Methyl iodide in the Greenland/Norwegian Seas and the tropical Atlantic Ocean: Evidence for photochemical production

CH3I was measured in open ocean waters during two cruises to the tropical Atlantic Ocean and a late fall cruise to the Greenland and Norwegian Seas (GSNS). In warm, tropical surface waters subject to high solar irradiance, average CH3I saturation anomalies were positive (1.5-7.7 pmol kg(-1)), indicating a sea-to-air flux. This contrasted with negative saturation anomalies (-0.65 +/- 0.02 pmol kg(-1)) measured in cold surface waters of the open ocean GSNS subject to low-light. High latitude oceans may therefore be a significant sink for atmospheric CH3I during the fall and winter. The locations and/or seasons where samples were analyzed were all characterized by relatively low biological production and the CH3I saturation anomaly along 19 degrees S decreased from 7.7 +/- 0.6 to 3.4 +/- 0.4 pmol kg(-1) when entering a more productive upwelling zone. Taken together these observations suggest a chemical, as opposed to biological, production mechanism for this compound in the open ocean. Within the open ocean of the GSNS, multiple linear regression between the observed CH3I saturation anomaly and variables including light intensity, water temperature, CFC-11 saturation (indicator of gas exchange and deep mixing), and distance from the Norwegian Coastal Current (indicator of coastal or southern sources) showed that light intensity was the only significant predictor, explaining 79% of the variance. Photochemical production may, therefore be dominant source of CH3I within the open ocean and this may have important implications for the large-scale, seasonal cycling of iodine between the ocean and the atmosphere.