IDENTIFICATION OF FRESHWATER-SALINE LAKE CYCLES WITH MAJOR ELEMENT OXIDE GEOCHEMISTRY, PLIO-PLEISTOCENE OLDUVAI GORGE, TANZANIA

Paleoanthropological discoveries at Olduvai Gorge and the possible correlations between African climate change and biological evolution have created the demand for high resolution paleoclimate records. Clay minerals, like those present in the lacustrine sediments at Paleolake Olduvai, are sensitive to both weathering in the drainage basin and the chemical composition of the lake water. These clay-rich sediments act as chemical sinks for a variety of lacustrine ions and can provide geochemical records of earth history. Due to hydrological closure, and thus likelihood to be highly responsive to climate change, Paleolake Olduvai is an ideal location in which to examine the East African paleoclimate.

The Olduvai Basin lies on the western margin of the eastern branch of the East African Rift in northern Tanzania. It falls between the Pliocene-Pleistocene Ngorongoro Volcanic Highland and the Serengeti Plain. The paleolake episodically flooded up to 15km across a broad lacustrine plain. The paleolake water was shallow, saline and alkaline. The study area, Locality 80, is in the deepest portion of Paleolake Olduvai west of the Fifth Fault in the central basin.

Studies have shown that Bed I and Lower Bed II of Paleolake Olduvai at Locality 80 are primarily composed of the authigenic lacustrine clay minerals illite, smectite and interlayered illite-smectite. A freshly excavated 40m section (1.92Ma- 1.76Ma) was logged and sampled in 1999. X-ray fluorescence analysis of 167 sediment samples and the sedimentation rates of Hay and Kyser (2001) were used to identify four apparent lake cycles beginning and ending with saline alkaline phases. Peaks in Al₂O₃/MgO ratios (the ratio of detrital to authigenic clay minerals), TiO₂ (detrital input) and P₂O₅ (paleo-productivity) abundances occur at approximately the same elevations within the stratigraphic section. Low values within these three parameters indicate saline alkaline conditions whereas high values represent fresh water conditions. Lake Cycles 1 and 4 completed in approximately 44,000 years and 42,000 years respectively, which is similar to the 41k.y. year cycle associated with Earth’s obliquity. Lake Cycles 2 and 3 span approximately 24,000 years and are similar to the 21k.y. precession cycle. Climate shifts had significant impact on flora and fauna at Paleolake Olduvai.