Why should we care about lake mud?
Some thoughts on my Lake Tanganyika research project:
The sediment deposited in Lake Tanganyika’s basin over the past several million years is largely composed of organic matter (Hartwell et al, 2002). Decaying algal remains, zooplankton excretions, and land-derived material all contribute to organic matter deposited in lakes, but the primary source is often phytoplankton biomass (Wetzel, 1975). Several physical and climatic factors influence the production, transport, deposition, and preservation of this organic matter, including: the degree of lake stratification, water temperature, oxygen levels, nutrient supply, and seasonal wind patterns. Understanding and quantifying these various influences in recent Lake Tanganyika sediment is important to paleoclimate studies, which attempt to reconstruct past environmental conditions based on the type and amount of organic matter present in ancient sediment. If we can better understand the factors determining sediment composition today, then we will be better able to understand regional and global climate changes recorded in the lake’s ~10 million year record.
Previous work has found that organic matter is a good paleo-indicator of depth, because it decreases with depth and has an almost inverse relationship to the amount of dissolved oxygen in the water (Jiminez, 2005). It is soluble in oxygenated water, so will tend to dissolve in shallow waters and will accumulate at higher rates as depth increases and oxygen levels decrease. Its presence or absence could help reconstruct paleo-oxycline levels.
However, the use of this proxy can be difficult because lakebed morphology also has a strong influence on organic matter abundances. In some places in Lake Tanganyika, terrestrial input is greater on sloped areas, which dilutes the organic matter (Hartwell et al, 2002). A quantitative understanding of slope on this proxy would be very useful.
Another difficulty with using organic matter as a paleoclimate proxy is that it also depends on seasonal winds and lake mixing depths. Oxygen levels do not decrease uniformly throughout the lake, but vary seasonally and with location. During the dry season, winds force surface currents northward; one model predicts that this will cause upwelling and high productivity in the southern part of the lake, which would lead to greater organic matter deposition. However, mixing reaches deeper in the southern lake and is predicted to cause high levels of organic matter dissolution (Tsuchida et al, 2002). Thus, factors such as seasonal winds and varying oxygen levels will also have to be taken into account when trying to understand the balance between productivity and preservation.
As climate is emerging as a major concern for the global community, studies such as this which enable high-resolution paleoclimate reconstruction are crucial. Lake Tanganyika contains an excellent record of past environmental changes, and could provide important information about the mechanisms of change on both regional and global scales.