The oldest known paleosol profiles on Earth: 3.46Ga Panorama Formation, Western Australia

摘要

AbstractSubaerial volcanic flank and floodplain facies of the 3.46Ga Panorama Formation have been recognized on the basis of trough cross-bedded sandstones, lapilli tuffs, and abundant nodularized barite sand crystals, like those noted in other Archean non-marine facies. These barite-nodular layers are interpreted as alluvial paleosols for the following reasons. They show different degree of bedding disruption scaled to nodule size beneath a sharp upper boundary, like desert soil profiles. They show multiple generations of cracking, clay skins, and up-profile destruction of feldspar and rock fragments, compatible with weathering of labile constituents of the parent material. Loss of alkali and alkaline earth elements and phosphorus up profile are also features of chemical weathering. Geochemical mass balance calculations (tau analysis) shows that the profiles lost mass and labile elements, unlike unaltered sediments and tuffs. Rare earth element analysis also shows light rare earth retention as in weathering, as opposed to sedimentation or hydrothermal alteration. Barite of nodules in the paleosols is mobilized to concentration under very acidic conditions (pH<3) and is very stable under less acidic conditions. These Archean alluvial paleosols may have formed by acid sulfate weathering by sulfuric acid, rather than the currently more common hydrolysis by carbonic acid. This archaic system of widespread acid sulfate weathering has since been marginalized to a few playa lakes, deep water tables, and sulfur springs.