| Summary: | The absorption of atmospheric carbon dioxide (CO2) by seawater and subsequent equilibrium reactions within this ionic medium give rise to a complex chemical system often referred to as the marine carbonate system. This system is influenced by physical and biogeochemical processes in the ocean. The marine carbonate system is a major component of the global carbon cycle and is, by virtue of its interaction with atmospheric CO2, of fundamental importance to the Earth’s climate. Accurate knowledge of the properties of the marine carbonate system is a prerequisite for understanding the chemical forcing and consequences of key biogeochemical processes such as biological production, organic matter respiration, or uptake of anthropogenic carbon. The assessment of the marine carbonate system builds on precise measurements by state-of-the-art analytical methods as well as an understanding of the underlying fundamental chemistry in terms of ionic interactions and equilibrium thermodynamics. This thesis focuses on different aspects of the marine carbonate system with emphasis on biogeochemical processes and thermodynamic modelling of the seawater ionic medium. A quantitative understanding of the equilibrium solution chemistry of seawater ultimately relies on accurate estimations of activity coefficients of all the various components that make up the solution. Activity coefficients of the carbonate system in sodium chloride solution of varying ionic strength were estimated by Monte Carlo simulations at different temperatures, as well as activity coefficients of chloride and sulfate salts of a simplified seawater electrolyte, suggesting that a complete Monte Carlo description of seawater activity coefficients may be achievable using the hard sphere approach with a very limited number of fitted parameters. Chemical speciation modelling showed that the measured excess alkalinity of Baltic seawater is consistent with an organic alkalinity derived from humic substances of terrestrial origin. In deep waters of the Baltic Sea, ...
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