Total mercury in sediments from mud volcanoes in Gulf of Cadiz

Abstract

Mercury emissions into the environment have bothnatural and anthropogenic sources. Natural sourcesinclude volcanic activity, erosion of mineral deposits (theearth’s crust contains 0.5 parts per million of mercury)(Gochfeld, 2003; Gustin, 2003), association with hydrocar-bons (Miedaner et al., 2005; Wilhelm et al., 2006), and vol-atilization from the oceans, whereas man related sourcesinclude metal smelting, coal combustion, chemical synthe-sis/use, and waste incineration (SEC, 2005). Between2700 and 6000 tons of elemental mercury are released peryear via natural means to the biosphere through degassingfrom the earth’s crust and oceans (Tchounwou et al., 2003).Industrial wastes and the combustion of fossil fuels add upto an additional 2000–3000 tons of mercury to the environ-ment (ATSDR, 1999). Natural and anthropogenic sourcesled to a significant enhancement in environmental exposureand deposition, increasing the existing global Hg pool cre-ated by past releases (SEC, 2005).Mercury has high affinity for suspended particles, whichcan lead to its extraction from the water column and itsaccumulation in the sediments. Thus, sediments functionas a deposit and also as a source of mercury to porewatersand biota (Ramalhosa et al., 2001, 2006; Ram et al., 2003).It is also well known that methylation processes, mediatedby bacteria in sediments, convert mercury into methylmer-cury, the most toxic lipophilic form (Heyes et al., 2006;Kim et al., 2006). This organic mercury is readily bio-available, accumulating along food chains due to bio-concentration and biomagnification (Baeyens et al., 2003;Gochfeld, 2003; Tchounwou et al., 2003). Therefore, mer-cury is now acknowledged as a global, diffuse and chronicproblem (SEC, 2005) due to its high toxicity to humans,ecosystems and wildlife.Mud volcanism and fluid flow processes at active conti-nental margins with high sedimentation rates, and in accre-tionary wedges such as in the Gulf of Cadiz (GC), have animportant role in the hydrological budgets, biogeochemicalcycles and physical properties of sediments. Fluid flow andmud volcanism also make a potentially important contri-bution to the geochemical budget of metals in the oceanand in the atmosphere. Fluid advection through the sedi-ments provides an efficient mechanism for the upwardtransport of reactive components and trace gases, wheremethane is one of the most important, and has an impacton the mineralization within the shallow sediments andon the chemistry and benthic biota. The widespread occur-rence of fluid escape processes has been recognized in avariety of different marine settings and throughout the geo-logical record (Campbell et al., 2002; Peckmann and Thiel,2004). The nature and composition of the expelled fluidsand materials from the fluid escape structures and mud vol-canoes (MV) can give valuable information on the geologyand geochemistry of the deeper sediments.The Gulf of Cadiz, located south of Iberia is an areawith extensive evidence of hydrocarbon-rich gas seepage,manifested by the presence of mud volcanoes, pockmarks,diapiric ridges and the formation of methane-related authi-genic carbonates (Gardner, 2000, 2001; Mazurenko et al.,2002; Pinheiro et al., 2003; Somoza et al., 2003). The firstmud volcanoes were discovered in this area in 1999 (Gard-ner, 2000, 2001) and since then they have been extensivelyinvestigated in the framework of several national and inter-national projects (INGMAR; MVSEIS; HERMES). Mostof the geochemistry studies dealing with mud volcanoes inthis area have focused either on the characterization of thegas composition in gas hydrates and in interstitial fluids, or