青海湖水体-沉积物界面锕系元素分布特征及环境指示意义研究

The natural radionuclides 231Pa and 230Th have potential advantage in tracing physico-chemical processes. Preliminary analysis of actinides in Qinghai Lake indicates that U concentration and isotopic distribution are consistent in spatial and depth profiles. The U concentration is about 13.6ppb, higher than that of seawater, (234U/238U) activity ratio is 1.889. The concentration of 230Th-xs showed obvious spatial distribution characteristics, but not for the depth profile. Then, what kind of factors affect the spatial distribution of 230Th, how about the Th and Pa distribution in water and particulate matter, the understanding about source and control mechanism of U in the water, and the particles in the water retention time and environmental significance, it is urgent for us to understand the above question. This paper aims to present measurements of dissolved and particulate (>0.45μm) 231Pa, 230Th, 232Th and 234U,235U,238U of dissolved from Qinghai Lake transect with multiple particulate type, combined with river and sediments of Qinghai Lake contribution in June and September, in conjunction with measurements of the mass concentration and major chemical composition of dissolved and particulate. With these measurements, we can understand the concentration and isotopic distribution of the water and particulates, assess the source of U of Qinghai Lake and the retention time of actinides in water, reveal particle sedimentation rate and sediment flux. There is an in-depth assessment of how particle types and flux affect the ultimate distribution of sedimentary 230Th and 231Pa, and evaluate the potential providing basin weathering history based on the disequilibrium of the U-Th.

锕系元素在定量化所参与的水体物理-化学过程具有显著的优势。青海湖水体锕系元素初步结果表明U含量和同位素呈现一致的分布特征，U含量约13.6ppb, 明显高于海水，(234U/238U)活度比约1.889。230Th-xs含量呈现明显的空间分布特征。那么230Th的空间分布差异到底受控于什么因素及环境指示意义的理解亟需我们结合湖水颗粒物，河水和沉积物钻孔深度研究来评估。基于此，本申请试图通过对青海湖27个深度剖面湖水和颗粒物、河水和沉积钻孔进行系统的U,Th和Pa的同位素分析，结合微量元素分析，评估青海湖水体U的来源，锕系元素在水体中的滞留时间，颗粒沉降速率和沉积物通量变化，深入理解锕系元素231Pa和230Th从水体到沉积物中的净化过程，进而评估湖泊沉积物U系不平衡特征是否能够提供流域古风化历史的潜力。

本项目通过对青藏高原东北部多个湖泊水体溶解态和颗粒物的铀系元素分布特征研究基于搭建了U-Th分析示踪多源过程的理论框架，该理论可以应用于核科学与安全，铀矿示踪，地球和行星表面风化过程研究。.基于建立的水体-颗粒物痕量U, Th同位素分析技术通过对多个湖泊的研究，结果表明:1）冰冻融过程对内陆湖泊水同位素分布和潜在环境指示的影响。不同季节的溶解U在整个水柱中的分布变化不大，为湖泊沉积物和/或生物壳的绝对年代测定和/或古水文提供了潜在的价值。2）钍（Th）同位素被应用于量化青海湖的碎屑源输入的贡献。通过青海湖水体溶解态Th含量的分析结果表明当大量外源补给河流混入青海湖西北盆地时，湖水的232Th含量和碎屑通量主要受布哈河和沙柳河中颗粒物的类型和含量影响。相反，在补给河有限的南部盆地，主要受大气尘埃的影响。3）对中国青藏高原东北青海湖的湖泊样品进行了铀地球化学研究，以评估湖泊化学的主要控制。结果表明：U、TDS和盐度的变异性部分由蒸发或不同的二次浓度途径控制；湖水的Th浓度由三个过程控制：（1）控制气溶胶粉尘的溶解，（2）控制河流和湖床沉积物的溶解，（3）清除水中的Th。4）柴达木盆地东北盐湖铀同位素分布特征及影响因素。选取柴达木盆地东北地区4个盐湖，探讨高蒸发背景下湖泊中铀同位素的分布特征和影响因素。结果证明：这些盐湖的U浓度随季节没有明显的变化，那么沉积物年龄和/或沉积速率可以由湖泊沉积物和/或生物碳酸盐决定，从而允许准确的古气候和古环境的重建。5）利用钍同位素定量分析了柴达木盆地克鲁克湖和托素湖的成岩输入。水体溶解态232Th浓度的关系顺序为巴音河>克鲁克湖>克鲁克-托素连通河>托素湖，证实了成岩物质来源的输入变化。在年际时间尺度上，克鲁克湖的成岩通量主要来自河流输入。相反，与粉尘贡献相比，托素湖成岩通量的河流贡献较小。因此湖泊沉积通量主要受区域尺度物源和地形的影响。