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OF 06–1053: Abstract

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Characterization of Organic Matter in Lake Sediments from Minnesota and Yellowstone National Park

By Walter E. Dean

Samples of sediment from lakes in Minnesota and Yellowstone National Park (YNP) were analyzed for organic carbon (OC), hydrogen richness by Rock-Eval pyrolysis, and stable carbon- and nitrogen-isotope composition of bulk organic matter. Values of δ13C of lake plankton tend to be around –28 to –32 parts per thousand (‰). Organic matter with values of δ13C in the high negative 20s overlap with those of organic matter derived from C3 higher terrestrial plants but are at least 10‰ more depleted in 13C than organic matter derived from C4 terrestrial plants. If the organic matter is produced mainly by photosynthetic plankton and is not oxidized in the water column, there may be a negative correlation between H-richness (Rock-Eval pyrolysis H-index) and δ13C, with more H-rich, algal organic matter having lower values of δ13C. However, if aquatic organic matter is oxidized in the water column, or if the organic matter is a mixture of terrestrial and aquatic organic matter, then there may be no correlation between H-richness and carbon-isotopic composition. Values of δ13C lower than about –28‰ probably indicate a contribution of bacterial biomass produced in the hypolimnion by chemoautotrophy or methanotrophy. In highly eutrophic lakes in which large amounts of 13C-depleted organic matter is continually removed from the epilimnion by photosynthesis throughout the growing season, the entire carbon reservoir in the epilimnion may become severely 13C-enriched so that 13C-enriched photosynthetic organic matter may overprint 13C-depleted chemosynthetic bacterial organic matter produced in the hypolimnon.

Most processes involved with the nitrogen cycle in lakes, such as production of ammonia and nitrate, tend to produce 15N-enriched values of δ15N. Most Minnesota lake sediments are 15N-enriched. However, some of the more OC-rich sediments have δ15N values close to zero (δ15N of air), suggesting that organic matter production is by nitrogen fixation, which further implies that nitrogen is limiting. Most lakes from YNP also have values of δ15N near zero.

 

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