现代长江沉积物地球化学组成的不均一性与物源示踪

摘要

长江沉积物从源到汇过程研究的关键是揭示长江沉积物的组成特征,准确识别其时空组成变化规律,这也是东部边缘海海陆相互作用研究和长江水系构造演化研究的关键.本文研究一些常用的元素地球化学参数在现代长江水系的河漫滩和悬浮沉积物中组成的时空变化特点.在稀土元素(REE)组成上,下游近河口段干流悬浮沉积物在季节性时间尺度上比较稳定,可以代表现代长江入海沉积物的平均组成,而长江主要支流和干流间REE组成变化较大,化学相态分析可以更好地揭示特定物源区的REE组成特征以及由矿物分异引起的REE参数变化.相比REE而言,悬浮物中Al/Ti与Zr/Rb比值更能敏感地指示不同物源区的源岩组成信息,且减弱水动力分选引起的粒级和矿物分异影响.长江入河口的干流悬浮物作为整个流域风化剥蚀细颗粒物质的平均混合,具有更好的源区平均组成的示踪特性；但在不同季节也存在不同的主导性源区,主要受控于流域内季风降雨区的迁移.在河流沉积物源汇过程重建中,要特别关注流域物质风化和输运的自然过程可能引起的这些地球化学参数变化.%The key for sediment source to sink study of the Changjiang River(Yangtze River)is to reveal the chemical composition of the Changjiang River sediment and to understand the compositional variations at different spatial and temporal scales,which are also important for the studies of land-sea interaction in the eastern marginal seas and of tectono-climatic evolutions of the Changjiang River.In this study,a total of 9 suspended and 25 fine-grained floodplain sediment samples were collected from the upper Jinshajiang River in Lijiang to near the Changjiang River mouth in March and April of 2003 and 2004,July and August of 2008,and July of 2009.In addition,51 suspended sediment samples were taken from the lower Changjiang mainstream near Nantong during the period from April 2008 to April 2009,with each sampling interval of every week.In the State Key Laboratory of Marine Geology,the analyses of grain size and elemental compositions were carried out on these samples.The main purpose of this paper is to investigate the heterogeneity of sediment geochemical composition of the Changjiang River sediments with emphasis on rare earth elements (REE)and other geochemical parameters.In particular,the REE fractionation parameters including(La/Yb) UCC,(La/Sm) Ucc,(Gd/Yb) UcC,Ce and Eu anomalies,ratios of Al/Ti and Zr/Rb,and chemical index of alteration CIA were discussed in this study.The REE compositions of the seasonal suspended sediments taken from the lower mainstream near the river mouth are quite stable,reflecting the average composition of the fine-grained Changjiang River sediment into the sea.In contrast,the REE compositions exhibit large variations in the floodplain and suspended sediments from the major tributaries and other localities of the mainstream.Chemical speciation analysis can better reflect the REE compositions in the sediments from unique provenances,and reveal the variability of REE fractionation parameters caused by mineral sorting during sediment transport.Compared to REE,the ratios of Al/Ti and Zr/Rb in the suspended sediments are more sensitive to the changes of sediment provenances by minimizing the influences of sediment grain size and mineral fractionations in relation to hydrological sorting.As the averaged mixture of finegrained sediments weathered from the whole Changjiang River drainage basin,the suspended sediments in the lower mainstream near the river mouth can be used as the modem representative of the Changjiang-derived sediments in the sea.Nevertheless,the dominant provenances of the Changjiang River sediments into the estuary and marginal sea change with seasons,mainly because of the seasonal shifts of monsoon precipitation fronts in the drainage basin.It has to be cautious in using some widely-documented geochemical proxies to reconstruct the source-to-sink pattern of fluvial sediments due to the influences of weathering and sediment transport processes.