Climatic and biotic changes around the Carboniferous/Permian boundary recorded in the continental basins of the Czech Republic
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The paper provides an overview of a several decades-long study of transitional Carboniferous-Permian (Stephanian C-Autunian) sedimentary successions in continental basins of the Czech part of the Bohemian Massif. These predominantly monotonous fluvial red beds intercalate with laterally widespread grey to variegated sediments of dominantly lacustrine origin. Both, fossil and climatic records show that apart from a generally known long-term climatic shift to drier conditions in Early Permian, the climate oscillated on several time scales throughout the study interval. Climatic indicators in the red beds part of the succession include palaeosols ranging between red vertisols and vertic calcisols suggesting strongly seasonal dry sub-humid climate. This is in agreement with the rarity of plant remains, which were mostly completely oxidised and only rarely preserved as plant impression in red mudstones or as silicified mostly gymnosperm woods in sandy channel fills. Silicification instead of coalification was the dominant fossilisation process during red-beds deposition. Even drier, possibly semi-arid climate may be indicated by spatially and temporarily restricted bimodal sandstones, dominated by well-rounded quartz grains and interpreted as eolian in origin. Periods of moist sub-humid (or even humid) climate were accompanied by formation of perennial lakes containing grey laminated mudstones, dark grey bituminous mudstones or limestones, muddy limestones, chert layers or even spatially restricted coals, some of them, however, of economic importance. Shorter climatic oscillations operating on a scale of tens to possibly hundreds of thousands of years are represented by transgressive-regressive lacustrine cycles followed by significant changes in lake water salinity reflected by boron content.

The fossil record indicates the presence of dryland and wetland biomes in basinal lowlands although their proportions varied significantly as the climate changed. During deposition of red beds, the alluvial plain was vegetated dominantly by dryland biome assemblages. The composition of these assemblages is indicated by fairly common silicified gymnosperm (cordaitalean and coniferous) woods in sandstone-conglomerate fluvial channel bedforms and by poorly preserved impressions of walchian conifer shoots and cordaitalean leaves in associated mudstone intercalations. This is in agreement with sub-vertical root rhizolites and haloes in calcic vertisols. Occurrence of ¡°wet spots¡± colonised by wetland assemblages is indicated by rather exceptional findings of silicified calamite stems in fluvial red beds associated with gymnospermous woods.

During the humid intervals parts of the basinal lowlands were occupied by lakes surrounded by broad belts of wetland biome floras. During the ¡°Stephanian C¡± most of these floras were dominated by tree ferns, calamites and sub-dominant pteridosperms. Local peat swamps were colonised by lycopsids including Sigillaria brardii, Asolanus camptotaenia and even some lepidodendrid lycopsids. In contrast, the fossil record of ¡°Stephanian C¡± dryland floras is rarely preserved in lacustrine sediments. The fossil record of ¡°Autunian¡± lakes, however, suggests increasing proportions of dryland elements, including conifers and peltasperms.

The response of lacustrine faunas to climatic oscillations around the Carboniferous-Permian transition is less prominent than that of plants. The origin of the transition between the local Elonichthys-Sphaerolepis and Acanthodes gracilis bio/ecozones around the Carboniferous/Permian boundary is impossible to deduce from the existing fossil record.

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