Material Fluxes on the Surface of the Earth (1994) / Chapter Skim
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Global Chemical Weathering on Glacial Time Scales
Global Chemical Weathering on Glacial Time Scales
Pages 46-60
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From page 46... ...
. It is only after we have determined how these factors have varied that we will be able to make an estimate of the changes in global chemical weathering rates on glacial time scales.
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From page 47... ...
, · soil moisture was significantly less than today in a large area of Eurasia south and east of the Fennoscandian ice sheet, and in a smaller area of North American south of the Laurentide ice sheet. These drier areas probably developed because of the combined effects of anticyclonic circulation over the ice sheets and of the ice sheets acting as cold traps for water vapor and reducing precipitation nearby; · the southwestern United States was significantly wetter than today, probably owing to a southward shift in storm tracks caused by the topography of the Laurentide ice sheet; · other regional changes occurred: soil moisture was probably reduced in equatorial regions, causing reduction in extent of ice-age tropical rain forests, but increased in some areas at higher southern latitudes; and · precipitation over ice sheets was reduced substantially compared to modern precipitation in the previously glaciated regions.
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From page 48... ...
Shelf Exposure As the Wisconsinan ice sheets grew, sea level fell, progressively exposing more and more of the continental shelf region (Figure 3.3) , which then became subject to glaciation at high latitudes and chemical weathering processes at lower latitudes.
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From page 49... ...
If so, then global chemical weathering rates actually could have been higher during the glacial period, although one must consider climate change as well. Rock Types Exposed A quick glance at a geological map confirms that the distributions of rock types exposed on the surface of the Earth is nonuniform.
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From page 50... ...
We will consider this coupled change below. Glacier Weathering anti Erosion Processes Temperature and Water Production Glaciers and ice sheets are among the most complex and variable systems on Earth.
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From page 51... ...
, allowing thick ice to have a low, warm surface. By simple analogy and by physical reasoning, it is likely that the Laurentide and Fennoscandian ice sheets were frozen to their beds in central regions until their sizes increased close to their maximum values (Hooke, 1977; Sugden, 19771.
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From page 52... ...
Based on this discussion, chemical weathering in contact with continental ice sheets can be summarized as follows: . large areas of ice sheets are frozen to their beds and slow or stop chemical weatherin~ esneciallv dllrin~ icesheet growth; · central ice-sheet regions with thawed beds produce |
From page 53... ...
surficial meltwater with CO2 concentration between that of ice and rainwater, and more atmospheric CO2 is available to this open system; however, high flow velocities and scarcity of supraglacial debris generally prevent rapid chemical weathering; and · glacial meltwater generally does not gain significant CO2 from soil processes, whereas soil processes in unglaciated regions often increase CO2 by an order of magnitude or more from rainwater values (Cawley et al., 19691. (In this respect, it is worth noting that much of the glacial discharge from the southern margins of the Laurentide and Fennoscandian ice sheets drained into climatically dry regions caused by the ice sheets, where soil processes producing CO2 may have been slowed compared to more humid regions.)
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From page 54... ...
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.
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From page 55... ...
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~.
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From page 56... ...
Some loess areas have higher soil moisture today than at any other time since the previous interglaciation, but almost all areas experienced soil-moisture levels higher than those prevailing during loess deposition at some time following deposition (e.g., Kutzbach, 1987; Webb et al., 1987; COHMAP, 19881. The pattern of loess deposition during glacial periods followed by enhanced chemical weathering and soil development on loess has been repeated numerous times, and probably for every Pliocene-Pleistocene Northern Hemisphere glaciation (Fink and Kukla, 19771.
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From page 57... ...
To do a convincing job of weighing these factors and determining a net change in the global chemical weathering rate, one would want to make a geographically based geochemical-cycling/climate calculation. In lieu of this, we can merely speculate that these changes in the global chemical weathering rate were roughly compensatory and thus that globally averaged chemical weathering rates during glacial times were similar to those during interglaciations (with large error bars)
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From page 58... ...
As our knowledge of oceanic changes in the rate of Quaternary carbonate accumulation improves, perhaps the need will arise to invoke changes in global chemical weather~ng rates to explain these observations. Finally, we have found that there is reason to believe that the rate of CO2 consumption during silicate weathering varies on glacial time scales, due both to the direct effects of glacial coverage and erosion, and to the indirect effect of glaciation on climate.
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From page 59... ...
The influence of continental ice sheets on the climate of an ice age, Journal of Geophysical Research 90(D1)
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From page 60... ...
(1972~. Deep erosion by continental ice sheets, Geological Society of America Bulletin 83, 1037-1056.
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