西安地区全新世古土壤下部木本植物根孔的发现与研究

Discovering and Research of Woody Plant Root Holes under Holocene Paleosol in Xian Areas

在西安南郊和东郊全新世古土壤下部发现了许多木本根孔及充填物 ,根孔直径多为 1～ 2cm ,大者为 5cm ,分布深度达 4m多。密集的木本根孔和其中乔木根孔的存在指示当时植被为繁茂的森林。根孔充填物的颜色、粒度和矿物组成均与全新世古土壤粘化层相同 ,表明充填物来自全新世古土壤。根孔以机械充填为主 ,后期有胶体化学充填。根孔的充填发生在全新世古土壤发育的中、后期 ,推测当时出现了气候突然变干的事件。根孔中分布深达 2m左右的光性粘士胶膜指示全新世古土壤具森林土壤特征 ,当时年均降水量可达 80

According to investigation into Hololcene paleosol in the southern and eastern suburbs of Xian, quantities of plant root holes filled with earth under the paleosol have been discovered. By X\|ray diffraction, paritcle size analysis and microstruture appraisal, the earth in root holes and the paleosol are studied. There all are the root holes from the paleosol to underlying Malan loess four metres deep, the intervals of root holes are 5～10 cm in upper Malan loess and 20～50 cm in middle Malan loess. The root holes are mostly round, flat and straight, and most of them are vertical, few are horizontal and lean. Diameters of the root holes are mostly 1～3 cm, few are 3～5 cm. Most root holes are filled with black\|brown earth, few horizontal ones keep empty. The color of earth, particle size and mineral are all same to the Holocene paleosol on the whole. According to diameter, shape and depth, these root holes were all formed by woody plants, and the root holes that are 2 m deep or so and 2 cm or more in diameters, were formed by trees. It is judged from concentrated root holes that vegetatoin at that time were dense. According to 6 samples X\|ray diffraction data of the earth in root holes and Holcence paleosol, quartz takes 50% or so, feldspar and mixed component of illite\|montmorillonite 15%, kaolinite 4%, chloorite 6%, calcite 3%. The mineral components of earth in the holes are basically same to paleosol, which shows that earth in the holes comes from upper Holocence paleosol. The existance of kaolinite shows that the weathering and soil formation process were fairly strong at that time, while existanece of calcite was the result of removing later. Particle size analysis of 6 samples shows that the particle size composition of the earth in root holes is basically same to that of the paleosol, clay particale is more than 50%, the particles less than 0\^001 mm are between 25%～35%. Compared with Malan loess around the root holes, the content of clay grain increases 20% in paleosol and the earth in the holes, which shows fairly strong clay grouting processes. Microstructure appraisal of 7 samples shows that optical clay film existsed in root holes and paleosol developed well and takes about 5%, which indicates that Holocene paleosol has the character of forest soil. According to data mentioned above, it is concluded that temperate forest prevailed, the paleosol was forest type, and annual mean precipitation was 800 mm or so during the development of Holocene paleosol in Xian area. The reason why root holes were filled with earth is that the climate became cold and dry, the plants became few and scattered, root system decomposing fast and formed root holes filling with earth, which should be an indicator for cold\|dry climatic event.