A ~1.25MA CONTINUOUS RECORD OF OSTRACODE PALEOECOLOGY FROM THE AFRICAN TROPICS AT LAKE MALAWI (Invited Presentation)

In 2005 the Lake Malawi Scientific Drilling Project collected a series of drill cores from Lake Malawi, the second largest and second deepest of the African rift valley lakes. Seven cores from two drill sites yielded over 600m of core. The longest of these cores penetrated ~381m below the lake floor, and provides a record of climate, ecosystem variability and paleohydrology for the southern African tropics extending back ~1.25Ma. The LMSDP team has used an interdisciplinary approach to accurately date these cores and infer paleoenvironmental changes through a combination of paleoecological, geochemical and sedimentological tools. A continuous and high resolution (~500 yr sampling resolution) interpretation of the fossil ostracode record from Lake Malawi has been an integral part of this effort.

The Malawian ostracode record is dominated by a few (Limnocythere, cypridopsine, Gomphocythere, Sclerocypris, and Ilyocypris) species, and is highly informative, based on both fauna and taphonomy. Drill Site 1, currently lying in 592m water depth, far below the oxygenated zone of the lake, episodically records a littoral benthic fauna (including ostracodes), illustrating the extraordinary and often rapid lake level fluctuations that have occurred in this lake. These changes are likely the result of eccentricity-forced precessional cyclicity. Cyclic changes alternate from zones devoid of ostracode fossils (when the drill site was below the oxicline), to a variety of ostracode-bearing states, including extremely rich monospecific intervals of cypridopsine juveniles (die off cohorts growing close to the oxicline),and monospecific Limnocythere or mixed Candonopsis/Limnocythere/cypridopsine assemblages, which accumulated in shallow water under alkaline/saline conditions. Based on all paleoecological evidence these changes signal shifts between a deep-water, relatively “blue-water” lake during periods of high precipitation (insolation maxima), to a shallow, saline and “green water” shallow lake during arid insolation minima. These changes are superimposed on longer-term tectono-geomorphic changes in the Malawi rift, including the elimination of influence of a major deltaic system near the drill site after ~700ka (disappearance of Ilyocypris).