Total mercury in sediments from mud volcanoes in Gulf of Cadiz.

Mercury emissions into the environment have both natural and anthropogenic sources. Natural sources include volcanic activity, erosion of mineral deposits (the earth’s crust contains 0.5 parts per million of mercury) (Gochfeld, 2003; Gustin, 2003), association with hydrocarbons (Miedaner et al., 2005; Wilhelm et al., 2006), and volatilization from the oceans, whereas man related sources include metal smelting, coal combustion, chemical synthesis/use, and waste incineration (SEC, 2005). Between 2700 and 6000 tons of elemental mercury are released per year via natural means to the biosphere through degassing from the earth’s crust and oceans (Tchounwou et al., 2003). Industrial wastes and the combustion of fossil fuels add up to an additional 2000–3000 tons of mercury to the environment (ATSDR, 1999). Natural and anthropogenic sources led to a significant enhancement in environmental exposure and deposition, increasing the existing global Hg pool created by past releases (SEC, 2005). Mercury has high affinity for suspended particles, which can lead to its extraction from the water column and its accumulation in the sediments. Thus, sediments function as a deposit and also as a source of mercury to porewaters and biota (Ramalhosa et al., 2001, 2006; Ram et al., 2003). It is also well known that methylation processes, mediated by bacteria in sediments, convert mercury into methylmercury, the most toxic lipophilic form (Heyes et al., 2006; Kim et al., 2006). This organic mercury is readily bioavailable, accumulating along food chains due to bioconcentration and biomagnification (Baeyens et al., 2003; Gochfeld, 2003; Tchounwou et al., 2003). Therefore, mercury is now acknowledged as a global, diffuse and chronic