APPLICATIONS OF τP2O5 AND τFE2O3 AS INDICATORS OF MICROBIAL BIOCYCLING AND REDOX PROCESSES IN PRECAMBRIAN PALEOSOLS AND PALEOSAPROLITES

The Paleoproterozoic Baraboo paleosol (1.7 Ga) is the most chemically mature Precambrian paleosol in the Lake Superior region, having formed under the influence of a warm, humid climate in a region of subdued topography (est. Mean Annual Temperature (MAT_PPM) and Mean Annual Precipitation (MAP_PPM) for Baraboo are 14.3 ºC and 1177 mm yr^-1). Mass-balance, assuming immobile Al₂O₃, indicates that the behaviors of P₂O₅ and Fe₂O₃ are different from those of the labile constituents, with both oxides displaying discontinuous % changes at depth that are larger than the % change variation for mean protolith. Despite the large variations in % changes displayed by the two oxides, their mass fluxes are effectively zero, indicating that they were partitioned in the profile, rather than simply removed, by pedogenic and microbial processes. P is depleted in upper regolith, enriched at the base of regolith, depleted at the top of saprolite, and enriched through most saprolite, possibly via primitive biocycling and bioconcentration. The spike in P₂O₅ at the regolith base is accompanied by a spike in Sr, which may be due to precipitation of an APS mineral, svanbergite. It is reasonable to assume that microbes were present and were probably critical to metabolic activities occurring within the Baraboo profile, and that solubilized P would have been an important “nutrient” for biosynthesis and growth. Total Fe is mostly enriched in the regolith of the Baraboo profile, whereas the upper saprolite is depleted of Fe and the lower saprolite is enriched. Enrichment of Fe in the Baraboo regolith might represent activities of acidophilic Fe²⁺-oxidizing bacteria, assuming that the reconstructed acidic paleo-pH estimates (4 to 5.5) are reasonably correct. The loss of Fe in the upper part of the Baraboo saprolite could record microbial-mediated anaerobic Fe³⁺-reduction, with microbial-mediated anaerobic Fe²⁺-oxidation occurring in the deepest part of the saprolite. Given the elevated levels of paleoatmospheric pCO₂ at 1.7 Ga, estimated from the bulk geochemistry of the Baraboo paleosol and the weak sun model, and attendant lower pO2, we infer that the upper regolith portion of the Baraboo paleosol was oxygenated and the lower saprolite portion was relatively oxygen-deficient.