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.

We conducted a detailed investigation of the evolution of methyl bromide concentrations, degradation rates, and ventilation rates for 26 days in a naturally contained, warm-core eddy of the North Atlantic Ocean. This is the first study of the oceanic cycling of methyl bromide in a natural, contained system with a complete suite of supporting measurements of physical and chemical variables. Methyl bromide concentrations in the mixed layer ranged from 2.3 to 4.2 nmol m-3, degradation rates ranged from 0.1 to 0.9 nmol m-3 d-1, net sea-to-air exchange rates ranged from 0 to 0.5 nmol m-3 d-1, and net loss rates through the thermocline were less than 0.1 nmol m-3 d-1. From a mass balance for methyl bromide in the mixed layer, we calculated production rates ranging from <0.1 to 1.3 nmol m-3 d-1. The median of this range, 0.48 nmol m-3 d-1, is higher than the ∼0.15 nmol m-3 d-1 necessary to maintain the reported global oceanic emission of 56 Gg yr-1. This is reasonable, because our study area was supersaturated in methyl bromide, whereas the ocean as a whole is undersaturated.

We conducted a detailed investigation of the evolution of methyl bromide concentrations, degradation rates, and ventilation rates for 26 days in a naturally contained, warm-core eddy of the North Atlantic Ocean. This is the first study of the oceanic cycling of methyl bromide in a natural, contained system with a complete suite of supporting measurements of physical and chemical variables. Methyl bromide concentrations in the mixed layer ranged from 2.3 to 4.2 nmol m⁻³, degradation rates ranged from 0.1 to 0.9 nmol m⁻³ d⁻¹, net sea-to-air exchange rates ranged from 0 to 0.5 nmol m⁻³ d⁻¹, and net loss rates through the thermocline were less than 0.1 nmol m⁻³ d⁻¹. From a mass balance for methyl bromide in the mixed layer, we calculated production rates ranging from <0.1 to 1.3 nmol m⁻³ d⁻¹. The median of this range, 0.48 nmol m⁻³ d⁻¹, is higher than the ∼0.15 nmol m⁻³ d⁻¹ necessary to maintain the reported global oceanic emission of 56 Gg yr⁻¹. This is reasonable, because our study area was supersaturated in methyl bromide, whereas the ocean as a whole is undersaturated.