Expedition 364 Chicxulub: Chicxulub Peak Ring Formation

The Chicxulub impact crater, Mexico, is unique. It is the only known terrestrial impact structure that has been directly linked to a mass extinction event, and the only terrestrial impact with a global ejecta layer. Of the three largest impact structures on Earth, Chicxulub is the best preserved. Chicxulub is also the only known terrestrial impact structure with an intact, unequivocal topographic "peak ring". Chicxulub's role in the K-Pg mass extinction and its exceptional state of preservation make it an important natural laboratory for the study of both large impact crater formation on Earth and other planets, and the effects of large impacts on the Earth's environment and ecology. Our understanding of the impact process is far from complete and, despite over 30 years of intense debate, we are still striving to answer the question as to why this impact was so catastrophic.

Expedition 364 is the first drill hole into an intact topographic peak ring, and the first to penetrate the offshore portion of the Chicxulub crater. Peak rings are a ring of hills that protrude through the crater floor within large impact basins on the terrestrial planets, and there is no consensual agreement on either their formational mechanism or the nature of the rocks that form them. Geophysical data indicate that the peak ring at Chicxulub is formed from rocks that have a low velocity and density, and one explanation for this is that they are highly fractured and porous. Immediately after impact the peak ring was submerged under water, and located adjacent to a thick pool of hot melt rocks. Hence, we would expect intense hydrothermal activity within the peak ring. This activity may have provided a niche for exotic life forms, in a similar way that hydrothermal vent systems do in the oceans. Drilling the peak ring will determine the origin, lithology, and physical state of the rocks that form it, allow us to distinguish between competing models of peak-ring formation, as well as document the hydrothermal systems and any associated microbiology. Immediately after impact the ocean is, locally, likely to have been sterile. We will use core through the post-impact sediments to examine the recolonization of the ocean, including: what biota came back first (benthic, dinoflagellates, specialists vs generalists), and how long did it take to return to normal conditions?

The proposed drilling directly contributes to IODP goals in the: Deep Biosphere and the Subseafloor Ocean and Environmental Change, Processes and Effects, in particular the environmental and biological perturbations caused by Chicxulub.

The publication procedure from IODP Expeditions is designed to gain maximum impact and timeliness. In addition to expedition reports, the science party are mandated to publish the results of their work within a strict time limit. We will ensure publication at the highest possible level of quality and visibility.

Other beneficiaries include:

1. The local population and next generation of young scientists in the Yucatan;
2. The mining and mineral exploration industries;
3. The insurance industry, regarding the financial consequences of impacts;
4. The general public.

1. The Yucatan government has invested significant funds in a new museum in the centre of Merida, Gran Museo del Mundo Maya (http://www.granmuseodelmundomaya.com), and a large Science Park just outside the city. We have met with the Director of Education for the Yucatan, as well as the Museum curators, and they propose to use the Chicxulub drilling for both tourism and educational purposes, including placing an exhibit with a replica drill core at one of these two locations.

2. The two other known terrestrial craters of a similar size to Chicxulub (Sudbury and Vredefort) are associated with significant mineral deposits. The peak ring may be rich in PGEs (or some other resource) and be of interest for future exploration.

3. The insurance industry has shown increasing support for studies of natural hazards, and impacts specifically, especially following the Chelyabinsk event in 2013 when videos of the meteor entry went viral. Simulations of impacts of various sizes provide critical user input into risk analysis studies leading, for example, to the development of new risk assessment tools for future impacts.

4. The public will benefit from outreach activities in Mexico, the UK and elsewhere (there are 30 science party members from 11 different countries). We propose to capitalize on the public's interest in the "death of the dinosaurs".