Methyl iodide production in the open ocean

Abstract

Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM-HAMOCC (MPIOM - Max Planck Institute Ocean Model HAMOCC - HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air-sea flux that ranges between 70 and 260 Gg yr-1. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean). © Author(s) 2014

Ideas and perspectives: climate-relevant marine biologically driven mechanisms in Earth system models

“…Some ESMs do consider DMS and N 2 O (Table 2), and their results suggest significant changes in the production with consequences for the climate system in the future (e.g., Six et al, 2013;Martinez-Rey et al, 2015). Other marine biologically produced SCCs (except CO 2 ) and aerosols are usually not included, but there are a number of recent modeling activities in which the pertinent processes have been implemented, and the climate impact of these substances has been partially evaluated (e.g., Kirkevåg et al, 2013;Stemmler et al, 2014Stemmler et al, , 2015Hossaini et al, 2015). The largest deficiency of ESMs in this respect is that primary production is still not sufficiently well represented, in particular in coastal regions (e.g., Schneider et al, 2008;Anav et al, 2013).…”

Conceptual Model of Iodine Behavior in the Subsurface at the Hanford Site

“…On a global level, phototrophs in marine environments have been shown to produce up to 10 6 to 10 10 g of methyl iodide (CH 3 I) per year (Stemmler et al 2014). In addition, CH 3 I is known to play a significant role in the global iodine cycle (Klick and Abrahamsson 1992).…”

Biogenic halocarbons from the Peruvian upwelling region as tropospheric halogen source

“…The occurrence of CH 3 I in the tropical oceans (up to 36.5 pmol L −1 ) has previously been attributed to a predominantly photochemical source (Richter and Wallace, 2004;Jones et al, 2010), explaining its global hotspots in the subtropical gyres and close to the tropical western boundaries of the continents Stemmler et al, 2014). Previous campaigns in the eastern Pacific obtained concentrations of up to 21.7 and 8.8 pmol L −1 (Butler et al, 2007), and around 1.0 to 1.5 pmol L −1 (Mahajan et al, 2012), but not directly in the upwelling.…”

“…However, biogeochemical modelling studies suggest that photochemistry may be more important for global CH 3 I production (Stemmler et al, 2014). Due to their much shorter lifetime in surface water and the atmosphere, fewer studies have investigated production processes of CH 2 I 2 and CH 2 ClI.…”