C-13 NATURAL-ABUNDANCE VARIATIONS IN CARBONATES AND ORGANIC-CARBON FROM BOREAL FOREST WETLANDS

C-13 natural abundance variations were measured in peat soil and vegetation from two contrasting boreal forest wetlands: an upland watershed basin and a permanently saturated lowland mire. Evidence of methane oxidation was shown in the permanently saturated wetland with deltaC-13 values as low as -97 parts per thousand in carbonate minerals found in floating peat mats. It is postulated that C-13 depleted CH4 is oxidized in the,mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Methane flux measurements during the summer of 1992 showed much lower fluxes in areas with floating peat mats relative to open water. Secondary carbonates in the basin peat have isotope compositions close to the deltaC-13 values of the peat organic carbon (25 parts per thousand), indicating their origin from fermentation and possibly from sulfate-reduction. In the upland basin peat deposits, the deltaC-13(PDB) Values of organic C were constant with depth, while the permanently saturated mire had localities of C-13 enrichment in deeper layers of the peat. The C-13 enrichment may reflect areas of intense CH4 production in which C-13 enriched residual substrate is left behind during the production of highly C-13 depleted CH4.