|Christian KOEBERL, Ao. Univ. Professor Dr.|
THE LAKE BOSUMTWI DRILLING PROJECT
The 10.5-km-diameter 1.07 Ma Bosumtwi impact crater was the subject of an multidisciplinary and international drilling effort of the International Continental Scientific Drilling Program (ICDP) from July to October 2004. Sixteen different cores were drilled at six locations within the lake, to a maximum depth of 540 m. A total of about 2.2 km of core material was obtained. Desite of some technical and logistical challenges, the project has been very successful and the first scientific results became available in early 2006. As part of this international project, the Austrian contribution in studying the drill cores consists of geochemical, petrographic, mineralogic, and petrophysical investigations.
The Bosumtwi impact crater is centered at 0632'N and 0125'W, and is almost completely filled by a lake. Lake Bosumtwi has been known to the scientific community since the beginning of the 20th century, but its origin was the subject of a controversy until the 1960, when petrological and isotope geochemical studies on tektites and impact glasses showed evidence for meteorite impact. It is one of 170 meteorite impact craters currently known on Earth. Bosumtwi is one of only four known impact craters associated with a tektite strewn field . It is a well-preserved complex impact structure that displays a pronounced rim and is almost completely filled by the 8 km diameter Lake Bosumtwi (Fig. 1). The crater is excavated in 2 Ga metamorphosed and crystalline rocks of the Birimian System; it is surrounded by a slight near-circular depression and an outer ring of minor topographic highs with a diameter about 20 km. Only limited petrographic studies of rocks found along the crater rim and of ejecta (suevitic breccias) are availble so far .
Fig. 1: Panoramic image of the Bosumtwi impact crater (view from the north rim) (Photo: CK).
Recent petrographic and geochemical work confirmed the presence of shock metamorphic effects and the presence of a meteoritic component in the Ivory Coast tektites and the breccias at the crater (e.g., [1,2]).
Insights into the deep structure of the crater and the distribution and nature of ejected material and post-impact sediments were obtained by recent geophysical work over the past seven years, which included aeromagnetic and airborne radiometric maps, multi-channel seismic reflection and refraction profiles, and land- and barge-based gravity and magnetic studies. The first magnetic field studies of the structure were conducted in 1960, and revealed a central negative anomaly of ~40 nT, attributed to a breccia lens below the lake sediments. Gravity measurements, collected around the lake at this time, reflected only the regional trends. In 1997 a high-resolution airborne geophysical survey revealed a halo-shaped magnetic anomaly . Seismic reflection and refraction data [4,5] defined the position of a 1.9 km-diameter central uplift situated northwest of the center of the lake.
The goal of the integrated drilling, rock property and surface geophysical study was to study the three-dimensional building blocks of the impact crater (delineate key lithological units, image fault patterns, and define alteration zones). Results from the Lake Bosumtwi scientific drilling project are important for comparative studies and re-evaluation of existing geophysical data from large terrestial impact sites (for example, Sudbury, Vredeford, Chicxulub and Ries).
Lake Bosumtwi is a closed-basin lake with very broad paleoclimate significance and a detailed paleo-environmental record. The lake is at an ideal geographical location to provide data on past interannual to orbital-scale variations in the West African monsoon and Sahel drought. Lake Bosumtwi has accumulated a detailed record of varved lake sediments that can be used to monitor both past local and Sahel rainfall variations. Rainfall over much of sub-Sahara Africa was highly correlated on centennial and longer time scales. Such data will benefit not only Ghana, were rainfall-dependent agriculture contributes to more than 50% of the GDP, but also the large populations of the entire sub-Sahara region of West Africa. A complex record of changes in lake level, lake chemistry, climate, and vegetation history has been documented earlier shallow piston core studies. More recent work has confirmed the potential of such paleoclimatic studies.
|ICDP Drilling Project - Planning|
All previous recent studies led to the realization that further investigations can only be obtained from deep drilling. Such drilling is desirable for several reasons, including 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 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.
The original proposal was to to obtain cores at nine locations in the crater lake, with core lengths ranging from 50 to 1035 meters, and total core length: 3 km sediments, 1 km impact-related rocks.
A number of questions can be answered by an integrated deep drilling project. The motivation for the drilling project included three main aspects:
The following tables list first- and second-order scientific questions that can only be answered by a deep drilling program. This is clearly an interdisciplinary program, because the impact, astrobiology, and paleoenvironmental goals can only be answered together.
Table 1: Bosumtwi Crater Drilling: Goals from the Impact Perspective
IMPORTANCE OF BOSUMTWI
Largest young impact structure known on Earth
Extremely well preserved (and well accessible)
Detailed geophysical site surveys available
Source crater of one of only four tektite strewn field
No crater of this has has ever been studied in such detail planetary perspectives
CRATER MORPHOLOGY AND GEOMETRY STUDIES
Determine crater depth (apparent and to basement)
Determine structure of central uplift
Characterize target stratigraphy (central uplift stratigraphy)
Measure post impact modification processes and effects (e.g., slumping)
STUDY OF CRATER FILL BRECCIA AND MELT ROCKS
Determine if melt rocks are present
Quantification of melt volume and breccia volume
Origin of various impact breccias, constraints on cratering process
Determine source rocks for tektites and fallout suevite
Determine meteoritic component in crater-fill breccia and melt rocks, and determine meteorite type
Occurrence of dyke breccias / pseudotachylitic breccias
Comparison between fall-out and fall-back breccias
Refine crater age from dating of impact melt
Internal breccia stratigraphy
Study clast population
Melt breccias occurrence and distribution
Origin of melt rocks
Search for 0.8 Ma Australasian tektite (microtektite) occurrences in crater-fill sediments
Provide ground-truth for previous aeromagnetic and other geophysical studies
Petrophysical data for interpretation of gravity and magnetic models and seismic studies
Obtain boundary conditions for modeling calculations
Vertical seismic profiling to obtain 3D image of crater subsurface
SHOCK METAMORPHISM STUDIES
Study of shock deformation within central uplift, including crater floor
Distribution through core of different grades of shock metamorphism: origin of ejecta clasts/melt,
implications for cratering physics
Carbon content in basement rocks and breccias
Search for impact diamonds
Search for shocked zircons
Paleomagnetic study of shocked rocks and melts
STUDY OF POST-IMPACT EVENTS
Search for possible evidence of other <1 Ma impact events (Australasian tektites) in post-impact sediments
Study the interface between fall-back breccia and lake-fill sediments
Climatic influences from impact-event
Study possible post-impact hydrothermal system
Examine hydrothermal mineralogy and isotope systematics
Metamorphic as well as hydrothermal overprint on crater floor and sub-crater strata
Thermal profile of post-impact heating
Search for chemical/fossil evidence of post-impact life forms (chemical signatures of bacteria; e.g.,
Post-impact carbon chemistry
Hydrocarbon generation (see also paleoclimatology goals)
Study of post-impact extreme environments and possible presence of extremophiles
Implications for regional impact-induced destruction and localized trauma to living systems
Climatic influences from impact-event
Biotic recovery after impact: speed of recovery, types and quantity of species returning after impact
Comparison of recovery with volcanic calderas (e.g., Crater Lake)
Comparison with numerical calculations of hydrothermal systems
Implications of data for impact craters of Mars and on Europa, as well as other comparative
Table 2: Bosumtwi Crater Drilling: Goals - Paleoenvironmental Perspective
IMPORTANCE OF BOSUMTWI
Longest & most cited continuous paleoenvironmental record in West Africa
One of very few long high-sedimentation-rate varved sediment records in the world
Proven recorder of hydrologic balance and terrestrial ecosystem variability (highly
relevant to human systems) the only such record in West Africa that is varved
Likely recorder of Sahel aridity and tropical Atlantic sea-surface temperature
variations - the only such record in West Africa or the western tropical Atlantic that is varved,
and thus highly relevant to human systems
Possibly the only high-resolution recorder of desert dust export from West Africa
Best possible record for study of long-term ocean-atmosphere-land surface interactions in
West + North Africa
Uniquely valuable record for reconstructing and understanding secular variations in
atmospheric radiocarbon, as well as the variations in solar output and ocean circulation that
cause these variations
West African location ideal for helping to understand environmental influences on human
and societal evolution over the last 1 million years.
DRILLING PROGRAM GOALS
ASTRONOMICAL-SCALE ENVIRONMENTAL VARIABILITY
Transfer astronomical theory to land, and to climate variability that impacts society
Determine lags and leads between insolation forcing, tropical African climate, and broader
Quantify the sensitivity of tropical climate to changes in global climate forcing (i.e., insolation)
Determine the relative influences of external astronomical forcing to impacts of
changes within the climate system (e.g., ice-sheets, ocean circulation change, atmospheric
trace-gas and aerosol changes)
MILLENNIUM-SCALE VARIABILITY AND ABRUPT CLIMATE CHANGE
Establish new level of geochronological precision using an integrated
varve-radiocarbon-paleomagnetic approach to study interannual to millennial-scale
variability over 106 years
Detail the impacts of late Quaternary abrupt change (i.e., Heinrich [H]and Dansgaard-Oeschger
[D/O] events) on tropical systems
Quantify roles of tropically regulated atmospheric trace-gases (e.g., CH4) and aerosol in
abrupt change dynamics
Measure impacts of H and D/O events on atmospheric radiocarbon, and use to unravel
associated ocean dynamics.
Determine the rates of ecosystem response and recovery to abrupt events
INTERANNUAL - TO CENTURY-SCALE CLIMATE VARIABILITY
Quantify response of regional (tropical and Sahel) interannual- to century-scale hydrologic
variability to full range of altered climate forcing
Test instrumental-based hypotheses regarding ocean forcing of West African monsoon
variability, including during interglacials warmer than the present day
Provide first detailed 40,000 record of solar-climate interactions
Provide first African record of interannual to decade-scale variations associated with H and D/O events
TROPICAL ECOSYSTEM DYNAMICS AND BIOGEOCHEMISTRY
Measure the rates and patterns of terrestrial ecosystem response to climate shifts over all
time scales of variability
Quantify Bosumtwi limnological response to the full range of possible climate variability
Define similarities and differences between present-day Bosumtwi systems (upland/aquatic)
and pre-anthropogenic natural system
Test role of tropics in modulating atmospheric trace-gas (e.g., CH4) and aerosol
(e.g., desert dust) concentrations
Examine relationships between climate variability, land-use and land-cover change, but
locally in Ghana, and also in the Sahel to the north
Produce record of dust aerosol export from Africa to S and N America; associated
ecological/ biogeochemical impacts
Establish environmental context for human population dynamics and societal change in
West Africa, and Africa
Differentiate human from non-human influences on regional land-cover, terrestrial ecosystems
and lake system
HYDROCARBON SYSTEM AND SOURCE ROCK DYNAMICS
Quantify processes of hydrocarbon generation
Refine understanding of lake sediment gas dynamics
Ground-truth geophysical interpretations and improve methods of geophysical exploration
in lacustrine systems
|ICDP Drilling Project - Operations|
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.
|Geophysics and Impact Results|
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 ) 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., ).
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.
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<
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.
 Koeberl C. et al. (1997) Geochim. Cosmochim. Acta 61, 1745-1772.
 Koeberl C. et al. (1998) Geochim. Cosmochim. Acta 62, 2179-2196.
 Plado et al. (2000) Meteoritics Planet. Sci. 35, 723-732.
 Scholz C.A. et al. (2002) Geology 30, 939-942.
 Karp et al. (2002) Planet. Space Sci. 50, 735-742.
 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|