L Kamp-Nielsen, University of Copenhagen, Hillerod, Denmark © 2008 Elsevier B.V. All rights reserved.
Net Deposition as an Aggregated Approach Further Reading
Sediments are particulate matter that can be or have been transported by fluid, wind, and glaciers and which might have been deposited as a layer of solid particles in dense suspension at the bottom of water bodies. The parts of sediments which have their origin outside the water bodies are called allochthonous sediments and have been transported by runoff from the drainage basin of the water body or by wet or dry deposition on the surface of the water body. Particles formed within the water body and the sediment are the autochthonous parts of the sediments and are transformations of dissolved elements to particles by chemical and biological processes. In the photic zone of rivers, lakes, coastal waters, and oceans, inorganic carbon as carbon dioxide or bicarbonate is fixed as particulate organic carbon by photosynthetic organisms, or fixed as inorganic carbonate in corals, for-amifers, and coccolithophorids. The dead and living organic particles can be processed through the aquatic food chain and sink to the bottom as living or dead particulate organic material. Due to increased pH, as a result of photosynthesis, the solubility product of calcium and magnesium carbonate can be exceeded and precipitation of carbonates may occur. Both the allochthonous and the autochthonous parts of the sediments are subject to further biogenetical processes in the sediment environment and other autogenic fractions are generated.
On a geological timescale, surface sediments are young structures, but they play an important role in the global and local cycling of elements like carbon, nutrients, and metals - all of which are important for the productivity in both terrestrial and aquatic ecosystems. For some conservative elements the sediments can be considered almost as a permanent sink, but for other elements (e.g., carbon, nitrogen, and phosphorus) the surface sediments act as important temporary sinks on an annual scale. Finally, the permanent settled material may be further metamorphosed to fossil fuels and sedimentary rocks.
After settling, the sediment undergoes a certain compaction and a concentrated environment develops with a high surface area, but with slow diffusional transport of dissolved gases and ions along strong concentration gradients. High concentrations of ions develop and stimulate precipitation reactions. Surface sediments become the habitat for intensive and diverse microbial metabolism dominated by anaerobic processes. To study this environment with high spatial and temporal heterogeneity, special equipment as microelectrodes with diameters of a few microns have been developed to measure oxygen, pH, sulfide, and other elements in sediments. And the very complex interactions of physical, chemical, and biological processes justify the intensive use of mathematical models in the study of sediment processes.
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