Stable isotope ratios in atmospheric CH 4 : Implications for seasonal sources and sinks

We report δ D and δ 13 C measurements of atmospheric CH 4 from air samples collected from two locations in the United States. They are the mid continental site Niwot Ridge, Colorado (40°N, 105°W), and a Pacific coastal site receiving strong westerlies, Montaña de Oro, California (35°N, 121°W). Data from multiyear approximately bimonthly sampling provide information relating seasonal cycling of CH 4 sources and sinks in background air, record long‐term trends in CH 4 mixing and isotope ratio related to the atmospheric CH 4 loading, and may indicate regional CH 4 sources. At Niwot Ridge, δ D‐CH 4 averaged −93.1 ± 3.0‰ from 1999 to 2001, while δ 13 C‐CH 4 averaged −47.22 ± 0.13‰ from 1995 to 2001 with distinct seasonal cycles in both isotope ratios. At Montaña de Oro, atmospheric CH 4 was observed to be more depleted in 13 C and D: Measured δ D‐CH 4 averaged −97.3 ± 3.7‰ from 2000 to 2001, while δ 13 C‐CH 4 averaged −47.26 ± 0.17‰ from 1996 to 2001, and seasonal cycles were larger than those observed at Niwot Ridge. Mixing ratios observed at Montaña de Oro were higher on average than at Niwot Ridge. At both sites, δ 13 C‐CH 4 was found to correlate poorly with mixing ratio, an indication that varying CH 4 sources are partly responsible for the δ 13 C‐CH 4 seasonal signal. In contrast, a strong anticorrelation exists between δ D‐CH 4 and mixing ratio, with maxima and minima approximately 6 months out of phase, indicating a sensitivity of δ D to sink processes. The dual isotopic constraint to atmospheric CH 4 seasonality implies that these midlatitude sites are annually influenced by a 13 C‐enriched CH 4 source(s) seasonally increasing in late spring and a 13 C‐depleted CH 4 source(s) seasonally increasing in late summer or early fall.