[meteorite-list] Meteorite Crater Drilling in Ghana Provides Extensive Samples -- and a Mystery

[Ron Baalke]

Austrian Science Fund (FWF)
Wien, Austria

Scientific contact:

Prof. Christian Köberl
Department of Geological Sciences
University of Vienna
Althanstraße 14
1090 Vienna, Austria
Tel: +43 -1 - 4277-531-10
E-mail: christian.koeberl at univie.ac.at

The Austrian Science Fund (FWF)
Stefan Bernhardt M. A.
Weyringergasse 35
1040 Vienna, Austria
Tel: +43 - 1 - 505 67 40-36
E-mail: bernhardt at fwf.ac.at

Distributor
PR&D - Public Relations for Research & Development
Campus Vienna Biocenter 2
1030 Vienna, Austria
Tel : +43 - 1 - 505 70 44
E-mail: contact at prd.at

October 18, 2004

Meteorite Crater Drilling Provides Extensive Samples -- and a Mystery

Vienna -- Drillings made in the Bosumtwi crater in Ghana, one of the youngest 
meteorite craters in the world, led to yet another mysterious finding -- the 
rock formation caused by the heat of the meteoric impact is only half as thick 
as expected. This is the first result from a large international drilling 
project, which will provide new information simultaneously to both the earth and 
climate sciences as a result of detailed planning. The technically very 
demanding drilling operations were carried out during the past weeks by a 
multi-national team led by an Austrian scientist, partially funded by the 
Austrian Science Fund (FWF) within the International Continental Drilling 
Program (ICDP). Over 2.2 km of drill core have been retrieved and that will 
provide important material for detailed analyses.

The Bosumtwi crater in Ghana (West Africa) is interesting in many respects. 
First of all, at an age of one million years it is one of the youngest and best 
preserved impact craters on earth. Second, along with only three other of the 
total 170 known impact craters on earth, it is associated with a special 
geological feature -- during the impact the uppermost rock layers were melted 
and formed glass (so-called tektites), which are scattered over a distance of 
over 1000 kilometres.

Climatic Archive Under Water

However, it is the third aspect that makes the Bosumtwi crater particularly 
interesting -- an eight-kilometre wide lake that fills the 11-km-diameter 
crater. The Austrian principal investigator, Prof. Christian Koeberl of the 
Department of Geological Sciences, University of Vienna, explains the 
significance of this lake: "Sediments have been deposited in this lake for a 
million years. According to the season, the sediment input is dominated by the 
Atlantic monsoon or by the dust from the Sahel and Sahara. Thus, this 
sedimentary layer offers a comprehensive archive of the climate of the last one 
million years in West Africa."

Based on Data

This sedimentary layer now overlies the rocks formed by the meteoritic impact. A 
two-step approach was taken to study both the climate record contained in the 
lake sediments and the rocks that formed during the meteorite impact event. The 
research team comprising members of seven nations has been drilling from June to 
October 2004. First, a U.S.-led part of the team extracted lake sediment cores. 
Fourteen cores from six locations yielded a total of 1850 metres of core 
samples, spanning the whole million-year climate history contained in the lake 
sediments.

The logistically very challenging drilling in the impact-derived rocks began 
under Prof. Koeberl's management at the end of August. Prof. Koeberl explained 
the difficult logistics, which were supported by the International Continental 
Scientific Drilling Program (ICDP): "In order to satisfy all the requirements of 
the project, our team comprised up to 10 drilling engineers, 10 geophysicists 
and 11 people just for the scientific analysis on site. Moreover, in such a 
remote area as the Bosumtwi crater, the efficient coordination of such a 
technically demanding project presented special scientific and logistical 
challenges. For example, to allow the launching and loading our floating 
drilling platform, we had to construct part of a road and a special pier."

However the effort proved worthwhile; within just five weeks the team managed to 
drill at two locations through the sedimentary layer and into the impact-derived 
rock to depths of 540 and 452 metres, respectively. This yielded over 350 m of 
"hard rock" core. Preliminary studies of the available cores resulted in a big 
surprise -- the layer with "impact melt rock", which formed by melting of 
country rock during the meteorite impact, is not as thick as expected from 
previous geophysical studies. But only the detailed analysis of all the samples 
in the 122 core boxes, which are currently on their way to Europe, will 
hopefully deliver the answer to the question -- why?

IMAGE CAPTION:
[http://www.fwf.ac.at/de/press/Images/pa1004.jpg (925KB)]
Satellite photos of the Bosumtwi crater in Ghana, West Africa