Biogenic silica deposition in Lake Malawi, East Africa over the past 150,000years
Abstract We report a 150,000 year record of the abundance of biogenic silica (BSi) in drill cores from the northern and central basins of Lake Malawi. The periods of highest diatom burial at both sites occurred around 65–69 and 51–60 thousand years ago (ka) after the termination of prolonged, intense drought in the region. These peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin. The droughts that preceded these BSi depositional episodes occurred at precessional frequency, corresponding to times of minimum austral spring (or summer?) insolation in the region prior to 60 ka. Subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity. However, biogenic silica records do not show significant sensitivity to precessional forcing. The BSi burial flux in the north basin displays strong millennial-scale variability since 50 ka, with peak values occurring during cold times in the Northern Hemisphere, as reflected in the Greenland ice core records. We observe no relationship between BSi and deposition of volcanic tephras in this system. The central basin core also displays millennial-scale variability in BSi abundance during the past 50 ky, of comparable magnitude to that in the north basin, but without systematic correlation to the north basin BSi or Greenland ice core records. In general, we conclude that BSi profiles from the north and (probably) south basins are more readily interpreted in terms of regional climate dynamics than are BSi profiles from more central locations in long, narrow tropical rift lakes. Diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low-latitude lacustrine systems.