长江下游沿岸山顶下蜀黄土年代学及地球化学物源示踪

The provenance of the Xiashu loess has been intensively investigated in recent years, but it remains unresolved because of the paucity of systematic chronological research and effective source discrimination method. Hilltop loess along the lower reaches of the Yangtze River area can represent an eastward extension of the Xiashu loess, succeeding the East Sea Island loess. Due to its special location close to riverside and seafront and geomorphic conditions, this hilltop loess stratum provides an ideal target for comparison and provenance analysis of the Xiashu loess. This project chooses hilltop loess sections on the Huangnishan Hill and Gushan Hill as the primary objective and employs chronological (Optically Stimulated Luminescence) and geochemical methods, in order to determine it's position in stratigraphic sequence of Xiashu Loess and compare the uniform stratum materials of hilltop loess and potential source areas, analyze the geochemical characteristics of different grain-size fractions, distinguish the component of distance and proximal sources, and track the provenance of hilltop loess. To >32μm grain-size fraction, detrital zircon U-Pb age distributions will be used to distinguish the proximal source materials. To <16μm grain-size fraction, which is usually deemed as long-distance transporting dust, the grain-size-independent and anti-weathering elements and Sr-Nd isotopic compositions will be used to analyze the distant source materials. This study will provide important information not only on establishing the sources of the Xiashu loess in different stages but also on revealing the driving mechanism of East Asian monsoon and its effects to paleo-climate and paleo-environment evolution in lower reaches of Yangze River area and East China.

对于下蜀黄土物质来源问题因缺乏系统的年代学研究和有效判源方法，至今仍存在不同观点。研究发现，长江下游沿岸低山顶部黄土地层滨江临海，是下蜀黄土向东延伸部分；特殊地理位置和地貌条件使其成为探索下蜀黄土物质来源的良好材料。课题拟以长江下游沿岸黄泥山、孤山等顶部黄土地层为研究对象，运用光释光测年法研究该地层地质年代，确定其在下蜀黄土地层序列中的位置，并与潜在源区同层位地层对比，分析不同粒级组分地球化学判源指标。对主要为近距离风力搬运的>32μm碎屑锆石颗粒分析U-Pb年龄特征；对能远距离搬运的<16μm组分分析其中不受后期改造和地球化学粒度效应影响的判源指标，如主、微量（含稀土）元素比值和Sr-Nd同位素组成等特征，由此识别山顶下蜀黄土中远、近源组分，探索其主要物质来源。这一研究对揭示不同阶段下蜀黄土主要物质来源，探索东亚季风对长江下游地区乃至中国东部古气候、古环境演变驱动机制具有重要的科学意义。

选择长江下游沿岸下蜀黄土剖面主要包括南通黄泥山、南京泰山新村、江阴长山、镇江圌山和靖江孤山等地下蜀黄土堆积地层为研究对象，综合运用分粒级稳定硅酸盐组分稀土、微量元素组成，主量元素组成，锆石U-Pb年龄，粒度等方法，对下蜀黄土物源进行分析。同时结合年代学研究结果，选用稳定地球化学物源识别指标与潜在源区相同层位地层对比，判别山顶下蜀黄土远、近源物质和主要物质来源。.下蜀黄土和洛川黄土<5µm组分元素地球化学特征的一致性，表明下蜀黄土<5µm细颗粒组分主要来自于中亚内陆干旱区的远距离输送。下蜀黄土下蜀黄土10–20 μm和20–50 μm组分元素地球化学特征和> 40 μm碎屑锆石U-Pb年龄分布与长江沉积相似，指示下蜀黄土在>10µm组分主要来自于长江下游泛滥平原的近源沉积。联合下蜀黄土分粒级物源示踪的结果和石英粒度组分含量的新方法定量估算下蜀黄土中远、近源含量。定量评估的结果显示下蜀黄土物源以长江泛滥平原的近源沉积为主，占比至少82.43%，但远源沉积的含量也不容忽视，占比约9.64%。下蜀黄土和古土壤层均是远、近源混合沉积，远、近源组分贡献量存在冰期-间冰期旋回变化，主要表现在冰期时，长江泛滥平原近源沉积的输送量明显增加；间冰期时，近源沉积的输送量明显减少。以当地泛滥平原的近源沉积为主的结论，指示了中国东部泛滥平原区冰期时广泛的干旱化程度的加剧，对中更新世全球降温事件的区域响应。细颗粒组分来自亚洲内陆干旱区的远距离输送，指示了中更新中亚沙漠地区干旱化程度的加强和东亚冬季风强度的增加。另外，下蜀黄土和桃花峪黄土近源沉积结果，指示长江和黄河下游黄土沉积主要属于高山源-河流传输模式。风尘黄土的物源研究可以加深对大气粉尘产生机制、黄土沉积模式的认识，对理解粉尘源区干旱化历史及其驱动机制具有重要指示意义。