HR: 0800h
AN: PP21A-1548    [Abstracts]
TI: Twentieth Century Ground-Truthing of Lake Malawi Climate Proxies
AU: * Brown, E T
EM: etbrown@d.umn.edu
AF: University of Minnesota Duluth, Large Lakes Observatory and Dept. of Geological Sciences, Duluth, MN 55812 United States
AB: Information on the climate history of the Lake Malawi basin has been derived from a number of proxy indicators. For example, enhanced accumulation of biogenic silica and of volcanic ash-derived material (rich in incompatible elements such as Nb), has been interpreted as an indicator of increased northerly winds. Over the past 25 kyr, such conditions appear to have been occurred during "cold" episodes of the Late Glacial as well as the Little Ice Age. At these times, it has been hypothesized that northerly winds resulting from southward ITCZ excursions led to increased wind-driven upwelling and enhanced delivery of volcanically-derived material to the lake from the Rungwe Volcanic field, the site of the only active volcanism in the basin. To evaluate the relationship of these proxies to historic changes in the lake basin we initiated a higher resolution (2.5-mm sampling intervals yielding ~1.5-year resolution) study of Malawi sediments deposited since the late Nineteenth Century. The observed range in Nb:Ti is small compared to that of the LIA or the Late Glacial. Since 1950 CE Nb:Ti appears unrelated to mean wind direction (based on NCEP reanalysis), but shows a significant inverse relationship with rainfall (NCEP) within the basin. There appears to be a relatively constant flux of weathered volcanic ash to the lakefloor that becomes diluted by river-borne terrigenous aluminosilicates during wetter periods. In addition, these sediments show a significant increase in Na:Ti around 1940 CE. This may be attributed to changing in land use in East Africa, as it may coincide with a time of increased deforestation and tillage during the British Colonial Period. Such agricultural practices would have mobilized less-altered material (with higher Na:Ti than surface soils) from deeper within soil sequences, which would subsequently have been eroded and brought into the lake.
DE: 1637 Regional climate change
DE: 1845 Limnology (0458, 4239, 4942)
DE: 1861 Sedimentation (4863)
DE: 9305 Africa
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005