The terrestrial environment crust and material cycling

Terrestrial environments consist of solid (rocks, sediments and soils), liquid (rivers, lakes and groundwater) and biological (plants and animals) components. The chemistry of terrestrial environments is dominated by reactions between the Earth's crust and fluids in the hydrosphere and atmosphere.

The terrestrial environment is built on continental crust, a huge reservoir of igneous and metamorphic rock (mass of continental crust = 23.6 X 1024g). This rock, often called crystalline basement, forms most of the continental crust. About 80% of this basement is covered by sedimentary rocks contained in sedimentary basins, with average thicknesses of around 5 km. About 60% of these sedimentary rocks are mudrocks (clay minerals and quartz—SiO2), with carbonates (limestones—calcium carbonate (CaCO3)—and dolostones—MgCa(CO3)2) and sandstones (mainly quartz) accounting for most of the rest (Fig. 4.1).

Mud, silt and sandy sediments form mainly by weathering —the breakdown and alteration of solid rock. Usually, these sedimentary particles are transported by rivers to the oceans, where they sink onto the seabed. Here, physical and biological processes and chemical reactions (collectively known as diagenesis) convert sediment into sedimentary rock. Eventually these rocks become land again, usually during mountain building (orogenesis).

The geological record shows that this material-transport mechanism has operated for at least 3.8 billion years. New sediments are derived either from older sedimentary rocks or from newly generated or ancient igneous and metamorphic rock. The average chemical composition of suspended sediment in rivers, sedimentary mudrock and the upper continental crust is quite similar (Table 4.1).

Fig. 4.1 Schematic cross-section of continental crust showing geometry and global average composition of sedimentary cover.

Table 4.1 Average chemical composition of upper continental crust, sedimentary mudrock and suspended load of rivers. Data from Wedepohl (1995) and Taylor and McLennan (1985).

Average upper continental crust (wt%)*

Average sedimentary mudrockt

(wt%)

Average suspended load (rivers)i (wt%)

SiO2

65.0

62.8

61.0

TiO2

0.6

1.0

1.1

AI2O3

14.7

18.9

21.7

FeO

4.9

6.5

7.6

MgO

2.4

2.2

2.1

CaO

4.1

1.3

2.3

Na2O

3.5

1.2

0.9

K2O

3.1

3.7

2.7

S

98.3

99.9

99.4

*A silicate analysis is usually given in units of weight% of an oxide (grams of oxide per 100g of sample). As most rocks consist mainly of oxygen-bearing minerals, this convention removes the need to report oxygen separately. The valency of each element governs the amount of oxygen combined with it. A good analysis should sum (S) to 100wt%.

fThis analysis represents terrigenous mudrock (i.e. does not include carbonate and evaporite components), a reasonable representation of material weathered from the upper continental crust. ^Average of Amazon, Congo, Ganges, Garronne and Mekong data.

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

Agent

Percentage of total 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 dust

0.2

Related to desert source areas and wind patterns, e.g. Sahara, major source for tropical Atlantic. Composition similar to average sedimentary rock. May have high (<30%) organic matter content

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 temperature 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 influence 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

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