Intensity of Chemical Weathering in the Riphean (1.75–0.6 Ga)

The paper compares the variations in several weathering indices (CIA, CIA–K, CIAcorr, MIA (o), MIA (r), and (ω) in clayey rocks of most representative Riphean shelf sedimentary sequences (1.75–0.6 Ga) of the Bashkirian Meganticlinorium (South Urals, a stratotype section) and the Uchur–Maya region (Southeast Yakutia, a Riphean hypostratotype) and their changes in paleosols of the same period from other world regions (USA, Australia, and Scotland). It is thus found that variations in intensity of chemical weathering, which are reflected in bulk chemical composition of the Riphean fine-grained clastic/clayey rocks of South Urals and Southeast Yakutia, are smoother than those in paleosols. This is likely due to the mineral and chemical composition of clayey rocks of shelf associations is more averaged than that of rocks with similar granulometry in continental sedimentation areas. Despite this, our results can be considered an additional argument in favor of a previous conclusion on approximately constant on-land weathering intensity over the last three billion years of the Earth’s geological evolution. The constant weathering intensity during the Riphean (i.e., over a period of >1 Ga) allows us to assume that various global and subglobal processes and events, such as the formation and breakup of supercontinents (including the presence of many continental blocks in the equatorial region at the peak of Rodinia assembly), the formation of large igneous provinces, the changes in atmospheric composition, and paleoclimatic perturbations, had a minor impact on weathering intensity. This issue however requires careful consideration in the future.