Chemistry Reference
In-Depth Information
estimates of the density (1 g cm
−3
) and specific surface area (0.1 m
2
g
−1
) of silicate
minerals in the weathering layer, the total exposed silicate mineral surface area is then
6.5
10
17
m
2
. Combining this value with the dissolution rates given above (10
−12
to 10
−11
mol Si m
2
s
−1
), the total flux of dissolved H
4
SiO
4
produced by weathering should be
comprised between 29 and 290 Tmol per year.
This extrapolation is fraught with uncertainties, yet it conveys a simple message.
Even with low estimates of the exposed silicate mineral surface area, the laboratory
dissolution rates predict weathering rates that are much higher than the total flux of
H
4
SiO
4
delivered by rivers to the oceans. The latter is on the order of 6 Tmol per year
(Tréguer et al. 1995). There are a number of reasons for this discrepancy (Velbel 1993;
Berner and Berner 1996). In contrast to experiments conducted in reactors, much of the
available mineral surface area in soils may only be intermittently in contact with soil
solution. This is particularly true in dry climates and in highly aggregated soils. Thus, the
mineral surface area that is actually dissolving is only a fraction of the total available
surface area. Soil formation also reduces the net production of silicic acid through the
precipitation of secondary silicate minerals. The total river flux of H
4
SiO
4
is further
reduced because of hydraulic short-circuiting in watersheds, internal continental
drainage, as well as biogenic silica retention in aquatic and terrestrial ecosystems.
Upscaling from laboratory dissolution rates to global weathering fluxes thus requires
a combination of approaches (Fig. 4). In the same way that biogeochemical cycles
respond to different forcings depending on the spatio-temporal scale of interest, the
different approaches yield information on processes and variables acting at variable
scales. Typically the interpretation of data and observations at a given scale relies on
process-based knowledge from the underlying scales. For example, climate may affect
continental weathering rates through the direct effect of temperature on mineral
dissolution rates, or via its effects on vegetation, rainfall and mechanical erosion. A
×
Figure 4.
Approaches for studying weathering rates: different approaches yield information on controls
acting at different scales, from the molecular to global scale. Typically, mechanistic understanding at
one scale helps rationalize observations made at higher scales.
