An eolian deposit-buried soil sequence in an alpine soil on the northern Tibetan Plateau: Implications for climate change and carbon sequestration

摘要

The accumulation of eolian dust plays an important role in the development of alpine soils on the northern Tibetan Plateau. However, little is known either about the nature of soil-forming processes in the region, or about the magnitude of soil organic and inorganic carbon (SOC and SIC) storage. Here we report the results of an investigation of a typical profile, consisting of a series of buried soils developed within eolian deposits, situated at an altitude of 4951 m in the Amdo zone of the northern Tibetan Plateau. Bulk density, pH, SOC and SIC content, grain-size distribution, magnetic susceptibility and mineralogical composition were measured at high resolution, and AMS C-14 dating was used to provide a chronology. Based on all of the analytical data we conclude that this alpine accretionary soil profile contains three buried soils beneath the present surface soil layer. The deepest and oldest soil is the basal paleo-weathering crust (Ferralsol) which underwent active soil formation from 5540 to 7615 yr. BP, or even earlier. The other soils, which are developed within the overlying eolian dust deposits, are Luvisols and they document the occurrence of two intervals of more humid conditions than the present day: from similar to 3455 to 5540 yr. cal BP and from similar to 2000-2500 yr. cal BP. Microscopic analysis of coarse particles from the profile reveals that high values of magnetic susceptibility at the base of the profile reflect the presence of coarse magnetite grains present as inclusions within quartz grains derived from the weathering of granitic gneiss. In addition, the magnetic susceptibility profile reflects the effects of winnowing of the eolian fraction by wind activity, and not the production of fine magnetic grains during pedogenesis. SOC is the dominant form of carbon in this alpine soil and the SOC density and average accumulation rate values within the entire interval of eolian deposits are 19.67 kg C m(-2) and 3.55 g C m(-2) yr.(-1), respectively. These remarkably high values indicate that this alpine accretionary soil is characterized by highly efficient SOC burial, and therefore that such soils are an important terrestrial CO2 sink. Likewise, the buried soil plays an important role in SOC storage. Finally, the characteristics of the buried soils indicate a long-term climatic trend towards aridity in the study region. This natural trend has promoted the processes of desertification and grassland degradation on the northern Tibetan Plateau, and these processes may have been exacerbated by human activity. (C) 2015 Elsevier B.V. All rights reserved.