Desilication in Archean weathering processes traced by silicon isotopes and Ge/Si ratios

The influence of continental weathering on oceanic seawater compositions in the Precambrian is poorly understood. Here we combine Si isotopes and Ge/Si ratios of a Mesoarchean paleosol (~2.95Ga) and overlying shales as proxies for weathering processes and Si mass transfer at the early Earth's surface. Results show that, as with modern soils, neoformation of secondary clay minerals in the paleosol was associated with the fractionation of Si isotopes and Ge/Si ratios in response to chemical weathering and soil desilication. Furthermore, the loss of Fe-bearing minerals, most likely Fe-bearing smectites, produced additional controls on Si and Ge mobility, through the coupled dissolution and re-precipitation of clay minerals under reductive conditions. Opposite fractionation behaviors are observed: neoformed secondary clay minerals acted as a 28Si and Ge sink, whereas the leaching of Fe-bearing minerals released 28Si and Ge into soil solutions. An attempt to estimate δ30Si and Ge/Si signatures of released dissolved elements into soil solution provided δ30Si and Ge/Si signatures of about +0.1‰ and 1.4μmol/mol, respectively. Furthermore, shales deposited shortly after paleosol formation display δ30Si and Ge/Si compositions that may be explained as mixtures of the recognized paleosol components. The shale record suggests that weathering-induced desilication and leaching of Fe-bearing minerals, to a lesser extent, might have been widely effective during the Mesoarchean for the transfer of Si from the continents to the hydrosphere. Comparison of Si isotopes recorded in detrital sediments and chemical precipitates over time might thus provide a useful supplementary tool to decipher the impact of putative increases in continental weathering and desilication, relative to changes in hydrothermal and continent-derived solute inputs to ocean and ocean cooling.SCOPUS: ar.j