Since the development of life on Earth was so deeply tied with the presence of oxygen in the ancient ocean and atmosphere, it is vital to understand when, how and why Earth’s oxygen concentration has risen and fluctuated since the Great Oxidation Event ( 2.4 billions years ago). To achieve these goals, the geoscience community needs to rely on indirect geochemical indicators sensitive to the presence or absence of oxygen: The paleo-redox proxies.

Among the redox sensitive trace elements (RSTE), molybdenum (Mo) has emerged as a powerful tool to reconstruct oxygen fluctuations in modern and ancient aquatic systems. Two reasons explain this popularity: (1) Mo enrichments in sedimentary records are correlated with a gradient toward euxinia; (2) Mo isotopes are supposed to capture the isotopic seawater signature and thus fingerprint the relative amount of oxygen. Many studies have used Mo systematics to investigate early Earth’s oxygenation, yet sometimes these two approaches led to ambiguous results.

Surprisingly, beside the reactions involving iron and manganese oxyhydroxides under oxygenated conditions, our knowledge regarding the possible pathways leading to Mo burial in presence of sulfide are quite limited and probably too simplistic. The irony of the situation being that Mo is mainly determined to detect past sulfidic conditions…

With the ambition to propose an updated model describing Mo burial pathways under non-oxygenated conditions, my group has explored Mo speciation in several anoxic and euxinic settings and developed new analytical methods. During my lecture, I will present our latest findings and hopefully will convince you of the importance of incorporating RSTE speciation (molecular geochemistry) in your future projects involving paleo proxies.