黄土沉积物原生组分磁化率与风力的物质分选

The magnetic susceptibility of the unaltered aeolian dust and wind sorting

本研究选取我国西北地区典型荒漠表土和黄土-古土壤序列样品为研究对象,探讨黄土沉积物及其潜在物源区表土的原生磁性矿物浓度在不同粒级间的变化规律、黄土剖面中粒度与磁化率的关系,以验证中国黄土沉积物中磁性组分在搬运过程中是否存在分选作用及其变化规律。来自西北地区表土的地质证据表明,磁性组分主要集中于>63μm和31~63μm粒径组分,并以后者的含量最高;与之相对应,黄土沉积中原生磁性组分主要集中于31~63μm粒径组分,但>63μm粒径组分不再是主要载磁组分,其磁性显著低于31~63μm粒径组分。同时,多个剖面的证据表明黄土沉积的原生磁化率与31~63μm粒径组分的含量是同相位变化的,进一步支持沉积物中原生组分的磁化率受到风动力的分选作用影响。需要强调的是,沉积物原生磁化率的主要载磁粒径组分为31~63μm的颗粒物,而并非最粗粒组分。本研究结果暗示,相较于沉积物的全样粒度,干旱半干旱地区风成沉积的磁化率在指示古风场强度方面具有潜力。

Loess is one of the most important continental geological records in the mid-latitudes,which is characterized by widespread,continuous deposit,and long history record.On the basis of loess record,the climate change can be reconstructed and compared both temporally and spatially.Magnetic susceptibility(X,mass-specific)is a widely used magnetic proxy for paleoclimate reconstruction from loess deposits.Previous studies have shown that variations of X correspond to loess and paleosol alternation in the sequences,with high values in paleosol and low values in loess.The subsequent studies suggested the aeolian deposit was magnetically enhanced due to in-situ pedogenic formation ferrimagnetic minerals which are closely linked with precipitation.However,studies from Alaskan loess,Siberian loess,Central Asian loess,revealed that variability of X values is dependent on the obvious change of magnetic properties of lithological material.Actually,recent studies had found that the lithological ferrimagnetic minerals were unstable in loess sequences,especially for the Chinese Loess Plateau loess.Previous studies put forward the'wind sorting'mechanism to explain the lithological magnetite variations in loess deposit.However,the particle-size dependence of aeolian dust still needs more systematic investigation.In this study,loess samples from Zhongliang(ZL)section,Axike(AXK)section,Kansu(KS)section,and Jiuzhoutai(JZT)section,and desert samples from Northwest China were collected to investigate their X behaviour of separated particle-size components(<2μm,2~8μm,8~31μm,31~63μm,>63μm)and the particle-size dependence characteristics of X in the loess sediments and their sources.Basing on this,our study tried to test whether original magnetic susceptibility of loess was influenced by wind sorting.The results showed that the ferrimagnetic minerals in desert samples mostly concentrate in the>63μm and 31~63μm components,especially in later one.In contrast,the ferrimagnetic minerals in all of the loess samples concentrate in the 31~63μm components,and the content of which is obviously lower in>63μm component.The parallel variations of original X and particle size of sediments were widely discovered in Chinese loess,with the strongest magnetic material enriched in 31~63μm component.Therefore,the lithological X is controlled dominantly by numerous of 31~63μm component.It indicates that,compared with the particle size of bulk sample,the X of aeolian deposit in the arid and semi-arid area has a much purer implication of wind intensity,due to which is only sensitive to magnetite content.