Geochemical characterization of loess-paleosol sequences:Comparison between the upper reaches of the Hanjiang and Weihe river valleys,China

摘要

This paper aims to compare the geochemical characteristics of loess-paleosol sequences in the upper reaches of the Hanjiang and Weihe river valleyswhich are located in the semi-humid temperate zone and humid subtropical zonerespectively. The Mituosi(MTS) profile in the upper reaches of the Hanjiang River valley and the Yaohecun(YHC) profile in the Weihe River valley were selected for this comparative research. The stratigraphic characteristicscompositionchemical weathering intensityleaching rates of Ca and Namobility of major elementsand transport features of Na and Fe were analyzed with respect to depth and compared between the two profiles. This study reached the following conclusions.(1) The composition of the loess-paleosol sequences in two regions are quite similar to the average composition of the upper continental crust(UCC)indicating that the loess in the two regions came from multiple sources and was mixed well. Thereforethe loess in the two regions is considered aeolian loess.(2) Compared with the loess-paleosol sequence in the Weihe River valleythe loess-paleosol sequence in the upper reaches of the Hanjiang River valley features a darker color; a higher chemical index of alteration(CIA) value; higher leaching rates of Na and Ca; higher migration ratio(relative to K) of AlSiMgand Na; and lower migration ratio of Fe and Ca. This evidence indicates that the loess-paleosol sequence in the humid subtropical environment experienced stronger chemical weathering intensity than the loess-paleosol sequence in the semi-humid temperate zone.(3) Both the YHC profile and MTS profile record a period of climate deterioration at 6000–5000 a BP. The period punctuated the mid-Holocene Climatic Optimum(8500–3100 a BP) in the study area.