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FIGURE 3.7 Modern CO2 sinks defined as regions of "silicate" exposure (Figure 3.6) that have high physical erosion rates (Snead, 1980; Broecker, 1983) and high runoff (UNESCO, 1978).
Knoll and James, 1987). Root respiration produces CO2 in the soil and, aided by bacterial decomposition of forest litter, generates CO2 pressures that are 10 to 100 times the atmospheric value (Cawley et al., 1969; Holland et al., 1986). In addition, roots aid in the physical breakdown of the bedrock. Organic acid production in forest soils also increases the solubility of minerals. These effects would be diminished if the areal extent of forests were diminished.
A marked effect of glaciation on forests is the loss of the temperate deciduous and evergreen forests (e.g., Broecker, 1983). These forests today cover a large fraction of North America, Europe, and Asia (Figure 3.8A); during glacial maximum they were largely removed, either from being overrun by glaciers or from increased aridity (Figure 3.8B).
The areal extent during glacial time of the tropical rainforests of South America and Africa was also reduced due to aridity. This reduction was partially offset by the expansion of tropical forests, especially in Oceania, into shelf regions that became exposed as sea level fell.
In total, it seems that the biological enhancement of chemical weathering rates was significantly reduced during glacial times due to restriction of the areas of temperate and tropical forests. However, these areas were largely replaced by grasslands; the enhancement factor of this ecosystem type should be less, but relative enhancements are poorly known. The implied reduction of chemical weathering rates should, over thousands of years, have tended to increase atmospheric CO2 concentrations due to (1) decrease in the carbonate ion concentration of the ocean, caused by the decrease in the rate of supply of alkalinity from weathering, and thus an increase in the equilibrium pCO2 (e.g., Broecker and Peng, 1987); and (2) an excess of CO2 production by volcanism over consumption by silicate weathering (e.g., Volk, 1987). This feedback is negative; it would tend to counter the climate perturbations producing glacial conditions.
Climate and Glacial Sediment Supply
Wet-based continental ice sheets probably suppress chemical weathering but speed physical erosion, as discussed above. Some results of the growth and decay of a large ice sheet include
removal of regolith from central regions, leaving fresh bedrock exposed or covered by a thin layer of glacial sediments;
ice-contact deposition of thick sequences of glacially transported sediment in marginal regions, often on top of older regolith, these glacially transported sediments contain much fresh mineral surface area formed by abrasion/comminution and may be deposited in seas, lakes, or on land; some of the terrestrial deposits are flooded by subsequent sea-level rise;
fluvial transport of glacially eroded and comminuted sediment into lakes and seas, although typically with significant aggradation of river channels and thus sediment storage [the common occurrence of glacial-age fluvial terraces along modern streams shows that such storage still is occurring (Schumm and Brakenridge, 1987)]; and
eolian transport of glaciogenic silt off outwash plains to be deposited as loess in adjacent regions, especially if dry or seasonally dry climates suppress vegetation on outwash and allow desiccation and easier wind erosion (Pye, 1984).
The net result is to increase the chemical reactivity of mineral surfaces in all glacially affected areas. Weathered regolith is thinned or removed beneath the center of the ice sheet to expose fresh bedrock, and comminuted regolith and bedrock are deposited over weathered regolith beneath and beyond the ice margins. Comprehensive data are not available on increases in mineral reactivity from ice-sheet glaciation, but such increases are likely to be significant.