Stable isotope ratios atmospheric CH4: Implications for seasonal sources and sinks

Abstract

We report δD and δ13C measurements of atmospheric CH4from 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 CH4sources and sinks in background air, record long-term trends in CH4mixing and isotope ratio related to the atmospheric CH4loading, and may indicate regional CH4sources. At Niwot Ridge, δD-CH4averaged -93.1 ± 3.0‰ from 1999 to 2001, while δ13C-CH4averaged -47.22 ± 0.13‰ from 1995 to 2001 with distinct seasonal cycles in both isotope ratios. At Montaña de Oro, atmospheric CH4was observed to be more depleted in13C and D: Measured δD-CH4averaged -97.3 ± 3.7‰ from 2000 to 2001, while δ13C-CH4averaged -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, δ13C-CH4was found to correlate poorly with mixing ratio, an indication that varying CH4sources are partly responsible for the δ13C-CH4seasonal signal. In contrast, a strong anticorrelation exists between δD-CH4and mixing ratio, with maxima and minima approximately 6 months out of phase, indicating a sensitivity of δD to sink processes. The dual isotopic constraint to atmospheric CH4seasonality implies that these midlatitude sites are annually influenced by a13C-enriched CH4source(s) seasonally increasing in late spring and a13C-depleted CH4source(s) seasonally increasing in late summer or early fall. Copyright 2007 by the American Geophysical Union.