An Introduction to Environmental Chemistry

68 Chapter Four

This suggests that rivers represent an important pathway of material transport

(Table 4.2) and that sedimentary mudrocks record crustal composition during

material cycling.

This chapter focuses on components of the solid terrestrial environment that

are chemically reactive. Silicates that formed deep in the crust, at high tempera-

ture and pressure, are unstable when exposed to Earth surface environments

during weathering. The minerals adjust to the new set of conditions to regain

stability. This adjustment may be rapid (minutes) for a soluble mineral such as

halite (sodium chloride, NaCl) dissolving in water, or extremely slow (thousands

or millions of years) in the weathering of resistant minerals such as quartz.

Although the emphasis is on understanding how minerals are built and how they

weather, it is clear that water—a polar solvent (Box 4.1)—exerts a major influ-

ence on the chemical reactions, while organisms, particularly plants and bacteria,

influence the types and rates of chemical reactions in soils.

Soils occupy an interface between the atmosphere and the lithosphere and the

biogeochemical processes that occur there are tremendously important. Soils are

a precious resource: indeed human existence—along with many other organisms

—is dependent upon them. Soils provide habitat for organisms and allow the

growth of vegetation, which in turn provides food and additional habitat for other

organisms. Soils thus support many global food webs and their associated

Table 4.2Agents of material transport to the oceans. After Garrels et al. (1975).

Percentage

of total

Agent transport Remarks

Rivers 89 Present dissolved load 17%, suspended load 72%.

Present suspended load higher than geological past

due to human activities (e.g. deforestation) and

presence of soft glacial sediment cover

Glacier ice 7 Ground rock debris plus material up to boulder size.

Mainly from Antarctica and Greenland. Distributed in

seas by icebergs. Composition similar to average

sediments

Groundwater 2 Dissolved materials similar to river composition.

Estimate poorly constrained

Coastal erosion 1 Sediments eroded from cliffs, etc. by waves, tides,

storms, etc. Composition similar to river suspended

load

Volcanic 0.3 (?) Dusts from explosive eruptions. Estimate poorly

constrained

Wind-blown 0.2 Related to desert source areas and wind patterns,

dust e.g. Sahara, major source for tropical Atlantic.

Composition similar to average sedimentary rock.

May have high (<30%) organic matter content