Application of particle size distribution throughout the soil profile as a criterion for recognition of newly developed geoforms in the Southeastern Caspian coast

Abstract

Abstract Soils vary depending on the type and intensity of interactions between the geoforms 1 and the soils. Coastal geoforms, due to special soil-forming features, contain less evolved but distinctive soils with different physico-chemical and morphological properties. The particle size distribution (PSD) is one of the most important indicators to understand the dominant conditions in sedimentary environments and determination of soil parent material. The purpose of this study was to use the PSD throughout the soil profile as an auxiliary analytical characteristic to identify the landforms in combination with surface topography and soil-forming factors. The estuarine geomorphic environment of the southeastern coast of the Caspian Sea, with about 480\xa0km2 area, a semi-arid climate (350\xa0mm precipitation, 17.6\xa0°C temperature) and coastal marine environment, was studied. Grid sampling consisting of 4 rows 6\xa0km apart and 20 points in each row 1\xa0km apart; 15 profiles; 61 auger holes, and 4 inaccessible points was performed. In total, 456 soil samples from soil profiles and auger points were collected. In addition to routine analyses, detailed PSD analysis (20 hydrometer readings) was done in all samples. Maps of the size fractions were prepared using ArcGIS 10.6 for each layer to illustrate 3D variations in PSD. Three types of granulometric curves including: 1) a sigmoid curve characteristic of free sediment accumulation, 2) a logarithmic curve characteristic of a torrential flow or splay deposit and 3) a parabolic curve characteristic of an accumulation forced by an obstacle obstructing the flow were determined across the study area and at different depths. The spatial (from the coast to a distance of 20\xa0km) and temporal (across different layers from the soil surface to a depth of 120\xa0cm) PSD variations were found to be significant in the study area. This information is useful in identifying lagoons, barriers, alluvial terraces and small islands in the coastal plain areas.