Carbon isotope ratios for chloromethane of biological origin: Potential tool in determining biological emissions

Chloromethane (CH3Cl) with a global atmospheric burden of 5.3 million t is the most abundant halocarbon in the atmosphere. However, the origin of ca. 50% of the estimated annual global input of 4 million t of the gas to the a tmosphere has yet to be determined. As the oceanic contribution to the glob al CH3Cl flux is now tightly constrained, an important terrestrial source i s either underestimated or unrecognized. It has recently been proposed that higher plants may represent a CH3Cl source of sufficient magnitude to reso lve the global budget imbalance. A potentially useful tool in validating CH 3Cl emission flux estimates is comparison of the carbon isotope ratio of at mospheric CH3Cl with those of CH3Cl originating from various sources. Here we report the first measurements of delta C-13 for CH3Cl produced biologica lly. The CH3Cl released by the higher plant species Batis maritima and Sola num tuberosum was dramatically depleted in C-13 with respect to plant tissu e (Delta C-13 = -36.8 parts per thousand and -34.5 parts per thousand, resp ectively); CH3Cl released by the fungus Phellinus pomaceus also showed sign ificant C-13 depletion with respect to the wood growth substrate (Delta C-1 3 = -17.9 parts per thousand). When reliable delta C-13 values for the othe r major sources of atmospheric CH3Cl become available, the distinctive isot opic signature of plant-derived CH3Cl should help constrain the contributio n to the atmospheric burden from this source.