宝鸡剖面S5古土壤磁化率变化机制

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。

THE MAGNETIC MECHANISM OF S5 PALEOSOL IN BAOJI SECTION

The magnetic susceptibility(MS)has been widely used as a proxy of summer monsoon,which is based on an important fact that the MS achieves the highest value in the most developed paleosol S5,while lower values in less weathered loess layer in the Central Chinese Loess Plateau(CLP).However,in Baoji loess section,the MS of S5 is the lowest value within the top 6 paleosols(S0,S1…S5),which contradicts above normal situation.Baoji(34°25'N,107°07'E) is located in the southern edge of CLP,north foot of Qinling Mountains.The present climate is warm temperate semi-humid with mean annual precipitation(MAP)of more than 700mm(up to 1100mm in Qinling areas),the rain distribution is uneven(60% rain falls between July and Sep.),and mean annual temperature(MAT)of 12℃(25.4℃ in July).Pedogenic condition in Baoji,such as precipitation,is better than the Central Loess Plateau,for example MAP 654mm in Xifeng(35°46'N,107°41'E) and MAP 555mm in Lingtai(35°33'N,107°31'E).In order to find out its mechanism,the samples were collected from the Baoji loess-paleosol sequence since S5,with sampling interval 30cm.The results of rock magnetism and geochemical measurements for Baoji S5 and S3 samples show,that ratio Rb/Sr of S5 samples was higher than that of S3,indicating that S5 was a better-developed paleosol unit,stronger chemical weathering than S3.The climate condition during S5 period was confirmed to be the warmest and wettest for the last 0.5Ma.The S3 paleosol was formed under oxidation environment,under such pedogenic conditions,maghemite and hematite were formed simultaneouly,the anti-ferromagnetic minerals are mainly hematite,superparamagnetic(SP)maghemite is the main contributor to the magnetic properties.However,the contents of ferrimagnetic minerals in S5 samples are lower than that of S3 samples.With pedogenic developing,the proportion of ferrimagnetic component reduces in S5 while the content of anti-ferromagnetic minerals(mainly goethite)increases.There are amounts of dark brown Fe-Mn film on the cranny surface of S5 paleosol,which indicates where the iron in S5 went to and why MS in S5 becomes lower.This phenomenon seems to suggest different pedogenic condition between S3 and S5 development.S3 pedogenic environment was under oxidizing conditions: so that extra fine grain ferrimagnetic minerals produce with pedogenic development; while S5 development in Baoji was under condition of moisture over much,pedogenesis actually occurs under waterlogged conditions.So that fine-grained magnetite,maghemite and hematite are gradually dissolved,and finally converted into goethite and limonite and other weak magnetic minerals.Such transformation of magnetic minerals,from high MS minerals(magnetite and maghemite)to low MS minerals(such as goethite)due to over moisture(gleying),results in the lower MS value for Baoji S5 paleosol.