Mineral magnetism of lacustrine sediments and Holocene palaeoenvironmental changes in Dali Nor area, southeast Inner Mongolia Plateau, China

摘要

Mineral-magnetic measurements and analysis of total organic carbon (TOC) and carbonitrogen (C/N) were performed on two AMS ~(14)C-dated lacustrine sequences recovered from Xiaoniuchang and Jiangjunpaozi (～42°N, 117°E). The two sites lie in the Dali Nor area of southeast Inner Mongolia Plateau where the semi-humid climate transforms into a semi-arid one. SIRM, low-frequency magnetic susceptibility (X_(lf)), IRM_(20mT), HIRM and IRM_(20mT)/SIRM may largely reflect the erosion of catchment materials and dilution effect of organic matter on deposited magnetic minerals. IRM_(-100mT)/SIRM and IRM_(-300mT)/SIRM likely indicate the aeolian activities. SIRM/X_(lf) positively correlated to sand percentage of sediments is diagnostic of changes of lake levels or effective humidity. Complemented by the data of TOC and C/N and of previously accomplished analyses of grain-size, loss-on-ignition (LOI) and pollen, the variations in mineral magnetism reveal Holocene palaeoenvironmental changes. Conditions were quite dry and cool before 9970 years BP and then humidified in Jiangjunpaozi. The effective humidity maximised prior to ～8000 years BP and subsequently declined in the two sites. The less wet but warmer conditions persisted in Jiangjunpaozi until 6600 years BP and Xiaoniuchang until 5600 years BP. Deterioration occurred since then and culminated during 4300-1300 years BP in Jiangjunpaozi and 3000-730 years BP in Xiaoniuchang, which was followed by a moderate but probably very short amelioration in the two sites.