Boletín de la Sociedad Geológica Mexicana Volumen 71, núm. 1, 2019, p. 21- 42 http://dx.doi.org/10.18268/BSGM2019v71n1a3

 Microscopic and tomographic studies for interpreting the genesis of desert varnish and the vesicular horizon of desert soils in Mongolia and the USA

Marina P. Lebedeva^1,3, Dmitry L. Golovanov^1,2, Vasily A. Shishkov³, Andrey L. Ivanov¹, Konstantin N. Abrosimov¹

¹ Dokuchaev Soil Science Institute, Russian Academy of Sciences, Pyzhevskiy pereulok, 7, 119017 Moscow, Russia.
² Lomonosov Moscow State University, Leninskie-gory, 1, office 2007, 119991 Moscow,Russia.
³ Institute of Geography, Russian Academy of Sciences, Staromonetnyj-Pereulok, 29, 119017 Moscow, Russia.

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Abstract

The composition and microfabric of the Av diagnostic surface horizon (crust) and desert varnish of soils formed on various parent materials covering ancient alluvial fans in extremely arid deserts of Central Asia (Trans-Altai Gobi, Mongolia) and North America (Mojave, USA) are analyzed. Application of a wide range of methods—morphological and micromorphological descriptions, scanning electron microscopy, electron microprobe analysis and microtomography, together with traditional physicochemical methods—has shown that desert varnish retains the same composition and relative contents of elements (Fe/Ti and Mn/Ba) independently from the initial composition of parent materials and geographical location of the studied regions. The absolute dominance of isolated closed (vesicular) pores, identified by micromorphology and confirmed by microtomography, can account for specific hydrophysical properties of the crust horizon; that is, low water-holding capacity and permeability. We explain both phenomena of desert pedogenesis—light-coloured crust horizon and dark-coloured desert varnish—the burst of microbiological activity and physicochemical processes after rare but heavy showers during the hot season. The development of iron-depleted zones around pores is considered the evidence of that phenomenon. Consequently, we propose that Mn²⁺ undergoes mobilization within the crust horizon, followed by its upward migration with capillary and film moisture flows, against the gravity gradient to the surface of desert pavement with its subsequent fixation at the oxygen barrier. The processes of sulphate reduction are indirectly confirmed by the mobilization of Ba²⁺, which is immobile in the presence of sulphates. These elements (Mn and Ba) accumulate within the lower part of two-layer films. These films are located in microcavities on the upper sides of rock fragments of desert pavements in the studied regions. Films on lower sides of rock fragments and the upper part of those two-layered films are enriched in Fe and Ti, and therefore acquire reddish-brownish colours. Despite its transitory character, the process, which we suggest to call ‘cryptosolodization’, includes five stages: (1) Fe, Mn and sulphate reduction with mobilization of Ba²⁺ from BaSO₄, (2) precipitation of FeS and migration of Ba²⁺ and Mn²⁺ to the stone surface, (3) coprecipitation of Ba and Mn at a combined oxygen-sorption barrier, (4) oxidation of S^2- to SO₄^2- with mobilization of Ti²⁺ from TiO₂ and (5) oxidation of Ti²⁺ and Fe²⁺ and their coprecipitation in the form of oxides/ hydroxides TiO₂ and Fe(OH)₃.

Keywords: desert soils, micromorphology, pedogenesis, mechanisms of development of desert varnish.