The difference of two laser diffraction patterns as an indicator for post-depositional grain size reduction in loess-paleosol sequences

Abstract

Chemical and physical processes within aeolian sediments result in a reduction of the particle grain size. In loess sediments the post-depositional grain size variation is due to a reduction of the dominating coarse silt fraction in favor of the clay and fine silt fraction. Generally, there are two post-depositional fractionation processes: (1) the chemical weathering of silt sized minerals like mica and feldspar as a result of hydration and hydrolysis, (2) the physical weathering of all minerals contained in the primary loess sample by cryogenic and fluvial relocation processes. There are many widely used proxies to estimate their vertical variation in thick sediment columns. However, there are complicating factors related to aeolian sorting effects, the distance to the source regions and carbonate dynamics, which reduces the sensitivity of common proxies to the chemical weathering. In this study, we present a simple and quick method using laser diffraction calculations of grain size distribution obtained by two optical models (the Fraunhofer approximation and the Lorenz-Mie theory) to highlight the enrichment of fine grained material by post-depositional chemical weathering processes. In contrast to the Fraunhofer approximation, the Lorenz-Mie theory considers the complex refractive index which depends on the mineral properties. Thus, the difference of the grain size distributions (ΔGSD) calculated with both models is sensitive to the mineral composition of the sample. In separated submicron mineral suspensions we found that different crystalline properties are reflected by repeatable signatures of the ΔGSD. This specific ΔGSD-signature also occurs in the submicron grain size range of bulk measurements of weathered loess samples. In contrast to previous studies, we obtain reliable laser diffraction results in the submicron range. Summarizing, we present the ΔGSD within the submicron range of bulk measurements as a suitable indicator for the chemical weathering degree of loess-paleosol sequences, which is virtually unaffected by cryogenic processes, weak relocation of inherited weathering products and synsedimentary processes (sheet wash, saltation or enhanced background sedimentation).

• GQT2
• Grain size analysis
• Laser diffraction
• loess
• silicate weathering
• soil formation

Resources

• http://dx.doi.org/10.1016/j.palaeo.2017.02.022

Bibliography

Schulte, P., Lehmkuhl, F. (2017): The difference of two laser diffraction patterns as an indicator for post-depositional grain size reduction in loess-paleosol sequences. – In: Palaeogeography, Palaeoclimatology, Palaeoecology, DOI: 10.1016/j.palaeo.2017.02.022