关中地区全新世大暖期的土壤与气候变迁

Soil Developed in the Holocene Magathermal and Climatic Migration in the Guanzhong Area

资料表明,关中地区全新世大暖期土壤中的CaCO_3受到了明显淋溶,部分土壤的CaCO_3淀积层脱离了粘化层,发育了淋溶性土壤所特有的风化淋滤母质层;该层土壤发生了明显的化学粘化作用,并有Fe_2O_3和Al_2O_3形成以及亚热带土壤所具有的淀积型红色光性粘土胶膜发育;粘土胶膜含量明显大于淋溶褐土而与黄棕壤接近。由此得出,全新世中期成壤作用比现今显著强,土壤应为发育弱的黄棕壤,当时气候比今明显暖湿;在全新世中期,来自东南的夏季降水气团能够较频繁地到达该区,秦岭已失去温带与亚热带气候分界线的作用,其南北两侧均为亚热带气候。

Based on the investigation of the Holocene soil in the Guanzhong area and data of the appraisal of soil microstructure from optical microscope and electronic microscope and chemical analysis, the subdivision of soil profile, soil type developed in the Holocene Megathermal were studied in the paper. The investigation shows that Holocene soil profile generally consists of organic horizon ( A) , clay grouting horizon( Bts) and illuvial horizon of CaCO3(Ck) , but some of soil profiles consist of organic horizon, clay grouting horizon, weathering-leaching parent loess horizon (Cl) which only occurs in leaching soil and illuvial horizon of CaCO3. The analysis of CaCO3 shows that CaCO3 content is low in clay grouting horizon and weathering-leaching parent loess horizon in the Holocene soil, and generally is less than 0. 8% , which indicats a stronger leaching process during the formation of the soil . The content difference of Al2O3 and Fe2 O3 from the upper to the lower of the Holocene soil profile is 1. 0% and 0. 7% respectively, indicating the formation of Al2O3 and Fe2O3 in soil formation process and a stronger chemical weathering process. There is reddish brown illuvial clay films, which is a characteristic of leaching soil, about from 5% to 10% in the middle of clay grouting horizon in the Holocene soil, and its content is more than luvic cinnamon soil and is close to weakly developing subtropical yellow-brown earth. Clay content in clay grouting horizon is from 38% to 58. 6% , and in the weathering-leaching loess horizon is from 32. 3% to 40. 7% , which is higher than that in unweathering loess horizon and is a result produced by stronger clay grouting. The appraisal from electronic microscope shows that transparent clay film in the Holocene soil was mostly composed of secondary clay mineral micro-crystals, and the micro-crystals was ranged directionally, which is the reason why clay films posses optical character that like a crystal. By x-ray diffraction, it is known that mineral of mixed layer of illite and askanite is dominant in micro-crystals in clay films. Based on above data, the soil in the Middle Holocene was leaching soil, and soil formation action was stronger at that time than at present , soil type should be weakly developed subtropical yellow-brown earth, the climate was obviously warmer and wetter at that time than at present; in the Middle Holocene, air masses bearing water in summer coming from southeast sea can frequently reached the area , and the Qinling Mountains had lost the function as the boundary between temperate zone and subtropical zone, and two sides of the Qinling Mountains both were subtropical climate.