LAKE MALAWI SEDIMENTS: THE MODERN RECORD

Our recent work has applied several proxies to examine climate history since 25 ka in Lake Malawi basin. In particular, the accumulation of biogenic silica and volcanic ash-derived material (rich in incompatible elements such as Nb), is proving to be a useful indicator of increased northerly winds, which appear to have been enhanced during “cold” episodes of the Late Glacial as well as the Little Ice Age. At these times, we hypothesize that northerly winds resulting from southerward ITCZ excursions increased wind-driven upwelling and brought volcanically-derived material to the lake from the Rungwe Volcanic field, the site of the only active volcanism in the basin.

In order to evaluate how these proxies relate to historic changes in the lake basin we initiated a high resolution (2.5-mm sampling intervals yielding ~1.5-year resolution) study of Malawi sediments deposited over the past 130 years. Measurements of Nb:Ti are unrelated to mean wind direction (based on NCEP reanalysis), but show a strong inverse relationship with rainfall (NCEP) within the basin. On this timescale, it appears that there is a relatively constant flux of weathered volcanic ash to the system that becomes more diluted by riverine terrigenous aluminosilicates during wetter periods. The observed range in Nb:Ti is far smaller than that of the LIA or the Late Glacial. Another signal recorded in these recent sediments is a significant increase in Na:Ti around 1940. We attribute this to changes in land use in East Africa, perhaps in association with development of agriculture during the British Colonial Period. Deforestation and tillage would have exposed less altered material (with higher Na:Ti than surface soils) from deeper within soil sequences to be eroded and brought into the lake.