T28 Core Analysis of Lake Sediments (Posters)

Sunday, 22 October 2006
8:00 AM - 12:00 PM

SCIENTIFIC DRILLING ON LAKE MALAWI: A LONG, HIGH-RESOLUTION RECORD OF CLIMATIC AND LIMNOLOGICAL CHANGE FROM SOUTHERN TROPICAL EAST AFRICA

Paper number: T28-17
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Lake Malawi is uniquely positioned to record a long and continuous record of southern hemisphere tropical climate change, and captures a record of climate conditions spanning 5 degrees of latitude. It 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's deep waters are anoxic and preserve 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 on a variety of time scales.

In 2005 the Lake Malawi Scientific Drilling Project collected a total of 623 m of core from seven holes at two sites on the lake. The deep Central Basin drill cores (592 m water depth) extended to 383 m below the lake floor, sampling more than ~1.5 million years of continuous lacustrine sedimentation. Sediment lithologies are highly variable and include finely laminated diatomaceous organic-rich mud, homogenous carbonate-rich mud, and near the base of cores at both sites, well-sorted fine-medium grained sand deposits, indicative of transgressive shoreface conditions.

High-resolution sampling and analyses of total organic carbon and d13C, sediment grain size, GRAPE density, magnetic susceptibility, and whole-core gamma measurements provide quantitative measures of lithology, that in conjunction with a regional stratigraphic framework developed from a dense suite of seismic reflection data, provide a high-precision record of lake level change over the duration of the sampled section. An age model has been developed for Site 1 that uses radiocarbon, luminescence, and Be-10 dates along with paleomagnetic excursion and reversal ages, to indicate a basal age for Site 1 of more than 1.5 Ma. Initial climate proxy measurements indicate that a series of high-amplitude, and high-frequency fluctuations in lake level, that resulted in lowstands of more than 500 m below the level of the modern lake during the past several hundred thousand years. This period of high climate variability ended about ~75 Ka with a long-term rise in water levels that culminated in the modern high-stand lake.