The lead cycle is the biogeochemical cycle of lead through the atmosphere, lithosphere, biosphere, and hydrosphere, which has been influenced by anthropogenic activities.
Lead (Pb) is a heavy trace element and is formed by the radioactive decay of uranium and thorium. In crustal rocks, it is present as the lead sulfide mineral galena. Natural sources of lead in the lead cycle include wind borne dust, volcanic outgassing, and forest fires. Natural weathering of rocks by physical and chemical agents can mobilize lead in soils. Mobilized lead can react to form oxides or carbonates. It can co-precipitate with other minerals by being occluded through surface adsorption and complexation.
Anthropogenic activities have accelerated lead mobilization to the environment. The majority of anthropogenic lead comes from non-ferrous metal manufacturing plants, mining and smelting of ores, stationary and mobile fossil fuel combustion platforms, and lead batteries. These activities produce very fine micron-sized Pb particles that can be transported as aerosols. Anthropogenic lead fluxes decreased from the 1980s to the 2000s as a result of global regulation and outlawing of leaded gasoline. However, the global production in lead has seen a steady rise in the 21st century.
Wet deposition removes lead from the atmosphere to the surface ocean. Precipitation leads to solubilization of aerosols and washout of particulates. Pb concentrations in the oceans is dependent on wet deposition and the concentration of Pb present in atmosphere. The main sink for lead is burial in marine sediments