Molybdenum (Mo) isotopes are efficiently removed under euxinic conditions and consequently may directly record the Mo isotopic composition of contemporaneous seawater in ancient organic-rich shales. Removal of Mo to sediment in other environments (i.e., anoxic and oxic) is less efficient and accompanied by a significant negative isotope fractionation, where Δ98MoSW-SED is typically 1 to 3 ‰ [1,2]. Because Mo in solution occurs primarily as the oxidized molybdate complex MoO42-, it is generally accepted that before the Great Oxidation Event (GOE) at ca. 2.3 Ga the transfer of Mo to the oceans was primarily in detrital form. This is in accordance with some available sedimentary data showing low concentrations and a narrow range of isotopic compositions corresponding to the crustal reservoir [3,4]. As atmospheric oxygen started to rise, Mo was chemically weathered from continental sources and transported to the oceans as molybdate. There, it was removed to sediments via several fractionating mechanisms, depending on the redox conditions. Consequently, Proterozoic and Phanerozoic black shales record a wide range of Mo concentrations and isotopic values, reflecting variations in the isotopic composition of seawater as determined by the mass balance between the different sinks [5,6].
In order to further explore the Mo isotopic record of Earth system redox evolution, we measured Mo concentrations and isotopic compositions of black shales from several Neoarchean and Paleoproterozoic sections (2.7 Ga - Belingwe Fm., Zimbabwe; 2.63 Ga - Jeerinah Fm., Western Australia; 2.52 Ga - Gamohaan Fm., South Africa; 2.32 Ga – Timeball Hill South Africa; 2.15 Ga - Sengoma Argillite Fm., Botswana; 2.06 Ga – Zaonega Fm., Karelia). The data suggest low levels of free O2 up to 400 Myr before the GOE, where elevated Mo concentrations together with large isotopic variations and high δ98Mo values are observed in sections dated 2.72 – 2.5 Ga. Moreover, early euxinic conditions are detected in the 2.63 Ga Jeerinah Formation. The 2.32 Ga Timeball Hill Formation, contemporaneous with the GOE [7], shows a dramatic increase in Mo transport accompanied by very strong fractionation effects, possibly pointing to rapid and large variations in free O2 levels. Post-GOE sections (2.15 – 2.05 Ga) indicate another increase in Mo transport to the ocean and development of widespread euxinia at 2.05 Ga. Overall, we show here that secular evolution of the oceanic Mo cycle tracks redox changes in the oceans and atmosphere and represents a powerful tool for refining our understanding of the Earth redox evolution.
