Originating from the Qinghai-Tibetan Plateau,the Yangtze River(YR) is the third longest(6300km)river in the world,covering a total area of 1.8× 106km2 and locating between 24°30′ ～35°45′N,90°33′ ～ 112°25′E.It is a key area in the study of "Source to Sink" processes by serving as a major pathway for erosion products from the continents to the oceans.In this study,we do a preliminary research on magnetic properties of suspended sediments from the main stream and major tributaries of the Yangtze River during flood season of 2011,with the purpose to trace sediment source using environmental magnetic approach.Sampling sites covering a large area of the Yangtze River drainage from Benzhilan in the upper reaches to Nanjing near the estuary and a total of 28 samples were collected.Environmental magnetic measurements were carried out on suspended particular matter(SPM),including magnetic susceptibility (X),anhysteretic remanent magnetization (ARM),isothermal remanent magnetization (IRM) of all samples and temperature-dependence of magnetic susceptibilities of representative samples.The results are as the following:(1) S-ratio of SPM samples are between 0.83 ～ 0.96,in average of 0.86,indicating that the magnetic mineralogy is dominated by low coercivity minerals (magnetite) in all samples,with a small proportions of high coercivity minerals (hematite and goethite).(2) Variation of HIRM suggests that the anti-ferromagnetic mineral contents of sediments from the upper reaches are higher than those in the middle and lower reaches,which is a reflection of the wildly distributed purple sandstone and shale within the Sichuan basin.(3) Affected by the vanadium-titanium magnetite around Panzhihua,concentration of ferrimagnetic minerals in the main stream increased rapidly near Panzhihua with X and SIRM arrives to 246 × 10-8 m3/kg and 48833 × 10-6 Am2/kg respectively.The high concentration was followed by a slightly decline downstream with a sharp drop at the Three Gorges Dam(TGD).This indicates an interception of the TGD on the magnetic minerals from the upper Yangtze River.(4)The high concentration of magnetic minerals in Yalong Jiang,Han Jiang and Xiang Jiang was controlled by the source rocks,industrial and agricultural activities,respectively.(5)xfd％ values of most samples are higher than 5％,suggesting the existence of superparamagnetic minerals.According to the variation of ARM/SIRM,grain size of magnetic mineral particles showed a fining trend downstream from source to sink.It is suggested to be determined by catchment lithology,geomorphology and hydrology of the Yangtze River.On the whole,though the magnetic properties of suspended sediment in the Yangtze River can indicate the source information to some extent,they are also controlled by weathering conditions and,in modern settings,anthropogenic impacts (such as TGD) which made this method restricted when applied to provenance studies.%通过分析长江干流及主要支流表层悬浮物的环境磁学参数特征(类型、含量及粒度)及其空间变化,探讨环境磁学对河流物源示踪的意义.结果表明,长江悬浮物磁性矿物特征以磁铁矿为主,含有少量赤铁矿及针铁矿.上游不完整反铁磁性矿物含量高于中下游,反映四川盆地广泛分布的紫红色砂页岩的影响.受攀枝花钒钛磁铁矿影响,干流磁性矿物含量上游高于中下游,表现为在攀枝花处迅速增加,向下游逐渐降低并在经过三峡后显著降低,说明三峡大坝对上游磁性矿物有明显的拦截作用.支流中雅砻江、汉江及湘江较高的磁性矿物含量分别受控于流域出露的源岩及工农业等人为影响.长江悬浮物xfd％大于5％,表明样品中含有超顺磁颗粒.受流域地势及气候影响,磁性颗粒的粒径从上游至下游逐渐变细.长江悬浮物磁性特征能够在一定程度上反映物源特征,但长江地质条件复杂,气候类型空间变异大,且流域人为影响较大(三峡大坝等),使得利用磁性矿物特征示踪物源受到限制.
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