extended abstracts - Geomorphic Processes and Geoarchaeology

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Geomorphic processes and geoarchaeology Lacustrine silts in the interval 5.80-5.20 m evidence the impulse of lake sedimentation dated to 11.0-10.5 ka BP (cal). It was followed by another phase of alluvial sedimentation (1.9-m thick sandy loams). The final establishment of lacustrine regime is marked by change to carbonate lacustrine silts at depth 3.30 m dated to 9.5 ka BP (cal). To estimate the age of the island the interruption of lacustrine sedimentation (the top of lake sediments at the land surface) should be dated. Radiocarbon dating of mollusk shells from a bank exposure gave dates 2-4 ka older than expected from stratigraphy and extrapolation of sedimentation rates. It is explained by contamination of lake carbonates by old carbon from Paleozoic marbles that compose a big portion of the lake catchment. Sediment composition excluded also luminescent dating techniques. Therefore to date the topmost lacustrine silts in the island geological correlation to a well dated lake core was undertaken. The lake core was divided into 7 sedimentation units based on composition of biological remains (mainly algae) and bulk chemical composition. The lower unit corresponds to underlying alluvial sediments. Sequence of lacustrine sedimentation units reflects succession of phases in lake development with different hydrological regime (more or less active renovation of water because of changing proportion between outflow and evaporation in water balance). Borders between units were radiocarbon-dated. Distinguishable changes of biological and chemical composition of units were used as a correlation tool. The top of lacustrine sediments in Por-Bajin Island were correlated to the second half of Unit V, which give the age estimation between 4-5 ka BP (cal). Given the above data, history of the Tere-Khol Lake and Por-Bajin Island has been reconstructed as follows. The lake appearance was pre-conditioned in the Late Glacial by active development of alluvial fans of big and small rivers that drained surrounding mountains. The fans protruded into the bottom of the Terekhol Basin and isolated its south-western corner. Shortly after the start of the Holocene ca 11,000 years BP the initial lake reservoir appeared probably during an interval of relatively high river runoff. During this initial phase the lake occupied a vast area that included also the location of the modern Por-Bajin Island that did not exist at that time. About 10,500 years BP the lake shrank considerably and the future Por-Bajin Island area changed to lowland terrain subject to overbank alluvial sedimentation. Probable reason for the lake shrinkage was thermokarstic (permafrost melting) subsidence. Lowering of the lake level favored incision of tributary small rivers that in turn resulted in cessation of overbank alluvial sedimentation in surrounding plain. The next phase of lake development was its final transgression and spread about its modern dimensions that is dated to ca 9,500 years BP. Since that moment the area of the Por-Bajin Island had been a part of lake bottom for about 5,000 years. Between 4,000- 5,000 years BP the lake bottom rose locally by 5-7 m to make the island. Mechanism of this rise was frost heaving favored by the lake winter freeze through during one of dry phases when water depth was even shallower than usually. In the emerged part of the lake bottom permafrost continued to accrete in subsequent years and led to formation of palsa – frost heave hill. The mechanism was as follows. Freezing of water-saturated sediments and attraction of additional water to the frost caused formation of ice lenses that considerably increased the total volume of sediments and finally had led to formation of a flat hill with the top 4-5 m above the lake and gentle sides sloping down to the lake bottom. This palsa island had existed in the lake for several thousand years before in the mid-VIII century it was chosen as a location for the fortress of Por-Bajin. 215
Geomorphic processes and geoarchaeology The above results and their paleogeomorphic interpretation allow arriving at the following conclusions: 1. The Tere-Khol Lake has natural origin and Early Holocene age (11,000 years). 2. The Por-Bajin Island is younger than the lake (4,000-5,000 years) and has cryogenic origin. 3. No land bridge existed between the island and lake banks at any moment during and after the fortress foundation (1250 years ago). The founders of Por-Bajin are likely to intentionally choose island location for the fortress to isolate it from surroundings. Hopefully this inference would help to better understand functions of this construction. The last conclusion imposes a problem of delivery of construction materials for walls and buildings which was risen by previous researches [1, 2]. Total volume and mass of artificial constructions are estimated at ca 70,000 m³ and 90,000 tons respectively. The question has been solved by detection of local sources of construction materials. Bulk chemical composition shows that outer and interior walls were made of the same lacustrine silts that compose the island. Two vast pits were found at southern and northeastern banks of the island that probably served for extraction of silts. Mining depth was shallow – 1.5-2 m – because of its limitation within the active layer above permafrost. After the pits were dug permafrost melting caused surface subsidence. Pits were inundated by the lake and now make inlets in the island coastline. This research contributes to the RFBR Project 09-05-00351. References 1. Klements, D.A. 1895. Archeologicheskiy dnevnik poezdki v Srednyu Mongoliu d 1891 godu. Sbornik Trudov Orkhonskoy expeditcii. Sankt-Peterburg, 76 p. (in Russian). 2. Vainstein, S.I. 1964. Drevniy Por-Bazhyn. Sovetskaya Ethnographia. No. 6, P. 113-124 (in Russian). 3. Arzhantseva. I., Inevatkina, O., Zav'yalov, V., Panin, A., Modin, I., Ruzanova, S., Härke, H. 2011. Por-Bajin: An Enigmatic Site of the Uighurs in Southern Siberia. European Archaeologist. The newsletter of EAA. Issue No. 35: Summer 2011, P. 6-11. 4. Harke, G. 2010. Letter from Siberia: Fortress of Solitude. Archaelogy, Vol. 63, No. 6. 5. Koshurnikov, A.V., Zykov, Yu.D., Panin, A.V. et al. 2008. Izuchenie merzlogo osnovania archeologicheskogo pamyatnika «Krepost' Por-Bajin» (Tuva Republic). Ingenernye Izyskania, No. 6, P. 28-31. SOIL AND CLIMATE CHANGES IN THE STEPPE ZONE OF RUSSIAN PLAINE DURING THE LATE HOLOCENE RECORDED IN PALEOSOILS OF THE ARCHAEOLOGICAL MONUMENTS Pesochina L.S. Institute of Physico-Chemical and Biological Problems of Soil Science, RAS, Pushchino, Russia, LSPesch @rambler.ru Paleopedological study became an important part of the complex investigations of archaeological monuments. The comparative analysis of recent and buried soils provides 216
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