by Christian Koeberl, Bernd Milkereit, Jonathan T. Overpeck, Christopher A. Scholz

This is a multidisciplinary proposal to drill the Bosumtwi impact structure in Ghana, West Africa. The Bosumtwi impact crater in Ghana (centered at 06
32'N and 01
25'W) is almost completely filled by Lake Bosumtwi. The crater has a rimto- rim diameter of about 10.5 km, while the lake has a diameter of about 8 km and maximum depth of about 80 m. The crater has an age of 1.07 Ma and was excavated in lower greenschist facies metasediments of the 2.1-2.2 Ga Birimian Supergroup. The Ivory Coast tektites and microtektites originated from this crater. Only recently, detailed geophysical studies were done. A first high-resolution aerogeophysical survey was conducted in early 1997. Measurements included: total magnetic field; aeroelectromagnetic; gamma radiation data for U-, Th- and K-contents, and total radiation. These data provide a much more detailed image of the crater structure than any previous data. Several projects have dealt with land- and lake-based geophysical measurements and surface geological and geochemical investigations regarding the subsurface topography of the structure. A shallow drilling project studied the near-surface composition of ejecta and weathered surface rocks for correlation with the aerogeophysical data (Austria/Ghana). Groups from Germany, the US, and Ghana have performed gravity measurements around the crater and over the lake, as well as refraction seismics with the goal to obtain a 3D tomographic model of the crater interior. Highresolution seismic studies were done that led to the discovery of a sediment-covered central uplift. Some shallow cores of lake sediments were also recovered. The results from these studies provided all the background work necessary to characterize the subsurface structure of the Bosumtwi crater that are necessary to define the targets for the deep drilling program proposed here.

• Deep drilling is desirable for several reasons, including 1) to obtain a complete 1 million year paleoenvironmental record in an area for which so far only limited data exist;
• 2) to study the subsurface structure and crater fill of one of the best preserved large young impact structures. Regarding the paleoenvironmental record, Bosumtwi is in a perfect location to reconstruct and investigate interannual through orbital-scale variations in 1) the West African monsoon,
• 2) hydrologic variations of the Sahel,
• 3) dust export from SW Africa, and 4) sea-surface temperature variations in the tropical East Atlantic. Understanding the full range of climate variability in this region over the last 1Ma will thus fill a major hole in our understanding of global climate dynamics, and thus also lead to an enhanced climate prediction capability over a broad part of the earth. In terms of cratering studies, Bosumtwi is one of only two known young craters of this size, and may have a crucial diameter at the changeover between a traditional “complex” crater with a central peak and a crater structure that has a central peak-ring system, maybe similar to that of the Ries crater in Germany (which is twice as large). Drilling will allow us to correlate all the geophysical studies and will provide material for geochemical and petrographic correlation studies between basement rocks and crater fill in comparison with tektites and ejected material.
• To obtain the goals outlined in detail in this proposal, cores at nine locations in the crater lake, with core lengths ranging from 50 to 1035 meters, need to be recovered; total core length: 3 km sediments, 1 km impact-related rocks. Drilling will be undertaken using the DOSECC/ICDP GLAD-800 lake drilling system.
• Drilling program goals (Impact Cratering Studies)  Crater Morphology and Geometry Studies  Study of Crater Fill Breccias and Melts  Geophysical Studies (Ground Truth for Seismic, Gravity, Magnetic, etc. studies)  Study Post-Impact Events  Astrobiological implications (extreme environments and extremophiles)  Regional impact-induced destruction, local trauma to living systems + climatic influence from impact event Drilling program goals (Paleoenvironmental Studies)  1 M.yr. high-resolution record: lake level change; Sahara/Sahel dust transport; geomagnetics; cosmic ray flux  Biological evolution within lake basin  High-resolution record of seasonal variability over several time windows  Sensitivity to climatic forcing  Catchment hydrological modeling  Environmental background to human occupation in West Africa

Africa, BOSUMTWI, Climate Change, Ghana, Global Environment, ICDP-2002/08, Impact Crater, Lake Drilling, Paleoclimatology

Latitude: 6.52227, Longitude: -1.42097