THE LAKE BOSUMTWI DRILLING PROJECT
| Christian KOEBERL, Ao. Univ. Professor Dr. |
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_ THE LAKE BOSUMTWI DRILLING PROJECT |
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| ICDP Drilling Project - Operations | ||||||||||||||||||||||||||||||||||||||||||||
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After the surface
studies were more or less exhausted in early 2000s, it was decided
to pool the efforts for drilling in the form of a multinational and
multidisciplinary study. This was submitted to ICDP as a preproposal
in 2000, which was followed by a workshop proposal submitted January
2001. This workshop was held successfully in Potsdam in September of
2001; as a result, a full proposal was submitted to ICDP January
2002, which was approved in mid 2002 with 70% of the total cost. The
PIs of that proposal (C. Koeberl, B. Milkereit, J. Overpeck, and C.
Scholz) had to raise the remaining funds from their own national
sources. The logistical preparations in Ghana and elsewhere started
in late 2002, with the very important help of the Government of
Ghana in the form of the Geological Survey Department of Ghana, and
the University of Science and Technology in Kumasi. After a lot of
logistical, financial, and technical challenges the drilling
operations started at the beginning of July 2004, and were completed
on October 2, 2004. Drilling was accomplished using the GLAD800
coring system, a joint operation of DOSECC and ICDP (Fig. 2a,b).
 Table 3. Scientific and operational staff who participated in the Lake Bosumtwi Drilling program. _
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| Geophysics and Impact Results |
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The new deep drill holes
LB07A and LB08A are tied to the potential field and seismic data
that define the Lake Bosumtwi impact structure (Fig. 4). Acquisition
of zero-offset and multi-offset VSP data in deep hardrock holes
LB07A and LB08A (Fig. 4) established a link with existing seismic
data. Slim-hole borehole geophysical studies provide crucial
information about the distribution of magnetized formations within
the breccia and help locate discontinuous melt units in the
proximity of the scientific drill hole(s). Information about the
distribution of magnetic susceptibility and remanance of breccias
and impact melt holds the key to an improved three-dimensional model
for the Bosumtwi crater and its thermal history. Multi-offset
vertical seismic (VSP) profiling support the integration of
conventional logs and existing grid of multi-channel seismic and
refraction seismic data. The offset VSP experiments are well suited
for the integration of core/laboratory data, logs, and conversion of
reflection seismic images from time to depth. By documenting the
distribution of magnetic susceptibility and the impact related
thermo-magnetic remanance, the distribution of the thermal effects
of the impact will be outlined. Combining the horizontal resolution
of the seismic surveys with the enhanced verticalresolution of the
borehole magnetic surveys provide an ideal set-up for 3D modeling
through data integration.
 Fig. 5. Seismic section (from [4]) with deep boreholes, showing in black the hard rock (impactite) core. The hard rock drilling phase, as well as borehole logging and geophysical studies, was completed on October 2, 2004. During that phase two boreholes, to depths of 540 and 450 m, respectively, were drilled in the deep crater moat, and on the outer flank of the central uplift as identified in seismic profiles. This represents about 200 m of impactites/breccias or fractured bedrock, with about 360 m of core having been recovered in total. Care was taken to make sure that all drilling operations took place on good-quality seismic lines (Fig. 5). In both cases casing was set through the lake sediment part of the section, and drilling, using diamond coring tools, started at the sediment/impactite (fallback suevite) interface. Drilling progressed in both cases through the melt rock / impact breccia layer into fractured bedrock. _ After completion of the drilling operations, the hard rock cores (122 core boxes) were shipped to the GeoForschungsZentrum in Potsdam, Germany, for scanning and documentation; a sampling party took place January 23 and 24, 2005. For updates and details, see http://bosumtwi.icdp-online.org/ |
| Paleoclimatic Studies at Bosumtwi. |
 Owing
to its impact crater origin, Lake Bosumtwi possesses several
important characteristics that make it well suited to provide a
record of tropical climate change. First, because of the great age
of the crater (1.07 Ma) and location in West Africa, the lake
sediments can provide a long record of change in North African
monsoon strength. Lake Bosumtwi lies in the path of the seasonal
migration of the Intertropical Convergence Zone (ITCZ), the
atmospheric boundary between NE continental trade winds and onshore
SE trade winds (Fig. 6). During summer months, the ITCZ migrates to
the north of Lake Bosumtwi and moisture-laden winds bring heavy,
monsoonal precipitation to western Africa. The reverse occurs during
winter months, as the ITCZ is displaced southward of Lake Bosumtwi
and dry, aerosol-rich NE continental trade winds (Harmattan)
dominate over southern Ghana._ Second, the high crater rim surrounding the lake results in a hydrologically-closed lake with a water budget extremely sensitive to the precipitation/evapotranspiration balance. Third, the steep crater wall and deep lake basin limit wind wave mixing of the water column. As a result, the deep water is anoxic, thereby limiting bioturbation and allowing for the preservation of laminated sediment varves and the potential for high resolution (annual) paleoclimate reconstruction (e.g., [6]). _  In
July and August 2004, a sediment drilling program was undertaken in
order to gain greater insight into the role of the tropics in
triggering, intensifying and propagating climate changes, as well as
in responding to global and high-latitude changes. Five drill sites
were occupied along a water-depth transect in order to facilitate
the reconstruction of the lake level history. At these five drill
sites, a total of 14 separate holes were drilled. Total sediment
recovery was 1,833 m. For the first time the GLAD lake drilling
system cored an entire lacustrine sediment fill from lake floor to
bedrock. Although detailed sedimentologic study is just beginning,
examination of the core catchers and core section breaks during
drilling provided glimpses of the paleolimnologic record recovered
in the cores (Figures 7 and 8). The complete 1 Ma lacustrine
sediment fill was recovered from the crater ending in impact-glass
bearing, accretionary lapilli fallout representing the initial days
of sedimentation. The lowermost lacustrine sediment is a bioturbated,
light-gray mud with abundant gastropod shells suggesting that a
shallow-water oxic lake environment was established in the crater.
Future study of the earliest lacustrine sediment will address
important questions related to the formation of the lake and the
establishment of biologic communities following the impact. Much of
the overlying 294 m of mud is laminated (Figure 9) thus these
sediment cores will provide a unique 1 million year record of
tropical climate change in continental Africa at extremely high
resolution. The shallow water drill sites consist of alternating
laminated lacustrine mud (deepwater environment), moderately-sorted
sand (nearshore beach environment) and sandy gravel (fluvial or lake
marginal environments). These sediments preserve a record of major
lake level variability that will extend the present Bosumtwi lake
level histories obtained from highstand terraces and short piston
cores further back in time. |
| Sampling Meeting |
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In late January 2005 a
sampling meeting (called a sampling party) was held at the ICDP
headquarters in Potsdam, Germany. Over a dozen research tems sampled
the two dep drill cores. Samples were distributed in the spring of
2005 and first scientific results became available in early 2006.
_ For further details, see >here< _ A summary of first results was published at the Lunar and Planetary Science Conference in Houston in 2006 _ The full program and the abstracts of the special session on the Bosumtwi crater drilling project at the 2006 Lunar and Planetary Science Conference can be found >here< |
| Acknowledgments |
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The project has been
supported by the Austrian FWF, the Austrian Academy of Sciences,
ICDP, the U.S. NSF, and the Canadian NSERC.We are particularly
grateful to the Geological Survey Department of Ghana (P. Amoako,
Director) and the University of Kumasi (A. Menyeh, Dean) for all the
logistical support, and to DOSECC (D. Nielson, President) for the
operational support, and we appreciate that the project would not
have succeded without the hard work of a dedicated group of DOSECC
drillers, the Kilindi captain, local Ghanaian scientists, students,
and workers, and a group of international scientists (Table 3). We
also would like to acknowledge the support and hard work of the ICDP
operational support group, in particular U. Harms, T. Wφhrl, and J.
Kόck.
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| References |
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[1] Koeberl C. et al.
(1997) Geochim. Cosmochim. Acta 61, 1745-1772.
[2] Koeberl C. et al. (1998) Geochim. Cosmochim. Acta 62, 2179-2196. [3] Plado et al. (2000) Meteoritics Planet. Sci. 35, 723-732. [4] Scholz C.A. et al. (2002) Geology 30, 939-942. [5] Karp et al. (2002) Planet. Space Sci. 50, 735-742. [6] Peck J. et al. (2004) Palaeogeogr. Palaeoclimatol. Palaeoceol. 215, 37-57. |
| Christian KOEBERL Althanstrasse 14 1090 Wien AUSTRIA phone: +43-1-4277-53110 fax: +43-1-4277-9534 |
