Soil and desert varnish development as indicators of landform evolution in central Iranian deserts

• Geomorphic position influences soil development in hyperarid Iranian desert.
• Soils contain evidence of a more humid paleoclimate in central Iran.
• Desert (or rock) varnish correlates with soil development and surface stability.
• Varnish cation and activity ratios decrease with increasing soil development.
• Magnetic susceptibility reveals hematite to be a dominant mineral in varnish.

Desert varnish (or rock varnish) and soils of Jiroft area, south central Iran play a crucial role in the evolution of landforms in this arid setting containing mantled pediments, rock pediments, inselbergs, alluvial fans, alluvial plains, and flood plains. Mantled pediment contains three stable, semi-stable, and unstable surfaces as indicated by desert varnish and soil development properties. However, alluvial fan could be divided into apex and mid fan settings based on slope position. Mid-fan settings include both stable and unstable surfaces, based on varnish properties. About 100 rocks containing varnish were selected randomly on each geomorphic surface. In addition, a representative soil pedon was described and sampled on each surface for a total of 10 pedons. Results of the study show that brown to black coatings of varnish with the cation exchange capacity (CEC) range of 8.5–21 cmolckg− 1 were composed of < 0.2–50 μm particles. Magnetic susceptibility of varnish samples fluctuated between 300 and 1855 10− 8 m3 kg− 1 and Si, Ca, Al, Fe, and Mg concentrations were 26.4, 3.9, 3.8, 3.1, and 2.2%, respectively. Age and development indices such as cation-ratio of desert varnish (with the range of 6.14 to 14) and activity ratio of Feo/Fed (with the range of 0.03 to 0.35) of soils decreased with increasing stability of geomorphic surface and soil development. Chlorite, smectite, illite, palygorskite, and kaolinite clay minerals exist in the clay fraction of studied soils. In addition, chlorite, smectite, vermiculite, sepiolite, illite, palygorskite, kaolinite together with zeolite, quartz, and feldspar tectosilicates were found in desert varnish. Cation-ratio of varnish and activity ratio of Feo/Fed in different geomorphic positions together with soil properties provide a simple method to study the evolution of landforms in this warm desert setting.