Mesoproterozoic calcareous paleosols from Montana

Abstract

Calcareous soils are widespread in aridlands today, and common as Phanerozoic paleosols, but rare among Precambrian paleosols. Their geological antiquity is relevant to reconstructing Precambrian atmospheric CO2 and soil microbiomes. This contribution documents in detail calcareous paleosols (Calcids) and other paleosols dated to 1460 Ma (Calymman) from the Spokane Formation of the Belt Supergroup near Wolf Creek, Montana. Dolomitic Calcids may be as old as Paleoproterozoic (2403 Ma), or perhaps Archean (2600 Ma), but these Mesoproterozoic paleosols are the oldest known calcitic Calcids. Chemical data and Bk metrics are evidence that these paleosols formed in a temperate, semiarid climate with pronounced seasonality. A stable isotope paleobarometer for the carbonate is evidence of atmospheric CO2 levels of 1798 ± 502 ppm, compatible with the greenhouse effect needed at that time to offset the faint young sun. Comparable levels of CO2 can also be calculated from consumption of cationic bases during soil formation. Redox element distribution in the paleosols is evidence of O2 levels of <1%, barely adequate for respiration of eukaryotes, but there is evidence of a microbial earth terrestrial ecosystem from phosphorus depletion, nodule distribution, and stable isotopic correlation.