Department of Earth System Science

Methyl chloride (CH₃Cl) is the most abundant halocarbon in the atmosphere, and constitutes a significant fraction of the total atmospheric halogen burden. Chemical reactions of CH₃Cl in seawater are slow, and it has been believed that the oceans are not an important sink for this compound. However, direct measurements of CH₃Cl degradation rates in coastal seawater (Bedford Basin, Nova Scotia), using a stable isotope incubation technique, indicate rapid loss attributed to microbial activity. A series of weekly measurements from March 2000 to May 2001 yielded degradation rates ranging from 0–30% d⁻¹, with an annual mean of 7.4% d⁻¹. If biological uptake of CH₃Cl occurs throughout the oceans at similar rates, the mean partial atmospheric lifetime of CH₃Cl with respect to oceanic removal could be a few years, rather than several decades as previously thought. This rapid removal would make the oceans a major sink for CH₃Cl and lower the overall atmospheric lifetime of CH₃Cl from the current estimate of 1.3 to about 1.0 years. Measurements of the degradation rate of CH₃Cl in open ocean waters are needed in order to quantify the oceanic uptake rate.