Eurasian drylands and the conceptual model of loess landscapes

At least 10% of the Earth surface and large areas of the Eurasian continent are covered by aeolian silt and sand deposits. The origin and distribution of these deposits is indicative for the (paleo-) environment and represent an important edaphic factor for eco-zone evolution, which in turn controls past and present land use.

Here we present a conceptual model of loess landscapes in relation to past and present drylands based on the spatial analysis of loess and loess facies' in Europe (Lehmkuhl et al., 2020). Corners of our conceptual 'loess-triangle' represent three eco-zones (nival, humid, and arid environments) peripheral to 'typical' loess formation (placed in the center). The modes of loess formation are controlled by climatic factors, namely water availability and temperature, which also constrain the prevailing vegetation: Mode 1 - Periglacial and tundra loess; Mode 2 - Temperate and subtropical loess; Mode 3 - Desert margin loess. In-between these three peripheral modes of loess facies we illustrate that ‘typical’, continuous and silt dominated loess formation takes place.

At the Chinese Loess Plateau, there is a gradual transition in grain-size from the humid regions in the south and southeast to the semi-arid and arid regions in the northwest (from Mode 2 towards typical loess and Mode 3). Reduced vegetation cover at desert margins, along dry riverbeds and lakes leads to increased aeolian deflation facilitating also mid- and long-distance transport of silt-sized particles. The resulting deposits range from silty loess to sandier loess in the direction of increasing aridity towards the deserts in Central Asia and in China. This transition towards the desert margin loess can be found also in eastern and southeastern Europe towards Central Asia, e.g. at the Sea of Azov and the Caspian Lowlands. Furthermore, in northern Mongolia and Siberia periglacial or mountain / tundra loess appears (Mode 1).

The concept also has relevance if used vertically: At higher elevations in semi-arid regions of central-eastern Europe and Asia (e.g. at the eastern margin of the Tibetan Plateau and the western part of the Qilian Shan) humidity controlled forest belts do not contain any loess deposits. However, in the rather high mountains and plateaus of arid Central Asia, mountain loess close to typical loess can be found. The uppermost boundary of loess is periglacial loess (Mode 1), whereas the lowermost parts are desert margin loess (Mode 3). For these regions, there are still debates on the influence of glaciers and deserts on loess formation.

Lehmkuhl et al. (in press). Loess landscapes of Europe – Mapping, geomorphology, and zonal differentiation. Earth-Science Reviews, Doi: https://doi.org/10.1016/j.earscirev.2020.103496