长江沉积物源示踪研究进展

近期物源分析方法发展较快,包括稀土元素、同位素地球化学与单颗粒碎屑矿物微区分析方法的广泛运用,以及分不同粒级组分或根据需要选择特定粒级组分进行物源分析逐渐代替了全样分析法,物源示踪效果得到明显的提高。长江作为连接青藏高原与西太平洋边缘海的最重要水系,河流沉积物从源到汇的现代与历史过程备受关注。长江沉积物源示踪研究进展包括:①建立和运用河流入海沉积物示踪端元模型,定性或定量地分析长江沉积物在海域的扩散与沉积分布规律,倾向于运用细颗粒组分矿物学、元素与同位素地球化学等方法,研究程度较高,今后需注意各端元值的时空变化,及受沉积过程的分异作用与早期成岩作用的影响;②建立和运用不同支流的物源示踪模型,研究晚新生代以来长江水系的演化历史,倾向于运用粗颗粒组分的物源分析方法,尤其是单颗粒碎屑矿物微区分析。由于长江流域面积巨大、区域地质复杂,建立支流域的精细物源分析指纹特征尚处在探索阶段,需注意运用碎屑锆石U-Pb定年与稀土元素、Hf同位素组成的综合物源分析法。综合运用多种物源分析法于长江中下游和三角洲盆地若干钻孔地层的研究,已较好地限定长江贯通的时间约在上新世晚期—早更新世之间,但仍存在较大争议,今后需在一些关键区域开展更多的深钻研究,提高物源精细示踪的效果、晚新生代地层测年的精度,并加强钻孔间的的对比研究。

Advances in Provenance Studies of Changjiang Riverine Sediments

The methods of provenance analysis have recently developed very rapidly with obvious increase in the effectiveness of the provenance discrimination.The trend development of the methodology includes wide usage of geochemical compositions of rare earth elements and isotopes and micro-analyses of single-grain detrital minerals to trace sediment source,and the replacement of traditional analysis of bulk samples by different-sized subsamples or typical-sized subsamples.Provenance studies of the Changjiang reverine sediments from their source to the sink system have recently attracted great interest,especially due to its highest importance in the linkage of the Tibet Plateau with the marginal seas along the West Pacific.Most of provenance studies of the Changjiang riverine sediments focus on two aspects.One is the attempt to construct the sediment-source discrimination model of the Changjiang River from others(typically the Huanghe River),and apply the model to trace transportation and distribution pattern of the Changjiang-sourced sediment in the East China marginal seas.The other is the attempt to set up the provenance distinguishing models of different tributaries or tectonic provinces within the Changjiang drainage basin,using the model to study the spatio-temperal variation in sediment delivery processes within the drainage system,and decode the evolution history of the Changjiang River network since the Late Cenozoic and its relationship with the uplift of the Tibet Plateau,gigantic geomorphological changes in East Asia,and evolving Asian monsoon system. The end-member model of the Changjiang-sourced sediments has been extensively studied in the last few decades through employing the methods of detrital mineral assemblages,elemental and isotopic composition,etc.,typically on the fine-grained fraction.Most of the effort is put on the discrimination between the Changjinag and the Huanghe riverine sediments.It is recommended that future studies should pay more attention to the impact of the spatio-temperal change in the end-member characters,transported fraction,and early diagensis on the end-member modeling discrimination of river-sourced sediments in the marginal seas.It is more effective to fingerprint the sediment source from different tributaries or tectonic units with the drainage basin through using the discrimination methods on the coarse-grained fraction,typically the micro-analysis of the single detrital mineral grains.It is very complex and still under its early stage to set up fingerprinting model for the discrimination of individual tributaries or tectonic units within the Changjiang river basin due to its huge drainage area,numerous tributary networks,and various geological and climatic environments.The effort in the near future is suggested to build up a bigger database,especially to synthesize data of detrital zircon U-Pb ages,REE and Hf isotopic compositions.The recent research tend to constrain the Changjiang running-through time on a narrower period of the Pliocene and the Early Pleistocene based on integrated provenance study of the Late Cenozoic sediments of several deep cores in the Changjiang middle-lower-reach and deltaic basins.The controversies are still significant on the sediment source discrimination and the timing of the Changjiang formation.Therefore,it still needs to refine the sediment fingerprinting methodology for the higher resolution,drill more deep cores typically in some key districts of the basins at the Changjiang middle and lower reaches and the surrounded seas,improve the dating accuracy and resolution of the ages of the Late Cenozoic strata,and promote comparison studies of the provenance analysis and chronological stratigraphy among the cores.