Acta Mineralogica-Petrographica, Abstract Series 4, Szeged, 2004
CLAY MINERALS IN LATE PLEISTOCENE-HOLOCENE SEDIMENTS FROM THE LAKE BAIKAL
NOVOTNA, K..' BEZDICKA, P.,1 GRYGAR, T.,1 HRADIL, D.,1 KADLEC, J.,2 PRUNER, P.2 1 Institute of Inorganic Chemistry AS CR [Üstav anorganické chemie AV CR], Rez, 250 68, Czech Republic
2 Institute of Geology AS CR [Geologicky üstav AV CR], Rozvojovä 135, Praha, 165 02, Czech Republic E-mail: katerina.michalova@post.cz
The profiles of the bottom sediments from Academician Ridge in the Lake Baikal are the most reliable continental climatic records of the Late Quaternary, which have become available in the 1990's. Their interpretation has mainly been based on biogenic proxies, diatom and pollen analysis, but only several attempts have been made to read their mineral record. The reason is the polymineral nature of the sediments, containing several classes of clay minerals, unweathered or partly weathered primary minerals from the lake catchment area, and amorphous silica from the diatom frustules. That hardly permits an unequivocal quantitative analysis using conventional tools. However, for any climatic interpretation of the sedimentary records some quantitative values, so called proxies, are required. The proxy must have a definite, implicit or explicit relation to the actual environmental conditions; let it be terrestrial weathering or syn- or post-depositional conditions.
The sedimentary profile VER98-1-13 obtained from GFZ Potsdam was analysed. The profile dating was based on the variations of the palaeointensity of the Earth magnetic field. The part of series covering the last about 200 thousands years was studied; this time span corresponds to the marine isotopic stages from MIS6 to MIS1 (presence).
The mineral assembly of the Lake Baikal sediments con- tains chlorite-vermiculite-smectite weathering sequence, pos- sibly including interstratified clay minerals, biogenic Si02, amorphous expandable clay minerals, amphiboles, feldspars, and quartz as major components (> 1-5%). Separation of clay
fraction in this material cannot be recommended because it is not a priori known what size fraction bears the environmental information, e.g. the chlorite weathering sequence can contain members with varying particle size. Conventional techniques of clay mineralogy are not suitable for such multicomponent mix- tures, containing more components with variable composition.
For the identification of the clay mineral components we used X-ray powder diffraction in heated support (HT XRD) at temperatures between room temperature and 250°C. HT XRD has not yet been systematically used for identification of expandable clay minerals, and so it was also applied to refe- rence clays and their homoionic forms. This qualitative me- thod was complemented with quantitative analysis of amount and kind of interlayer (exchangeable) cations that seems to be the most reliable proxy-yielding method in the given complex matrix of the sediments. The resulting estimates of content of expandable clay minerals and the ratio between individual interlayer cations were compared to the magnetic suscepti- bility that have both direct and indirect relations to the actual climate.
Acknowledgements
The sedimentary profile was kindly supplied by Dr. Hedi Oberhaensli from GeoForschungs Zentrum Potsdam, Ger- many. The work was supported by the Grant Agency of Aca- demy of Sciences of the Czech Republic, project no.
IAA3032401.
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