宝鸡剖面S5古土壤磁化率变化机制

摘要

Climate change is closely related to people's livelihood ,Holocene is the nearest geologic epoch we are living. Holocene climate change is therefore particularly important to human beings. It is known that aeolian loess deposit in China is one of the best geological recorders for paleoclimatic change. As position of Holocene loess is on the top layer of loess sequence, it may be easily disturbed by human activities. That is why the achievement of studying paleoclimatic change based on Holocene loess is not high so far. Here we report a continuous and integrity Holocene loess section in Baicaoyuan( 36°14. 390'N , 105°08. 032'E) ,Huining County,Gansu Province. This high resolution section is located in the northwest edge of Loess Plateau. The total thickness of the section we sampled is 2. 2m, 110 samples were sampled with 2cm interval. Among them, the thickness of L0 is lm,with pale yellow color, loose, massive,many grass roots and wormholes. The thickness of S0 is 0. 96m,with black brown color,hard texture,white calcium toadstools,many grass roots and wormholes; the thickness of L1 is 0. 24m, with many pores showing looser than L0, its color is also light than that of L0. The section recording the evolution of Holocene climate change can be beautifully correlated with oxygen isotope of stalagmite in Sanbao and Hulu. This record showing the cold and dry Younger Dryas event is during about 12.5 ~ 11.5ka period; the Holocene Climate Optimum (with higher temperature and precipitation) is during about 10. 5 ~ 5. 2ka period. However, climate is not stabilizing during the Holocene Climate Optimum; it becomes dry and cold gradually since 5. 2ka.%运用岩石磁学和地球化学相结合的方法对宝鸡剖面S5和S3古土壤的磁性特征进行了详细对比分析,结果表明S5古土壤层Rb/Sr比值较高,S5古土壤形成期气候温暖湿润,成壤程度比S3时期强.S5层亚铁磁性矿物含量低于S3层,反铁磁性矿物主要为针铁矿；而S3层的反铁磁性矿物主要为赤铁矿.S5层土体表面及土壤空隙中可见大量黑褐色的铁锰胶膜分布,由于沉积后土壤长时期处于偏干或偏湿的氧化、还原交替环境中,细粒的磁铁矿和/或磁赤铁矿、赤铁矿溶解转化成褐铁矿、针铁矿等弱磁性矿物,主要转化成在局域湿润环境下能够稳定存在的针铁矿,这种磁性矿物的转化可能导致了成壤强的S5古土壤超顺磁亚铁磁性矿物含量的减少和磁化率的降低.