Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Abstract

Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ 34 S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ 34 S VCDT (Vienna Canyon Diablo troilite) values between +5‰ and −5‰, while the rims show δ 34 S VCDT values between −20‰ and −24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34 S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.