Weathering products, accumulation

Bormann et al. (1998) attempted to determine whether rates of weathering of primary minerals were being underestimated due to failure to include weathering products accumulating in the biomass and in soil. They compared two sandbox ecosystems (large monitored lysimeters) at Hubbard Brook, NH, one with red pine and the other relatively nonvegetated (containing sporadic lichens and mosses). 

Figure 1. Supercritical flow reactor. Key (I) Mettler balance (2) flask with 1 0 (filtered and deaerated) (3) HPLC pump (4) bypass (three-way) valve (5) feed cylinder (6) weather balloon with feed solution (7) probe thermocouple (type K) (8) ceramic annulus (9) Hastelloy C-276 tube (10) entrance cooling jacket (11) entrance heater (12) furnace coils (13) quartz gold-plated IR mirror (14) window (no coils) (15) guard heater (16) outlet cooling jacket (17) ten-port dualloop sampling value (18) product accumulator (19) air compressor (20) back-pressure regulator and (21) outflow measuring assembly.

On a time series of Quaternary marine terraces in northern California, Brimhall et al. (1992) conducted the first mass balance analysis of soil formation over geologic time spans. This analysis provided quantitative data on well-known qualitative observations of soil formation (i) the earliest stages of soil formation (on timescales of 10 -10 yr) are visually characterized by loss of sedimentary/rock structure, the accumulation of roots and organic matter, and the reduction of bulk density and (ii) the later stages of soil development (>10 yr) are characterized by the accumulation of weathering products (iron oxides, silicate clays, and carbonates) and the loss of many products of weathering. 

Another consequence of increased weathering is the accumulation of weathering products, (i.e., Fe and Al oxides) close to the root surface (Courchesne and Gobran, 1997 Martin et al., 2004). These secondary minerals, alone or in combination with organic matter, increase the potential cation- (CEC) or anion-exchange (AEC) capacity of the rhizosphere materials (Gobran and Clegg,... 

The particular secondary minerals formed by neoformation are controlled by the leaching intensity of the local soil environment, and to what extent the environment is confining, allowing the more soluble weathering products (silica, base cations) to accumulate. The temperature and moisture conditions also have an effect on the specific secondary minerals that form. To understand in general terms the complex balance of factors that dictates which secondary minerals tend to form, it is first necessary to appreciate the chemical factors that limit solubility of two weathering products—alumina and silica—the major constituents of clay minerals. 

The last-mentioned factor influencing weathering rates of minerals in soils (see list above), efficiency of removal of soluble weathering products, warrants further comment. In confined soil environments, dissolved silica and base cations accumulate from weathering reactions. So, for example, the neoformation of kaolinite from smectite ... 

For weathering steels corrosion product accumulated with time leading to the retardation of corrosion rate with time, whereas for mild steel corrosion rate increased with time. 

If the permeability of the parent material is high, expanded mica and vermiculite are produced by weathering due to the accelerated removal of mono- and divalent cations and soluble silica. In moderately drained soils, the removal of soluble weathering products and, consequently, the decomposition of mica are delayed. As a result of ion accumulation in poorly drained soils, the conditions are favorable for pedogenic formation of minerals. 

In arid environments, where the soluble products of weathering are not completely removed from the soil, saline solutions may circulate in the soil as well as in rock fractures. If upon evaporation the salt concentration increases above its saturation point, salt crystals form and grow (Goudie et al, 1970). The growth of salt crystals in crevices can force open fractures. Salt weathering occurs in cold or hot deserts or areas where salts accumulate. Boulders, blocks. 

The materials that accumulate to form sedimentary rocks are (i) products of disintegration (weathering) of rocks, (ii) volcanic ejecta, (iii) insoluble decomposition products, (iv) precipitated substances from aqueous solutions, and (iv) bio-organically derived substances. 

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