Studies of the El´gygytgyn Impact Crater

The Elgygytgyn impact crater, located in central Chukotka, NE Russia, is a 3.6 million year old impact structure, 18 km in diameter, which is partly filled by a lake with a diameter of 12 km and a water depth of 170 m. The El`gygytgyn crater was formed in the volcanic strata of Late Cretaceous age. Thus, the El`gygytgyn is the only known terrestrial impact structure where it is possible to investigate shock metamorphism in siliceous volcanic rocks. The sediment record in the crater is thought to preserve a complete unprecedented record of Arctic climate evolution, starting nearly a million years prior to the first major glaciation of the Northern Hemisphere. Site surveys confirmed that the crater was never glaciated during the Late Cenozoic and that the lake sediments sensitively reflect regional and hemispheric climate change on millennial time scales. During the last few years this remarkable structure has become the major focus of a multi-disciplinary and international deep drilling project. The science themes the defined the project focused on (1) the unique paleoclimatic history and potential of the lake sediments and (2) impact studies of the crater origin and bedrock stratigraphy. A full drilling proposal was submitted to the International Continental Scientific Drilling Program (ICDP) in January 2005, received enthusiastic reviews, and was accepted for funding (contribution to the drilling operations) in the summer of 2005. The PI of the present proposal is one of 4 PIs of the international drilling project ad mainly responsible for the impact science of the project. Further funding for the drilling and logistical costs was raised from the US National Science Foundation, the German Ministry for Science and Research, and the Austrian Ministry for Science and Research. Partly because of the complexity of the project and the inaccessibility of the location, currently the logistical/drilling costs stand at over 9.5 million US$. Two deep cores to 800 m depth are planned to be drilled in February-May 2009 to retrieve a continuous paleoclimate record from the deepest part of the lake and into the underlying impact breccias and bedrock. Studies of the impact rocks offers the planetary science community with the opportunity to study a well preserved crater uniquely found in igneous rocks like those on Mars. The work that is covered by this proposal comprises the main scientific study to result from the ICDP drilling project on the impact-derived rocks (i.e., the hard-rock cores) within the international drilling project; the post-impact sediments will be studied by research groups mainly in Germany and the USA. The cores should include abundant impact breccias and melt rocks, although it is not yet sure how abundant melt rocks will be and if they form thick layers or not. In the bottom of the core we expect a series of variably shocked rocks, allowing to determine the dependence of shock levels with. Melt rocks will provide an opportunity to search for a meteoritic component and to possibly detect the type of impactor. Also we will obtain a good understanding of the variety of target rocks, which will then be compared with the target rocks found elsewhere in the region. This study will lead to an improved understanding of the formation and effects of medium-sized impact craters. Impact craters of the size of the El`gygytgyn crater are known to form on Earth about once per million years. Such impact events have a disastrous effect on local climate, agriculture, etc., and will lead to massive loss of life. As El`gygytgyn is a relatively young and well-preserved example of a medium-sized crater, important results for the understanding of local and regional effects of such a geological catastrophe will follow from the drilling project.

The Elgygytgyn crater formed 3.6 million years ago by the impact of a large meteorite or asteroid, about 1 km in diameter. The crater and the lake that fills most of it are of scientific interest for two main reasons. First, this is the only known meteorite impact crater that formed in acid volcanic rocks and thus it offers the unique possibility to study the impact and shock effects on such rocks, which has implications for comparative planetology. Second, the about 300 meter thick deposit of lake sediments that were laid down on top of the impactites constitutes a unique climate archive of the largely unknown Arctic climate history. Investigating the lake sediment drill cores will provide important constraints on cause and effect of climate changes in the polar regions. Such work has great implications for the understanding of future climate change. The logistically very challenging drilling project (an ICDP project) was successfully concluded during the first half of May 2009. As hoped for and anticipated, an almost complete section of impact breccias was recovered underneath the lake sediments. Immediately below the lake sediments is an about 60 meter thick layer of so-called suevites. These are breccias that contain impact glasses and are composed of fragments representing a variety of different rocks that are cemented together in a fine-grained matrix. Such rocks occur on Earth only in meteorite impact craters. Underneath the suevites, the drilling encountered broken and fractures volcanic basement rocks, which were shocked, brecciated, and uplifted during the impact event. During formation of a meteorite crater, basement rocks are first pushed downwards, but then rebound and formerly deep-lying rocks are uplifted to close to the surface and form a so-called central peak. Impact craters with a central peak are also called complex craters. The formation of such a crater, and the central peak, is a very rapid process within about a minute, a mountain of several kilometers in diameter is uplifted out of the ground by 1 to 2 kilometers. The over 200 meters of impactite drill cores that were recovered will help to understand the details of the crater formation process. In total the drilling reached a depth of 517.3 meters below the lake floor, or a total depth, from the lake surface, of 687.3 meters. The impact breccia studies in Vienna included complete mineralogical, petrographic, and geochemical analyses. In addition, a detailed study of the shock behavior of the volcanic basement rocks was done, and various attempts were made to distinguish volcanic clasts/melts (formed about 90 million years ago) from those remelted during the impact event 3.6 million years ago. Further work was done to determine the composition and nature of the meteorite that formed the crater; the results indicate either a chondritic or a ureilitic meteorite.