Weathering natural systems

Note that the models aim to define general descriptive bulk parameters for an aquifer. Values for bulk adsorption or weathering will reflect the weighted range for a variety of phases present in complex, natural systems, and provide an overall measure of the behavior of the aquifer. In contrast, individual phases can be examined in the laboratory to determine the specific processes involved. 

So far we have discussed weathering rates and rate laws from laboratory experiments on pure minerals. These laboratory studies are meant to provide insight for natural systems (rates and variables that affect these rates). We may first try to compare laboratory and field results. 

From the temporal scale of adverse effects we come to a consideration of recovery. Recovery is the rate and extent of return of a population or community to a condition that existed before the introduction of a stressor. Because ecosystems are dynamic and even under natural conditions are constantly changing in response to changes in the physical environment (weather, natural catastrophes, etc.) or other factors, it is unrealistic to expect that a system will remain static at some level or return to exactly the same state that it was before it was disturbed. Thus the attributes of a recovered system must be carefully defined. Examples might include productivity declines in an eutrophic system, re-establishment of a species at a particular density, species recolonization of a damaged habitat, or the restoration of health of diseased organisms. 

Some of the extensive literature surrounding a relatively simple reaction, exchange on silica will be summarized in this section and contrasted with that for magnesium oxide in the next section. This reaction proceeds via O -centers on both catalysts, but the sites, modes of production and the reaction mechanisms themselves are very different. This reaction is very useful for illustrating the possible effects, on mineral catalysts, of processes common in natural systems, i.e., artifacts of mineral formation and weathering and electronic excitation. 

Figure 11 Measured dissolution rate for plagioclase from Panola granite, Georgia, USA compared to published dissolution rates of other plagioclase samples. Panola plagioclase was dissolved either as fresh unweathered samples, or as naturally pre-weathered samples (see text). Also plotted are dissolution rates for plagioclase under ambient conditions in the laboratory at near-neutral pH for freshly ground samples from other localities (solid symbols) and for samples weathered naturally in other field localities and then dissolved in the laboratory (open symbols). All rates were normalized by BET surface area. Dashed line is a fit to all field and laboratory data, including field data from systems weathering for periods of time 1 yr (data not shown). Figure adapted from White and Brantley (2003), and all data are attributed in that paper.

Mass-balance studies are widely considered to be the most reliable means of making quanta-tive determinations of elemental transfer rates in natural systems. Garrels (1967) and Garrels and Mackenzie (1967) pioneered the use of mass-balance calculations for mineral weathering in their classic study of Sierra Nevada springwaters. These waters were chosen because a careful set of water analyses and associated primary igneous rock minerals and the soil mineral alteration products were known. Since the actual compositions of the minerals were not known, Garrels and Mackenzie used the theoretical formulas for the minerals. 

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 ... 

Although weathering (see Volume 5) plays an important role in the neutralization of acids, the rates at which base cations are released in natural systems from weathering are not well known. 

Several simple experimental systems that simulate some aspect of the groundwater environment have been used to study the breakdown of individual minerals. These kinetics studies have encompassed quartz (Brantley et al., 1986), feldspars (Holdren and Berner, 1979 Holdren and Speyer, 1985), pyroxenes and amphiboles (Berner and Schott, 1982 Schott and Berner, 1985), carbonates (Berner, 1978), and glasses (White, 1983). The relative stability observed in laboratory weathering is consistent with field-based observations however, experimental rates appear to be faster than those in natural systems. 

P limitation on N2 fixation is widely observed in aquatic systems in terrestrial ecosystems there is good evidence for it from agricultural, pastoral, and some natural systems (Eisele et al, 1989 Smith, 1992 Crews, 1993). The model estimates P availability within a mass-balanced P cycle, with inputs via weathering, outputs via leaching, and a labile adsorbed fraction. Nonfixers are given priority for P, in proportion to the amount of fixed N available. If not enough available P is present to match available N, nonfixers are P limited. If P remains available after nonfixers have taken up what they can, N2 fixers can use it — at a lower C P ratio than that of nonfixers (Pate, 1986), and up to the overall limit to NPP set by light or water availability (Vitousek and Field, 1999). Limitation by other elements (e.g., Mo Silvester, 1989) could be treated similarly. 

In natural systems, chemical reactions often start far removed from equilibrium, then progress along towards a final stable (or metastable) state. Chemical weathering is a... 

---