CHEMICAL PROCESSES AT THE SEDIMENT WATER INTERFACE
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abstract
In natural waters, the sediment-water interface is the site where gradients in physical, chemical and biological properties are the greatest. Both chemical and microbiological transformation processes are responsible for cycling elements between water and sediments. This paper discusses the various chemical transformations that take place during early diagenesis in sediments, and which are fueled by supply rates of organic carbon and electron acceptors. In particular, our current knowledge of the cycling of the electron acceptors O, N, Mn, Fe and S is assessed, and the important role of transport reactions is described. The elemental fluxes across the sediment-water interface can be described from first principles only if the coupling of physical, chemical and biological processes is better understood. Large gradients in chemical potentials of many chemical species across this interface often lead to steep gradients in proton and electron activities. The redox sequence commonly observed in time and space also leads to changes in the partitioning of solutes between carrier and solution phases. Such processes can cause rapid back-diffusion of released species to the overlying water column or their removal onto secondary carrier phases within the sediments. Physical transport systems such as resuspension or bioturbation of particles control the physical boundary conditions for chemical reactions immediately above or below the sediment-water interface. Because of the complex coupling of the different chemical, physical and biological processes, chemical reactions within this region must be considered an integral part of a three-dimensional network of interactions. Hence, the apparent coexistence of chemical species considered 'incompatible' by thermodynamic models can be a consequence of the three-dimensional nature of redox gradients. The microbiological, chemical and physical interactions of dissolved, colloidal and particulate organic carbon, and iron and manganese oxides play a crucial role in the early diagenetic reactions at the sediment-water interface. Their effects on diagenetic reactions, and on trace elements in solution and adsorbed to particles, are discussed. Furthermore, we emphasize the way in which hydrodynamics can control elemental fluxes in environments with high carbon rain rates. 1990.
