No abstract

[1] Oceanic emissions of gaseous organic iodine-atom precursors have the potential to significantly affect atmospheric chemistry and climate, however there is currently considerable uncertainty associated with quantifying their sources. We present sea-air fluxes calculated from simultaneous air and seawater measurements of a comprehensive range of volatile organic iodine compounds (VOICs), including CH 3 I and the less commonly reported dihalomethanes CH 2 ICl, CH 2 IBr and CH 2 I 2 , made during two cruises in the Atlantic Ocean between 15-58°N. The combined dihalomethane flux provides a global iodine source (∼0.33 ± 0.19 Tg I y −1 ) comparable to that of CH 3 I, and a surface iodine atom source 3-4 times higher. However, a 1D atmospheric model reveals that, in the tropical east Atlantic Ocean in the vicinity of Cape Verde, even these combined VOIC fluxes are capable of supporting only ∼10-25% of the observed IO levels, and suggests that a substantial (340-640 nmol I m −2 d −1 ) additional photochemical source of iodine is required.

Abstract. Air-sea fluxes and bulk seawater and atmospheric concentrations of bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) were measured during two research cruises in the northeast Atlantic (53-59 • N, June-July 2006) and tropical eastern Atlantic Ocean including over the African coastal upwelling system (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) • N May-June 2007). Saturations and sea-air fluxes of these compounds generally decreased in the order coastal > upwelling > shelf > open ocean, and outside of coastal regions, a broad trend of elevated surface seawater concentrations with high chlorophyll-a was observed. We show that upwelling regions (coastal and equatorial) represent regional hot spots of bromocarbons, but are probably not of major significance globally, contributing at most a few percent of the total global emissions of CHBr 3 and CH 2 Br 2 . From limited data from eastern Atlantic coastlines, we tentatively suggest that globally, coastal oceans (depth <180 m) together contribute ∼2.5 (1.4-3.5) Gmol Br yr −1 of CHBr 3 , excluding influences from anthropogenic sources such as coastal power stations. This flux estimate is close to current estimates of the total open ocean source. We also show that the concentration ratio of CH 2 Br 2 /CHBr 3 in seawater is a strong function of concentration (and location), with a lower CH 2 Br 2 /CHBr 3 ratio found in coastal regions near to macroalgal sources.

Oceanic gas hydrates have been measured near the seafloor for the first time using a seagoing Raman spectrometer at Hydrate Ridge, Oregon, where extensive layers of hydrates have been found to occur near the seafloor. All of the hydrates analyzed were liberated from the upper meter of the sediment column near active gas venting sites in water depths of 770-780 m.Hydrate properties, such as structure and composition, were measured with significantly less disturbance to the sample than would be realized with core recovery. The natural hydrates measured were sI, with methane as the predominant guest component, and minor/trace amounts of hydrogen sulfide present in three of the twelve samples measured. Methane large-to-small cage occupancy ratios of the hydrates varied from 1.01 to 1.30, in good agreement with measurements of laboratory synthesized and recovered natural hydrates Although the samples visually appeared to be solid, varying quantities of free methane gas were detected, indicating the presence of occluded gas a hydrate bubble fabric and/or partial hydrate dissociation in the under-saturated seawater.