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the variability of natural processes and fluxes in terms of fluctuations and changes occurring on different time scales;
gaps in the understanding of the natural variability of surficial processes and material fluxes; and
how the natural variability might be incorporated into the modern process ''baselines" to be used in models of future change.
This study concentrates on fluxes that move material on the surface of the Earth and are continuous or occur frequently. We have not attempted to evaluate the rate of fluxes of material associated with volcanic eruptions and mass wastage that are discontinuous and are infrequent. We recognize that, in the long term, catastrophic events might have a cumulative impact of the same order of magnitude as the more frequent and continuous processes, but the historical record contains an inadequate sampling of such events. Neither have we attempted to evaluate fluxes of volcanic gases, hydrothermal fluids, or the fluids expelled from sediments along subsection zones. Studies of these fluxes are still in their infancy, and their variability in time and space is poorly known. Preliminary extrapolations indicate that for some materials, these fluxes may be of the same order of magnitude as those from better-known sources.
Although we are concerned primarily with natural fluxes of surficial materials, human influence is important. For example, because of modern agricultural and construction practices, many river particulate loads measured over the past 30 years probably do not reflect natural processes. The storage-transport processes that are important today may not be those important on either shorter or longer time scales. For dissolved materials, humans are already a very effective geologic agent. They increase sources through construction, mining, deforestation, and agricultural activity, including fixation of atmospheric nitrogen. Human activity modifies the sinks for detritus through dam, reservoir, and coastal construction, and for nutrients through the induced eutrophication of lakes. These changes make determination of the roles of natural processes in the modern biogeochemical cycles increasingly difficult.
The natural fluxes of materials on the surface of the Earth are a function of the rates of weathering of rocks and sediments, transport, and deposition. "Weathering" originally referred to the alteration of rocks exposed to atmospheric agents; in recent years it has taken on a broader meaning to include the effects of all processes operating at the interface between the solid earth and the atmosphere, hydrosphere, and cryosphere. Transport is effected by gravity and by moving water, air, or ice. Deposition is a result of gravitational, chemical, or biological processes, or a combination of these processes acting on the transported materials. Weathering is typically very slow; characteristic rates are those of soil formation — on the order of thousands to hundreds of thousands of years. Transport is typically rapid, operating on the scale of minutes to years for gravitational movement, hours to days for eolian transport, days to months for fluvial transport, and decades to millennia for glacial transport. Transport need not be direct from the site of weathering to the site of ultimate deposition, but materials may
FIGURE 1 Some erosional and depositional environments during a glacial lowstand of sea level.