Continuously increasing δ98Mo values in Neoarchean black shales and iron formations from the Hamersley Basin

A similar Mo-isotope pattern was previously found in contemporaneous black shales and carbonates of the Griqualand West Basin, South Africa. The consistent and concomitant increase in δ98Mo after 2.54 billion years ago suggests a more homogenous distribution of seawater molybdate with uniform isotopic composition in various depositional settings within the Hamersley Basin and the Griqualand West Basin. The modeling of the oceanic Mo inventory in relation to the Mo in- and outflux suggests that the long-term build-up of an isotopically heavy seawater Mo reservoir requires a sedimentary sink for isotopically light Mo. The search for this sink (i.e. adsorption onto Mn-oxides in well oxygenated surface oceans and/or subaerial environments or incomplete thiomolybdate formation in weakly sulfidic settings) remains debated, but its relevance becomes more important closer to the Great Oxidation Event and is probably related to already weakly oxidizing conditions even prior to the 2.5 Ga “whiff of oxygen”.