My Decomposition of Combined Effects in Chemical Denudation

An outline of a hypothetical decomposition of effects that generates the observed field rate is shown in table 6-1.

The estimates of a typical field chemical denudation rate and a computed two-dimensional rate for common rock-forming CaMg silicates have been previously discussed. The estimated magnitudes of each effect in table 6-1 are, at best, informed "guesstimates." The hydrologic factors are presumed to be the macropore effect of most water flushing through a soil bypassing reactive mineral grains, along with the apparently common condition that most of the soil water is usually close to saturation. The latter effect would reduce dissolution rates because reactive mineral surface in contact with saturated solution will not dissolve, being at chemical equilibrium. The combined hydrologic effects are presumed to reduce dissolution rates by a factor of 10_1 to 10~3. The inferred effect of pCO2 soil elevation is perhaps the most robust because pCO2 soil levels have been extensively measured. A factor of3 to 6 is computed, using the previously derived expression, in this case applied to a realistic soil on present Earth, i.e., R is proportional to (pCO2 soil/pCO2 atmosphere)". The soil effect assumes a plausible combination of a range of grain size of CaMg silicate minerals and thickness in a soil; for the example previously given, 1 mm grain size, 1 m thick, gives an elevation in surface area over a two-dimensional surface of 6000 times. The microenvironmental effects of enhanced dissolution from mineral contact with roots, with organic acid levels and lower pH than bulk soil solution, are probably the biggest guess because little research has focused on quantifying these effects and their impact on the observed chemical denudation rate. I would guess a factor of 10 to 102.

The multiplied result of all these factors ranges from 3 X 102 to 6 X 103 compared with the ratio of field chemical denudation rate to computed two-dimensional rate ranging from 102 to 104. Therefore, these two ratios agree. I will not make too much of this agreement given the uncertainties involved. Of all the above effects, the magnitude of the hydrologic and microenviron-mental effects are the most uncertain. However, given these uncertainties, there is little justification to claim, as Drever (1994) did, that "there is not an orders-of-magnitude acceleration attributable to microenvironments." In conclusion, I claim that a plausible decomposition of combined effects occurring in soils can be made that is consistent with relatively high biotic

Decomposing Combined Effects in Chemical Denudation

Effect

Estimated Magnitude

Hydrology: macropore (bulk ol soil water near saturation) Microenvironment: organic acids, chelation, direct microbial dissolution ofsilicate grains pCO2 soil elevation (a = 0.25-0.4,100 PAL in soil)

"Soil effect" (increase in potentially reactive surface area/land area)

10-M0-3

102-104

10-102

Range of combined effects: 3 X 102to 6 X 103, compared with (R/R2D) = 102-104 The proposed rate of chemical denudation (R) is assumed to be 10~2 mm yr_1 (tropical/ saprolitic denudation of granitic rock).

The two-dimensional geometric rate for CaMg silicates (R2D) at 1 PAL CO2, 25°C, pH = 5.5): 10~4-10~6 mm yr_1 (assumes roughness factor of 10, i.e., the ratio of BET to geometric surface area; see White 1995)

enhancement of weathering. This should be taken as a challenge to the weathering community to include this possibility in their research program. Only detailed studies of specific field sites will clarify the relative importance ofeach postulated effect and their combined influence on observed chemical denudation rates in soils and watersheds.

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