Evidence From Late Cenozoic Sediments In Jianghan Basin:Implication For The Evolution Of The Three Gorges

During recent years,evolution history and deposition development of large-scale river systems,especially originated from the inland mountains and poured into the marginal seas,have been paid more and more attention.The Yangtze River,as the most important linkage between the Tibet Plateau and East Asian marginal seas,delivers great volume of water,sediment and relevant chemicals from its headwater regions and tributaries to the East China Sea,significantly affecting regional geochemical cycles,environment evolution,and depo-systems development in these areas.Thus,investigation of the source to sink transport pattern of the Yangtze River materials allow us to better understand these phenomena.In the past few decades,variously source tracing methods have been performed on the modern Yangtze River sediment and geo-historical deposit.However,diverse source tracing methods resulted in diverging consequences,the unified cognition remains confusion,which generally owing to the limitation of the source tracing method.Thus,more new source tracing methods should be applied to provide more significance recognition.Based on “source to sink” process of the Yangtze River.In this study,three provenance indicators including Cd content,clay minerals and the Electron Spin Resonance(ESR)signal intensity were integrated performed to sediments from Zhoulao(ZL)Core,depocenter of the Jianghan basin,Middle Yangtze River,China.Moreover,the multiple centers electron spin resonance(ESR)dating method on quartz grains was initially performed this method on ?3 Ma B.P.borehole sediments from the ZL core to provides a precise chronology framework for provenance studies.The main conclusions of this thesis are listed as below:1.The multiple centers electron spin resonance(ESR)dating method results show that: 1).The multi-centers ESR dating method on quartz grains shows great potential in chronological framework establishment of Plio-Pleistocene borehole sediment and yields a consistent estimated age compared with the palaeomagnetic dating results.2).For middle Pleistocene samples(i.e.?0.13-?0.8 Ma),the Ti-Li center provides a more precise age,while the estimated ages of the Al center are overestimated.For early Pleistocene samples(i.e.?0.8-?2.6 Ma),both Al center and Ti-Li center deduce highly consistent estimate ages,indicating a favorable dating range for the multiple centers ESR dating method.Meanwhile,the Al center exhibits a promising prospect for dating Pliocene samples,whereas the Ti-Li center is more suitable for middle-early Pleistocene(?0.13-?2.6 Ma)sediments dating.3).SSE function could provide a similar fitness equal to the Ti-2 function for the Ti-Li center and an even better goodness-of-fit superior to the EXP+LIN function for the Al center at a low artificial addition dose(?20000 Gy).Marked Cd anomalies were detected along the Yangtze River system,which was reported mainly dominated by natural factors rather than anthropogenic activity.The Cd levels of the sediments in the ZL core were analyzed and two units were identified(0–160 and 160–300 m depth).The markedly higher Cd levels from 160 m to the top,consistent with the change in the clay-mineral assemblage,is interpreted as indicating the input of sediment from above the Three Gorges.According to the results from multicenters ESR dating method on quartz grains(1.7Ma at 160 m),we conclude that the incision of the Three Gorges valley occurred no later than 1.7 Ma.For clay minerals,three sections were diagnosed on the basis of variations in claymineral contents and its formation mechanisms was demonstrated.In this study,sediments of section 1 and 2 were derived from internal materials within the Jianghan Basin and its indicators were coupled with the climate change,whereas the sediments of section 3 were dominant controlled by provenance superior to climate change.We interpret this change in sediment provenance at 160 m depth was resulted from the cutthrough of the Three Gorges,which brought sediments from the upper stream.The quartz ESR provenances indicators,including natural E' center intensity,thermal E' center intensity,natural E' center intensity VS thermal E' center intensity and thermal E' center intensity VS quartz crystallinity(CI)all reveals two obvious end-member components.From 300 m to 160 m,the ESR index of borehole sediments is consistent with that of modern sediments in the middle and lower reaches of the Yangtze River.From 160 m to the top,the quartz ESR provenance tracer of borehole sediments is consistent with that of modern sediments in the upper reaches of the Yangtze River.Based on the distribution characteristics of relevant indexes of modern sediments in the Yangtze River Basin,we speculated that this phenomenon was mainly caused by the change of provenance.Before the incision of Three Gorges,sediments of the ZL core were mainly fed by the proximal,which obvious show a higher value of ESR signal intensity.After the incision,sediments from the upper reaches,which were characterized lower value of ESR signal intensity gradually transport to the Jianghan Basin and become a dominant contributor for deposit in ZL core.Combined with the ESR dating method on quartz grains(1.7Ma at 160 m),we conclude that the incision of the Three Gorges valley occurred no later than 1.7 Ma.Above all,the multi-centers ESR dating method on quartz grains shows great potential in chronological framework establishment of Plio-Pleistocene borehole sediment and yields a foundation for further provenance studies.Comprehensive analysis of multiple indicators including natural E' center intensity,thermal E' center intensity,natural E' center intensity VS thermal E' center intensity and thermal E' center intensity VS quartz crystallinity(CI)synchronous diagnosed the provenance shift at 160 m,which means the Yangtze River has been connected at least 1.7 Ma.