The surficial geofluxes, as understood in this volume, are the fluxes of solid, gaseous, and dissolved materials occurring at the Earth's surface. This chapter deals specifically with the geofluxes associated with chemical weathering (or weathering, for short) — an assemblage of processes that are related to biogeochemical reactions between waters and minerals or biogenic solids in the surficial sections of the continental crust. Some, and sometimes practically all, of the products of weathering are transported by running surface water and groundwater, which amounts to a release of the crustal materials to the surficial environment, and their subsequent transport on the continents and off the continents to the oceans.
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.
Water flow is the main agent of material transport from the continents to the oceans and it accounts for the major fraction of the masses of solid and dissolved materials transported from the continents to the oceans: about 90 ± 5 percent of the global material flux from land to the oceans is due to water runoff from the continental surface, and the remaining 10 ± 5 percent of the flux are carried by winds (Garrels and Mackenzie, 1971; Goldberg, 1971; Lerman, 1979; Drever, 1988; Meybeck, Chapter 4, this volume; Milliman and Syvistski, Chapter 5, this volume; Rea et al.,Chapter 8, this volume).
Water as a transport agent of continental materials is unique in its continuous movement in a cycle from the oceans to the atmosphere and land, with two-way precipitation and evaporation flows, its occurrence on the Earth's surface as a liquid, gas, and solid; and its ability to react chemically with minerals and biogenic materials, dissolving or altering them in the process. The dual characteristic of water as a substance interacting chemically with the crustal solids and as transport agent carrying dissolved and solid materials is its most important feature in the context of the surficial geofluxes. The flows of ions and electrons occurring in chemical reactions between water and solids underlie such macroscopic processes as dissolution and precipitation, oxidation and reduction, ion exchange, recrystallization of minerals, and the entire complex of biological primary production and respiration.
Ideally one would like to be able to estimate the magnitudes of the surficial geofluxes under different environmental conditions for the past, present, and future by relying on conceptual and mathematical models of the surficial weathering and transport processes. For example, an adequate model for estimation of the weathering geofluxes should ideally be based on fundamental knowledge of such processes as the mechanical disaggregation of rocks by moving ice sheets, mineral dissolution and precipitation rates under different temperatures, chemical composition of flowing waters as controlled by inorganic chemical reactions and biological processes, and the principles of fluid transport by water and wind. Such straightforward estimates, however, derived from "bottom-up" models based on fundamental data and leading to reliable conclusions on regional, continental, or global scales have been achieved more often as exceptions rather than as rules. The complexity of the surficial environment and changes occurring within it on different physical and time scales make many of the estimates based on fundamental principles useful only as orientational values.
On a time scale of some 16,000 yr, from the peak of the last glaciation about 18,000 yr ago to early human times that preceded the spreading of agriculture and the later industrial activity, a number of major environmental changes were likely to affect the surficial releases by the weathering geofluxes. These changes included the reduction of the ice-sheet cover of the continents in the Northern Hemisphere (Kump and Alley, Chapter 3, this volume), the rise of global sea level by some 80 to 120 m; inundation of the continental shelves; an increase in the continental drainage area; changes in the areal exposure of crystalline and carbonate rocks (Meybeck, 1984); possible changes in the volumes of water discharge from land to the oceans; broad