Simplified schematic of the lead cycle. All values indicated are fluxes with unit of Mg/yr; the values have been obtained from Cullen and McAlister (2017).[1] The size of the arrows are approximately proportional to their flux. The major reservoir for lead is the crust and mantle with a concentration of 11–14.8 ppm.[1] The natural sources (green arrows) of lead in the atmosphere are volcanic eruptions, plant exudates, forest fires, extraterrestrial particles, radioactive decay, and physical and chemical weathering of rocks.[2] The major anthropogenic sources (red arrows) are mining and smelting of ores, non-ferrous metal production, stationary fossil fuel combustion platforms, and mobile fossil fuel combustion platforms.[1] The sinks (blue arrows) of lead are wet deposition of aerosols on to the ocean water surface[3] and the subsequent burial in deep sediments. Since lead is toxic to life, there are no predominant metabolic pathways.[1]