Natural sources of metals to the environment

Almost all metals present in the environment have been biogeochemically cyc led since the formation of the Earth. Human activity has introduced additio nal processes that have increased the rate of redistribution of metals betw een environmental compartments, particularly since the industrial revolutio n. However, over most of the Earth's land surface the primary control on th e distribution of metals is the geochemistry of the underlying and local ro cks except in all but the worst cases of industrial contamination and some particular geological situations. Fundamental links between chemistry and m ineralogy lead to characteristic geochemical signatures for different rock types. As rocks erode and weather to form soils and sediments, chemistry an d mineralogy again influence how much metal remains close to the source, ho w much is translocated greater distances, and how much is transported in so lutions that replenish ground and surface water supplies. In addition, dire ct processes such as the escape of gases and fluids along major fractures i n the Earth's crust, and volcanic related activity, locally can provide sig nificant sources of metals to surface environments, including the atmospher e and sea floor. As a result of these processes the Earth's surface is geoc hemically inhomogeneous. Regional scale processes lead to large areas with enhanced or depressed metal levels that can cause biological effects due to either toxicity or deficiency if the metals are, or are not, transformed t o bioavailable chemical species.