Weathering transport interaction

Figure 9-3 portrays a hypothetical model of how chemical weathering and transport processes interact to control soil thicknesses. The relationship between soil thickness and rate at which chemical weathering can generate loose solid material is indicated by the solid curve. The rate at which transport processes can potentially remove loose solid weathering products is indicated by horizontal dotted lines. The rate of generation by chemical weathering initially increases as more water has the opporhmity to interact with bedrock in the soil. As soil thick-... 

Figure 6-3 portrays a hypothetical model of how chemical weathering and transport processes interact to control soil thicknesses. The relationship between soil thickness and rate at which chemical... 

In addition to runoff, rivers transport products of upland weathering to the oceans, forming a key link in the tectonic cycle of uplift and erosion. This interaction will be explored further in Section 6.6. 

Fig. 9-3 Conceptual model to describe the interaction between chemical weathering of bedrock and down-slope transport of solid erosion products. It is assumed that chemical weathering is required to generate loose solid erosion products of the bedrock. Solid curve portrays a hypothetical relationship between soil thickness and rate of chemical weathering of bedrock. Dotted lines correspond to different potential transport capacities. Low potential transport capacity is expected on a flat terrain, whereas high transport is expected on steep terrain. For moderate capacity, C and F are equilibrium points. (Modified with permission from R. F. Stallard, River chemistry, geology, geomorphology, and soils in the Amazon and Orinoco basins. In J. I. Drever, ed. (1985), "The Chemistry of Weathering," D. Reidel Publishing Co., Dordrecht, The Netherlands.)...

Traditionally, books dealing with sewer systems have been devoted to hydraulics and pollutant transport phenomena. In this context, urban drainage and wet-weather performance, as well as the sewer s interaction with treatment plants and receiving waters, were main focal points. 

Even though paleoaltimetric data from internal structural elements of orogenic belts and plateaus represent much needed complementary information to those derived from surface deposits or weathering products we caution about the uncritical use of stable isotopic data from deeper Earth environments in paleoaltimetric studies. It is highly desirable to obtain reliable thermometric, structural, and isotopic tracer data before attempting any paleoaltimetric reconstruction in such environments, as uncertainties exist about the fluid pathways and mechanisms responsible for fluid transport into the ductile crust. Maybe more importantly, it is imperative to document that the timing of meteoric water-rock interaction can be dated precisely, especially within thermally and kinematically rapidly evolving tectonic environments such as extensional detachment systems. 

In addition to the influence of the gas-water equilibria discussed above, the presence and concentration of different compounds in surface water, seawater, groundwater, and the like depend on physicochemical processes such as weathering, adsorption, ion exchange, redox processes, and precipitation reactions. These reactions and processes are affected by the interactions among the different dissolved species and those with suspended constituents and sediments. Physical processes such as water flow, transport phenomena, evaporative processes, and others can also determine the composition and transformations of the different compounds. 

In this chapter, we have tried to review the recent literature on trace elements in rivers, in particular by incorporating the results derived from recent ICP-MS measurements. We have favored a field approach by focusing on studies of natural hydrosystems. The basic questions which we want to address are the following What are the trace element levels in river waters What controls their abundance in rivers and fractionation in the weathering - - transport system Are trace elements, like major elements in rivers, essentially controlled by source-rock abundances What do we know about the chemical speciation of trace elements in water To what extent do colloids and interaction with solids regulate processes of trace elements in river waters Can we relate the geochemistry of trace elements in aquatic systems to the periodic table And finally, are we able to satisfactorily model and predict the behavior of most of the trace elements in hydrosystems ... 

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

The Mo concentration measured in a soil also is affected by the influence of geochemical processes on its mobility, transport, and deposition. The extent of its mobilization and transport from source rocks is determined by mineral stability, which in turn is affected by the weathering environment. Once it is mobilized from the mineral source, the transport, deposition, and availability of Mo to organisms are dependent on its interactions with other soil components (such as clays, organic matter, microbes, and Fe and Mn oxyhydroxides) and the chemistry (pH, Eh, and other ion concentrations) of the soil solution. Molybdenum associated with clay minerals, oxyhydroxides, and organic matter represents the available fraction. 

The chemical composition of air depends on the natural and man-made sources of the constituents (their distribution and source strength in time and space) as well the physical (e. g. radiation, temperature, humidity, wind) and chemical conditions (other trace species) which determine transportation and transformation. Thus, atmospheric chemistry is not a pure chemistry and also includes other disciplines which are important for describing the interaction between atmosphere and other surrounding reservoirs (biosphere, hydrosphere, etc.). Measurements of chemical and physical parameters in air will always contain a geographical component, i. e., the particularities of the locality. That is why the terms chemical weather and chemical climate have been introduced. For example, diurnal variation of the concentration of a substance may occur for different reasons. Therefore general conclusions or transfer of results to other sites should be done with care. On the other hand, it is a basic task in atmospheric chemistry not only to present local results of chemical composition and its variation in time, but also to find general relationships between pollutants and their behavior under different conditions. 

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