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

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. Corg, N, δ13Corg, δ15N, and n-alkanes (and their compound-specific δ13C) were analyzed to characterize organic matter composition and sources. In modern soils a systematic increase in Corg and N was observed with precipitation. The δ15N decreased from about 6 to 4‰ pointing to systematically more degraded organic matter in semi-desert soils. The leave-wax specific ratio of (nC31 + nC33)/(nC27 + nC29)-n-alkanes was >1 for semi-desert soils and <1 for the forest ecosystem. The δ13Corg showed no systematic trend in this gradient. In loess and palaeosol profiles, contents of Corg, N and n-alkanes dropped about a factor 10 compared to modern soils. The n-alkane ratio and δ15N ratios remained on comparable levels as did the compound-specific δ13C in n-alkanes. However, bulk δ13Corg was altered from about −27 in modern soil to −23‰ in loess-palaeosols. Systematically higher Corg and N values were observed in palaeo-topsoils compared to loess and subsoil. Stable C isotopes varied rather unsystematically within loess-palaeosol sequences, while δ15N revealed trends within palaeosols, however, in contrasting directions with palaesol depth. The (temporal) average n-alkane 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.