全新世安塞剖面的磁学特征变化历史及其受控因子分析

黄土高原黄土是第四纪古气候研究的主要对象之一，磁化率是黄土古气候研究的主要代用指标。然而，黄土地层磁学特征的主控因子依然存在争论。本文选取黄土高原中部的安塞黄土剖面为研究对象，对地层进行系统的磁学特征分析。安塞剖面在早全新世至中全新世早期(~10000~8500 a)黄土层中磁性矿物含量较低且呈递增趋势，中全新世(8000~3000a)古土壤层中磁性矿物含量较高，在中全新世中期(7000~5000a)达到最高随后呈递减趋势，晚全新世(3000 a以来)黄土层中磁性矿物含量很低并呈递减趋势。此外，地层的磁学特征表现出3次快速变化：6500 a前后，地层中磁性矿物含量迅速增加；4500 a前后和2700 a前后，地层中磁性矿物含量快速降低。通过与全新世降水和气温等气候因子对比发现降水量是控制安塞剖面中次生强磁性矿物含量变化的关键性气候因子，低温并不能增加地层中强磁性矿物含量，而高温则可能限制地层中强磁性矿物的形成。土壤中强磁性矿物含量变化对气候好转响应迟缓，对气候恶化响应迅速。

Holocene variations of paleomagnetic characteristics along the loess section of Ansai and their controlling factors

The loess in west China is widely recognized as one of the most important continental records of Quaternary paleoclimate and paleoenvironment. Numerous studies have been devoted to the reconstruction of the histories of Asian monsoon, atmospheric circulation, and uplifting of the Tibetan Plateau. Magnetic susceptibility is one of the most important paleoclimatic proxies. As previous studies suggest, pedogenesis may produce a large amount of fine-grained particles with strong magnetism to enhance the magnetic susceptibility of soil units, and as the result from a sequence characterized by alternated sequence of paleosol layers of higher magnetic susceptibility and loess layers of lower magnetic susceptibility. However, the main factors controlling over the formation of the magnetic particles are still in debate. In this paper, the Holocene loess section in Ansai of the central Chinese Loess Plateau, is investigated for the variation in magnetic characteristics, and the main controlling factors over the formation of secondary magnetic particles discussed upon the basis. The results show that during the time of 10000-8500 a, the concentrations of ferrimagnetic minerals were low upon an increasing trend; while during the period from 8000-3000 a, the concentrations of ferrimagnetic minerals were high. It reached its peak in 7000-5000 a. Then, the ferrimagnetic minerals concentrations turned to a decreasing trend with the lowest value after 3000 a. All of the magnetic parameter curves have three rapid changes, a sharp increase in ferrimagnetic minerals concentrations at ~6500 a and two sharp decreases at ~4500 a and ~2700 a, respectively. By comparing the magnetic susceptibility with the records of paleo-precipitation and temperature in Holocene, it is proposed that precipitation is the key climatic factor controlling the concentration variation of secondary ferrimagnetic minerals. Hot weather may limit the formation of secondary magnetic minerals owing to the decrease in effective moisture in soil. However, the relatively cool weather can not either help to increase the concentration of strong magnetic minerals in loess soils. It is also found that the variation in magnetic minerals concentrations responds slowly to climate improvement, but rapidly to the deterioration of climate.