HR: 14:10h
AN: PP13C-03    [Abstracts]
TI: Initial Results of Scientific Drilling on Lake Malawi, East African Rift
AU: * Scholz, C A
EM: cascholz@syr.edu
AF: Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuseq, NY 13244 United States
AU: Johnson, T C
EM: tcj@d.umn.edu
AF: Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, 10 University Drive, 204 RLB, Duluth, MN 55812 United States
AU: King, J
EM: jking@gso.uri.edu
AF: Graduate School of Oceanography, University of Rhode Island, Bay Campus, South Ferry Road, Narragansett, RI 02882 United States
AU: Cohen, A S
EM: acohen@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St., Tucson, AZ 85721 United States
AU: Lyons, R P
EM: rplyons@syr.edu
AF: Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuseq, NY 13244 United States
AU: Kalindekafe, L
EM: lkalindekafe@yahoo.com
AF: Geological Survey of Malawi, P.O. Box 27, Zomba, 0000 Malawi
AU: Forman, S L
EM: slf@uic.edu
AF: Dept. Earth. and Envir. Sci., Univ. of Ill at Chicago, Chicago, 845 W. Taylor St., Chicago, IL 80607-7059 United States
AU: McHargue, L R
EM: mchargue@physics.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St., Tucson, AZ 85721 United States
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, Weeks Hall for Geological Sciences 1215 West Dayton Street, Madison, WI 53706 United States
AB: Lake Malawi is one of the largest and oldest lakes in the southern hemisphere and in the tropical latitudes, measuring more than 580 km in length, with a maximum depth of 700 m, and with an estimated age of more than 7 Ma. The lake water column is anoxic below 200 m, thus preserving an extensive record of laminated (varved) sediments. The lake's hydrologic budget is hypersensitive to minor changes in precipitation:evaporation, and signals of lake level and limnologic change respond markedly to regional climate variations. During February-March 2005 a scientific drilling program was conducted on the deep, open waters of Lake Malawi, using a converted local fuel barge outfitted with a deep-water-capable geotechnical drilling rig, a portable dynamic positioning system, and a suite of sampling tools designed for deep lake coring. A total of 623 m of core was recovered in seven holes at two sites on the lake. The high-resolution North Basin site (water depth 359 m) was triple-cored, providing redundant sampling of the sedimentary section representing the past ~75-100 kyr; the Central Basin deep site (592 m water depth) was double cored over the period of the past ~200 kyr, and single-cored to 380 m, covering perhaps 1.5 million years. The North Basin high-resolution site targeted a hemipelagic sequence, which in core samples consists of alternating zones of laminated and homogenous silty mud. At the base of these cores we observe well-sorted medium-grained shoreface sand deposits, that we interpret to be a transgressive surface deposited following a severe (500 m) lake lowstand. Density, magnetic susceptibility, TOC, and C/N profiles closely follow lithologic changes, and correspond to periods of enhanced or diminished productivity and dramatically varying lake level. Initial dating of the North Basin site (total core length 38 m) is underway using a combination of Radiocarbon, Tephra Ar-Ar, and Optically-Stimulated Luminescence methods, and initial results indicate a near-linear sedimentation rate in the uppermost sediments of 0.35-0.5 mm/yr. The deep Central Basin site also shows pronounced cyclical signals in lithostratigraphy and physical properties, and dating is underway on the deep 380 m-long core using OSL, paleomagnetics, and Be-10. Ostracode assemblages, along with other paleoenvironmental indicators, suggest highly variable lake conditions, with some intervals of considerably higher salinities and alkalinities, and much lower lake levels than present.
UR: http://malawidrilling.syr.edu
DE: 4215 Climate and interannual variability (1616, 1635, 3305, 3309, 4513)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4914 Continental climate records
DE: 4942 Limnology (0458, 1845, 4239)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005