This work investigates how peatlands can be used as archives of past environmental conditions to record changes in atmospheric deposition of some (especially lithophile and chalcophile) elements and any water–rock–organic matter interactions followed by the sedimentation or leaching of others. We have provided a detailed description of both the distribution of various groups of elements and their chemical species in the entire depth of peat deposits. The study analyzes the features of the peat sediment formation and element distribution in the 0–310 cm depth core sample of peat deposits of the Ubinskoye peat bog located in the forest-steppe zone of Western Siberia. The study reveals a profound diagenetic transformation of water and peat chemical composition. Element speciation investigated using the modified Tessier sequential extraction procedure showed the vertical transition of an oxidative geochemical environment to a reducing one with the formation of geochemical barriers for variably valent elements. Computer modeling calculations of saturation indices of pore solutions in relation to a number of minerals allowed us to estimate the degree of equilibrium of the system and the direction of its transformation. Early diagenetic processes lead to the deposition of authigenic minerals. Therefore, barite forms on the redox line, while pyrite is found in the reducing environment. With depth, the content of Ca, Mg and Sr increases, which leads to the formation of authigenic carbonates.