Precambrian paleosols on the Great Unconformity of the East European Craton: An 800 million year record of Baltica’s climatic conditions

Abstract

Abstract Meso- and Neoproterozoic paleosols, collected from different areas of the East European Craton: Belarus, Estonia, Lithuania, and Ukraine, offer a chance to examine continental weathering sequences from early to advanced stages of weathering on a variety of different parent materials such as gneisses, granites, gabbros and amphibolites. They were studied using quantitative XRD of the bulk rock, XRD and Mossbauer of the clay fractions, microscopic, geochemical, carbonate stable isotopes and carbonate U-Pb geochronology methods. These paleosol profiles are on average 10\xa0m thick, reddish coloured, and many of them are characterized by a well-developed and well-defined alteration sequence with uppermost horizons approaching the lateritic stage, as indicated by the Chemical Index of Alteration values reaching 90. The dominant type of weathering leads towards kaolinite and a Fe-oxide/hydroxide mineral assemblage through a smectitic intermediate stage. In the paleosol profiles developed on mafic parent materials, dioctahedral smectite is the first weathering product at the base, it dominates in the middle-upper horizons and it later becomes unstable and alters into kaolinite; whereas in paleosols developed on felsic parent materials kaolinite forms already at the initial stage of weathering, as a result of Na-plagioclase dissolution. Kaolinite content in the uppermost horizons reaches 34\xa0wt% in the best developed profile, and Fe-minerals (hematite and goethite) show a clear increasing trend towards the top, reaching 12\xa0wt%. It is likely that uppermost kaolinite-dominated horizons, which are lacking in some profiles, have been eroded. Such paleosol composition and ferric composition of smectites indicate oxidative weathering and are interpreted to represent a warm and humid climate, which seems to have prevailed on the EEC over the Meso- and Neoproterozoic, except well-documented glacial periods. The δ13C signatures of the pedogenic carbonates document microbial processes in the paleosols, which is also indicated by the elevated U/Th values in their top layers. One paleosol was dated with in situ pedogenic calcite U-Pb geochronology at 655\xa0±\xa045\xa0Ma (2σ), confirming the estimate based on its stratigraphic position. The Proterozoic paleosol profiles, and dioctahedral smectite in particular, remained unaltered for over 900\xa0Ma until the Paleozoic, when they were affected by low-temperature (