ИСТИНА

Soil chronosequence studies aimed at estimation and comparison of rates of the processes in defined conditions course a lot of methodological and interpretative problems. In the proposal of RAISIN project paleosols are mentioned as “chronometers”, that implies the bodies capable to measure time. The questions of the key importance for studies of rates of soil processes: how to estimate the time spans? 14C analysis is the most important method used to date paleosols. At the same time estimations of possible age of soil formation (rates of soil-forming processes) based on 14C dates of soil humus is a complicated problem. Any soil while exposed (so to say “alive”) is an open system where organic material accumulates and is renewed continuously. The information on specific 14C activity in humus allows usually inferring that a certain soil is not younger than the obtained date. Actually the calibrated 14C age obtained from a bulk sample of buried horizon is made up of an “own age” of soil humus depending on rates of humus rejuvenation before its burial and the time passed after the burial. Furthermore rates of humus rejuvenation are depth- and environmentally-dependant (depend on a type of soil formation dictated by the environmental conditions). Age of soil humus in contemporary surface soils according to limited estimations available now may vary between first hundreds and 5-8 thousands of years (Chichagova, 1985; Alexandrovskiy, Chichagova, 1998). The problem of rejuvenation rates is not that significant for Pleistocene paleopedology and totally negligible for Prequarternary paleosols, but it is of a great importance for Holocene paleopedology and evolutionary pedology dealing with more or less short-term climate fluctuations and evolutionary trends. We try to contribute the problem of 14C dating and rejuvenation of soil humus reporting our experience in analyzing a set of radiocarbon dates obtained for surface and buried soils and sediments in a small intermountain Terekhol’skaya basin, Tyva republic, South of Siberia (51ºN., 97ºE., about 1300 m a.s.l.). Our database includes in total about 160 14C dates, 60 of them were obtained from surface and buried soil humus horizons. Following palaeo-archives which cover about 13000 yrs. were studied there: lake sediments in bottom cores and on palsa-islands, soils of palsa-islands, palesol-sedimentary sequences including on delta-alluvial fan of a small river, surface and buried soils of a lake terraces. We were lucky to discover Late Holocene surface soils with a known date of exposure (the beginning of soil formation) in one of palsa-islands of the Terekhol’ Lake. There were soils developed on lacustrine sediments exposed by a quarry on the area of early medieval archaeological site. The quarry is related to the time of the archaeological site functioning. Basing both on historical and archaeological context and wiggle-match 14C dating archaeological wood we know that the sediments in the quarry were exposed between 1160 – 1180 cal yrs. BP. Two radiocarbon dates were obtained from the subface of humus horizons of these soils. Basing on this set of data a rejuvenation coefficient was calculated for the soils of the locality. Then the age of surface soils on islands was estimated from their radiocarbon dates and calculated rejuvenation coefficient. The calculated age of these soils (and islands themselves) appeared to be in a good correspondence with their age based on a biostratigraphical correlation between a well dated underwater column of sediments and columns of lacustrine sediments on the islands. Bulk samples, samples from the subface of humus horizons, samples of the top 2 cm of the A and Ab hor. and cryogenic humus wedges were dated in surface and buried soils. The radiocarbon dates from the top of Ab hor. allow approaching the date of their burial. The dates from bulk samples appeared to be acceptable only if the horizon is thin and supposedly existed for a short time. Dating of wedges allowed some conclusions on cryogenic phases of soil development. A correction for lifetime rejuvenation of soils buried in mid Holocene was done. As a result the age of these soils was estimated as a whole Holocene. Failed experience of mollusk shells 14C dating will be overviewed from the point of reservoir effect. References: Alexandrovskiy, A.L., Chichagova, O.A. The 14C age of humic substances in paleosols, 1998, Radiocarbon 40 (2) , pp. 991-997 Chichagova O.A., 1985: Radiocarbon dating of soil humus. Moscow: "Nauka". (In Russian).