For example, the rate of sea level rise in the next century could become 1 cm per year, a rate not unlike that at which sea level rose at the end of the last ice age. Study of the coasts drowned at that time will provide information relevant both to biotic response to a future sea level rise and to the anticipated acceleration of sedimentation in the lower reaches of river systems.
Geomorphologists and geochemists have been using particular isotopes, produced in the atmosphere and in rocks by cosmic radiation, to determine ages of landforms and to date events such as floods, landslides, fault movements, lava and debris flows, and the onset of glaciation. The application of accelerator mass spectrometry to carbon-14 (14C) dating provides a means of dating samples both older and smaller—by a thousand times—than the type of sample conventionally used.
Various new methods are being developed to determine the age of a landform, measuring the time elapsed either since the rocks forming it were deposited or since they were exposed by erosion. These methods represent a breakthrough in quantifying landform dynamics, permitting more precise dating of key events in the evolutionary development of landforms and yielding erosional rates for the various features of a landscape.
The Himalaya, for example, have long been considered not only the highest but also the fastest rising mountain range in the world (Figure 3.1). Just how fast they are rising is something we are only beginning to understand. Uplift rates may be as great as 5 mm per year—5 km per million years—in places along the front of the mountain belt during the past 20-million-years. Crystals of microcline were eroded from the surface and deposited in the sediments of the Bengal deep-sea fan south of Sri Lanka only a few million years after they became cool enough to stop losing radiogenic argon by diffusion, which occurred at a depth of 5 km inside the mountain belt.
The interaction of the atmosphere and hydrosphere—in the form of groundwater—with the rocks at the surface is very complex, partly because organisms ranging in size from bacteria to trees are involved but largely because the relevant time scales vary so widely. Water has a residence time as short as a single storm and typically cycles on an annual scale; trees have lives of decades to centuries; and the minerals formed in the weathering processes can have residence times in the soil of as high as thousands to millions of years. Varieties of soil are strongly controlled by local climate, and with the growing interest in global change soils are being looked at anew to learn what they record about past climatic changes on time scales from decades to millions of years.
Soils lie at the interface between the geosphere, biosphere, and atmosphere. They have unique properties that derive from the intimate mixing of partly weathered geological substrata, dead organic matter, live roots, microorganisms, and an atmosphere high in carbon dioxide, nitrogenous gases, and moisture. Ions of potassium, sodium, calcium, magnesium, sulfur, and phosphorus are released from minerals by hydrolytic weathering. Many of these ions, as well as carbon and nitrogen, are also released from dead organic matter by microbial digestion. The geological composition of the soil passively influences soil biota through ionic deficiencies or release of toxic elements. The soil biota actively influence weathering rates through respiration and production of organic acids.
Soils are open systems that gain and lose energy and matter as they evolve through time (Figure 3.2). Gains, losses, translocations, and transformations occur continuously in the soil column; the relative magnitudes of these processes determine the types of horizons formed along the column. If the vectors of these processes remain relatively constant, the intensity of their expression in horizons is time dependent. Time-dependent soils are useful for correlating geographically separated geomorphic surfaces. This property of soils has been widely used in studies concerned with the rates and timing of tectonically and climatically driven geomorphological processes.
In the past, most research on rates and pathways of soil development centered on careful chemical, physical, and microscopic dissection of soil horizons developed during known time intervals. Future research will emphasize collection of in situ measurements to document fluxes of gases and solutes, rates of mineral weathering, and biological interactions. Already, these studies reveal the physiology of soil to be modulated by complex feedback mechanisms that are nonetheless subject to catastrophic breakdown under external influences such as fire, erosion, or dramatic climatic change. If we are to understand the contribution of soils to ecological stability, we must also study the effect of anthropogenic disruptions, such as toxic chemical discharges, on soil processes. Soils are great purifiers, but we do not know what level of disruption constitutes a lethal dose.