River sediments chemical composition

MacKenzie and Carrels (1966) approached this problem by constructing a model based on a river balance. They first calculated the mass of ions added to the ocean by rivers over 10 years. This time period was chosen because geologic evidence suggests that the chemical composition of seawater has remained constant over that period. They assumed that the river input is balanced only by sediment removal. The results of this balance are shown in Table 10-13. 

In some sediments, downcore variations in the bulk chemical composition are interpretable as records of temporal shifts in the elemental composition of the sinking POM. Such shifts are caused by changes in the production of sinking POM, which are in turn the result of fluctuations in the abundance and diversity of the overlying plankton community. In nearshore sediments, fluctuations in river runoff and lateral transport can lead to shifts in the supply rate of terrigenous organic matter. An example of a nearshore sediment core in which such fluctuations have been recorded is shown in Figure 23.18. 

Many important processes in the environment occur at boundaries. Here we use the term boundary in a fairly general manner for surfaces at which properties of a system change extensively or, as in the case of interfaces, even discontinuously. Interface boundaries are characterized by a discontinuity of certain parameters such as density and chemical composition. Examples of interface boundaries are the air-water interface of surface waters (ocean, lakes, rivers), the sediment-water interface in lakes and oceans, the surface of an oil droplet, the surface of an algal cell or a mineral particle suspended in water. 

By comparing the actual composition of sea water (sediments + sea -f- air) with a model in which the pertinent components (minerals, volatiles) with which water has come into contact are allowed to reach true equilibrium, Sillen in 1959 epitomized the application of equilibrium models for portraying the prominent features of the chemical composition of this system. His analysis, for example, has indicated that contrary to the traditional view, the pH of the ocean is not buffered primarily by the carbonate system his results suggest that heterogeneous-equilibria of silicate minerals comprise the principal pH buffer systems in oceanic waters. This approach and its expansion have provided a more quantitative basis for Forchbammer s suggestion of 100 years ago that the quantity of the different elements in sea water is not proportional to the quantity of elements which river water pours into the sea but is inversely proportional to the facility with which the elements in sea water are made insoluble by general chemical actions in the sea. 

The main components of marine sediments are inorganic aluminosilicate minerals which are usually accumulated on the sea floor by river and other geological activities, and also skeletons and shells of marine organisms (mainly calcium carbonate and silica) [2]. Of course, some metal salts or particulates which precipitate from seawater form new minerals, e.g. manganese nodules [2]. The chemical compositions of the three principal types of sediments in the ocean are shown in Table 12 [105], Most of the sediments found in the deep-sea floor are mixtures of these three principal minerals. Study of the sediments in the oceans and seashores can provide important data related to geochemical, oceanographical or biological circulation and deposition of elements, formation and distribution of marine sediments, and exploitation of marine resources. 

The hydrological cycle interacts with the cycle of rocks. Minerals dissolve in or react with the water. Under different physicochemical conditions, minerals are precipitated and accumulate on the ocean floor and in the sediments of rivers and lakes. Dissolution and precipitation reactions impart to the water constituents that modify its chemical properties. Natural waters vary in chemical composition consideration of solubility relations aids in the understanding of these variations. This chapter sets forth principles concerning reactions between solids and water. Here again the most common basis is a consideration of the equilibrium relations. 

Since the chemical composition of alluvium obviously depends upon the source of the material, one can only make general statements about plant nutrient contributions that are related to alluvial processes. Soils vary widely as to their plant nutrient contents, as shown in Table 9.4. It should be recalled that, at most, only 5 to 10% of these totsil amounts is likely to be solubilized in any given year. Using the figure of 0.45 Tg of sediment per day carried by the Mississippi River at St. Louis, and assuming 5% dissolution of plant nutrients contained in the sediment, an estimated 90—2000 Mg of available nutrients is transported daily by this river alone. The sediment plus contained nutrients is either laid over or extends existing deposits. For example, the Po River extends its delta by about 8 X 10 m y". ... 

The geological record shows that this material-transport mechanism has operated for at least 3.8 billion years. New sediments are derived either from older sedimentary rocks or from newly generated or ancient igneous and metamorphic rock. The average chemical composition of suspended sediment in rivers, sedimentary mudrock and the upper continental crust is quite similar (Table 4.1). 

In the hypothetical terrestrial system depicted in Fig. 14-4, the P eroded from the land is eventually transported to the estuaries. As in lakes, soils, and rivers, many chemical and biological processes act to control the transport of P within and from the estuary (Lucotte and d Anglejan, 1988 Jonge and Villerius, 1989). Dissolved P may be removed from solution onto the particulate phase and deposited in the sediments. On the other hand, the change in the solution composition may cause P to be released from the particulate load. The P that is transported from the estuaries to the ocean in particulate form will rapidly settle to the sea floor and be incorporated into the sediments. The dissolved P will enter the surface ocean and participate in the biological cycles. Determining what proportion of P that is transported out of the estuary is reactive is a critical step in the elucidation of the marine P budget (Froelich et al., 1982). 

Forstner, 1984 Forstner, 1988). Usually, the chemical compositions of these sediments are close to those of the regoliths and C-horizons in soil profiles of a given area. As in soils, trace elements can be contained in the mineral phases or adsorbed on them (Bourg, 1988). In this paper, we discuss river sediments in connection with naturally contaminated catchments (Section 3). In this field, a good knowledge of typical natural trace element ranges is particularly important in order to assess a possible anthropogenic contribution. 

The lanthanide patterns for many modem sediments are similar to that of the post-Archean shales. The chemical composition of suspended particulate matter in some of the world s major rivers was examined by Martin and Meybeck (1979) (table 27, fig. 45). The patterns are very similar to PAAS although slightly enriched in total lanthanides (about 1.1-1.4 times). The cause of the enrichment is probably related to grain size. 

Chemical composition of seawater is controlled by various processes. River water input and sedimentation have been considered as important processes controlling chemical composition of seawater. However, there are other processes controlling chemical composition of seawater. They are seawater cycUng at mid-oceanic ridge, input of volcanic gas, evaporation, weathering of oceanic cmst, aerosol fall, anthropogenic pollution etc. (Wolery and Sleep 1976 Holland 1978). 

Mg " > Ca and Cl > S04 > HCOa, respectively that are different from those of river water. When river water inputs to seawater, it mixes with seawater, resulting to a removal of base metals from mixed solution to sediments. For example, base metal elements in polluted river water inputting to Tokyo Bay, and organic matter settle onto seawater bottom in the bay. The removal of base metals and other elements from seawater near the coast significantly influences chemical composition of seawater. 

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