Diversity and peculiarities of soil formation in eolian landscapes – Insights from the mineral magnetic records

Abstract

Magnetic properties of loess-paleosol sequences are widely utilized as sensitive, easily-obtained and reliable proxy archives of past climate change in continental settings. However, soil formation in aeolian landscapes may occur under different and changing conditions of dust sedimentation, which will affect the growth and development of the pedogenic magnetic component. This phenomenon is explored in our study by analyzing depth variations of the following magnetic characteristics: frequency dependent magnetic susceptibility, anhysteretic magnetic susceptibility and the ratio between anhysteretic susceptibility and isothermal remanent magnetization acquired in a 100 mT back field (χarm/IRM-100 mT), along three loess-paleosol profiles from NE Bulgaria. A conceptual model for the time evolution of the grain size of the pedogenic magnetic fraction with soil depth is proposed, which is build upon the mechanism of soil formation – accretional or in stable land surfaces, or a combination of the two. Another magnetic ratio – isothermal remanence acquired in a 300 mT back field divided by low field magnetic susceptibility (IRM-300 mT/χ) is utilized as a proxy for the changes in the ratio of silt to clay fractions, indicative of changes in dust sedimentation regimes during glacial-interglacial transitions. According to the proposed conceptual model, verified by the magnetic signature of three Holocene soils from NE Bulgaria, discrimination between accretional soils and soils developed without dust additions during soil forming period can be done. Accretional soils are characterized by parallel changes in grain size sensitive magnetic proxies. Soils, developed at stable landscape conditions show gradation of the depths at which maximum enhancement of various proxies occurs with deepest occurrence of the maximum in χfd, followed by depth of maximum χarm and χarm/IRM-100 mT. Another implication of the new conceptual model is that the mean coercivity of the pedogenic component of accretional soils will be higher than that of soils developed without eolian input at equal temperature conditions because of the soils' thermal gradient and different depths, at which pedogenic minerals form in the two settings. The results revealed that the Holocene soils (So) has developed under different soil-forming conditions than paleosols from the same sites.