CHANGE OF SEDIMENT COMPOSITION IN THE YANGTZE RIVER SUBAQUEOUS DELTA AND ITS ENVIRONMENTAL IMPLICATIONS

Delta deposits receives sediment supply from both the river basin and the marine, which can alter sediment composition in accordance with their relative importance. Furthermore, the composition of delta deposits can be influenced by the varying contribution of different tributaries within the river basin as a result of climate change and human activities. Therefore, the time-varying sediment composition of delta deposits contains information on the dynamical process of sediment transport along the catchment-estuary-sea water continuum, which can be used to decipher the complicated delta evolution processes. The Yangtze River is the third largest river in the world, and the huge drainage basin (1.8 million km²) consists of multiple large tributaries(e.g, Jinsha River, Jialing River and Han River). These tributaries contribute to the formation of the Yangtze River delta. Furthermore, the recent southern migration of the Yellow River to northern Jiangsu Province(1128 ~ 1185 A.D.) can deliver sediment southward to the Yangtze River delta and participate in the formation of the delta. However, the temporal influence of different tributaries and the Yellow River on the sediment composition of the Yangtze River delta remains poorly studied, although fluvial sediment discharge to the sea by both the Yangtze and Yellow River has fluctuated significantly over the past 1000 years from historical documents and instrumental observations. In this study, core A3-1(31.64°N, 122. 36°E, water depth 16 m, 158 cm in length) and A6-6(30. 80°N, 122. 81°E, water depth 27 m, 180 cm in length) from the Yangtze River subaqueous delta has been investigated by Optical Stimulated Luminescence(OSL) dating, grain size and geochemical analyses. Our results reveal that cores A3-1 and A6-6 cover different depositional periods, with the former ranging from 1070±50 a to 1180±30a, and the latter between 49±9a and 198±21 a. Such a marked age difference reveals the heterogeneity in spatial deposition. These two cores have significant differences in the compositions of major and trace elements. Compared to A3-1, A6-6 has higher contents of element (e.g., Al, Fe, Mg, K, Ca, Cu, Cr, Co, Cd, V, U and Th), while A3-1 has higher concentration of Na and Zr. Grain size is an important factor affecting the elemental contents in the two cores. Using Al normalization to reduce the effect of grain size, we find that Core A3-1 has higher content of Na and lower content of Ca. Core A3-1 also has lower ration of Cr/Th compared to A6-6. The ternary plot of Ca/Al-Na/Al-Ti/Al distinguishes the sediment composition of A3-1 and A6-6 clearly. Compared with sediments form the main tributaries in the Yangtze River catchment and the Yellow River, temporal change in provenance of subaqueous delta sediments are discussed. One reason is that the contribution of different tributaries change with time. Core A3-1(1000~ 1200 a B.P.) is more affected by the Han River, while Core A6-6(50~220a B.P.) has a dominant Jialing River influence. Alternatively, there exists the impact of southward migration of the Yellow River in the past millennium. High content of Ca in Core A6-6 in the past 200 years may reflect significant contribution from the Yellow River due to enhanced sediment discharge by the Yellow River as a result of engineering regulation over the past ca. 500 years. Further provenance tracing study is needed to solidify our work. Nevertheless, our study reveals that composition of delta sediments varies markedly over time. Analysis of elemental composition can provide information on the relative influence of river and marine forcing on delta construction, which can be helpful for interpreting roles of climate change and human activities in delta evolution.