ИСТИНА

Numerous data on soil development with time are based on soil chronosequence studies, in particular those obtained in California (e.g. Harden, 1982, 1988; Muhs, 1982; McFadden and Weldon, 1987; Harrison et al.,1990). Chronosequence studies are formulated around a group of soils on uniform geomorphic surfaces of different age. Soils within a chronosequence are supposed to vary in age, while all other soil-forming factors are presumably held constant (Jenny, 1941; Stevens and Walker, 1970; Yaalon, 1975; Huggett, 1998; Shaetzl et al., 2006). This condition is hardly to be accomplished for the long-term chronosequences (n•104 years and more) since the older a surface soil, the higher a probability that at least ones it has survived crucial environmental change resulted in evolution of soil profile driven by evolution of a set and strength of soil forming processes. This circumstance is not always taken in consideration while studying soil chronosequences as a source of information on rates of soil processes. This study was conducted for three profiles of desert soils formed on dated alluvial fans and lacustrine beach-ridge deposits to estimate a contribution of ageing and possible environmental change to morphological portray of soils. These profiles were among those of demonstrated during INQUA RAISIN Workshop-2014 in Panamint Valley. The youngest soil (profile PAN-23) Avk-Bwk-Bck-Ck Skeltic Cambic Calcisol (Endosalic, Yermic) is dated as 12-13 ka. The soil profile has very insignificant differentiation in color and structure, high share of skeletic material (generally <80% of the bulk) with thin calcareous pendants on rock fragment. Av horizon is loose and thin, desert varnish is not clearly developed. At micromorphological level Av horizon has crystallitic B-fabric with rather even micritic impregnation; vesicular porosity of Av horizon is better developed in zones richer in fine material. Fragmented clay intercalations occur mostly in Av horizon, signs of clay illuviation are very faint in B horizons. In Bwk horizons within looser microzones calcareous pendants looks as freshly formed, though in denser microzones gravels with calcareous pendants are incorporated in a matrix (possibly re-deposited pedofeatures). Two phases of carbonate accumulation were distinguished in Bk horizons: one occurred before the illuvial clay redistribution, another followed minor accumulation of illuvial clay in Bk horizon. 30 ka soil profile PMF-1 has following sequence of horizons: Avk-Btk-Bky-Bkwz-Ck Skeletic Luvic Calcisol (Gypsiric, Endosaliic, Yermic). Vesicular horizon is better developed and thicker, desert pavement and varnish (black from the surface, pumpkin-red from the bottom of rock fragments) is well developed. Av horizon is subdivided into upper, coarser and looser part with obvious aeolian input (dust within some of vesicular pores), and denser lower part. The last one combines micro-zones nearly free of carbonates with iron-clay coatings in pores, and clayey zones with micritic impregnation. Av horizon is underplayed by close to the surface Btk horizon with numerous clay intercalations and thick dusty clay coatings and insignificant accumulation of carbonates in small micritic nodules. Under Bt horizon skeletic material prevails over fine earth. Within fissures inside rock fragments we find combined clay-microsparitic infillings where the clay illuviation and carbonate accumulation features do not combine a uniform sequence. There is no clay coatings in lower horizons, where only multilayered calcareous infillings inside rock fragments were found. Lithogenic residual gypsum with clear features of destruction also occurs here. The oldest profile PAN-28 (70-85 ka) Avk-ABvk-Btky-BCky-CBky, Skeletic Luvisols (Yermic, Cutanic) has strongly developed desert pavement and varnish. The material is generally more clayey and richer in iron oxides (redder in color), much more weathered. Av horizon has no features related to fresh aeolian accumulation, but rich in clay coatings, small nodules of iron oxides and fresh aggregates composed of microsparitic needles in pores. Below Av horizon a series of Btk horizons is found with uneven distribution of carbonates (it has generally low calcareous), extremely abundant clay coatings and infillings owed to illuviation process. There are zones nearly free of carbonates with well developed granostriated b-fabric. In soils of studied chronosequence manifestation of the features related to desert environment, such as desert pavement, desert varnish, vesicular horizon definitely increases with age of the surface (soil). Rubefecation is also reveals time-dependance, though here we cannot exclude also purely geological reasons of the process, s.a. hydrothermal pre-treatment of primary rocks before their fluvial sedimentation. However soils of the chronosequence demonstrate not only quantitative but also qualitative differences in morphology which hardly could be explained only by ageing of soils and soil forming materials. As it was mentioned earlier, clay illuviation features (regarded by Harden, 1982, to arise due to ageing processes) do not occur in Holocene soils of Central California, but develop systematically in soils older than 40,000 years. In studied soils Bt horizons with abundant clay illuviation coatings were described in two older profiles dated as about 30 ka and 70-85 ka. We consider Bt horizons to be inherited from the last two maxima of the Wisconsin Glacial Episode: Tahoe and the Tioga phases. Estimates of paleo-temperatures and paleo-precipitation in southwestern North America during the last glacial indicate a significantly cooler (−9 °C below avg.), wetter (+700 mm above avg.) climate compared with nowadays (Barbour, 1988; Heusser, 1998). .