The origin and significance of pedogenic dolomite from the Upper Permian of the South Urals of Russia

Pedogenic carbonate nodules from six sections spanning the continental Permian–Triassic boundary in the South Urals, Russia, were analysed. Morphological, petrographic, SEM and XRD analyses have demonstrated that many of the latest Permian palaeosols are dolomitic. This dolomite forms the microcrystalline (5–16 μm) groundmass of the nodules. Later diagenetic phases, represented by coarser crystalline textures, were identified as calcite. Isotopic analysis of the microcrystalline dolomite has revealed it to be similar in isotopic composition to authigenic dolomite forming today in saline soils in Alberta, Canada. These data indicate that the dolomite found in these nodules is pedogenic, and formed in equilibrium with the atmosphere. Upper Permian pedogenic dolocretes in the studied sections are most frequent in (a) palaeosols that formed on palaeo-highs and (b) in the latest Permian period (Changhsingian), which may indicate that there was an increase in seasonality and evaporation in the South Urals region at this time. The presence of only calcitic palaeosols in the earliest Triassic may reflect a subsequent dramatic change in the basin conditions, possibly relating to the Permian–Triassic mass extinction, which stopped the conditions that are necessary for dolomite formation.