MOLYBDENUM GEOCHEMICAL CYCLE IN THE ARCHEAN

Molybdenum is a redox-sensitive element; its behavior in shales may be used to reconstruct the paleoredox conditions of the oceans. We have investigated the relationships among Mo, organic C (C_org), and S contents in 113 black shale samples, 3.3 to 2.2 Ga in age, from South Africa (the Fig Tree, West Rand, Platberg, Wolkberg, Chuniespoort, and Pretoria Groups) and Australia (the Fortescue and Hamersley Groups).

Positive correlations exist among Mo, C_org, and sulfide S contents, but the degree of Mo enrichment in the examined samples is generally lower than in highly carbonaceous Phanerozoic shales. For example, the Mo/C_org ratios are typically low for the Archean-Paleoproterozoic shales when compared to the Devonian black shales. Our calculations on the weathering flux of Mo, based on the oxidation kinetics of pyrite, suggest Mo would have been quantitatively transferred from the continents to the oceans if the average soil age was greater than 10⁴ years and if the atmospheric pO₂ was higher than 10^-6 atm, i.e., >0.0005 % PAL (present atmospheric level); this pO₂ value lies far below that of the atmosphere around 2.5 Ga (~0.1% PAL according to Rye and Holland, 2000, or ~1 PAL according to Ohmoto, 1996). Therefore, the lower Mo/C_org ratios for the Archean-Paleoproterozoic shales, compared to the Phanerozoic ones, do not imply that the Archean-Paleoproterozoic atmosphere had a lower pO₂ value than today.

We suggest the geochemical cycle of Mo during the Archean-Paleoproterozoic time was essentially the same as today: Mo-bearing minerals were quantitatively oxidized during weathering, dissolved Mo was transported to the oceans, and it was fixed by C_org and sulfide S in locally anoxic marine environments. These suggestions further imply that the Phanerozoic-style redox structure of the oceans (i.e., a generally oxic ocean with localized anoxic/euxinic environments where bacterial sulfate reduction is active) may have already developed in the Archean.