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The evaluation of sedimentological and multi-element analysis reflected on loess derivates – Examples from Asia, Middle and Eastern Europe.

Jens Protze1, Melanie Mendes1, Frank Lehmkuhl1, Holger Kels1
1 Chair of Physical Geography and Geoecology, RWTH Aachen University

P 2.8 in Löss und terrestrische Archive

 

During the last few years, both, the grain size measurement by laser diffraction as well as the detection of element contents by x-ray fluorescence (XRF) became a standard procedure also in Quaternary sediments. But up to now, there is no uniformity in sample preparation and measurement. Because of the diversity of techniques and instruments used in the loess community a reasonable comparability is hindered. In fact, it is important to explain the methodological modus operandi in detail. Further, the methodological knowledge shows deficits quiet often. This leads quite often to a miss-interpretation of data in application-oriented studies.

In this study the result of a methodological comparison of preparation and measurement techniques on loess and loess-like sediments from Middle- and Eastern Europe as well from Central and High Asia are presented.

The grain size characterization was performed by a laser diffraction particle size analyser LS 13 320 (Beckmann Coulter). The preparation technique is in accordance to Machalett et al. (2008). Each sample was measured in two to three different concentrations each repeated twice in minimum, in dependence on to the macroscopically visible texture. The evaluation of results is taking the sediment origin and their degree of weathering into account. Both optical models, Mie- and Frauenhofer-theory, are drawn. For the latter the refraction and adsorption indices are mentioned by Özer et al. (2010). The degree of roundness was proofed by different shape factors. To review the so called unrepresented clay contents of laser diffraction measurements a set of samples are audited by an x-ray diffraction using a sedigraph (Micromeritics 5100).

Evaluating the differences in measurement techniques for multi element-analysis by X-ray fluorescence, result of a portable instrument (PEDXRF Niton Xlt 792) were compared to them of a stationary energy-dispersive polarization instrument (EDPXRF AMETEK SPECTRO Xepos). In principle, effects of texture and mineral content should be excluded. Due to these effects, different sample preparations, namely powder samples, pressed and melted pellets, were compared depending on the regarded element. Up to now there is no common application in the Quaternary sciences for handling powder and press samples. Powder samples show serious disadvantages due to their inhomogeneity and a resulting selective measurement for several elements. In contrast, pressed pellets have a more homogenous surface. Beside these two facts, test series had to be done to find the best available technique for preparation. Most important factors are the proportion of sample weight to the binding material, the duration and the amount of pressure.

The results of the study show distinct variations for both techniques depending on sample preparation, supporting the decision of the appropriate optical model.

The qualified determination of grain size distribution allows distinguishing between loess and loess like sediments depending on their state of weathering and their composition. Main factor is the shape of the single grain.

Adopting the most appropriate optical model for laser diffraction data, clay contents were brought more into line to the clay contents, which are measured by X-ray diffraction particle size analysis.

The results show clearly that an inappropriate sample preparation leads to a misinterpretation of XRF-data. In this case element contents merely reflect texture and mineralogical differences of the samples.

Providing an appropriate sample preparation, the combination of the introduced methodologies submits new possibilities challenges for the interpretation of loess and loess-like sediments. Insights into the degree of weathering, the provenance and genetic aspects can be achieved. For example, the content of Rubidium (Rb) is representing a tonphile element and this correlates very good with clay contents determined by the laser diffraction. The amount of Rubidium along the course of profile compared with the vertical variations in clay content give further information about the decomposition of the sediment.

 

References:

Machalett, B., Oches, E. A., Frechen, M., Zöller, L., Hambach, U., Mavlyanova, N. G., Marković, S. B. & Endlicher, W. (2008): Aeolian dust dynamics in central Asia during the Pleistocene: Driven by the long-term migration, seasonality, and permanency of the Asiatic polar front. - Geochemistry Geophysics Geosystems, 9 (8): 1-22.

Özer, M., Orhan, M. & Işik, N. (2010): Effect of particle optical properties on size distribution of soils obtained by laser diffraction. - Environmental & Engineering Geoscience, 16 (2): 163-173.

Letzte Änderung 26.07.2012