Marine Emissions of Halogenated Trace Gases From The Tropical Ocean

Oceanic bromoform (CHBr3) and dibromomethane (CH2Br2) are the largest contributors to organic bromine in the atmosphere, while atmospheric organoiodine is significantly influenced by marine methyl iodide (CH3I) and diiodomethane (CH2I2). Halogenated hydrocarbons (halocarbons) and their degradation products are involved in ozone chemistry in both the troposphere and stratosphere. With decreasing anthropogenic atmospheric halocarbons, the impact of naturally produced halocarbons on atmospheric processes will likely increase. Many uncertainties still exist with regard to their production and degradation in the ocean, as well as to their emissions. While macroalgae have been identified as important sources of them, microalgae were shown to be halocarbon producers as well. Hence, oceanic upwelling systems might play a crucial role for open ocean emissions. The tropical ocean has not only been hypothesized to contribute largely to global halocarbon emissions, but it also may contribute to their transport into upper atmospheric layers. They may be transported in significant amounts into the tropical stratosphere by tropical deep convection. This thesis aims at reducing some of the uncertainties regarding halocarbon emissions from the tropical ocean to understand their role in a future climate. Two campaigns are covered here: MSM18/3 onboard RV Maria S. Merian investigating the Eastern tropical equatorial Atlantic during the cooling season in June and July 2011, and DRIVE (Diurnal and RegIonal Variability of halogen Emissions) onboard RV Poseidon, which focused on the Mauritanian upwelling region in June 2010. Oceanic and atmospheric halocarbon data, biological, meteorological and oceanographic parameters were collected to investigate impact factors on halocarbon emissions. The oceanic and atmospheric data were also included in the most complete halocarbon database so far, HalOcAt (Halocarbons in the Ocean and Atmosphere). Manuscripts, prepared and published on the basis of this data set, include the first manuscript (Hepach et al., in prep) that focuses on the first measurements of CHBr3, CH2Br2, CH3I and CH2I2 in the surface and the water column of the equatorial Atlantic during the Atlantic Cold Tongue (ACT) season. The second (Hepach et al., 2014) and third manuscript (Fuhlbrügge et al., 2013) cover oceanic and atmospheric abundances of CHBr3, CH2Br2 and CH3I in the Mauritanian upwelling region on a diel and regional scale. While the second manuscript investigates impact factors on emissions of these compounds, the third manuscript analyzes meteorological constraints on atmospheric halocarbons. The fourth manuscript (Ziska et al., 2013) uses the HalOcAt database to determine global emissions of CHBr3, CH2Br2 and CH3I, and estimates global contributions from different regions. In the fifth manuscript (Stemmler et al., 2013), depth profiles of CH3I measured during the DRIVE campaign are used to validate modeled profiles from the tropical open ocean using the General Ocean Turbulence Model (GOTM). The transport of emissions of CH3I into the stratosphere is calculated in the sixth manuscript (Tegtmeier et al., 2013), indicating that CH3I from the DRIVE campaign is entrained in small amounts into the stratosphere. Both upwelling systems, the Mauritanian upwelling and the equatorial Atlantic, were shown here to be source regions for CHBr3 and CH2Br2, contributing to the large emissions of these compounds from the tropical ocean. While CH3I has been found to be ubiquitously distributed in the Mauritanian upwelling region hinting towards photochemical formation there, strong implications for biological formation were found in the ACT. This agrees well to the modeled depth profiles of CH3I indicating they may be influenced both by photochemical and biological formation of this compound. Although it has been hypothesized that the tropical ocean may not contribute to CH2I2 emissions to the atmosphere due to its very rapid photolysis, CH2I2 could be detected in low concentrations in the surface water of the ACT. The first determination of diapycnal fluxes of CHBr3, CH2Br2, CH3I and CH2I2 in the ACT indicate that their production takes place within the mixed layer regardless of deeper biomass maxima, which may be very important for their emissions. In the Mauritanian upwelling, oceanic halocarbon production was identified as the main driver of halocarbon emissions with wind speed having impact on a diel scale. For the first time, the height of the Marine Atmospheric Boundary Layer (MABL) has been found to indirectly impact halocarbon emissions due to its decreasing and increasing effect on atmospheric halocarbons. Together with enhanced emissions of halocarbons, the largely elevated atmospheric halocarbons above the Mauritanian upwelling could be explained solely by local emissions in contrast to previous hypotheses. This process could be of importance in other coastal upwelling systems as well

New biostratigraphic data from the Early Pleistocene tyrrhenian PALEOCOAST (Western Umbria, Central Italy)

Plio-quaternary marine deposits are largely documented in western Umbria (central Italy), although they still lack biostratigraphic definition. Contrary to published data, Early Pleistocene deposits outcrop more extensively than previously reported in the Orvieto area. A composite biostratigraphic succession, almost continuous from the top of the G. gr. crassaformis Zone to the top of the Gl. cariacoensis Zone, can be reconstructed in offshore clay sections. Nannofossil assemblages and marker events (bmG, tCm, blG, tHs, tlG) from the MNN16a to MNN19e subzones have been documented. Lower shoreface - transition to offshore sections as described, are characterized by poor planktonic assemblages; nevertheless, they are still referable to the same stratigraphic interval. Deposits can be partially inserted into the "Chiani - Tevere" depositional cycle, also documented in this area. Moreover, marine conditions persist in the area from the base of the Gelasian to the top of the Calabrian, and it can be modelled as a peripheral, survival sea-branch, cut-off from the main river supply and from continental influence. However, Zanclean to Piacenzian deposits occur in a small area, localized around the town of Orvieto, so the former distinction of superimposed depositional cycles can only be speculative

Numerical modelling of methyl iodide in the eastern tropical Atlantic

Methyl iodide (CH3I) is a volatile organic halogen compound that contributes significantly to the transport of iodine from the ocean to the atmosphere, where it plays an important role in tropospheric chemistry. CH3I is naturally produced and occurs in the global ocean. The processes involved in the formation of CH3I, however, are not fully understood. In fact, there is an ongoing debate whether production by phytoplankton or photochemical degradation of organic matter is the main source term. Here, both the biological and photochemical production mechanisms are considered in a biogeochemical module that is coupled to a one-dimensional water column model for the eastern tropical Atlantic. The model is able to reproduce observed subsurface maxima of CH3I concentrations. But, the dominating source process cannot be clearly identified as subsurface maxima can occur due to both direct biological and photochemical production. However, good agreement between the observed and simulated difference between surface and subsurface methyl iodide concentrations is achieved only when direct biological production is taken into account. Production rates for the biological CH3I source that were derived from published laboratory studies are shown to be inappropriate for explaining CH3I concentrations in the eastern tropical Atlantic

Senescence as the main driver of iodide release from a diverse range of marine phytoplankton

The reaction between ozone and iodide at the sea surface is now known to be an important part of atmospheric ozone cycling, causing ozone deposition and the release of ozone-depleting reactive iodine to the atmosphere. The importance of this reaction is reflected by its inclusion in chemical transport models (CTMs). Such models depend on accurate sea surface iodide fields, but measurements are spatially and temporally limited. Hence, the ability to predict current and future sea surface iodide fields, i.e. sea surface iodide concentration on a narrow global grid, requires the development of process-based models. These models require a thorough understanding of the key processes that control sea surface iodide. The aim of this study was to explore if there are common features of iodate-to-iodide reduction amongst diverse marine phytoplankton in order to develop models that focus on sea surface iodine and iodine release to the troposphere. In order to achieve this, rates and patterns of changes in inorganic iodine speciation were determined in 10 phytoplankton cultures grown at ambient iodate concentrations. Where possible these data were analysed alongside results from previous studies. Iodate loss and some iodide production were observed in all cultures studied, confirming that this is a widespread feature amongst marine phytoplankton. We found no significant difference in log-phase, cell-normalised iodide production rates between key phytoplankton groups (diatoms, prymnesiophytes including coccolithophores and phaeocystales), suggesting that a phytoplankton functional type (PFT) approach would not be appropriate for building an ocean iodine cycling model. Iodate loss was greater than iodide formation in the majority of the cultures studied, indicating the presence of an as-yet-unidentified "missing iodine" fraction. Iodide yield at the end of the experiment was significantly greater in cultures that had reached a later senescence stage. This suggests that models should incorporate a lag between peak phytoplankton biomass and maximum iodide production and that cell mortality terms in biogeochemical models could be used to parameterise iodide production

Meteorological constraints on oceanic halocarbons above the Peruvian Upwelling

During a cruise of R/V METEOR in December 2012 the oceanic sources and emissions of various halogenated trace gases and their mixing ratios in the marine atmospheric boundary layer (MABL) were investigated above the Peruvian upwelling. This study presents novel observations of the three very short lived substances (VSLSs) – bromoform, dibromomethane and methyl iodide – together with high-resolution meteorological measurements, Lagrangian transport and source–loss calculations. Oceanic emissions of bromoform and dibromomethane were relatively low compared to other upwelling regions, while those for methyl iodide were very high. Radiosonde launches during the cruise revealed a low, stable MABL and a distinct trade inversion above acting as strong barriers for convection and vertical transport of trace gases in this region. Observed atmospheric VSLS abundances, sea surface temperature, relative humidity and MABL height correlated well during the cruise. We used a simple source–loss estimate to quantify the contribution of oceanic emissions along the cruise track to the observed atmospheric concentrations. This analysis showed that averaged, instantaneous emissions could not support the observed atmospheric mixing ratios of VSLSs and that the marine background abundances below the trade inversion were significantly influenced by advection of regional sources. Adding to this background, the observed maximum emissions of halocarbons in the coastal upwelling could explain the high atmospheric VSLS concentrations in combination with their accumulation under the distinct MABL and trade inversions. Stronger emissions along the nearshore coastline likely added to the elevated abundances under the steady atmospheric conditions. This study underscores the importance of oceanic upwelling and trade wind systems on the atmospheric distribution of marine VSLS emissions

Impact of the marine atmospheric boundary layer conditions on VSLS abundances in the eastern tropical and subtropical North Atlantic Ocean

During the DRIVE (Diurnal and Regional Variability of Halogen Emissions) ship campaign we investigated the variability of the halogenated very short-lived substances (VSLS) bromoform (CHBr3), dibromomethane (CH2Br2) and methyl iodide (CH3I) in the marine atmospheric boundary layer in the eastern tropical and subtropical North Atlantic Ocean during May/June 2010. The highest VSLS mixing ratios were found near the Mauritanian coast and close to Lisbon (Portugal). With backward trajectories we identified predominantly air masses from the open North Atlantic with some coastal influence in the Mauritanian upwelling area, due to the prevailing NW winds. The maximum VSLS mixing ratios above the Mauritanian upwelling were 8.92 ppt for bromoform, 3.14 ppt for dibromomethane and 3.29 ppt for methyl iodide, with an observed maximum range of the daily mean up to 50% for bromoform, 26% for dibromomethane and 56% for methyl iodide. The influence of various meteorological parameters - such as wind, surface air pressure, surface air and surface water temperature, humidity and marine atmospheric boundary layer (MABL) height - on VSLS concentrations and fluxes was investigated. The strongest relationship was found between the MABL height and bromoform, dibromomethane and methyl iodide abundances. Lowest MABL heights above the Mauritanian upwelling area coincide with highest VSLS mixing ratios and vice versa above the open ocean. Significant high anti-correlations confirm this relationship for the whole cruise. We conclude that especially above oceanic upwelling systems, in addition to sea-air fluxes, MABL height variations can influence atmospheric VSLS mixing ratios, occasionally leading to elevated atmospheric abundances. This may add to the postulated missing VSLS sources in the Mauritanian upwelling region (Quack et al., 2007)