Archives for the Month of March 2006 on Ramblings of a Geologist
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.
Stories out of Mud
I think the big shifts in my sediment core data are recording a climate signal.
Between 1100 - 600 cm depth (roughly 3,000 years ago) there is a large increase in magnetic susceptibility, total inorganic carbon, % fine-grained material, and delta 13C (13 Carbon isotope). When water temperatures increase, CaCO3 becomes less soluble (in other words, it likes to be in colder water, and starts to "rain" out when temperatures increase). I think that air temperatures increased around 3k years ago, causing CaCO3 to precipitate out of the lake water -- an event called a "whiting." Sediments from whitings tend to be very fine-grained (consistent with my grainsize data) and can contain iron (in the form of siderite, an iron-carbonate).
Previous work supports this idea (which doesn't necessarily mean it's right, but is encouraging). Mudroch (1982) found that fine-grained, carbonate-rich sediments in a core from Lake Erie's central basin contained mollusc species that lived in much warmer waters (15-23 degrees C) than those found in coarser, low-carbonate sections (4-12 degrees C).
Graham and Rea (1980) also saw a sudden increase in fine-grained carbonate-rich sediments at roughly the same time, indicating that this is a regional rather than a basin-specific signal/event (points to climate).
I still have a lot of questions. Why does our % water decrease and then increase again with depth? Where does the Fe come from -- is there really siderite in our sediment? Why are there sections of the core with mostly whole, articulated mollusc shells, and other sections with shell hash? Does it fit the story? Why are my radiocarbon dates out of order stratigraphically? Turbidity currents/gravity flows?
Lake research in Tanzania
This summer I will be doing tropical lake research on Lake Tanganyika, Tanzania. I'll be working with other students and faculty from the US and from the University of Dar es Salaam, which is located in the capital of Tanzania. The Kigoma field station will be used by people in our research group to study things like climate change, human impact on nutrients and erosion, evolution of lacustrine organisms, and several other topics.
My research project will use lake sediments to get a high-resolution record of recent climate change and environmental impacts. Specifically, I'll be studying controls on organic matter distribution and accumulation in different areas over time, and will also evaluate organic matter's relationship to grainsize and lake-bottom morphology.
To do this, I'll be measuring organic and inorganic matter abundances in cores taken in a transect roughly parallel to shore, in water about 100 m deep. I'll analyze total organic carbon (TOC) and total inorganic carbon (TIC), do smear slide descriptions, and perform laser particle size analysis, probably using a Spectrex Laser Particle counter. TOC will probably be determined using the loss-on-ignition (LOI) method, which can be useful, but can also give inflated organic matter concentrations if there is a lot of volatile non-carbon matter in the sediment.
We'll use a multicorer (which grabs sediment samples down to about 60 cm depth) and a gravity corer, which recovers sediment to about 2 m depth. I haven't used a multicorer or a gravity corer before, but the principles are simple: drop a heavy tube over the side of a boat, let it hit the bottom and sink into the mud, then pull it back up. Will have to do some more reading on specifics.