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Title: SOURCE APPORTIONMENT OF LAKE BED SEDIMENTS TO THEIR WATERSHEDS: A NEW METHODOLOGY

Author
item Kelley, David
item NATER, EDWARD - UNIVERSITY OF MINNESOTA

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Lake bottom sediments offer researchers the means to reconstruct changes in vegetation, climate, and landscapes over time for broad areas of land. Because sediments in certain lakes are one of the few things on the Earth's surface that are stable and unchanged over thousands of years, they can provide valuable information about how the land has changed with time. Minerals, plant pollen, spores, wood fragments, shells, etc., are all preserved to different degrees when buried in successive layers of mud in ponds, lakes, and marshes. Many techniques have been developed over the years to enable scientists to determine not only how old something buried in the mud is, but to identify where the item might have come from and how it got there. Additionally, based on the item's physical and chemical properties, scientists can reconstruct the conditions under which the item might have developed or grown. This paper describes a new technique that allows minerals found in lake sediments at different depths to be traced back to their different origins on the landscape. It also describes methods to determine how these original sources changed over time. Used properly, this technology should provide scientists with another tool to study the Earth's changing surface.

Technical Abstract: This contribution presents a methodology for the reliable apportionment of sediments to their watershed sources. Sediment samples were collected from three known sources of sediment load to a depositional basin. The sediments were subjected to separation techniques to isolate their heavy silt fractions, which were analyzed for their elemental signatures via ICP- -MS. By comparing the elemental composition of heavy minerals from each river system, particularly rock trace elements and rare earth elements, signature profiles were established for each source watershed. To test the veracity of using these profiles as source fingerprints, known proportions of heavy minerals from each source were blended and the composite samples analyzed for their elemental composition. Actual composite sample proportions were compared to their apportionments calculated statistically using the USEPA Chemical Mass Balance (CMB) air quality receptor model. Actual vs. calculated results showed that the technique could apportion sediments to their watershed sources reliably (within 7%). The technique was then applied to the dated sediment record of an actual basin, Lake Pepin, to apportion its sediments to their contributing watersheds. Results indicate that the vast majority of sediments deposited in the basin originated historically and recently in the Minnesota River watershed, and that the sediment load changed over time as anthropogenic impacts in the watershed increased. This fingerprinting technique should be applicable to similar investigations where the determination of sediment provenance and identification of relative changes in source apportionment over time are desired.