Material Fluxes on the Surface of the Earth (1994) / Chapter Skim
Currently Skimming:
Surficial Weathering Fluxes and Their Geochemical Controls
Surficial Weathering Fluxes and Their Geochemical Controls
Pages 28-45
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 28... ...
The formation of new minerals in soils as a by-product of weathering of the primary source rocks provides a temporary storage reservoir that modifies the surficial runoff fluxes in response to environmental changes, of magnitudes comparable to those that occurred in postglacial time. The role of weathering reactions as geoflux buffers is compatible with the rates of soil formation and the rates of chemical denudation on global, continental, and regional scales.
|
From page 29... ...
In this volume, various aspects of material transport on global or continental scales have been addressed for the dissolved and suspended materials in rivers (Meybeck, Chapter 4; Milliman and Syvitski, Chapter 5~; for sediment transport, crustal denudation, and river flows in Glacial times (Andrews and Syvitski, Chapter 7; Kump and Alley, Chapter 3; Baker, Chapter 6~; for eolian transport (Rea et al., Chapter 8~; and for the chemical, biogenic, and detrital material fluxes in the oceans (Dymond and Lyle, Chapter 9; Hay, Chapter 1; Martin and Sayles, Chapter 10; see also Schneider and Kellogg, 1973; Milliman, 1993~. In view of the importance of continental waters to the weathering geofluxes, the present chapter addresses the following aspects of the weathering releases: · the residence times of surficial water flows on the continents; · the main mechanisms of chemical weathering of surficial crustal rocks and soils; · the dependence of weathering processes on the major environmental changes, comparable to those that occurred since the latest glaciation peak, 18,000 yr ago; · the magnitudes of variation in the weathering geofluxes that are controlled by water-rock chemical interactions and water flow; and · the resistance to change displayed by the surficial geofluxes when the surficial environment is exposed to major environmental perturbations.
|
From page 30... ...
Specific water discharge values for 5°-wide latitudinal zones of the globe have been compiled for water flows from the continents to the oceans, as well as to the internal drainage basins, by Baumgartner and Reichel (1975~. Specific discharge is taken equal to the difference between the annual amounts of atmospheric precipitation and evaporation, and as such it includes both the surficial runoff and the groundwater flow.
|
From page 31... ...
Drainage basin area (ADB) Water residence times Freshwater lakes All)
|
From page 32... ...
Residence Times on a GiobaZ Scale Global discharge to the ocean includes the rivers that drain the major lakes and lake systems, as well as river and land-surface flows that do not pass through big lakes in their drainage basins. Estimates of the two kinds of discharge may be arrived at as follows.
|
From page 33... ...
Summary of Water Flows and Residence Times Mean residence times of water on the land surface, partitioned into a river flow component passing through lakes and a component running off directly to the oceans are of the same order of magnitude: a mean of 5.5 yr for the lake outflow and 6.5 yr for an upper-end estimate of direct runoff. The mean residence time for lake-draining rivers does not well represent the very wide range of lake residence times that extends up to several centuries and their geographically uneven occurrence.
|
From page 34... ...
Weathering Flux Equations Mass and Linear Weathering Rates Based on the conclusions of the preceding section, the flux of dissolved materials in global runoff is treated as a measure of bedrock and soil weathering. The rate of weathering is usually expressed as mass loss per unit area of the geographic rock surface per unit of time: W = Cq = A ~ (2.8)
|
From page 35... ...
, literature data for Adirondack Mts., New York, and Cascade Mts., Oregon NOTE: Conversion of the weathering rates in units of glm21yr to units of vertical recession of the land surface, in millimeters per 1000 yr (mrnllca) , was done by dividing the mass rate by a rock density value of about 2.5 g/cm3; see Eq.
|
From page 36... ...
, S 3hs (2.14) A r The mean particle sizes for a weathered rock and a residual soil on it may be taken from the data in Figure 2.6: (rock)
|
From page 37... ...
Mineral Dissolution Rates (RJ Terminology For a discussion of weathering processes in mineral-water systems, a brief clarification of some of the concepts and terms is given: · Dissolution is a process of transfer of mass from a solid to the aqueous phase. A rate of dissolution is measured in units of mass per unit of surface area of a dissolving mineral per unit of time, commonly in units of mol/cm2/s.
|
From page 38... ...
For carbonates, the experimentally determined rates of dissolution for calcite and dolomite (Plummer et al., 1978; Chou et al., 1989) , shown in Figure 2.7, are significantly higher than those for silicates; this feature finds its reflection in the weathering rates of carbonate rocks, which are generally higher than those of crystalline silicate rocks.
|
From page 39... ...
concluded that field dissolution rates, based on the best reasonable estimates of the mineral surface areas, are much slower than the rates indicated by laboratory results. The weathering rates of sandy-silty soils, volcanic rocks and crystalline silicate rock terrains (Table 2.2)
|
From page 40... ...
Further refinement, such as inclusion of the individual dissolution rates of limestones, dolostones, and different types of silicate rocks, is not justified for this simple computation. By using the ranges of carbonate and silicate dissolution rates from Figure 2.7, and the preceding estimates of the carbonate outcrop area and the percentage contributions of rock types, the weathering rate W is W (10-9 )
|
From page 41... ...
Table 2.4 lists explicit equations for alkalinity and the auxiliary parameters defining it in terms of the hydrogen-ion concentration, and either total dissolved inorganic carbon (CT) or the CO2 pressure at equilibrium with the solution (POOP.
|
From page 42... ...
Because the residence time of surface runoff from the continents is short, months to years, the effects of higher dissolution rates might be expected to show in the runoff on time scales comparable to those of the runoff residence times. Although historical data on dissolved loads of streams and rivers in preindustrial times are difficult to come by, some modern analogues to the magnitudes of past changes in weathering are provided by chemical surveys of lakes over the past 40 to 60 yr.
|
From page 43... ...
In judging by the acidity or alkalinity values of surface land waters at the present time, newly formed acidity may be effectively neutralized in carbonate-mineral terrains, but it may persist for longer periods in waters that are in contact with crystalline silicate rocks. ACKNOWLEDGMENTS It is a pleasure to acknowledge James Drever's (University of Wyoming)
|
From page 44... ...
(1988~. Weathering and erosion in the humid tropics, in Physical and Chemical Weathering in Geochemical Cycles, A
|
From page 45... ...
(1988~. Influence of acid rain on weathering rates, in Physical and Chemical Weathering in Geochemical Cycles, A
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.