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Geochemical behavior of molybdenum and its isotopes in the sedimentary environment - A bibliographic review

Abstract:

Numerous studies have been devoted to trace metals and their value in assessing the paleoredox conditions of ancient depositional settings. Among them, molybdenum (Mo) is frequently cited as an effective paleoredox proxy for sediments and sedimentary rocks. Different mechanisms that imply Mo removal from the sea and its later concentration in sediments are widely discussed in the literature. Due to the capacity to form bond of the covalent type and its atomic properties, the dissolved Mo in the ocean (MoO42-), under anoxic-euxinic (rich in H2S) conditions, reacts to form thiomolybdates that subsequently are scavenged by forming bonds with sulfur-rich organic molecules, Fe-Mn oxides (ferromanganese nodules) and iron sulfide (pyrite); this adsorption drives an isotope fractionation of Mo that is marked in δ98/95Mo, with an average of -0.7‰ in oxic conditions and with an average of 2.3‰ in anoxic-euxinic conditions. The use of Mo concentration as a marker of paleoredox conditions needs to be complemented with other independent markers of redox conditions (e.g. U, V, TOC, Re, Os, Fe). Mo isotopic composition seems to be a promising proxy to reconstruct and detect changes in the paleoredox status of the marine sedimentary environments.