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

Abstract

We present Mo-, C- and O-isotope data from black shales, carbonate- and oxide facies iron formations from the Hamersley Group, Western Australia, that range in age from 2.6 to 2.5billion years. The data show a continuous increase from near crustal δ98Mo values of around 0.50‰ for the oldest Marra Mamba and Wittenoom formations towards higher values of up to 1.51‰ for the youngest sample of the Brockman Iron Formation. Thereby, the trend in increasing δ98Mo values is portrayed by both carbonate facies iron formations and black shales. Considering the positive correlation between Mo concentration and total organic carbon, we argue that this uniformity is best explained by molybdate adsorption onto organic matter in carbonate iron formations and scavenging of thiomolybdate onto sulfurized organic matter in black shales. A temporal increase in the seawater δ98Mo over the period 2.6–2.5Ga is observed assuming an overall low Mo isotope fractionation during both Mo removal processes. Oxide facies iron formations show lowest Mo concentrations, lowest total organic carbon and slightly lower δ98Mo compared to nearly contemporaneous black shales. This may indicate that in iron formation settings with very low organic matter burial rates, the preferential adsorption of light Mo isotopes onto Fe-(oxyhydr)oxides becomes more relevant.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 2.54billion 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 2.5Ga “whiff of oxygen”.