Aftermath: The first day of the Cenozoic

Gravity anomaly map of the Chicxulub impact structure (From Wikimedia Commons)

In the late ’70, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that an asteroid collided with the Earth and caused one of the most devastating events in the history of life. In 1981, Pemex (a Mexican oil company) identified Chicxulub as the site of this massive asteroid impact. The crater is more than 180 km (110 miles) in diameter and 20 km (10 miles) in depth, making the feature one of the largest confirmed impact structures on Earth. The Cretaceous–Paleogene extinction that followed the Chicxulub impact was one of the five great Phanerozoic mass extinctions. The impact released an estimated energy equivalent of 100 teratonnes of TNT, induced earthquakes, shelf collapse around the Yucatan platform, and widespread tsunamis that swept the coastal zones of the surrounding oceans. The event also produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Global forest fires might have raged for months. Photosynthesis stopped and the food chain collapsed.

The Chicxulub impact site is the only known impact structure on Earth with an unequivocal peak ring but it is buried and only accessible through drilling. In April to May 2016, a team by International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) drilled the Chicxulub peak ring offshore. The core recovered during the expedition provides a window into the immediate aftermath of the impact.

Timeline of events recorded inside the impact crater.

The recovered core was divided into 4 Units. The Unit 1 is 111.63-m-thick postimpact sedimentary rock. The Unit 2 is 104.28-m thick and dominantly suevite. The Unit 3 is 25.41-m-thick impact melt rock, with some clasts present. The Unit 4 consists of shocked granitic target rocks, preimpact sheet intrusions, and intercalations of suevite and impact melt rock. There are high abundances of charcoal in Unit 1. The charcoal likely originated from impact-related combustion of forested landscapes surrounding the Gulf of Mexico. Data indicate that Chicxulub impact released sufficient thermal radiation to ignite flora up to 1,000 to 1,500 km from the impact site. The upper few centimeters of the unit 2 contain abundant reworked Maastrichtian planktic foraminifera that indicate redeposition of sediments that were unconsolidated at the time of the impact.

The lack of evaporites in the recovered sedimentary section, supports the impact generated sulfate aerosol production and extinction mechanisms, including global cooling and limitations on photosynthesis. Core samples also revealed that a high-temperature hydrothermal system was established within the crater but the appearance of burrowing organisms within years of the impact indicates that the hydrothermal system did not adversely affect seafloor life. These impact-generated hydrothermal systems are hypothesized to be potential habitats for early life on Earth and other planets.

 

References:

Sean P. S. Gulick, Timothy J. Bralower, Jens Ormö, Brendon Hall, Kliti Grice, Bettina Schaefer, Shelby Lyons, Katherine H. Freeman, Joanna V. Morgan, Natalia Artemieva, Pim Kaskes, Sietze J. de Graaff, Michael T. Whalen, Gareth S. Collins, Sonia M. Tikoo, Christina Verhagen, Gail L. Christeson, Philippe Claeys, Marco J. L. Coolen, Steven Goderis, Kazuhisa Goto, Richard A. F. Grieve, Naoma McCall, Gordon R. Osinski, Auriol S. P. Rae, Ulrich Riller, Jan Smit, Vivi Vajda, Axel Wittmann, and the Expedition 364 Scientists. The first day of the Cenozoic. PNAS, 2019 DOI: 10.1073/pnas.1909479116

Morgan, J. V., Gulick, S. P. S., Bralower, T., Chenot, E., Christeson, G., Claeys, P., … Zylberman, W. (2016). The formation of peak rings in large impact craters. Science, 354(6314), 878–882. doi:10.1126/science.aah6561

Christopher M. Lowery et al. Rapid recovery of life at ground zero of the end-Cretaceous mass extinction, Nature (2018). DOI: 10.1038/s41586-018-0163-6

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