by Sean P.S. Gulick, Joanna Victoria Morgan, Christian Koeberl, Jaime Humberto Urrutia Fucugauchi

The Chicxulub impact crater, Mexico, is unique. It is: 1) the only known terrestrial impact structure that has been directly linked to a mass extinction event, 2) the only one of the three largest impact structures on Earth that is well-preserved, 3) the only terrestrial crater with a global ejecta layer, and 4) the only known terrestrial impact structure with an 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. We propose to drill Chicxulub to address several questions, including: 1) what is the nature of a peak ring, 2) how are rocks weakened during large impacts to allow them to collapse and form relatively wide, flat craters, and 3) what caused the environmental changes that led to a mass extinction? The first two questions represent fundamental gaps in our knowledge. Despite over 30 years of intense debate, we are still striving to answer the third question. This proposal is a modified version of IODP proposal 548-Full3 that remains the highest-ranked, drillable mission specific platform proposal in the system. A hazard survey was carried out in 2013; a tender process for a drilling platform and drilling operation will be undertaken in 2014 and drilling could commence as early as Spring, 2015. The science goals of this ICDP proposal are by necessity the same as those in IODP proposal 548-Full3 but, here, we propose that ICDP co-fund the drilling. ECORD provided a provisional costing of $15.8 - 19.7 million for the drilling, which is a major increase over previous estimates and now may exceed the ECORD budget (~$17 million). There are large uncertainties in this budget and the exact costs will be unknown until the tender process is complete. Funding from ICDP will likely be critical to whether drilling is able to go ahead as planned. IODP and ICDP both welcome collaboration on futuredrilling projects but currently there is no pathway in place to achieve this goal. Hence, we are asking for co-funding to be provisionally agreed by ICDP and that ICDP representatives meet with the ECORD facility board to determine the details of that joint funding. In this way, ICDP can negotiate the optimum way forward for Chicxulub and future joint projects that best allows both IODP and ICDP to achieve their science plans and program strategies. The proposed drilling was agreed at an ICDP-IODP workshop, and continues the transect of the crater started The proposed drilling was agreed at an ICDP-IODP workshop, and continues the transect of the crater started by ICDP drill hole Yaxcopoil-1, and directly contributes to several goals in the new ICDP Science Plan.

• Hole Chicx-03A samples material that forms a topographic peak ring, and reveals the lithological and physical state of these diagnostic features, including porosity, fracturing and extent of shock effects. We will test the working hypotheses that peak rings are formed from:
• 1) overturned and uplifted basement rocks,
• 2) megabreccias, or 3) some other material. If the peak ring is formed from uplifted basement rocks, we can distinguish whether the rocks have been uplifted from the upper crust or deeper and whether the rocks arehighly fractured, porous, and/or contain thick zones of pseudotachylitic breccia, as seen at Vredefort and Sudbury. Implications include direct evidence of physical mechanisms of large impact processes and planetary resurfacing. Hole Chicx-04A penetrates enigmatic dipping reflectors that dip from the outer edge of the peak ring inwards. We suggest three working hypotheses for the cause of the dipping reflectivity:
• 1) The dipping reflectivity beneath the peak ring at Chicxulub is a lithologic boundary between uplifted basement lithologies and younger Mesozoic sediments;
• 2) the dipping reflectivity is a thrust fault formed during peak ring emplacement;
• 3) the dipping events are the result of vigorous hydrothermal circulation in the wake of peak ring emplacement that deposited hydrothermal minerals along a hydrothermal conduit. The origin of this reflectivity could well be some combination of these processes, such as a fault formed during peak ring emplacement that then served as a conduit for fluids post-impact. Implications include potential niche for chemosynthetic life possibly important for evolution of life on Earth.

CHICXULUB2, Cretaceous/Tertiary Boundary, Gulf Of Mexico, Yucatan

Latitude: 21.45015, Longitude: -89.94937