MOLYBDENUM SEQUESTRATION MECHANISMS UNDER EUXINIC CONDITIONS: IMPLICATIONS FOR THE UTILITY OF MOLYBDENUM AS A PALEOREDOX PROXY

The sequestration of molybdenum (Mo) in restricted basins is an important, but poorly constrained process. Because of the differing behavior of Mo under oxic versus anoxic conditions, its enrichment and isotopic composition in ancient marine sediments have been used as effective tools for determining palaeoceanographic redox conditions. While it is known that molybdate (MoO₄^2-) is the prevalent form of molybdenum in oxic waters (i.e., including oceans and terrestrial water bodies) and thiomolybdate (MoO_xS_4-x^2-) is the prevalent form under euxinic conditions (i.e., deep anoxic and sulfidic zones of the ocean, and some meromictic lakes), the potential reduction and sequestration mechanisms of Mo following thiomolybdate formation remains to be revealed. Previous studies of various euxinic sediments have shown evidence that both abiotic and biological processes may contribute to the Mo removal from aqueous phases; however, it is controversial which pathway (i.e., cellular association, organic matter complexation, or adsorption/incorporation with iron-sulfide) is the primary contributor to significant Mo abundances in the rock record. It is also unknown if the primary sequestration pathway of Mo has changed through geological history, and if the evolution of early life may have played a role. Mechanistic understanding into the major pathway(s) of Mo removal will enable us to reconstruct ancient ocean chemistry more accurately, improve our ability to measure the timing and intensity of the climatic fluctuations associated with the major oxygenation and deoxygenation events throughout Earth’s history, and shed light on biogeochemical factors that may have influenced and/or been influenced by evolutionary trends during the early stages of life on Earth. The objective of this review is to examine the current proposed mechanisms of Mo sequestration in euxinic environments, to identify existing caveats and controversies, and to propose avenues for future research aimed at resolving the remaining questions regarding the utility and precision of Mo as a paleoredox proxy.