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Geochemistry of Diagenesis of Organogenic Sediments: An Example of Small Lakes in Southern West Siberia and Western Baikal Area
Authors:G A Leonova  A E Mal’tsev  V N Melenevskii  L V Miroshnichenko  L M Kondrat’eva  V A Bobrov
Institution:1.Sobolev Institute of Geology and Mineralogy, Siberian Branch,Russian Academy of Sciences,Novosibirsk,Russia;2.Trofimuk Institute of Petroleum Geology and Geophysics,Siberian Branch of the Russian Academy of Sciences,Novosibirsk,Russia;3.Institute of Water and Ecological Problems, Far East Branch,Russian Academy of Sciences,Khabarovsk,Russia
Abstract:Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO42? and Fe2+ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO42? concentration in the pore waters.
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