Grainsize Dependence and Environment Significance of Trace Elements from ZL Core in the Jianghan Basin

Key words: 
• the Jianghan Basin /
• trace element /
• grainsize dependence /
• palaeolake /

Abstract: Element content can be affected by the grain size of sediments. Previous studies revealed that elements, content of which can be easily affected the grain size distribution are usually concentrated in finegrain fractions. The grain size effect makes it more complicated to decipher useful information on provenance via the elemental compositions of sediments. Provenance of the late Cenozoic sediments in the Jianghan Basin has been highlighted for a long time. Many provenance indictors have been taken to decipher information on the provenance of the sediments of the Jianghan Basin. Provenance study of the sediments in the Jianghan Basin plays an important role in reconstructing the evolutionary history of the Yangtze River and has been highlighted for a long time. Among these researches, geochemical methods have been largely adopted, e.g. the elemental compositions of the sediments. Unfortunately, it is still controversial that how the grain size affect the elemental compositions of the sediments. In this study, we presented the elemental compositions of the late Cenozoic sediments of the Jianghan Basin and tried to decipher some information on the correlation between the elemental compositions and the grainsize effect and the sedimentary environment of the Jianghan Basin.
The correlation between the 23 elements and the grainsize effect revealed that only 6% of the 23 elements show correlation higher than 0.3, while 13%, 35% and 46% of the 23 elements show correlation of 0.2~0.3, 0.1~0.2 and <0.1, respectively. The elements of V,Sc,Cr,Zr,Co,Sr,Ba,Nb,Hf,Ta,Hf,Th,U are relatively concentrated in the grain size fraction of 5~7, in contrast the elements of Li,Rb,Cu,Cs are concentrated in the grain size fraction of 10~12. The content of Be, Y and Pb are not obviously affected by the grainsize effect. It means that the grainsize effect is not the main factor that affects the elemental compositions of the late Cenozoic sediments of the Jianghan Basin. Based on that, we conclude that the elemental compositions can be effective provenance indicators. The grainsize effect can be ignored when the elemental compositions are used to trace the source of the late Cenozoic sediments of the Jianghan Basin.
The average variation coefficient of the 23 elements is 0.23, of which the Sr, Zr, Ba and Hf are the most fluctuant elements while the Y has the lowest variation coefficient. Curiously, the elements with higher variation coefficients are also enriched compared with the UCC.
The lacustrine sediments with lowest Sr content in the core sediments around the depth of 287 m indicate the weak chemical weathering conditions. The lowest V/Ni ratio indicates the weak oxidizing or reduction conditions. Meanwhile content of the lithophile elements are low throughout the core sediments, indicating less terrigenous input. Besides the color of these sediments is steel gray, which accords well with the elemental characteristics. We propose that around 2.6~2.43 Ma BP, the Jianghan Basin was controlled by arid climate. Rivers around the basin were with small water discharge inducing the development of salt lake. All of these factors could result in less terrigenous input. Sediments around the depth of 214 m are characterized by highest Sr content. The Sr/Ba ratios are relatively low while the V/Ni ratios are generally high and fluctuant. Most lithophile elements are with peak values of content. All of the evidence indicates that during this period the Jianghan Basin was controlled by humid climate. The water discharge of the rivers was sufficient especially the upper reaches. Under this condition the basin received large amounts of terrigenous input. As a result of this weather condition and the river hydrodynamic conditions, the lake in the Jianghan Basin was under oxidizing environment. Based on those evidence, we made a reasonable assumption that large lake developed under the control of the Yangtze River during this period.
Trace elemental compositions of the sediments revealed that most of the lithophile and chalcophile elements are relatively enriched compared to the Upper Continental Crust (UCC). The siderophile elements are depleted compared with the UCC. It probably indicates that the most of the sediments of the Jianghan Basin derived from the upper Yangtze rather than the materials from the deep earth in the Yangtze block. The lacustrine sediments of the Core Zhoulao around 287m show lowSr, and V/Ni ratios and high Zr/Sr and Rb/Sr ratios, which indicate that salt lake largely developed in the Jianghan Basin under the cold and dry weather between 2.6 ~ 2.43 Ma B.P.. In contrast, freshwater lake developed around 2.1 Ma B.P.. It is consistent with the late Cenozoic pollen assemblages in the Jianghan Basin, and the sedimentary records in the Subei Basin and Yangtze delta. We conclude that the elemental compositions in the Jianghan Basin might record the global cooling around 2.5 Ma B.P. and the intense uplift of the Tibetan Plateau around 1.4~1.2 Ma B.P.