基于宇宙成因核素剖面法测年的川西龙日坝冰水扇研究

Study on Outwash of Longriba in West Sichuan Province Based on Cosmogenic Nuclide Profiles

冰水扇等快速堆积的混杂沉积,多由粗大砾石组成,成层性差,少砂层夹层,缺有机质,用释光和^(14)C等方法测年难度很大。因此,探讨对这类沉积进行定年的方法,并进一步分析其古环境及区域对比意义的工作显得迫切而现实。选取青藏高原东部四川红原龙日坝一个冰水沉积剖面,尝试利用2种宇宙成因核素剖面法:Monte Carlo法和指数拟合法,对冰水扇的形成年代进行定年和分析。研究结果表明,龙日坝冰水扇堆积的结束年代为(36.7±4.6)ka或(35.2±3.7)ka,核素继承量为11.3^(+4.0)/_(-4.0)×10^(4)atmos/g,表面侵蚀速率为0.5^(+0.3)/_(-0.5)cm/ka。剖面密度分析和Monte Carlo控制实验结果显示,核素继承高估了暴露年龄,而不完全暴露则低估了暴露年龄。与冰碛垄宇宙核素测年相比,核素继承是冰水扇定年的主要误差来源。龙日坝冰水扇形成在冰阶(MIS 3b)—间冰阶(MIS 3a)过渡阶段,反映降水和冰川融水增加导致河流搬运能力增强,沉积物通量增加,冰水扇沉积的环境特征。宇宙成因核素剖面法有利于详细分析测年的误差来源,可以作为混杂沉积精确定年的有效手段。

It remains a big challenge in dating the outwash and other rapid accumulated mixed depositions using conventional methods,such as Optically Stimulated Luminescence and ^(14) C,because these depositions are mainly composed of coarse gravels with poor stratification,lacking sand interlayers and organic matters.Therefore,it is urgent and meaningful to develop a method to date such kinds of deposits so that we can analyze their paleoenvironment and compare regional differences.In this paper,we selected an outwash deposition profile at Longriba,located in Hongyuan county,Sichuan Province,eastern Tibetan Plateau and tried to date and analyze it by two Cosmogenic Nuclide Profile dating methods:the Monte Carlo calculator and the expansional regression.Results showed that the age of the outwash was(36.7±4.6)ka or(35.2±3.7)ka,the nuclide inheritance was 11.3^(+4.0)/_(-4.0)×10^(4) atmos/g,and the post erosion rate at the surface of the outwash was0.5^(+0.3)/_(-0.5) cm/ka.We also analyzed the experimental error using density analysis method and Monte Carlo control experimentations,revealing that the exposure prior to deposition yields exposure ages that are too old and incomplete exposure due to post-depositional shielding yields exposure ages that are too young.In contrast with the ages of boulders by cosmogenic nuclide dating,our results point out that the experimental error is dominated by nuclide inheritance for the outwash.The comparisons of regional climate data indicate that the higher precipitation and glacial melt water resulted in higher transport capacity of the river,increased high sediment flux of the river during the glacial(MIS 3b)to interglacial(MIS 3a)transition,so that the outwash aggradation occurred.The Cosmogenic Nuclide Profile dating method is better to deeply analysis the error source of dating,so that it can be used as an effectively method for accurately dating the rapid accumulated mixed depositions.