Isotopic Composition of Marine-Derived Methyl Bromide

Methyl bromide (CH₃Br) is the largest source of atmospheric bromine, which catalytically destroys stratospheric ozone 50-60 times more efficiently than chlorine. Despite much attention that has been focused on CH₃Br in recent years, it has proven difficult to constrain its global budget. It is of particular interest to compare the magnitude of the anthropogenic source with that of the largest naturally occurring source, which is believed to be marine-derived. One way to estimate the relative strengths of these sources is to use stable carbon isotopes as a tracer. The viability of this approach relies on the distinctiveness of the isotopic composition of each source and, in the case of marine-derived CH₃Br, on the extent to which isotopic fracionation occurs during chemical degradation in the water column. Once produced in the water column, CH3Br undergoes ion exchange (CH₃Br + Cl- \rightarrow CH₃Cl +Br-) and hydrolysis (CH₃Br + H2O \rightarrow CH₃OH +HBr), which changes the isotopic composition of the remaining CH₃Br. In this study we measured the fractionation factor (k12/k13) for both ion exchange and hydrolysis of CH₃Br in sea water, and then used a simple air-sea exchange model to determine the extent of fractionation that occurs in the water column. Using a range of 20-30 per mil for the isotopic composition of marine biologically produced methyl bromide, the predicted isotopic composition of CH₃Br emitted to the atmosphere is significantly enriched and further distinguished from the average anthropogenic signature of -54.4 per mil. An isotopic study may therefore prove to be a useful tool for constraining the methyl bromide budget.