Factors influencing methyl iodide production in the ocean and its flux to the atmosphere

• Main Author: Richter, Uwe
• Format: Thesis
• Language: English
• Published: 2004
• Subjects: North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/59369/; https://oceanrep.geomar.de/id/eprint/59369/1/d1024.pdf

Methyl iodide is an atmospheric trace gas and a major source of atmospheric iodine. Recent estimates of methyl iodide sources and sinks indicate that anthropogenic sources are neglectible. The major source of atmospheric methyl iodide are emissions from the ocean. The production pathways of methyl iodide in the ocean are, however, poorly understood. The known oceanic sources can only account for about 10 % of the ocean-atmosphere flux, whereas the sources for the remaining 90 % are not known. In this work a measuring system, consisting of an equilibrator connected to a 2-dimensional gas chromatograph, equipped with two electron capture detectors, was developed. This system was used on four cruises in the tropical and North Atlantic to measure the seawater concentrations and the atmospheric dry gas mole fractions of methyl iodide, and the ocean-atmosphere flux was calculated. A mean flux of ∼22 nmol m-2 d-1 was calculated from all cruises. A global annual flux of 2.89 Gmol a-1 was estimated using this flux. The limiting factor for the methyl iodide flux seems to be the production in the ocean, because the measured fluxes were relatively uniform and independent from observed changes in the surface water concentrations and wind speeds. Incubation experiments were done in the tropical Atlantic to test the hypothesis that methyl iodide is produced in the surface water by a photochemical pathway, instead direct biological production by phytoplankton or bacteria. During the experiments untreated, filtered and poisoned seawater was incubated at ambient water temperature, either in the sunlight or in the dark. The production of methyl iodide observed in all incubations kept in the sunlight was five times higher than the production from incubations kept in the dark. No significant difference was observed between untreated, filtered and poisoned samples. This strongly indicates a photochemical production with no direct influence by biota. The mean photochemical production rate from all incubations was 0.12 nmol m-3 h-1. A ...