Portal de Periódicos da CAPES

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

2016; Elsevier BV; Volume: 429; Linguagem: Inglês

10.1016/j.quaint.2016.02.048

1873-4553

Ali Shahriari, Farhad Khormali, Melanie Bläsing, Stefan Vlaminck, Martin Kehl, Manfred Frechen, Alireza Karimi, Eva Lehndorff,

Groundwater and Isotope Geochemistry

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.