High Amplitude
Climate Variability in a Tropical Rift-Lake: Correlation of Drillcore and
Seismic Reflection Data in Lake Malawi, East Africa
Lyons, R.P.1, C.A. Scholz1,
J.W. King2, A.S. Cohen3, T.C. Johnson4 (1)
Syracuse University, Syracuse, NY (2) University of Rhode Island, Kingston, RI
(3) University of Arizona, Tuscon, AZ (4) University of Minnesota, Duluth, MN
Lake Malawi is one of the world's
largest lakes, extending ~580km along the East Africa Rift System from 9-14
degrees S. The 700m deep lake has a permanently stratified water column that is
anoxic below 250m. The water budget is dominated by evaporation and direct
rainfall onto its surface; accordingly lake level is highly sensitive to minor
shifts in climate.
Initial deep basin-scale multi-channel
seismic data were collected in the mid-80's as part of
Project PROBE, which focused on the large scale changes of the rift basin over
broad time scales. Since then three high-resolution, high-density,
single-channel seismic reflection surveys were collected on Lake Malawi between 1992 and 2001.
An additional 4 second multi-channel seismic data set was collected in 2001 in
conjunction with the single channel data. These studies revealed a series of
lowstand delta deposits in the North Basin located in water depths
as deep as 500 m.
In February 2005, the Lake Malawi
Scientific Drilling Project recovered 623m of continuous drillcore from Lake Malawi. Toplap and downlap
surfaces from the lowstand deltas are correlated to the two drill site
locations, one in the North Basin, and one in the Central Basin, using lithologic and
physical properties logs from the recovered drillcores. An age model
constructed using paleomagnetic and radiometric techniques constrains the
timing of the drastic changes in lake level interpreted from the
seismic-drillcore correlation, and the cores provide a continuous record of
hydrologic, limnologic and climate change in the East African Rift System over
the past ~1.5 Ma.