English: Figure 1. 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). All the green arrows indicate natural processes and are sources of lead, while red arrows indicate lead emissions due to anthropogenic activity, and the blue arrows downwards indicate the sinks for lead. 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. It is the most abundant heavy metal owing to its radiogenic nature and abundant isotopes as a result of decay of Uranium and Thorium in the crust. The natural sources (green arrows) of lead in the atmosphere are due to volcanic eruptions, plant exudates, forest fires, extra-terrestrial particles, radioactive decay, and physical and chemical weathering of rocks. Global estimates of natural lead emissions is approx. 12,000 Mg/yr, and the lead reaching ocean from continental runoff due to natural weathering is estimated to be 295,000 Mg/yr. The anthropogenic emissions (red arrows) of lead have caused disruption to the natural cycle and it may be seen that the fluxes are very high. The major anthropogenic sources are mining and smelting of ores, non-ferrous metal production, stationary fossil fuel combustion platforms, and mobile fossil fuel combustion platforms. The runoff from anthropogenic sources is significantly large, 919000 Mg/yr; this value corresponds to the dissolved lead concentration in major rivers of the world. The dissolved concentration mainly depends on the speciation of lead with other radicals/complexes such as carbonates, sulphates, hydroxyls, and other organic ligand complexes. The sinks (blue arrows) of lead are wet deposition of aerosols on to the ocean water surface and the subsequent burial in deep sediments because of formation of inorganic and organic complexes. Since lead is toxic to life, there are no predominant metabolic pathways. However, lead can be concentrated intracellularly by marine organisms such as phytoplankton. Both these processes serve as sink for lead, however adsorption of lead to particulates is the major sink pathway.