans became extinct, carbon uptake by photosynthesis in the oceans was reduced by half. This state lasted several tens of thousands of years before calcareous algae spread again. However, it took several million years until the fauna and flora had recovered and the carbon cycle had reached a new equilibrium.
The researchers found decisive data for this during an excursion to the Netherlands, where a particularly thick layer of rock from the Cretaceous-Palaeogene boundary is preserved in a cave. “In this cave, an especially thick layer of clay from the immediate aftermath of the impact accumulated, which is really quite rare” says Henehan. In most settings, sediment accumulates so slowly that such a rapid event such as an asteroid impact is hard to resolve in the rock record. “Because so much sediment was laid down there at once, it meant we could extract enough fossils to analyse, and we were able to capture the transition,” says Henehan.
Most of the work was done at his former place of work, Yale University. Now, at the GFZ, he is using the infrastructure here and hopes that this will provide a major impetus for his work. “With the femtosecond laser in the HELGES laboratory, we are working to be able to measure these kind of signals from much smaller amounts of sample,” says Henehan. “This will in the future enable us to obtain all sorts of information at really high resolution in time, even from locations with very low sedimentation rates.”
Funding: The main part of the study was done at Yale University, with financial support from the Yale Peabody museum.
Contacts and sources:
Josef Zens
Citation: “Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact,” http://www.pnas.org/cgi/doi/10.1073/pnas.1905989116
Source: http://www.ineffableisland.com/2019/10/asteroid-that-killed-dinosaurs.html
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