LAUSR

Abstract This paper is devoted to a study of the nature and origin of the mineral matter in the Vasyugan Mire in Western Siberia. Based on mineralogical and geochemical analyses, new results were obtained concerning the mineral composition within the bottom part of the peat deposit for different landscape zones in the Vasyugan Mire. This part of the peat deposit represents a transition sequence from peat through organo-mineral sediments (OMS) to basal loams and is characterized by a gradient change in the physicochemical and environmental conditions with depth. In the same sequence, pH and peat ash yield increase, while moisture decreases. The clay minerals identified in the basal layers of the mire, consisting of loam, OMS and peat, include illite, chlorite, kaolinite and smectite. All these clay minerals found in the OMS and peat layers are believed to have been inherited with little change from the underlying loam layer during the initial transient stage of mire development. That this is indeed the case is strongly suggested by the extraordinary correspondence in the mean content of over 40 trace elements between the OMS and loam layers. In the overlying peat layer, however, some elements, including U, W, Sb, Sn, Mo, Zn, Cd, Nb, Ta, Br and B, are present in higher concentrations than in the OMS and loam. Scanning electron microscope observations suggest that this can be at least partly accounted for by the precipitation of sulfide phases in the strongly reducing and acidic environment of the peat, although whether by inorganic or microbial processes is unknown. The age of the Vasyugan Mire within the Holocene succession is discussed and it is concluded that it developed either during the Atlantic Period (8 ka–6 ka BP) or during the upper part of the Sub-Boreal Period (4 ka BP).