Sr-Nd isotope and REE compositions of surface sediments from the three Gorges Reservoir: Implications for source identification and apportionment

摘要

Sediment deposition has already been considered to be a potential risk to the general running of the Three Gorges Reservoir (TGR). Identifying the sediment sources and their contributions to the TGR is of great significance for the sustainable management of reservoir operation. In this study, surface sediments of the TGR and its main tributaries were sampled in the flood (June to September), dry (November to next February) and even (March to May, October) seasons, 2017-2018, together with surrounding upland soils, and analyzed for Sr-Nd isotopes and rare earth elements (REE). The geochemical results showed that ε_(Nd)(0) values and ~(87)Sr/~(86)Sr ratios of surface sediments from the TGR mainstream varied from -12.21 to -9.75 and from 0.720241 to 0.726655, respectively. All sediment and upland soil samples generally showed the LREE enrichment and significant Eu depletions. Three potential sources were identified for the TGR sediments and ranked in the order: regional source (adjacent tributaries) > distal source (upstream tributaries) > proximal source (upland soil), based on the Sr-Nd isotopes and REE parameters. Isotopic mixing model coupled with Monte-Carlo simulations was firstly applied to quantify source contributions to the TGR sediments among different seasons. Through Monte-Carlo model simulations, the regional source contributed approximately 40 to the TGR sediments during the flood and even seasons but approximately 50 during the dry season. The contribution from distal sources arrived at about 30 in the three seasons, and 30 from proximal sources during flood and even seasons and 20 in the dry season. Dam construction was the primary factor affecting the sediment deposition in the TGR, and it would further reduce sediment supply from the distal source with the operation of new increasing dams in the upper Yangtze River Basin. Hence, more attention should be paid to the regional sediment supply of the TGR in the future.