Chemical reactions in marine sediments

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Chemical reactions in marine sediments and the resulting fluxes across the sediment–water interface influence the global marine cycles of carbon, oxygen, nutrients and trace metals and control the burial of most elements in marine sediments. On very long time scales these diagenetic reactions control carbon burial in sedimentary rocks and the oxygen content of the atmosphere. Sedimentary deposits that remain after diagenesis are the geochemical artifacts used for interpreting past changes in ocean circulation, biogeochemical cycles and climate. Constituents of marine sediments that make up a large fraction of the particulate matter that reaches the sea floor (organic matter, CaCO 3 , SiO 2 , Fe, Mn, aluminosilicates and trace metals) are tracers of ocean physical and biogeochemical processes when they formed, and of diagenesis after burial. Understanding of sediment diagenesis and benthic fluxes has evolved with advances in both experimental methods and modeling. Measurements of chemical concentrations in sediments and their associated porewaters and fluxes at the sediment–water interface have been used to identify the most important reactions. Because transport in porewaters is usually by molecular diffusion, this medium is conducive to interpretation by models of heterogeneous chemical equilibrium and reaction kinetics. Large chemical changes and manageable transport mechanisms have led to elegant models of sediment diagenesis and great advances in understanding diagenetic processes. We shall see, though, that the environment does not yield totally to simple models of chemical equilibrium and chemical kinetics, and laboratory-determined constants often cannot explain the field observations.