Mid-Holocene soil water and vegetation in the Xi'an area of the southern Chinese Loess Plateau

摘要

This paper investigates mid-Holocene palaeo-vegetation and soil water in the Xi'an area of the southern Chinese Loess Plateau (CLP). A well-dated (8.5-6.0 ka BP and 5.0-3.1 ka BP) loess-palaeosol (unit S-0) was logged, sampled and analyzed at four sites in the Xi'an area of the southern CLP, from which we report soil micromorphological (type and occurrence of root pores), particle size and geochemical data (CaCO3 and Sr concentration) to: (i) infer changes in soil water content, (ii) infer the limits of gravity water and the types of hydrologic cycle, and (iii) reconstruct the palaeovegetation. Numerous woody plant root holes were identified (2-4 mm diameter) within the illuvial horizon underlying palaeosol S-0, the morphology of which is consistent with those produced by woody plants. The clay-rich argillic horizon of palaeosol S-0 was found to contain high concentrations of kaolinite, which in concert with the geochemical indices, indicates that soil gravity water attained a depth of 2.2-2.8 m, whilst soil water content was similar to 20-25%. This implies that pedogenic processes at this site were more intense during the mid- compared to late Holocene loessic soils elsewhere on the CLP, and ultimately attests to relatively wet climatic conditions (e.g. up to 800 mm yr(-1) rainfall), the hydrologic cycle type of precipitation-soil water-groundwater, and the vegetation of deciduous broad-leaved forest. We propose that combined analyses of root-channel morphology and mobile element geochemistry can be used to infer changes in palaeo-hydrology and ultimately, the strength of the Asian Summer Monsoon.