Oxygen and carbon stable isotope composition of authigenic carbonates in loess sequences from the Carpathian margin and Podolia, as a palaeoclimatic record

摘要

Samples were collected from the outcrops situated in the north and east of the Carpathian Margin, in the transition zone between the area of loess covers of Eastern and Western Europe. The chosen localities include two zones of temperate climate with slightly different prevailing wind directions. These loess-palaeosol sequences formed and develop in different time intervals: Early Quaternary Epoch (Podolia, Ukraine) and Late Quaternary Epoch (Carpathian Foothills, Poland).rnThe isotopic analysis was carried out on many types of authigenic soil carbonates, including nodules, rhizoliths, rhizocretions, calcified root cells and bioclasts. The carbon isotopic compositions (δ~(13)C) of different types vary markedly. These differences seem to be more important than the variations within one type of carbonate at a particular section.rnThe δ~(13)C values for authigenic carbonate cements (rhizoliths, rhizocretions and nodules) vary in a narrow range from about -11‰ to -8‰, suggesting that calcite cements were precipitated under roughly similar conditions, with a considerable supply of CO_2 derived from biomineralization of organic matter, predominantly from C_3 plants. The δ~(18)O record of the calcite cements within the upper Quaternary deposits exhibits relatively slight variability, suggesting that δ~(18)O values in soil water were homogeneous.rnThe significant depletion of ~(13)C as well as δ~(18)O in calcite from calcified root cells (-12‰ and -7‰, respectively), and to a lesser extent from calcitic globules with respect to catcite cements, points to different environments of formation. The morphology of calcite crystals replacing root cells, and their carbon and oxygen isotopic composition indicate that they may precipitate inside the root cells from fluids with organic compounds.rnIn general, the temperature reconstructions yield an average MAT about 11 ℃ for the upper Quaternary sediments deposited during MIS 2. The estimated MAT for sediments of MIS 3-5 range from 12 to 14 ℃. Mean annual temperatures estimated for the lower Quaternary of Podolia fall in the range from 9 to 11 ℃ with two single minimums (about 7 ℃) probably corresponding to short-lived environmental changes. The MAT curve throughout the lower part of lower Quaternary is more variable and shows several local shifts (up to 3 ℃).