渭河宝鸡市区段河床沉积物磁性特征分析

利用环境磁学的方法可以有效地示踪河流沉积物的物源，对渭河宝鸡市区段干流及其支流河床沉积物全面系统的磁学特征分析表明：渭河干流和支流沉积物的磁性矿物含量总体相对较高，磁性矿物类型主要以亚铁磁性矿物磁铁矿和磁赤铁矿为主，同时存在不完全反铁磁性矿物赤铁矿，磁晶粒以PSD（假单畴）和MD（多畴）颗粒为主。渭河干流沉积物的磁性特征在空间上表现出了一定的差异，根据χ_ARM/χ（非磁滞剩磁磁化率/磁化率）值的变化规律将渭河宝鸡市区段沉积物的来源分成了两段，第一段从宝鸡峡到神龙大桥，沉积物χ_ARM/χ 值逐渐减小，磁性颗粒逐渐变粗；第二段从神龙大桥到千河入渭口，沉积物χ_ARM/χ 值逐渐增大，沉积物的颗粒呈现出由粗变细的特征，指示了渭河宝鸡市区段河流沉积物存在一定的物源差异，渭河宝鸡市区段干流沉积物的物质来源主要以上游携带为主导，从神龙大桥向下游，支流对河床沉积物具有一定的贡献，北岸比南岸流域贡献的物质相对较多。

Magnetic property analysis of sediments at Baoji Section ofWeihe River

The provenance of the river sediment can be efficiently traced by using the method of environmental magnetism. According to the analysis of the river sediment's complete and systematic magnetic properties in the main stream and tributaries at Baoji section of Weihe River,the content of magnetic mineral in Weihe River sediments and tributaries is very little. The χ(Magnetic susceptibility)mainly reflects the total content of the sediment magnetic mineral and the variation's range of χ_lf is 36.4×10^-8～378.4×10^-8 m³·kg^-1 in the main stream of Weihe River,the average value of χ_lf is 116.9×10^-8 m³·kg^-1,and the variation's ranges in Qingjiang River, Jingling River and Qian River respectively are 36.8×10^-8～189.4×10^-8 m³·kg^-1,54.2×10^-8～168.1×10^-8 m³·kg^-1 and 54.2×10^-8～168.1×10^-8 m³·kg^-1. SIRM(Saturation isothermal remanent magnetization)can well reflect the magnetic mineral concentration of the river sediment. The variation's range of Weihe River main stream sediments is 2.9×10^-3～20.3×10^-3 Am²·kg^-1,the average value is 8.6×10^-3 Am²·kg^-1,the variation's range of three main tributaries respectively are 13.2×10^-3～28.0×10^-3 Am²·kg^-1,7.9×10^-3～9.2×10^-3 Am²·kg^-1 and 6.9×10^-3～ 9.6×10^-3 Am²·kg^-1,the magnetic mineral content of Qingjiang River exceeds that of Weihe River's main stream,Jinling River and Qian River sediments. The χ_ARM(Anhysteretic remanent magnetic susceptibility)reflects the content of the ferrimagnetism mineral,the variation's range of main stream sediments is 3.1×10^-8～ 572.1×10^-8 m³·kg^-1,its average value is 135.6×10^-8 m³·kg^-1,which is low in general. The variation's range of Qingjiang River,Jinling River and Qian River sediments respectively are 323.4×10^-8～632.7×10^-8 m³·kg^-1, 251.7×10^-8～552×10^-8 m³·kg^-1 and 81.8×10^-8～190.7×10^-8 m³·kg^-1. In addition,according to the hysteresis loop of representative samples and correlation analysis of SOFT and χ_lf(r²=0.91),the main magnetic minerals of samples are low coercivity maghemite,maghemite and high coercivity of antiferro magnetic minerals hematite. The FORC(First- Order Reversal Curve),King Plot and Day diagram indicate that the magnetic mineral in Weihe River sediments and tributaries are mainly pseudo-single domain(PSD)and multiple domain(MD)grains. The magnetic concentration of sediments in Weihe River shows the difference,on the basis of the variation tendency of χ_ARM /χ,the sediment sources can be divided into two parts in Weihe River different section,the value of number of sediments χ_ARM /χ gradually decreases in the first part which is from Baoji George to the Shenlong Brige,the magnetic particles becomes coarser,in the second part which is from Shenlong Brige to the border on Qian River and Weihe River,the χ_ARM /χ gradually increases,the sediment particle changes from coarse to fine,which clearly indicates that there are some differences in sediment provenance of the Weihe River in Baoji section. The source of main stream sediment in Baoji section of Weihe River is mainly the upstream carry-dominated,from the Shenlong Brige to the downstream,branch stream make a contribution to riverbed sediment,relatively,north shore contributes more substance than south.