Aquatic Sciences Meeting, Albuquerque 2001

SS03 Chemosynthetic Processes at Oxic-Anoxic Interfaces (Environmental Connections)

Date: Tuesday, February 13, 2001, Time: 4:30:00 PM

Location: Mesilla

Lehmann, M, F, Geological Institute ETH, Zürich, Switzerland, lehmann@erdw.ethz.ch

Bernasconi, S, M, Geological Institute ETH, Zürich, Switzerland, stefano@erdw.ethz.ch

Warthmann, R, J, Geological Institute ETH, Zürich, Switzerland, warthmann@erdw.ethz.ch

McKenzie, J, A, Geological Institute ETH, Zürich, Switzerland, sediment@erdw.ethz.ch

Hypolimnetic methane oxidation: Constraints from natural abundance level measurements of carbon and nitrogen stable isotope ratios

The carbon and nitrogen isotope compositions of organic matter have widely been used to trace the source of organic material to the water column and finally to the sedimentary archive. Large amounts of methane are produced in the anoxic sediments of eutrophic Lake Lugano (Switzerland). The methane diffuses up into the water column where it can serve as an energy source for methylotrophic bacteria. Bottom water seston shows very negative carbon isotope compositions (-60 permil), indicating a large contribution from chemosynthesis from 13C-depleteted methane. Information from culture experiments, in combination with in-situ isotopic data, demonstrates that the methylotrophs are aerobes that preferentially use ammonium over nitrate as their nitrogen source. The apparent fractionation for bacterial ammonium metabolism is approximately 13 permil. Both the carbon and nitrogen stable isotopic ratios of seston allow us to trace the oxic-anoxic interface in the water column that rises during periods of stagnation. Bulk isotopic measurements of organic matter in the sediment do not, however, permit the recognition of these unique isotope signals in the sedimentary record.

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Aquatic Sciences Meeting, Albuquerque 2001

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