长江口沉积物化学相态分析及主泓变迁的指示

Chemical Phases of Sediments in the Changjiang Estuary and the Indication for the Geomorphological Evolution of Changjiang Estuary

元素地球化学是沉积物源判别和环境研究的重要手段,但河口海岸地区沉积动力环境复杂多变,人类活动影响强烈,全岩沉积地球化学的示踪研究存在局限性和多解性。选择长江下游干流悬浮物、东海陆架表层沉积物以及长江口具有一百多年沉积记录的ZK6孔,通过化学相态分析(1 N HCl处理),探究酸溶态微量元素组成特征及其对河口环境变迁的指示。相较于钻孔全岩样品,酸溶态Sr/Ba比能更可靠地反映河口古盐度和海陆相沉积环境的变化。ZK6孔沉积物酸溶态稀土元素(REE)主要赋存于Mn氧化物中,Mn、ΣREE含量、Ce/Ce*以及Sr/Ba比在1899—2007年间呈三段式变化,主要反映长江河口流路分汊和主泓位置改变引起的河口沉积环境变化,进而影响河口环境中活跃元素和次生组分在沉积地层中的保存记录。该研究对今后深化认识复杂河口环境下微量元素地球化学行为以及微量元素示踪海洋环境变化具有借鉴意义。

Elemental geochemistry is an important means for provenance discrimination and palaeoenvironmental reconstructing in marine and continental environments.However,a study based on the bulk sedimentary geochemistry is significantly influenced by the complex sedimentary dynamics in estuarine and coastal areas,as well as the influence of human activities.The Changjiang(Yangtze River)Estuary,as an important depocenter for the riverine sediments,is featured by the active hydrodynamic and depositional processes.Due to the combined effects of runoff and tidal currents,erosion and siltation processes interact,which causes the main stream channel to swing frequently,and the sedimentary environment of the estuary is constantly changing.Therefore,it is an ideal place to investigate the geochemical behavior of elements in a complex estuary environment.However,the evolution of geomorphology and sedimentary environment of the Changjiang Estuary in the historical period is mainly based on the studies of estuarine geomorphology and sedimentary dynamics,while few studies focus on the application of sedimentary geochemistry for tracing channel evolution and sedimentary environment.This study collected the suspended sediment samples in the main Changjiang stream,the seafloor sediments on the East China Sea(ECS)shelf,and the sediment samples of core ZK6 drilled from the Changjiang Estuary.In the lab,all of these samples were treated with 1 N HCl,and then the compositions of major and trace elements in the leachates and residues,as well as in the bulk samples,were measured.We then discuss their implications for the evolution of estuarine environment over the last hundred years.The Sr/Ba ratios in the leachates of the sediments in core ZK6 varied from 1.2 to 1.9,which is significantly higher than the average of the suspended samples in the main Changjiang stream(Sr/Ba=0.25)and significantly lower than the average of the seafloor sediments on the ECS shelf(Sr/Ba=8.0).The average Sr/Ba ratios in the residues of the above three fractions are similar,with values of 0.19,0.19,and 0.33,respectively,and do not have an environmental indication.The average Sr/Ba ratios in the above three bulk samples are 0.21,0.31,and 0.62,respectively,indicating that the environmental response sensitivity is significantly lower than that of the leachates,although it yields the increase from land to sea.Therefore,compared to the traditional bulk composition,the Sr/Ba ratio in the leachates is a more reliable geochemical indicator for paleo-salinity and can well distinguish between terrestrial and marine sedimentary environments.The total rare earth element(ΣREE)concentration in the leachates of the sediments in core ZK6 ranges from 59.9μg/g to 90.9μg/g,with an average of 75.2μg/g.The leachates account for 34%-53%of the bulk REE concentration,with an average of 43%,with the middle REE(MREE)more easily leached than the light REE(LREE)and the heavy REE(HREE).The Upper Continental Crust(UCC)-normalized REE patterns in the leachates of the sediments in core ZK6 are consistent with the composition of the modern Changjiang sediments,with significant enrichment of MREE.TheΣREE concentration is positively correlated with Mn content in the leachates of the sediments in core ZK6(R~2=0.91).We infer that Mn oxides are the main host of acid-leachable REE in core ZK6.The weak positive correlation of theΣREE concentration in the leachates with the bulk Al/Si of the sediments in core ZK6 indicates the weak grain size effect on REE composition.Mn andΣREE concentrations and cerium anomaly(Ce/Ce*)are significantly negatively correlated with the palaeo-salinity proxy,Sr/Ba,in the leachates of the sediments in core ZK6 and were significantly higher than that of the seafloor sediments on the ECS shelf.The results indicate that the salinity change and the combined action of runoff and tidal current in the Changjiang Estuary can significantly affect the redox environment of bottom water,the coagulation process,and the intense deposition in the low salinity region,further controlling the contents of Mn oxide andΣREE in the sediments.Mn andΣREE concentrations,Ce/Ce*,and Sr/Ba ratios show three-step variations during the period of 1899-2007:from 1899 to 1948,the Sr/Ba ratio in the leachates first decreased and then remained relatively stable after the mid-1920s,while the Mn andΣREE concentrations and Ce/Ce*increased first and then maintained relatively high values;from 1948 to 1971,the Sr/Ba ratio increased,while Mn andΣREE concentrations and Ce/Ce*decreased rapidly;after 1971,the Sr/Ba ratio decreased slightly at first,then remained relatively stable;Mn andΣREE concentrations and Ce/Ce*increased significantly at first and then showed a decreasing trend.The variations of Mn andΣREE concentrations,Ce/Ce*,and Sr/Ba ratios during the period of 1899-2007 mainly indicate the evolution of the estuarine sedimentary environment caused by the geomorphological evolution in the Changjiang river mouth,which affects the preservation of active elements and authigenic components in the sedimentary strata in the estuarine environment.This study suggests that even in the large estuary where the sedimentary dynamic process is complex and the river-sea interaction is intense,some indicators based on sediment element geochemistry can still indicate the evolution of the estuary environment.