Asteroid dust found within the dinosaur-killing crater

25/02/2021 09:01

Sixty-six million years ago, a catastrophic mass extinction completely reshaped life on our planet. The (non-avian) dinosaurs vanished from the Earth’s surface, together with many other species and families, including the ammonites and mosasaurs. The first clue to better understand this sudden and global disappearance of life was found in sediment layers near Gubbio in Italy and Caravaca in Spain, where a very thin clay layer represents the boundary between the Cretaceous and the Paleogene Periods. In the early 1980s, scientists measured in this clay layer remarkably high concentrations of iridium, a rare metal that occurs in high concentrations in meteorites but in very low concentrations in the Earth’s crust. This clay layer was thus explained as having formed from dust that was produced following the impact and vaporization of an asteroid of approximately 12 km in diameter. This finding was later strengthened by the discovery of the 180 to 200 km sized Chicxulub impact crater, buried underneath the surface of the Yucatán Peninsula in México. Now, more than 40 years later, scientists have uncovered the final piece of evidence that ties the global mass extinction to the asteroid impact. An international team of researchers led by scientists from the Vrije Universiteit Brussel have traced back the global asteroid dust layer to within the Chicxulub impact crater in Mexico. “The circle is now finally complete”, comments Steven Goderis, a geochemistry professor at the Vrije Universiteit Brussel and lead author of the study.

Lift boat L/B Myrtle used to drill the core. Foto credit: The University of Texas at Austin, Jackson School of Geosciences.

In May 2016, a discontinuous ring of hills that surrounds the center of the Chicxulub impact structure in Mexico, called a peak ring, was drilled by a science team from the International Ocean Discovery Program (IODP) and the International Continental Scientific Drilling Program (ICDP) Expedition 364. Approximately 835 meters of rock were brought to the surface, which have provided a tremendous amount of new information on the processes that occurred in the crater region during and right after the asteroid impact. The extracted drill core also recorded in great detail the time interval when the crater transitioned from a dynamic environment with returning ocean water and tsunami waves to much quieter conditions. Based on an extensive geochemical analysis of this part of the drill core, the highest concentrations of iridium were found in a clay-rich interval in sediments that cover the crater peak ring, just below limestone from the earliest Paleogene.

“Iridium is an element that is fairly difficult to measure in this context due to its low concentrations. This is why we combined the results from four independent laboratories around the world to make sure we got this right” clarifies Steven Goderis. The iridium concentrations that were measured in the drill core are in agreement with those previously measured in locations around the Gulf of Mexico. “It is quite remarkable that we found concentrations this high within the impact structure itself. In the first hours to months after the impact, the crater was a highly turbulent environment affected by tsunami, oscillating waves, and earthquakes. In addition, hydrothermal fluids venting from deeper in the crater to the surface also passed through the iridium layer but did not substantially change it. Luckily, the iridium layer was preserved, in part due to the unique position of the drill core in a depression on the elevated peak ring. After circling the Earth in the atmosphere for several years, the deposition of this iridium-rich dust may have taken up to a couple of decades after the impact event”, summarizes Goderis.

Therefore, the atmospheric settling of this asteroid dust places important time constraints on the deposition of the crater rocks just below this iridium layer. “This part of the Chicxulub impact basin returned to a relatively low-energy environment in a much shorter time than previously expected. Together with other recorders of time, such as microfossils and helium-3 concentrations, the iridium layer captures the timing of the recovery of life during the years to millennia after the impact, indicating a highly complex biological response to the rapidly changing environment at ground zero”, observes Sean Gulick, research professor at University of Texas at Austin and co-chief scientist of the drilling expedition. The IODP-ICDP Expedition 364 drill core thus contains an exceptionally detailed record of the processes associated with the formation of the Chicxulub crater and the recovery of life.

The detection of such well-defined iridium anomaly within the Chicxulub crater will undoubtedly also revitalize research on the Cretaceous-Paleogene mass extinction. “With this discovery, we are able to place precise time constraints on the products that formed as the result of the asteroid impact, better than ever before. Within the crater, we observe a 130-meter-thick pile of molten, brecciated and fine-grained rocks that were probably deposited in less than twenty years, with most of it deposited in the first day, which is astonishingly fast. At many different locations around the world, this 20-year time interval is represented by a much thinner rock layer, which consists of shocked, melted and condensed material that was ejected from the crater. By comparing these different locations, we will better understand the exact mechanisms related to the Chicxulub impact that led to the global mass extinction”, explains Pim Kaskes, an FWO-PhD researcher at the Vrije Universiteit Brussel working on the drill core. “The preservation of the iridium layer within the crater is just fantastic, it constitutes the indisputable evidence that the impact and the extinction are closely linked”, concludes Philippe Claeys, geology professor at the Vrije Universiteit Brussel and a 30 years Cretaceous-Paleogene research veteran.

In addition to the Vrije Universiteit Brussel, the following Universities and institutes have been involved in the study: University of Padova, Japan Agency for Marine-Earth Science and Technology, Natural History Museum in Vienna, Lund University, University of Notre Dame, Université Libre de Bruxelles, Katholieke Universiteit Leuven, Arizona State University, University of Vienna, Universität zu Köln, Ghent University, Utrecht University, Tokyo Institute of Technology, Florida State University, HNU Neu-Ulm University of Applied Sciences, Lunar and Planetary Institute, Durham University, Pennsylvania State University, University of Texas at Austin, Imperial College London, Vrije Universiteit Amsterdam, and University of Alaska Fairbanks.

The study is published in the scientific journal Science Advances.

Full reference: S. Goderis, H. Sato, L. Ferrière, B. Schmitz, D. Burney, P. Kaskes, J. Vellekoop, A. Wittmann, T. Schulz, S. M. Chernonozhkin, P. Claeys, S. J. de Graaff, T. Déhais, N. J. de Winter, M. Elfman, J.-G. Feignon, A. Ishikawa, C. Koeberl, P. Kristiansson, C. R. Neal, J. D. Owens, M. Schmieder, M. Sinnesael, F. Vanhaecke, S. J. M. Van Malderen, T. J. Bralower, S. P. S. Gulick, D. A. Kring, C. M. Lowery, J. V. Morgan, J. Smit, M. T. Whalen, IODP-ICDP Expedition 364 Scientists, Globally distributed iridium layer preserved within the Chicxulub impact structure. Sci. Adv. 7, eabe3647 (2021).

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