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Pavel Kabanov, Paleontological Institute RAS (Russian Federation)
Tatiana V. Alekseeva, Institute of Physical Chemical and Biological Problems of Soil Science RAS (Russian Federation)
Andrey O. Alekseev, Institute of Physical Chemical and Biological Problems of Soil Science RAS (Russian Federation)
Veronika A. Alekseeva, Moscow State University (Russian Federation)
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Numerous unconformities in the Podolskian (C2p)-Myachkovian (C2m) epeiric-sea carbonate succession of the East European Craton (EEC) are marked by subaerial exposure profiles, primarily paleosols. In the outcrops of Moscow Basin, the paleosol profiles principally consist of the subaerially altered (dispersely karstified, penetrated by rhizocretions and Microcodium) limestone and its thin (<0.3 m) and partly truncated terrestrial clay blanket (topclay). The limestone/topclay interface is usually capped by a thin discontinuous crust of beta calcrete. The solution pits and small (< 3 m deep) karst sinkholes occur at C2m unconformities but unknown in C2p. The topclay consists of in situ clayey epipedons (Ti) and resedimented terrestrial clays (Ts). The latter are mostly regarded as playa sediments. Chert horizons occur low in paleosol profiles and probably represent subsurface duric soil horizons.
The C2m unconformities of S.Dvina-Pinega area (northern EEC) are mostly devoid of topclays, beta calcretes, and root traces. In that area, the entire succession is penetrated by sepiolitic weathering crust.
The C2p paleosols of the southern Moscow Basin reveal palygorskitic composition of topclays. The C2m paleosol clays are smectitic-illitic with minor chlorite.
Multiproxy study of the lower C2p Sennitsa Creek Paleosol at Gory reveals calcareous Ti truncated by non-calcareous Ts. The low alumina/bases and Ba/Sr ratios attest to low leaching activity. In Ti, high alkali, Mn and Sr, presence of fine-grained gypsum, opal, minor sepiolite, and characteristic peak in magnetic succeptibility are consistent with semiarid to arid pedogenic environment. The crystal structure of Ts and Ti palygorskites has been modified by acid attacks as revealed by IR spectra and presence of corroded detrital quartz. These acid events are linked to early oxidation of hydromorhic pyrites. The IR spectra also manifest presence of humic substances. The total organic carbon in the clay fraction is 0.5-1.5 %.
This study reveals the 'great carbonate plain' landscapes lacking fluvial runoff and covered by monotonous soils and shallow short-living playas. Palygorskitic C2p topclays, shallow (less than 30 cm) occurrence of pedogenic carbonates in the soil profile, and details from Sennitsa Creek paleosol attest to hot semi-desert comparable to modern aridlands with precipitation < 300 mm/yr. The shift from palygorskitic to smectitic paleosol clays across the C2p/C2m boundary and deeper soil carbonates in Myachkovian paleosols record general humidisation of lowstand/glacial phases.
In spite of harsh arid to semiarid climate, the late Moscovian emerged plains were vegetated as evidenced by root traces and biologically templated beta calcretes. This xeromorphic flora included bushy or low tree perennial plants. The great majority of surveyed late Moscovian paleosols comply to lithic (rendollic) haplocalcids or rendzic calcic leptosols.
Supported by RFBR grants 07-05-00737 and 08-04-01552.
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