NEOGENE THROUGH QUATERNARY CLIMATE VARIABILITY IN LAKE ALBERT, EAST AFRICAN RIFT, FROM XRF ANALYSIS OF EXPLORATION WELL CUTTINGS

Lake Albert is the northern most rift lake in the western branch of the East African Rift System. It forms the headwaters of the White Nile River and is one of several large lakes in the rift valley. Because the lake is a key source of the White Nile River, fluctuations in its lake level have affected local flora and fauna along the White Nile River valley. Determining the timing and extent of these fluctuations can increase our understanding of past African climate, as well as provide information on the environmental background of early hominins.

Long term changes in African climate are thought to be driven by events such as the closing of the Indonesian seaway and/or uplift of central Africa through the late Cenozoic. These events and shifts in climate are likely recorded in the sediments of Lake Albert. The well cuttings in this study, taken from exploration wells on the southeastern shore of Lake Albert, are possibly the first continuous record of the Neogene through Quaternary climate variability in continental equatorial Africa.

Samples have been analyzed using an ITRAX scanning XRF instrument at the Large Lakes Observatory, University of Minnesota Duluth. Changes in elemental abundances through time have provided information on sediment chemistry and past environmental conditions. For example, changes in Ca/Fe, indicates changes in the abundance of lacustrine carbonate, perhaps indicative of past lake level conditions. Preliminary results also show consistent trends in elemental abundances in two separate exploration wells, suggesting systematic basin-wide changes in lake chemistry. At intervals of relatively high Ca/Fe, a fraction of powdered sediment reacted with dilute HCl, supporting the results from XRF analysis. The scanning XRF results for one well have also been compared to discrete results from an ARL 8410 XRF and are found to be significant with an R² value of .8-.9.

The variability seen in Ca/Fe also tracks with Si/Ti ratios (used as an indicator of biogenic silica) and K/Al ratios (an indicator of certain clay minerals formed by the weathering of feldspars) at certain intervals. These trends in elemental abundance, combined, can be used to reveal the past chemistry of the lake.