Biogenic Silica Profiles in the Sediments of Large Tropical Lakes: Examples from East Africa
Profiles of the abundance or mass accumulation rate of biogenic silica in sediment cores can provide important insight into the past productivity of diatoms and the climatic influence on a lake basin through time. While primary production of diatoms in the overlying waters can influence the burial rate of diatoms, other factors need to be considered as well. The geochemical mass balance of dissolved silica in the lake system is one such factor that plays an important role in controlling the rate at which diatoms can be buried in the lake sediments. Examples are provided from Lake Malawi and Lake Victoria, East Africa, that demonstrate how the rates of input and outflow of dissolved silica are related to biogenicsilica profiles in the sediments, and how conclusions were drawn on past environmental conditions in these large tropical lakes.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.