A 1,200,000 YEAR PALEOECOLOGICAL RECORD FROM OSTRACODES, CHARCOAL, AND OTHER PALEOCLIMATE INDICATORS FROM LAKE MALAWI, EAST AFRICA

Drill core records from Lake Malawi provide a long and detailed record of climate and limnologic change for the African southern subtropics. Core MAL05-1B (380m-long) was drilled at ~11°S, 34.5°E in central Lake Malawi in 592m water depth. The most recent dates suggest a basal age of ~1.2 Ma, making Core MAL05-1B the longest continuous lacustrine record of East African paleoclimate. Throughout the length of the core, we collected one cm-thick sub-samples every 16cm (intra-sample time average of ~32 years, time step between samples of ~505 years) for coarse residue analyses of ostracodes, other invertebrates, charcoal and mineralogical/sedimentological indicators of lake level and redox. For ostracodes we collected taxonomic data at the generic level, as well as ostracode abundance and taphonomic condition.

The presence of un-transported ostracodes in Core 1B indicates times when the lake bottom was oxygenated, and thus significantly shallower than at present. The assemblage data for the section of core analyzed thus far (231.12 – 381.194 mblf) suggest a transition from a relatively diverse saline/alkaline, littoral assemblage toward the base, to a more Cypridopsis-rich assemblage above 272m (~870 ka), indicative of deeper water conditions. These conclusions are supported by other indicators in the core record: from ~338 –347 mblf (~1.08 – 1.11 Ma) there are abundant littoral indicators such as: ooids, rounded and sorted quartz sand, framboidal pyrite and authigenic siderite (where co-occurring, indicative of anoxic marsh conditions), and the ostracode genera Limnocythere and Ilyocypris; all indicating an extremely shallow lake environment (and in the case of Ilyocypris probably indicative of nearby deltaic inputs). This interpretation is supported by the pollen data over the same interval, suggesting an arid, vegetation-poor watershed at this time.

Upon completion of the ostracode assemblage analysis of the entire 1B core, we intend to quantify water chemistry, productivity, and water depth variation through the entire record using a training set from modern Lake Malawi surface sediments. Using this information we hope to generate a quantitative interpretation of local hydrological change in the Lake Malawi basin, as well as generate a ~1,200,000 year record of tropical East African paleoclimate.