TOWARDS A PALEO-METHANE FLUX PROXY: COMBINING CARBON AND HYDROGEN ISOTOPE RATIOS OF BIOMARKERS (Invited Presentation)

Methane from terrestrial peatlands strongly influences the atmospheric concentration of methane, but the relationship between peatlands and atmospheric methane is poorly constrained. While the total concentration of atmospheric methane has been estimated from ice cores, there is currently no method of reconstructing past methane flux from peatlands or any other environment. We have developed a technique for reconstructing this flux using δ¹³C and δD of sedimentary biomarkers. Methanotrophic bacteria, in symbiosis with Sphagnum, oxidize methane produced at depth, contributing ¹³C-depleted CO₂ to the carbon pool used by Sphagnum. Thus, Sphagnum biomarkers are depleted of ¹³C relative to typical C3 vegetation. While methane flux is related to the amount of methane-derived CO₂ used by Sphagnum, the δ¹³C of Sphagnum biomarkers is also influenced by the thickness of water film around its photosynthetic cells. A thicker water film (higher water content) acts as a barrier to diffusion, causing the Sphagnum to be less selective of the light isotope. To disentangle the competing effects of methane and water film thickness, we constrain Sphagnum water content using δD of Sphagnum and vascular plant biomarkers. The contrast in δD between Sphagnum and vascular plant biomarkers can be used to estimate evaporative water loss from Sphagnum, equivalent to Sphagnum water content. We also validate down-core estimates of methane flux from several different microhabitats using instrumental measurements made by the FluxNet Canada program.