HR: 0800h
AN: B51C-0222    [Abstracts]
TI: Determination of in Situ Rates of Methane Production and Oxidation From Terrestrial Wetlands
AU: * Shoemaker, J K
EM: jshoemak@fas.harvard.edu
AF: Harvard University Organismic and Evolutionary Biology, 305 Hoffman 20 Oxford St, Cambridge, MA 02138 United States
AU: Schrag, D P
EM: schrag@eps.harvard.edu
AF: Harvard University Earth and Planetary Sciences, 20 Oxford St, Cambridge, MA 02138 United States
AB: Wetlands are responsible for over 70% of non-anthropogenic methane emissions. We present a method, using the δ¹³C of CO₂ in pore water, to obtain the in situ rates of methanogenesis occurring beneath the wetland surface. This method allows us to distinguish methanogenesis from methane oxidation during escape, both of which contribute to the net methane flux. The δ¹³C of CO₂(aq) - the dominant form of DIC in acidic natural waters - reflects the processes occurring at that location modified by transport of gas from surrounding depths. Methane production and oxidation are imprinted in the δ¹³C signature of the aqueous CO₂ with heaviest values at depth resulting from the fractionation associated with methane production. We measured δ¹³C profiles with depth along with CO₂ and CH₄ concentrations from Sallie's Fen in Barrington, NH. Although the δ¹³C profiles varied considerably between locations and seasons, the logarithmic shape of the curves showed that methane production was restricted below a certain depth in the sediment - sometimes as shallow as 30 cm. Using a one-dimensional diffusion-reaction model, we are able to estimate rates of methane oxidation and successfully reproduce features present in the data's seasonal cycle. Features of the data not reproducible by the model indicate the importance of alternate gas transport routes such as ebullition and plant-mediated transport. The model also provides evidence for low-level oxygen availability during the winter-spring transition and narrow zones of very high productivity at depths of 60-70cm during the winter. We suggest that this method provides insight directly into the processes that determine methane fluxes from natural wetlands and has great potential for improving our understanding of the biogeochemistry of these systems.
DE: 1890 Wetlands (0497)
DE: 4806 Carbon cycling (0428)
SC: Biogeosciences [B]
MN: Fall Meeting 2005