Projekt: Subproject 3.2.1 Impact of biogenic carbonates on pH buffering in an acidifying coastal sea (North Sea)

We propose to investigate the influence of changing pH and carbon dioxide partial pressure, and alkalinity on carbonate dissolution in surface sediments of the Wadden Sea and the consequences for and relation to the carbonate system of the North Sea. Our hypothesis is that biogenic carbonates at the surface of intertidal sediments and the upper sediment layers play -to different extents- a role in modifying tidal waters that exchange with the shallow North Sea. This adds to carbonate fluxes caused by benthic organic matter degradation. The absolute and relative importances will change as the North Sea carbonate system will acidify in the future. One important aspect will be the reactivity (reaction rates and thermodynamic stability) of different biogenic carbonates and the experimental quantification of the different dissolution rates in pure and seawater media under different controlled laboratory conditions, i.e. pH, partial pressure of carbon dioxide (PCO2-stat and free-drift), and carbonate undersaturation. The reactive surface of the carbonate will be characterized to extract specific dissolution rates from experimental data on the development of liberated major minor and trace cations and the dissolved carbonate species. Experiments will be carried out at different fixed CO2 partial. This approach will include in parallel experiments geomicrobiological effects on microbial degradation of the organic matrix compared to purely abiotic reactions. Experimental laboratory-based approaches will be compared to field in-situ transformation experiments and will be followed by microscopic (e.g., SEM-EDX), inorganic geochemical (ICP-OES, photometry, microsensors) and stable isotope (C, O; irmMS) approaches. Experiments will also be carried out in collaboration with Dr. Hoppem! a (3.2.3) regarding benthic mesocosm experiments at AWI. Field work will additionally characterize the sources and surface textures of different carbonate fractions in surface sediments with geochemical methods and SEM-EDX. These results will allow to evaluate the time-dependent corrosion of biogenic carbonates as a function of burial time. Application of microsensors to characterize the chemical gradients will be carried out in collaboration with Dr. D. deBeer (MPI-MM; 3.2.2). From the side of carbonate formation, the influence of a changed carbonate system on growth of Mytilus and Crassostrea is planned to be assessed in a later phase of the project. Finally, pelagic measurements and experimental results will link the alkalinity and DIC exchange with the coastal North Sea and the possible ecosystem consequences via biogeochemical modelling. Parameters of the dissolved carbonate system (TA, DIC, PCO2, pH) will be measured in collaboration with PD Dr. Bernd Schneider (IOW) while d13C(DIC) will be determined via irmMS. These results will be provided for a collaborative integration in Theme 5 (Pätsch et al.) and the inclusion in the modelling of the North Sea carbonate system. The approach provides the base for a quantitative understanding of the role of the carbonate system in the water column on preservation, destruction and temporal authigenesis in the intertidal surface sediments, and the role of benthic metabolisms.