第四纪黄土物源的时空差异研究现状及展望

Progress and prospect of research on temporal and spatial differences of the Quaternary loess provenance

黄土物源研究对揭示第四纪以来东亚大气环流格局的演化和构造—气候之间的相互作用具有重要意义。目前已应用多种物源示踪方法对黄土物源开展了大量研究,但对黄土物源时空差异规律及其动力学机制尚未取得较为一致的认识。在综合分析黄土物源研究现状的基础上,重点从影响^(87)Sr/^(86)Sr值和^(143)Nd/^(144)Nd值组成及单颗粒碎屑锆石U-Pb年龄谱构成的因素角度,分析了第四纪黄土的物源及可能变化,获得以下认识:(1)第四纪黄土沉积物的^(87)Sr/^(86)Sr值变化幅度可达0.002580~0.004949,远远超出实验室分析测试误差(<0.000018)或由Sr同位素衰变带来的影响(2.6 Ma时段小于0.000026,1个冰期旋回小于0.000001);^(143)Nd/^(144)Nd值亦发生了较显著变化(0.000095~0.000240),其变化幅度远大于实验室的分析测试误差(<0.000010)及衰变导致的^(143)Nd/^(144)Nd值(<0.000013)变化。因此,在黄土中检测到的Sr-Nd同位素组成的变化应具有较明确的地质意义。(2)第四纪黄土沉积物中无论全岩还是主要粒径组分的^(87)Sr/^(86)Sr值和^(143)Nd/^(144)Nd值的变化都与气候代用指标变化不同步,难以从气候变化角度进行解释,可能更多地反映了源区的变化。(3)与Sr-Nd同位素示踪体系相比,单矿物或单颗粒的物源示踪体系对物源区变化的响应更为敏感,在追踪第四纪黄土沉积物源区时空差异方面具有较明显的优势。(4)基于碎屑锆石U-Pb年龄谱对黄土物源的有限研究揭示黄土高原的黄土在地质历史时期可能发生了原始物源区显著的时空分异,特别是不同剖面都反映出了1.2 Ma前后黄土主要源区的变化,表明通过锆石U-Pb年龄谱研究黄土物源时空差异规律具有揭示岩石圈、大气圈、水圈耦合作用过程及历史的巨大潜力。

The study of provenance of loess on the Chinese Loess Plateau is critical for revealing evolution patterns of atmospheric circulation in East Asia and the interaction between tectonics and climate evolvement.By now,numbers of studies on provenance of loess sediments have been carried out based on different tracing systems.However,the temporal and spatial variation of loess provenance and its dynamic mechanism have not been well understood.Based on the comprehensive review of the previous studies on the Chinese Loess Plateau,the provenance and possible changes of the Quaternary loess were analyzed from the perspective of factors affecting the composition of^(87)Sr/^(86)Sr and^(143)Nd/^(144)Nd,and the U-Pb age spectrum of single grain detrital zircons.The following conclusions are obtained.(1)The variations of SrNd isotopic compositions in loess during the Quaternary have exact geological significance,for the reason that the^(87)Sr/^(86)Sr in Quaternary loess changed significantly with a range of 0.002580~0.004949,which is beyond the influence of analysis error(<0.000018)and the maximum radioactive decay effect(<0.000026 during the 2.6 Ma period and<0.000001 during a glacial-interglacial interval);the variation of^(143)Nd/^(144)Nd(0.000095~0.000240)during the Quaternary is also beyond the influence of laboratory analysis(<0.000010)and radioactive decay effect(<0.000013).(2)The variations of^(87)Sr/^(86)Sr and^(143)Nd/^(144)Nd both in bulk sediments and in the dominant grain-size fraction of loess sediments are of significance of provenance change because they are hard to be interpreted solely regards of climatic changes given the provenance of loess sediments is stable.(3)Relative to the Sr-Nd isotope tracer system,the tracing systems based on single-mineral or single-grain measurement(e.g.,zircon U-Pb age)is robust in detecting variation of the loess sediment provenance.(4)A limited study of the provenance of the loess based on the detrital zircon U-Pb age spectrum reveals that the original provenance of the loess in the Chinese Loess Plateau may have undergone significant spatial and temporal differentiation in geological history.In particular,different sections reflect the changes of the main provenance of the loess before and after 1.2 Ma.It is indicated that the study of the temporal and spatial variation of loess provenance by zircon U-Pb age spectrum has great potential to reveal the coupling process and history of lithosphere,atmosphere and hydrosphere.