We use the TM2 three-dimensional atmospheric tracer model with a methane source-sink budget based on existing literature to simulate small spatial and temporal variations in the 13C/12C ratio of atmospheric methane. The results show that &dgr;13C varies markedly with wind direction everywhere outside the extratropical Southern Hemisphere (ETSH). Within the ETSH, both methane mixing ratio and &dgr;13C have regular seasonal cycles with differing and latitude-dependent phases. Phase diagrams constructed from these seasonal cycles, showing changes in &dgr;13C versus changes in mixing ratio, have elliptical shapes. The slope of the major axis of these ellipses is determined by the kinetic isotope effect (KIE) of the single atmospheric methane removal process used in the model. The ellipse eccentricity is determined by seasonal variation in the source &dgr;13CH4, which is dominated by the biomass burning source because of its isotopic enrichment relative to other sources. Comparison of the model results, for a KIE based on CH4+OH oxidation, with observations in the South Pacific region shows significant discrepancies in both the ellipse major axis slopes and eccentricities. We suggest that this is an indicator of an additional sink process that discriminates strongly against 13CH4. Such a sink could be active chlorine in the marine boundary layer. ¿ 2001 American Geophysical Union