Smith, M. A. Northwestern University-Department of Geological Sciences, email@example.com
Hollander, D. A. Northwestern University-Department of Geological Sciences, firstname.lastname@example.org
ANAEROBIC PROCESSES AS A POTENTIAL CONTROL ON CARBON-ISOTOPE CYCLING IN SEVERELY EUTROPHIC LAKES: A CASE STUDY OF LAKE MENDOTA (WISCONSIN, USA)
Sedimentary and water-column geochemical studies are used to evaluate the influence of anaerobic processes on the carbon-isotope cycle in severely eutrophic Lake Mendota. Studies of other lakes demonstrate that organic-carbon accumulation rates increase, and that sedimentary organic and inorganic carbon becomes enriched in 13C, as eutrophication intensifies. In Lake Mendota organic-carbon accumulation rates display a general correspondence with historic water-column nutrient levels, but there is a long-term trend towards 13C-depletion in sedimentary organic and inorganic carbon with progressive eutrophication. Bacterial biomarkers (e.g. hopanols and FAMEs) display extreme 13C-depletion diagnostic of chemoautotrophy and methanotrophy, and increased accumulation rates during periods of enhanced cultural eutrophication. Furthermore, isotope mass-balance calculations indicate that FAMEs deposited during summer predominantly originate from methanotrophic biomass. Cumulatively, the biomarker evidence indicates that the contribution of 13C-depleted organic carbon increases as seasonal anoxia and long-term eutrophication intensify. Therefore, anaerobic carbon-cycling processes (methylotrophy, chemoautotrophy, fermentation and methane-oxidation), while not the dominant source of organic carbon to the sediments, have contributed sufficient amounts of 13C-depleted biomass to influence the isotopic composition of bulk sedimentary organic carbon in Lake Mendota.
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