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Faculty for Biology, Chemistry and Earth Sciences

Department Soil Ecology - Prof. Dr. Eva Lehndorff

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Khormali, F; Shahriari, A; Ghafarpour, A; Kehl, M; Lehndorff, E; Frechen, M: Pedogenic carbonates archive modern and past precipitation change – A transect study from soils and loess-paleosol sequences from northern Iran, Quaternary International(552), 79-90 (2020), doi:10.1016/j.quaint.2019.12.011
Stable carbon and oxygen isotope compositions in soil carbonates and in soil organic matter provide insights into (paleo) climatic conditions, especially in arid and semiarid regions where carbonate is abundant in soils. We here investigate δ13C and δ18O isotope composition of bulk carbonate, pedogenic carbonate nodules and organic carbon from modern soils developed along a precipitation gradient (Entisol, Inceptisol, Mollisol and Alfisol) and paleosols formed within loess units in northern Iran. δ13C values of organic carbon in the modern soils ranged from −19.5‰ in the arid and semiarid regions to −24‰ in the more humid regions with forest vegetation. δ13C values of bulk carbonate in the modern soils ranged from −12.1‰ at the wettest sites to −0.8‰ at the drier sites and thus, a 550 mm increase in precipitation leads to a ~11‰ decrease in δ13C values of bulk carbonate. In the modern soils, δ13C values of pedogenic carbonate nodules in the semiarid areas show higher values by ~3–4‰ more than the regions with forest vegetation. Unlike δ13C values of pedogenic carbonate nodules the δ18O values of pedogenic carbonate nodules of modern soils in this study did not display pronounced changes across the sections. The change in δ13C of carbonates and organic carbon is consistent with the present above-ground vegetation, consisting of a mosaic of habitats with differences in the proportion of C3 and C4 vegetation tending to larger C4 contribution with increasing aridity. The δ13C values of carbonates coupled with climate data of the region confirm seasonal bias in the formation of carbonate nodules in the modern soils seen in studies elsewhere. The δ13C values of pedogenic carbonate nodules in the paleosols range from −10‰ in humid areas to −7.2‰ in semiarid areas. The trends in δ13C composition in modern soils are also visible in the δ13C composition in the paleosols, suggesting that in the foothills of the Alborz and Iranian loess plateau, a precipitation gradient existed also in the last interglacial.
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