Micromorphological characteristics and environmental significance of the Holocene aeolian loess-paleosol section in the Zoige Basin

Abstract

A typical Holocene aeolian loess-paleosol section was identified in the Zoige Basin on the northeastern (NE) Tibetan Plateau. Detailed micromorphological observations and systematic studies were conducted using a Leica DMRX polarizing microscope, and microstructure identification and quantitative analysis were performed using Leica Qwin3.2 software. The results revealed the following findings. (1) In the Zoige Basin of the alpine-cold region, the micromorphological characteristics include coarse grain sizes, mainly ranging from 100 to 200 μm, with quartz and plagioclase as the main mineral types. The secondary minerals consist mainly of small amounts of secondary calcites and secondary clay minerals. The organic matters includ humus, plant residues, and charcoal fragments. The main void type is stacked voids with rough walls. (2) Compared to loess, the coarse grain parameters (area, perimeter, length/width ratio, roundness, and equivalent circle diameter) in the paleosol decrease significantly, and the unstable minerals exhibit obvious signs of weathering. The porosity and humus content increase. In modern soil, coarse grain parameters increase, unstable mineral boundaries become evident, and the porosity decreases. These characteristics suggest a variation in the intensity of weathering and pedogenesis following a weak-strong–weak pattern, indicating the climatic evolution in the Zoige Basin from dry-cold to warm-wet and back to dry-cold since the Holocene. (3) Compared with the Loess Plateau, the loess-paleosol micromorphology of the Zoige Basin in the alpine-cold region is characterized by significantly higher unstable mineral content, coarser grain size, less secondary mineral content, and mainly vesicular voids with rougher walls. These characteristics indicate significant differences in weathering and pedogenesis between the two regions owing to variations in geographical environments. The hydrothermal conditions of the Zoige Basin in the alpine-cold region were lower than those in the Loess Plateau, resulting in less intense weathering and pedogenesis. These results could provide the micromorphological evidence for reconstructing the Holocene paleoclimate evolution in the NE Tibetan Plateau.