Biomarkers in modern and buried soils of semi-desert and forest ecosystems of northern Iran

摘要

In Northern Iran mean annual precipitation and vegetation vary significantly over short distance from a semi-desert to a forest biome. These ecosystems likely responded differently on past climate changes. We here aim at i) testing the applicability of biomarkers (leave-derived n-alkanes, their stable carbon isotope composition, and C and N stable isotopes of soil organic matter) in loess-derived soils to identify and differentiate past ecosystems, and ii) elucidating the variability of these biomarkers in palaeosols. We sampled modern topsoils and palaeosol horizons within an ecological gradient covering a range in mean annual precipitation from 200 to 750 mm from the Kopet Dag semi-desert to the Hyrcanian forest on the footslopes of the Alborz Mountains. C-org, N, delta C-13(org), delta N-15, and n-alkanes (and their compound-specific delta C-13) were analyzed to characterize organic matter composition and sources. In modern soils a systematic increase in C-org and N was observed with precipitation. The delta N-15 decreased from about 6 to 4 parts per thousand pointing to systematically more degraded organic matter in semi-desert soils. The leave-wax specific ratio of (nC(31) + nC(33))/(nC(27) + nC(29))-n-alkanes was >1 for semi-desert soils and <1 for the forest ecosystem. The delta(1)3C(org) showed no systematic trend in this gradient. In loess and palaeosol profiles, contents of C-org, N and n-alkanes dropped about a factor 10 compared to modern soils. The n-alkane ratio and delta N-15 ratios remained on comparable levels as did the compound-specific delta C-13 in n-alkanes. However, bulk delta C-13(org) was altered from about -27 in modern soil to -23 parts per thousand in loess-palaeosols. Systematically higher C-org and N values were observed in palaeo-topsoils compared to loess and subsoil. Stable C isotopes varied rather unsystematically within loess-palaeosol sequences, while delta N-15 revealed trends within palaeosols, however, in contrasting directions with palaesol depth. The (temporal) average nalkane ratio for all palaeosol horizons of one site systematically followed the modern precipitation gradient indicating that in all periods of soil formation a climatic gradient developed. (C) 2016 Elsevier Ltd and INQUA. All rights reserved.