Organic geochemical records from Lake Malawi (East Africa) of the last 700years, part II: Biomarker evidence for recent changes in primary productivity

摘要

Relatively few well-dated and high-resolution paleoclimate records of the past few centuries presently exist from tropical East Africa. Here, we examine the bulk and molecular geochemical records of two varved sediment cores from Lake Malawi, which together provide a continuous record of environmental variability in East Africa of the last 730years. We observe a number of changes in the aquatic ecosystem of Lake Malawi, which are likely attributed to both natural climatic forcing and anthropogenic activities. Biomarkers of dinoflagellates (dinosterol) and bacterivorous ciliates (tetrahymanol) display increased accumulation rates from ~1900AD to the present, while a simultaneous decrease in accumulation rates of diatom biomarkers (isololiolide/loliolide) is observed. Increased accumulation rates of retene, a compound derived from conifers, are also noted since ~1930AD and likely reflect increased soil erosion due to deforestation of the Lake Malawi watershed. Spectral analysis of the high-resolution TOC record indicates a periodicity of 204years, similar to the 206year cycle noted in 14C and 10Be records, suggesting a link between East African climate and solar forcing.