杭州湾地区全新世古河口湾沉积物源示踪

Estuary as the seaward portion of a drowned river system which receives sediment from both fluvial and marine sources and which contains facies influenced by tide, wave and fluvial processes is distributed widely around the world throughout geological history. Their fills are critical stratigraphic archives for unraveling earth-surface processes, depositional history, source to sink process of terrigenous materials, and the interaction between terrestrial and coastal marine processes in the coastal settings, as well as for documenting environmental and climatic changes. The Holocene paleo-estuary in the Hangzhou Bay area is a world-famous tide-dominated estuary, and has a close relationship with the paleo-Yangtze River. Although there has been much descriptive documentation of the sedimentary characteristics, facies architecture, and depositional model for the paleo-estuarine sediments, few studies have involved the provenance tracing of the sediments and their coupling relationship with the paleo-Yangtze River sediments. In this study, with the research idea of the Source to Sink scientific theme for the “NSF Margins Program Science Plans 2004” as the guidance, based on the detailed analyses on the element, Nd isotope, detrital zircon U-Pb age and Hf isotope composition of the paleo-estuarine sediments/fluvial sediments adjacent the bottom of paleo-estuary from the typical cores SE1, SE2 and SE3 in the study area, we aim to 1) determine the provenance of the paleo-estuarine sediments by comparing their geochemical characteristics with those of the paleo-Qiantang River and Yangtze River sediments, and then investigate the distribution range, accumulation process and sedimentary contribution of the paleo-Qiantang River and Yangtze River sediments especially the latter in the different time scale; 2) discuss the controlling factors on the variations of the Source to Sink relationship for the paleo-estuarine sediments; 3) reveal the controlling effects of the provenance on the formation and evolution of the paleo-estuary and sand bodies. These results will be of great significance in in-depth knowledge of the “fate” and diffusion patterns of the postglacial Yangtze River sediments, as well as of the quantitative research of the sedimentary system and the inversion of the sedimentary-dynamic processes in the paleo-estuary. The study is also going to provide more scientific basis for the exploration and exploitation of shallow-biogenic gas, and for understanding the material flux of the Qiantang River and Yangtze River sediments, as well as the source-sink relationship in the future for the study area. Meanwhile, this study should fill the deficiency of the previous research, enrich and perfect the theory system for the same type of estuary around the world, and then provide a typical research example for the paleo-estuary in a certain extent.

本项目以国际大陆边缘科学计划的沉积物从源到汇的研究理念为指导，通过采用当前成熟的物源示踪方法，包括元素—同位素地球化学、锆石U-Pb年代学等，在对钱塘江源和长江源沉积物物源示踪指标深入认识基础上，准确判别杭州湾地区全新世古河口湾沉积物的物质来源；探讨不同时间尺度内钱塘江源和长江源沉积物，特别是长江源沉积物在古河口湾内的分布范围、堆积过程和沉积贡献；揭示古河口湾沉积物源汇关系变化的控制因素，以及沉积物物质来源对古河口湾及其内部砂体形成演化等方面的控制作用。这对冰后期长江沉积物的“归宿”和扩散规律，以及古河口湾内沉积体系的定量化研究和沉积动力过程反演等方面的深入认识都具有重要意义；也为杭州湾地区浅层生物气的勘探和开发、未来钱塘江源和长江源沉积物的物质通量预测和源汇关系认识提供更多科学依据；同时将会丰富国内外同类河口湾的理论体系，对古老地层中的河口湾研究也有一定指示作用。

本项目以国际大陆边缘科学计划的沉积物从源到汇的研究理念为指导，通过采用当前成熟的物源示踪方法，包括重矿物、元素地球化学、锆石U-Pb年代学等，在对钱塘江源和长江源沉积物物源示踪指标深入认识基础上，准确判别杭州湾地区全新世古河口湾沉积物的物质来源。结果显示钱塘江古河口湾的沉积物在重矿物、元素地球化学和锆石U-Pb年代学等方面均与现代长江沉积物显著不同，而与钱塘江沉积物非常相似，表明钱塘江古河口湾沉积时期长江和钱塘江下切河谷相互分离，两者之间无直接的物质交换，钱塘江古河口湾的沉积物主要来自口外已沉积的老钱塘江沉积物的再搬运。该研究成果对冰后期长江沉积物的“归宿”和扩散规律，以及古河口湾内沉积体系的定量化研究和沉积动力过程反演等方面的深入认识都具有重要意义；也为杭州湾地区浅层生物气的勘探和开发、未来钱塘江源和长江源沉积物的物质通量预测和源汇关系认识提供更多科学依据；同时将会丰富国内外同类河口湾的理论体系，对古老地层中的河口湾研究也有一定指示作用。