The age of buried soils of the sand dune near Kuehtanar stream (South-East of Gorny Altai)

Ovchinnikov I.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

yet@ngs.ru

A section of 10-meters-thick aeolian sands, forming the dune, was uncovered in the deflationary kettle on the left bank of Chuya river near Kuehtanar stream. The dune is lying directly on a moraine, coursing from South-East to North-West. V. V. Butvilovsky previously studied this region [1], obtaining two radiocarbon dates: 2425±30 y. a. (SOAN-2377), 1610±25 y. a. (SOAN-2376) from humus horizons of the soils. In the section of this dune, three humus horizons of buried soils of low thickness were studied, samples for ¹⁴С from which were correctly derived. The dating was proceeded following the technique, developed in the laboratory of Cenozoic geology, palaeoclimatology and mineralogical indicators of climate of IGM SB RAS [2] under the supervision of Ph. D. in geology-mineralogy Orlova L. A. (Table).

Aeolian deposits are composed mainly by polimictic sand with approximately the same mineralogical composition (grains of yellow argillites, serpentinites, quartz, ultrabasic rocks, mica), indicating a single source of the original material. The following strata are marked out from the top downward.

1. Consertal grey sand composed by interlaced layers of fine- and coarse-grained carbonaceous sand with rare gravel (less then 5 mm sized) inclusions. The lower boundary of the layer is clear in it’s color and density, and is 0,7 meter thick.

2. Two juxtaposed poorly developed buried soils. Humus horizon of the upper soil represents a dark-grey with brownish shade, dense carbonaceous sandy loam. This horizon contains small pieces of coal of woody and shrub vegetation. This layer’s thickness is up to 10 cm. Accumulative horizon of the second below-ground soil is a brownish-grey, carbonaceous, densified, up to 5 cm thick sandy loam. The total thickness of both soil profiles is equal to 0,5 m. Two samples were taken from the upper soil: coals (sample SOAN – 8411) and humus rich dark-grey with brownish shade sandy loam (sample. SOAN – 8412). Coals (sample SOAN – 8413) and humus rich brownish-grey sandy loam (sample SOAN – 8414) were taken from the second below-ground soil.

3. The third below-ground soil is separated from the two superincumbent ones by polimictic, consertal, carbonate rich, 0,3 m thick sand, with clear, rough lower boundary.

4. Humus horizon of the third soil represents a dark-grey with brownish shade, dense carbonaceous sandy loam, including decomposed root remains, with twisted upper and lower boundaries. A carbonaceous horizon, composed of spots and stripes, is clearly identified in the soil profile. Two burrows measuring up to 5 cm were seen in the middle part of the horizon. The profile’s thickness is equal to 0,5 m. Coals (sample SOAN – 8415) were taken from the third below-ground soil.

5. Sand stratum including layers of fine-grained, polimictic, carbonaceous, greyish-yellow sand, alternating with layers of medium- and coarse-grained, grey sand, containing small gravel. Down the section, increase of the fine-grained sand layers’ thickness is noted. The stratum’s thickness is equal to 8 m.

Thus, during the forming of the dune, fastening of its surface occurred three times. These corresponded to the forming of buried soils, which developed during warm time intervals. Received radiocarbon dates allow us to report the presence of three periods of climate warming during late Holocene: ~ 2-3 century AD; ~7-9 century AD; ~ 13-15 century AD (Table). Dates of similar age are derived from buried soils in the dunes of Cisaltai Plain. That seems to be the evidence of simultaneous forming of soils on aeolian complexes of Altai high mountains and Cisaltai Plain during late Holocene [3].

The study was supported by RFBR Grant № 10-05-00673а and by interdisciplinary integration projects of SB RAS № 53, 147.

Lab code and №	Radiocarbon dates, bp	Calibrated date, cal AD
SOAN – 8411	730 ± 35	1300 ± 80 years
SOAN – 8412	450 ± 40	1445 ± 20 years
SOAN – 8413	1385 ± 35	650 ± 20 years
SOAN – 8414	1300 ± 50	830 ± 40 years
SOAN – 8415	1860 ± 30	155 ± 65 years

Table. Radiocarbon dates

References:

1. Butvilovsky V. V. Palaeogeography of the last glaciation and Holocene of Altai: event-catastrophic model. – Tomsk: University of Tomsk, 1993. – 253 p. (In Russian)

2. Orlova L. A., Panychev V. A. Below-ground soils and the problem of their radiocarbon dating reliability // Problems of Pleistocene stratigraphy and palaeogeography of Siberia: To the XI INQUA Congress in USSR, Moscow, 1982. Proceedings of IGG SB AS USSR, issue. 521. - Novosibirsk: Science. Sib. branch, 1982. - P. 28-36 (In Russian)

3. Zykin V. S., Zykina V. S., Zazhigin V. S., Orlova L. A., Chirkin K. A., Smolianinova L. G., Balakin P. V. The Quarternary of the South of West Siberia: stratigraphy, environmental and climatic changes. // Fundamental problems of the Quarternary: results of study and main research trends: Materials of the VI All-Russian conference on Quarternary study (Novosibirsk, October, 19-23 2009). - Novosibirsk: Publishing house of SB RAS, 2009. - P. 238-241 (In Russian)