Lake Bosumtwi

Compilation of sediment holes drilled on Lake Bosumtwi, Ghana, Africa, July - August 2004
Date	Hole #	Water Depth	Total Hole Depth (meas from water line	Cored from (ft)	Cored to (ft)	Reamed from (ft)	Reamed to (ft)	Total core (ft)
7/8/2004	1-A	149' 11"	179' 5"	149.92	179.42			29.50
7/9/2004	1-B	149' 11"	215' 8"	179.42	215.67			36.25
7/10/2004	1-C	149' 11"	313' 1"	215.33	383.42			168.08
7/11/2004	1-D	149' 11"	472' 0"	380.17	406.17			26.00
	1-D	149' 11"				406.17	429.83	0.00
	1-D	149' 11"		429.83	472.00			42.17
7/12/2004	1-E	141' 9"	220' 6"	141.75	220.50			78.75

7/13/2004	2-A	212' 10"	321' 1"	212.83	321.08			108.25
7/14/2004	2-B	215' 3"	497' 4"	215.25	497.33			282.08
7/16/2004	2-D	215' 3"				215.25	265.00	0.00
	2-D	215' 3"	494' 1"	265.00	494.08			229.08

7/18/2004	3-A	247' 7"	681' 0"	247.58	681.00			433.42
7/20/2004	3-B	253' 0"	685' 0"	253.00	685.00			432.00
7/22/2004	3-C	243 ' 3"	263' 0"	243.25	263.00			19.75
7/22/2004	3-D	244 ' 3"	659' 4"	243.25	659.33			416.08

7/25/2004	4-A	232' 5"	454' 4"	232.42	454.33			221.92
7/25/2004	4-B	227' 5"	443' 11"	227.42	443.92			216.50
7/26/2004	4-C	235' 11"	294' 11"	235.92	294.92			59.00
7/26/2004	4-D	235' 11"	617' 7"	294.92	617.58			322.67
7/26/2004	4-E	235' 11"	1035' 4"	617.58	703.83			86.25
	4-E	235' 11"				703.83	706.58	0.00
	4-E	235' 11"		706.58	1035.33			328.75

8/1/2004	5-A	245' 10"	736' 0"	245.83	736.00			490.17
8/5/2004	5-B	249' 0"	1215' 9"	249.00	1215.75			966.75

8/28/2004	6-A	174' 0"	554' 0"			174.00	554.00	0.00
							Total (ft.):	4993.42

Compilation of hard rock holes drilled on Lake Bosumtwi, Ghana, Africa, Sept - October 2004
Date	Hole #	Water Depth	Total Hole Depth (measured from water line)	Cored from (ft)	Cored to (ft)	Casing Advance from (ft)	Casing Advance to (ft)	Total core (ft)
9/7/2004	7-A	226' 4"	1798' 4"			226.33	1090.00	0.00
9/8/2004		226' 4"		1085.00	1798.33			713.33

9/23/2004	8-A	239' 6"	1480' 8"			211.40	773.00	0.00
9/25/2004		239' 6"		772.98	1480.70			707.72
							Total (ft.):	1421.05

DOSECC drilled the Bosumtwi impact structure from July to October 2004. Almost 5,000 feet of lake sediment core, and over 1,400 feet of hard rock core, was collected from 14 separate drill holes at five different sites across the lake. The project was completed using DOSECCs GLAD800 coring system.

This project was unique in that two teams of scientists, one studying the paleoclimate history and the other, meteorite impact processes, collaborated on the drilling. The project was also unique for the GLAD800 system because we collected lake sediment and then transitioned the system and collected underlying impact rock by diamond coring.

Lake Bosumtwi, Ghana. Photo by C. Koeberl

Locations of drill sites superimposed on lake bathymetry. The drilled sediment holes are red, the impact rock holes are green. Figure courtesy of ICDP.

GLAD800 on Lake Bosumtwi, Ghana. Photo by C. Koeberl

Link to the ICDP Lake Bosumtwi Project website here.

Because Lake Bosumtwi is one of a very few long, annually laminated sediment records in the continental tropics, it presents a unique opportunity to document tropical hydrologic and terrestrial ecosystem variability on time scales that vary from annual to hundreds of thousands of years. In order to gain greater insight into the role of the tropics in triggering, intensifying and propagating climate changes; scientific drilling for the recovery of long sediment records from Lake Bosumtwi was undertaken in July and August, 2004 using the GLAD800 lake drilling system. During the sediment drilling phase, five drilling sites were occupied spanning a depth transect with a total of 14 separate holes. Total sediment recovery was 1.833 km.

The complete 1 Ma lacustrine sediment fill was recovered ending in mircotekite-bearing, post-impact, accretionary lapilli fallout at site 5. This accretionary lapilli unit represents the initial post-impact sedimentation and provides an important age constraint for the overlying sedimentary sequence. The initial lacustrine sediment is characterized by a bioturbated, light-gray mud with abundant gastropod shells suggesting that a shallow-water oxic lake environment was established in the crater. The future study of the earliest lacustrine sediments will address important questions related to the formation of the lake and the establishment of biologic communities following the impact.

Most of the overlying 294 m of mud is laminated thus these sediment cores will provide a unique 1 million year record of tropical climate change in continental Africa at extremely high resolution. The drill sites are arranged along a depth transect in order to facilitate the reconstruction of the lake level history. The shallowest water site has a sediment sequence consisting of alternating laminated lacustrine muds (deepwater depositional environment), moderately-sorted sands, nearshore beach depositional environments) and sandy gravels (likely fluvial or lake marginal environments). These sediments preserve a record of major lake level variability that will greatly advance the present Bosumtwi lake level histories obtained from highstand terraces and shorter piston cores. The deepest water site consists almost entirely of laminated mud with lithified, carbonate-rich layers at about 152 m blf possibly related to a major lake level fluctuation. All of the drill sites will be used to produce a basinscale stratigraphic framework needed for placing the paleoclimate records into a proper depositional framework.

3D Perspective of theBosumtwi Impact Structure (courtesy of Chris Scholz)