The Environment Of Central Guizhou Reflected By The Geochemical Characteristics In Soil Profile Of Sancha River Since 3000y

The vegetation coverage in Karst area can reflect the severity of karst rocky desertification and we can also understand the forming and developing process of rocky desertification in karst area. As the local vegetation coverage is closely related to the regional climate changes, the regular rules of these changes can be obtained. Guizhou is located in the center of Southwest China karst ecosystem, which is the world's largest karst ecosystem. The fragile eco-environment restricted the local economic development. Understanding the natural factors and artificial influence in the changes of vegetation coverage in Guizhou will help us realize the mechanism of fragile ecosystems.The soil profiles of Sancha River in Anshun Guizhou are the main studying objects. We discussed their stable isotopes (δ13Corg,δ15Norg)and elemental geochemistry characteristics, in order to unveil vegetation and climate changes which are reflected by soil profile in the late Holocene. Then, we come up with the following conclusions:(1)The leaching and deposition of Mg, Ca, Sr in the soil profile clearly reflected that the deposition layer (B layer) has not been influenced on the whole. The correlation of other elements in the soil profiles reflects the geochemical characteristics of various elements. This outstanding peak of high concentrations at a distance of 27cm from the top probably reflects a event caused by human activity and transfer of heavy metals in the area.(2)Considering element changes in different levels of soil profiles, we can use dating results to establish a more reliable time scale. During the formation process of soil carbonates, organism would absorb CO2 from old carbonate dissolution to synthesize organic substance, while the 14C activity in ancient carbonate is 0, which makes organic carbon dating results ofδ13Corg older. In order to establish a reliable time scale, and considering this influence, we pushed forward the age of the soil about 400y. Then the deposition of soil profile is approximately from Oyr BP to 3300yr BP, formed in the late Holocene.(3)The C/N ratio of soil profile illustrates the main sources of organic matter is terrestrial plant-based.δ13Corg values of the soil profile indicate that the C3 plant was the dominant vegetation during the late Holocene in the studied area. Theδ15N values of the part above the 22 cm line strongly reflect contamination of sewage fertilizer which had nitrogen isotopic enrichment, but the irregularity of theδ13Corg and 815NOrg values of the part below the 137 cm line may reflect the influence of seasonal swelling of the river. And theδ15Norg values of the part between 22 cm and 137 cm lines reflect that the soil profile is of natural soil.813Corg andδ15Norg values have similar variation trend, demonstrating thatδ13Corg andδ15Norg are all from the organic matters of decomposed plants.(4)The value ofδ13Corg and 815Norg is lighter in 3300 yr BP to 2400 yr BP, which indicates that the C3 plant was dominant vegetation during that time in studied area. Theδ13Corg andδ15Norg values from 2800 yr BP to 2400 yr BP reflect that the monsoon weakened gradually and the climate turned cold and dry, which happened during the transition time from mid-Holocene to late Holocene. The heavier values ofδ13Corg andδ15Norg from 2400 yr BP to 1500 yr BP reflect the cold and dry environment in late Holocene.(5) From 1500 yr BP to 0 yr BP, the values ofδ13Corg andδ15Norg increased gradually even faster, which reflect the dual effects of human activity and climatic conditions. Especially after 500 yr BP, the value ofδ13Corg gets much heavier because of the massive immigration in Ming dynasty in central-western Guizhou. And the greatly increasing population combined with the demands of house and farmland, led to the damage of local vegetation.(6) The record ofδ13Corg can reliably reflect the vegetation and climate changes of late Holocene in the studied area. What's more, the good comparision with the value of theδ13C of the stalagmite records of Zhijin cave after 750 yr BP indicate the value ofδ13Corg reflects the impact of human activity. Compared with the stalagmite records of Dongge cave, the value ofδ13Corg in soil profile makes a good record of the changes in vegetation, climate and environment. The influence of human activities on the regional environmental changes must be considered, so as to give constructive advice about integrated management of karst region's fragile eco-system.