Halocarbons produced by natural oxidation processes during degradation of organic matter

Volatile halogenated organic compounds (VHOC) play an important role in atmospheric chemical processes—contributing, for example, to stratospheric ozone depletion1,2,3,4. For anthropogenic VHOC whose sources are well known5, the global atmospheric input can be estimated from industrial production data. Halogenated compounds of natural origin can also contribute significantly to the levels of VHOC in the atmosphere6. The oceans have been implicated as one of the main natural sources7,8,9,10, where organisms such as macroalgae and microalgae can release large quantities of VHOC to the atmosphere11,12. Some terrestrial sources have also been identified, such as wood-rotting fungi13, biomass burning14 and volcanic emissions15. Here we report the identification of a different terrestrial source of naturally occurring VHOC. We find that, in soils and sediments, halide ions can be alkylated during the oxidation of organic matter by an electron acceptor such as Fe( III): sunlight or microbial mediation are not required for these reactions. When the available halide ion is chloride, the reaction products are CH3Cl, C2H5Cl, C3H7Cl and C4H9Cl. (The corresponding alkyl bromides or alkyl iodides are produced when bromide or iodide are present.) Such abiotic processes could make a significant contribution to the budget of the important atmospheric compounds CH3Cl, CH3Br and CH3I.