• European Marine Science
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• Journal of Geophysical Research Atm... close

Methyl iodide has been measured during two field campaigns in the Atlantic region in 2002. The first took place in July–August at 2300 m on the island of Tenerife, while the second was a shipborne, east–west crossing of the Tropical Atlantic at 10°N in October–November of the same year. Both campaigns were periodically impacted by dust, advected from Africa in trade winds. Unexpectedly, during these dust events, methyl iodide mixing ratios were observed to be high relative to other times. Backward calculations with the particle dispersion model FLEXPART show the origin of the dust storms as Mauritania and southern Algeria for the ground‐ and ship‐based campaigns, respectively. The dust‐laden air traveled from its source above the marine boundary layer to the measurement region. On the basis of the field data correlations and the simulations, we suggest that dust‐stimulated emission of methyl iodide has occurred. To test this hypothesis, dust samples were collected from the identified source regions and added to filtered Atlantic seawater. This rapidly produced methyl iodide. Further tests established that the addition of iodide increased the yield and that iodide with H2O2 was greater still. This was found for both sterilized and nonsterilized samples. We conclude that there is an abiotic methyl iodide production mechanism that can occur via substitution, analogous to those in soil, rather than radical recombination. This may occur when dust contacts seawater containing iodide or when marine water vapor condenses on dust containing iodide. This hypothesis appears to be consistent with recent long‐term methyl iodide data sets from Tasmania and may help resolve current uncertainties in the iodine cycle.

Although often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation. International audience