Lake Malawi contains a long continuous sedimentary record of climate change in the southern hemisphere African tropics. We develop a stratigraphic framework of this basin over the last ~. 150 ka by integrating several vintages of seismic-reflection data with recently acquired drill cores. In the seismic-reflection data set, we document three lake-level cycles where progradational delta seismic facies and erosional truncation surfaces mark the basal boundary of each sequence. The clinoform packages and their down-dip, time-equivalent surfaces can be mapped throughout each basin, where each major lowstand surface was followed by a transgression and highstand. On several occasions, lake level dropped as much as 500 m below present lake level (BPLL) in the North Basin and 550 m BPLL in the Central Basin, resulting in a 97% reduction of water volume and 89% reduction of water surface area relative to modern conditions. Evidence for these lake-level fluctuations in the drill cores includes major changes in saturated bulk density, natural gamma ray values, and total organic carbon. During lowstands, density values doubled, while total organic carbon values dropped from ~. 5% to 0.2%. Coarse-grained sediment and organic matter flux into the basin were higher during transgressions, when precipitation, runoff, sediment supply, and nutrient input were high. This sedimentation pattern is also observed in seismic-reflection profiles, where coarse-grained seismic facies occur at the bases of sequences, and in the drill-core data where the highest total organic carbon values are observed immediately above lowstand surfaces.
Bibliographical noteFunding Information:
General contracting and barge modifications during drilling operations were carried out by the University of Rhode Island and Lengeek Vessel Engineering, Inc. We thank ADPS Ltd. and Seacore Ltd. for marine operations and drilling support, respectively, on the Viphya drilling vessel. DOSECC, Inc., the Geological Survey of Malawi, Malawi Department of Surveys, and Malawi Lake Services provided technical and logistical support. Drill-core processing and analysis were performed at the National Lake Core Repository at the University of Minnesota. Support for seismic data acquisition was provided by the Lacustrine Rift Basin Industrial Associates of Syracuse University. Landmark Graphics Corporation provided seismic data processing and interpretation software, P. K. Cattaneo provided assistance with seismic-reflection data acquisition and processing, and R. Latimer assisted with synthetic seismogram generation. Funding for scientific drilling on Lake Malawi was provided by the U.S. National Science Foundation and by the International Continental Scientific Drilling Program. Scientific analysis of drill-core samples has been supported by the U.S. National Science Foundation. We thank the government of Malawi for permission to conduct this research. S. Colman, an anonymous reviewer, journal editor F. Kop, and M. McGlue provided helpful reviews and comments that greatly improved the manuscript.
Copyright 2011 Elsevier B.V., All rights reserved.
- East Africa
- Lake Malawi
- Scientific drilling
- Seismic stratigraphy
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