Current Research in EEPS Seminar: Dr. Sean Gulick, University of Texas at Austin
Life and Death by Impact: Drilling for Clues
The most recent of Earth’s five largest mass extinction events occurred 66 Ma, coeval with the impact of a ~12 km asteroid, striking at ~60 degrees into what is today the Yucatán Peninsula, México, producing the ~200 km-wide Chicxulub crater. This impact, by some estimations, drove the extinction of 75% of life on Earth at the genus level including all non-avian dinosaurs. Proposed kill mechanisms include thermal effects caused by the reentry of fast ejecta into Earth’s atmosphere, dust and sulfate aerosols reducing Earth’s solar insolation, ocean acidification, and metal toxicity due to the chemical make-up of the impactor. In 2016, 835 m of core was recovered from the Chicxulub impact structure through IODP-ICDP Expedition 364 at Site M0077. Analyses done on these cores, downhole logs, and geophysical site survey data have led to a series of advancements to our understanding of impact cratering processes and to how the Chicxulub impact affected the Earth’s environment leading to the Cretaceous-Paleogene mass extinction.
The ~200 km diameter Chicxulub impact structure contains a fully preserved ring structure, terrace zone, peak ring (ring of mountains surrounding the center of large impact basins), melt sheet, and impactite sequence. Integration of seismic imaging and Expedition 364 drill cores demonstrate that peak rings form in a manner consistent with the dynamic collapse model. Implications are that significant vertical transport occurs during large impacts as in the case of Chicxulub where the peak ring granitoids originated at ~10 km depth. The weakening mechanism that allows rocks to deform in a fluid-like rheology often employed in numerical impact models is acoustic fluidization, but observational evidence for this process has been lacking. Expedition 364 cores of the peak ring exhibit consecutive stages of fracturing and shear deformation that in general increase in localization and decrease in spacing on the timescale of crater formation (minutes). These observations imply a sudden loss of cohesion, followed by a gradual recovery of shear strength, which is consistent with models of impact-induced acoustic fluidization and leaves the peak ring rocks in a permanently and pervasively fractured, distended state. Seismic images show the peak ring is underlain by Cretaceous sediments transported as slump blocks but also 100s of meters of seismically indistinct material that extends to beneath the crater floor and in places reaching 2 km thick. We interpret this seismic facies as a breccia derived from transient cavity wall collapse subsequently overlain by a melt-bearing breccia called suevite that record dynamic impact processes in the immediate aftermath of the impact.
With 100% core recovery, IODP-ICDP Expedition 364 with its 100% recovery provides a very high temporal resolution record of this impact aftermath. Site M0077 includes 130-m of impact melt rock and suevite, covered by < 1m of micrite-rich carbonate deposited over the subsequent weeks to years. The presence of terrestrial soil derived biomarkers and a reflected rim-wave tsunami deposit at the top of the 130-m sequence implies that the poorly sorted suevite may have been deposited through a combination of density flows and melt-water interactions, and the well sorted suevite deposited in a flooded crater within the first day of the Cenozoic. The K-Pg mass extinction demonstrates that impact events can have global consequences. Impacts cause mass extinction by inducing rapid physical and chemical changes to the atmosphere and/or oceans that cannot be accommodated by adaptation or motility. In the case of Chicxulub and the K-Pg extinction it appears that the presence of sulfur-rich evaporites, and other volatiles, was a key, but not unique, ingredient driving the extinction. Site M0077 however also provided evidence for rapid return of life to the crater environment and generation of a subsurface habitat.