Little is known about the factors driving the cycle of the climate-active gas dimethylsulfide (DMS) and of its precursor the metabolite dimethylsulfoniopropionate (DMSP) in sea ice. To date, studies have focused on biotic factors, linking high DMSP concentrations to the high biomass of sympagic communities, and to physiological adaptations to the low temperatures and high salinities of the brine habitat. This thesis presents an approach integrating biotic and abiotic factors, investigating the influence of ice growth processes and brine dynamics on the DMS cycle. First, brine dynamics from growth to melt are explored based on ice temperature and salinity profiles measured in the Arctic. A strong but brief desalination phase is identified in spring. Using calculated proxies of permeability (brine volume fraction) and of the intensity of brine convection (Rayleigh number), this phase is shown to correspond to full-depth gravity drainage initiated by restored connectivity of brines on warming. Full-depth gravity drainage is crucial for the vertical transfer of DMS-compounds at the ice-ocean interface. This physical background is then used to investigate the spatio-temporal variability of DMS in Arctic sea ice during a year-round survey in Amundsen Gulf. The influence of processes such as scavenging and brine convection on the DMS cycle is shown, and the first combined measurement of DMS, DMSP, and dimethylsulfoxide (DMSO), a compound acting as source/sink for DMS through photo-chemical and bacterial processes, is presented. DMSO is shown to dominate the dimethylated sulfur pool in surface ice when the snow cover is low. Based on correlations with irradiance, it is suggested that this DMSO originates from photo-chemical oxidation of DMS trapped in impermeable ice. Finally, the spatio-temporal variability of DMS in Antarctic sea ice is investigated during another year-round survey in McMurdo Sound. Platelet crystals growth under the influence of ice-shelf waters are shown to favor the incorporation of strong DMSP producers, to increase the environmental stress on cells, and to favor the accumulation of DMS,P by reducing permeability. The increase of permeability on warming is shown to trigger strong release of DMS in the ocean and a vertical redistribution of DMSP in the ice cover.

Proefschrift Universiteit van Amsterdam. / Met bibliogr., lit. opg. - Met samenvatting in het Nederlands.

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Recent field observations suggest that the radiative forcing of aerosol and clouds in the Arctic may be seasonally regulated by the oceanic emissions of the climatically-important biogenic trace gas dimethylsulfide (DMS). However, the validity of the proposed argument is challenged by the limited spatio-temporal coverage of these earlier studies in this difficult-to-access region. In particular, little is known about the pan-Arctic distribution of the oceanic DMS emissions, its temporal variability, and the impacts of sea-ice biogeochemistry on these emissions. In this dissertation, I investigated these unexplored subjects through numerical modelling. Using a one-dimensional (1-D) column modelling framework, I developed a coupled sea ice-ocean biogeochemical model and assessed the impacts of bottom-ice algae ecosystems on the underlying pelagic ecosystems and the associated production and emissions of DMS. The model was calibrated by time-series measurements of snow and melt-pond depth, ice thickness, bottom-ice and under-ice concentrations of chlorophyll-a and dimethylsulfoniopropionate (DMSP), and under-ice irradiance obtained on the first-year landfast sea ice in Resolute Passage during May-June of 2010. Many of the model parameters for the DMSP and DMS production and removal processes were derived from recent field measurements in the Arctic, which is advantageous over the previous Arctic-focused DMS model studies as their model parameters were based on the measurements in extra-polar regions. The impacts of sea-ice biogeochemistry on the DMS production in the underlying water column and its potential emissions into the overlying atmosphere were quantified through sensitivity experiments. To extend the study domain to the pan-Arctic, I implemented the sea-ice ecosystem and the coupled sea ice-pelagic DMS cycling components of the 1-D column model into a three-dimensional (3-D) regional modelling framework. A multi-decadal model simulation was performed over the period 1969-2015 using realistic atmospheric forcing and lateral boundary conditions. The results of the simulation were evaluated by direct comparisons with available data products and reported values based on field and satellite measurements and other model simulations. The decline of Arctic sea ice was successfully simulated by the model. The magnitude of the pan-Arctic sea-ice and pelagic annual primary production and their general spatial patterns were comparable to other model studies. The mean seasonal cycle and the spatial distribution of the model-based surface seawater DMS climatology within the pan-Arctic showed some similarities with in situ measurement- and satellite-based climatologies. However, at the same time, the comparison of the DMS climatologies was challenged by the bias in the measurement-based climatology, emphasizing the need to update this data product, which was created almost a decade ago, by incorporating data acquired during the recent field campaigns. The analysis of the modelled fluxes of DMS at the ice-sea and sea-air interfaces revealed different responses to the accelerated decline of sea ice over the recent decades (1996-2015). There was no trend in the pan-Arctic ice-to-sea DMS flux due to the counteracting effect of vertical thinning and horizontal shrinking of sea ice that drove ice algal production. In contrast, the pan-Arctic sea-to-air DMS flux showed a consistent increase (about 40 % over the last two decades) driven by the reduction of sea ice cover that promoted outgassing and biological productivity. This finding suggests that the climate warming in the Arctic causes an increase in DMS emissions, and encourages further exploration of the biological climate regulation in the Arctic. / Graduate

Accurate simulations of the global radiative impact of anthropogenic emissions must employ a tropospheric chemistry model that predicts realistic distributions of aerosols of all types. The need for a such a comprehensive yet computationally efficient tropospheric chemistry model is addressed in this research via systematic development of the various sub-models/mechanisms representing the gas-, aerosol-, and cloud-phase chemistries. The gas-phase model encompasses three tropospheric chemical regimes - background and urban, continental rural, and remote marine. The background and urban gas-phase mechanism is based on the paradigm of the Carbon Bond approach, modified for global-scale applications. The rural gas-phase chemistry includes highly condensed isoprene and a-pinene reactions. The isoprene photooxidation scheme is adapted for the present model from an available mechanism in the literature, while an a-pinene photooxidation mechanism, capable of predicting secondary organic aerosol formation, is developed for the first time from the available kinetic and product formation data. The remote marine gas- phase chemistry includes a highly condensed dimethylsulfide (DMS) photooxidation mechanism, based on a comprehensive scheme available in the literature. The proposed DMS mechanism can successfully explain the observed latitudinal variation in the ratios of methanesulfonic acid to non-sea-salt sulfate concentrations. A highly efficient dynamic aerosol growth model is developed for condensing inorganic gases. Algorithms are presented for calculating equilibrium surface concentrations over dry and wet multicomponent aerosols containing sulfate, nitrate, chloride, ammonium, and sodium. This alternative model is capable of predictions as accurate for completely dissolved aerosols, and more accurate for completely dry aerosols than some of the similar models available in the literature. For cloud processes, gas to liquid mass-transfer limitations to aqueous-phase reactions within cloud droplets are examined for all absorbing species by using the two-film model coupled with a comprehensive gas and aqueous-phase reaction mechanisms. Results indicate appreciable limitations only for the OH, HO₂, and NO₃ radicals. Subsequently, an accurate highly condensed aqueous-phase mechanism is derived for global-scale applications. / Ph. D.

The ocean emission and subsequent oxidation of dimethylsulfide (DMS) provides a source of sulfate in the atmosphere, potentially affecting the amount of solar radiation reaching the Earth's surface through both direct and indirect radiative effects of sulfate aerosols. DMS in the ocean can be quite variable with season and location, which in turn leads to high spatial and temporal variability of ocean DMS emissions. This study tested currently available observational and empirically-based climatologies of DMS concentration in the surface ocean. The exploration of the existing parameterizations mainly reveals the limitations of estimating DMS with an empirical model based on variables such as chlorophyll and mixed layer depth. The different algorithms show significant differences in spatial pattern, and none correlate strongly with observations. There is considerable uncertainty both in terms of the spatiotemporal distribution in DMS concentration and flux, as well as in the global total DMS flux. The present research investigates the influence of DMS on sulfate aerosols and radiative fluxes given different DMS climatologies in the fourth generation of the Canadian Global Atmospheric Climate Model (CanAM4.1). In general, the response in the radiative flux seems to follow the variation in the global mean flux of DMS linearly. Differences in the spatial and temporal structure of oceanic DMS have only a secondary effect on the radiative changes. The overall response of the atmosphere to the presence or absence of structure of DMS in space and time is distinctly smaller compared to the possible uncertainty of this response associated with the magnitude of the annually averaged global flux. / Graduate / 0425 / 0725 / 0416 / jetesdal@uvic.ca

The eutrophication of the Southern Bight of the North Sea has been benefitting to the prymnesiophyte Phaeocystis globosa (P. globosa). This species is a known high dimethylsulfoniopropionate (DMSP) producer whose bloom accounts for 95% of spring phytoplankton biomass. An increase in DMS(P) and its oxidation product dimethylsulfoxide (DMSO) cellular contents have been frequently observed in cellular stress conditions. To test this, we have first analysed the natural distribution of DMS(P,O) cellular contents in the North Sea. Secondly, we have measured DMS(P,O) cellular contents in monospecific cultures of several key species of the North Sea and their responses to salinity variations. Our main working hypothesis is that DMSP acts as an osmoregulator and/or as an antioxidant, depending on the species. The DMS(P,O) annual cycle in the Southern Bight of the North Sea revealed a seasonality linked to the spring phytoplankton communities succession: (1) colonial diatoms (reappearing in autumn), (2) Chaetoceros spp. (3) P. globosa, (4) large-size summer diatoms (mainly Guinardia spp.), and (5) dinoflagellates. Spatial gradients of DMS(P) were related to those of phytoplankton biomass, itself related to the inputs of nutrients from the Scheldt estuary. It also discharges suspended matter in which DMSO may have been produced by anaerobic oxidation of DMS. Laboratory measurements confirmed a large variability in DMSP cellular contents between the six studied diatoms (Nitzschia closterium, Skeletonema costatum, Thalassiosira rotula, Chaetoceros socialis, Chaetoceros debilis, and Guinardia delicatula), low producers in comparison with P. globosa and even more with Heterocapsa triquetra (Dinoflagellate). In particular, communities 2 and 4 have lower DMSP cellular contents than community 1 (N. closterium, S. costatum and T. rotula). Senescence induces a decrease in DMSP/DMSO suggesting an oxidative stress caused by nutrients and/or light limitation in DMSP producers. In S. costatum, DMSP seems to play an osmoregulatory role and is oxidised into DMSO in hyposaline conditions. In P. globosa and H. triquetra, an oxidative stress appears in hypo- and hypersaline conditions diverging from their salinity optimum. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished