If we speculate that wet-based glaciation occupied only one-quarter of the Laurentide region for one-quarter of the glacial period (possible but unproven numbers), then the wet-based erosion rate was about 1 mm/yr, similar to mountain-glacier rates (Boulton, 1979).

It must be noted, however, that some geological observations on land are not consistent with 120 to 200 m of glacial erosion in North America over the past 3 m.y., but seem to indicate only glacial stripping of regolith with little bedrock erosion (e.g., Gravenor, 1975; Sugden, 1978). If so, then glacial denudation rates are similar to interglacial rates, but glacial rates are controlled by physical processes whereas chemical weathering is more important during interglacials.

Analogy to the modern Antarctic ice sheet is not very helpful. Documented sedimentation of material transported by the ice sheet is quite slow, indicating slow subglacial erosion. However, data from the best-studied drainage basin of West Antarctica suggest a subglacial erosion rate of O(0.1 to 1 mm/yr), with deposition localized at the grounding line beneath several hundred meters of floating ice shelf (Alley et al., 1989). The generality of this result is unknown.

To summarize, then,

  • ice in wet-based accumulation zones of glaciers and ice sheets erodes and transports unconsolidated materials efficiently; the cumulative effect of midlatitude ice sheets in eroding bedrock remains uncertain;

  • sediments of demonstrable glacial origin from the Laurentide ice sheet indicate denudation rates similar to subaerial values in the region today; and

  • an order of magnitude more sediments than those of demonstrable glacial origin may have been produced by the Laurentide ice sheet and deposited in marine environments.

COUPLED CHANGES

From glacial times until the present, virtually all of the important factors in global chemical weathering experienced significant changes. The planet warmed, the glaciers receded and exposed new materials to weathering, the continental shelves flooded, and precipitation and runoff fields experienced significant perturbations. The net effect of all of these changes on the global rate of chemical denudation is difficult to assess. In this section we will examine some of the coupled changes that occurred, glacial to recent, and finally make an assessment of their net effect.

The Climate/Rock Type Connection

The highest chemical weathering rates on the present Earth occur in regions of high rainfall, high relief, and with limestone and evaporite (e.g., the Yangtze and Brahmaputra rivers) or recent volcanic lithologies (Berner and Berner, 1987). These regions do not occur at high northern latitudes today (or in the recent geologic past) so they did not experience direct glacial coverage during the Wisconsin glaciation. In fact, few of these regions of high chemical denudation fell in zones of decreased soil moisture 18 ka (compare Figures 3.1 and 3.5). Thus there is no clear reason to suspect that glacial, global chemical weathering rates were reduced relative to today, on this basis.

In terms of a climate feedback, silicate weathering is paramount, for as shown above, it is during the weathering of this rock type that net CO2 consumption occurs (averaged over thousands of years). Walker et al. (1981) and Berner et al. (1983) have argued that the balance of CO2 consumption during silicate weathering and production during volcanism regulates atmospheric CO2 concentrations, and thus climate, on time scales longer than a few thousand years. This time scale represents the residence time of atmospheric CO2 with respect to recycling by the carbonate-silicate geochemical cycle (about 3000 yr). Of course, on these short time scales one must consider ocean/atmosphere exchange to be fast, so that perturbations in weathering and volcanism on thousand-year time scales are damped. However, the residence time of the entire ocean/atmosphere carbon reservoir with respect to the geochemical cycle is on the order of 100,000 yr, so we must consider changes in this cycle as potential climate modifiers on glacial time scales.

Where is CO2 being consumed today? For a region to qualify as a significant CO2 sink it should be dominated by silicate exposures and be in a region of high runoff and high physical erosion. By excluding those regions of Figure 3.6 that (1) lie in areas where runoff averages less than 500 mm/yr and (2) are in areas undergoing >100 tons/km2 of physical erosion per year, a map of CO2 sinks is produced (Figure 3.7).

Rapid consumption of CO2 occurs over a relatively restricted part of the land surface. The more important regions include the southeastern United States, the Andes, and southeast Asia. Regions typically assumed to be areas of high chemical weathering (e.g., Amazonia) do not appear, for they have developed thick soil mantles, and chemical weathering rates are actually low.

Of these areas, none fall within a region of higher soil moisture at 18 ka (Figure 3.1), and some probably experienced drier conditions (especially those in southeast Asia and South America). This would suggest that perhaps CO2 consumption rates were somewhat diminished by glacial climates, a negative feedback.

Forest Coverage and Weathering

The presence of forests on land enhances chemical weathering rates by their effect on the soil environment (e.g.,



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