Influence of HCl pretreatment and organo-mineral complexes on laser diffraction measurement of loess–paleosol-sequences

The influence of different sample pretreatment methods on grain size distributions in particle size analysis has been subject to controversial discussions. Standard sample preparation typically comprises the disaggregation of aggregated and agglomerated particles into single primary particles, i.e., the organic binding material is oxidized by hydrogen peroxide (H2O2) and the contained carbonates are dissolved by hydrochloric acid (HCl). The aim of this study is to evaluate the effects of HCl treatment on grain size analyses of Late Pleistocene and Holocene loess–paleosol-sequences investigated by a Beckman Coulter LS 13320 laser particle analyzer. For this purpose, samples from two different sections with different weathering degrees and sedimentary genesis were measured: (1) the Suohuoduo section on the northeastern fringe of the Tibetan Plateau (China) containing loess and paleosols, and (2) a vibracore from Düsseldorf-Grafenberg (Germany) containing calcareous loess and intercalated interglacial, interstadial and periglacial soils and soil sediments. All samples were pretreated with hydrogen peroxide and sodium pyrophosphate. Subsequently, the samples were prepared with and without the addition of HCl. There is no significant association of the HCl-induced grain size modifications after the HCl treatment with the calcium carbonate content. Conversely, a distinct dependence of the modification of grain size distributions on the content of organic matter, the weathering degree of the sediment, and the presence of stable aggregates as well as organo-mineral complexes was observed. Consequently, pretreating post-depositionally modified aeolian sediments with HCl may result in misleading grain size distributions and should be avoided in standard analyses of loess–paleosol-sequences. However, the HCl-induced modification of grain size distributions provides an indication of the existence of stable aggregates or organo-mineral complexes.