【作者】 徐毅；

【导师】 张经； 瞿建国；

【作者基本信息】 华东师范大学 ， 海洋化学， 2007， 硕士

【摘要】 本文通过Mg(OH)2-阳离子交换树脂法对样品中的硅元素进行富集分离,并采用单接收器电感耦合等离子质谱仪测定了长江口及邻近东海区域中溶解态和颗粒态样品中硅稳定同位素的组成。通过以上研究,初步认识研究区域内硅稳定同位素的分布特征及其与相关生物化学因子之间的联系,为今后深入研究长江口及邻近海区的初级生产力变化、水团运动等生物地球化学过程提供一些帮助。在质谱仪中分辨率工作条件下,硅同位素质量峰可以和相邻多原子干扰峰完全分开,确保了测量信号的准确性。通过对质谱仪进样条件、检测条件和数据处理条件的选择,采取增加测量次数的方法抵偿系统瞬间波动造成的偏差,通过增加质量窗口扫描范围可以避免质量漂移对测量信号的影响。在最佳分析条件下,硅同位素标准物质的比值重复测量精密度RSD小于0.05%,硅同位素标准物质GBW04421和GBW04422的δ29Si和δ30Si值长期重现性分析结果分别为-0.04±0.04%。,-0.06±0.06‰和-1.42±0.12%。,-2.71±0.1‰(1σ),与相关文献中报道的结果范围一致(Ding et al.,2005)。完善了溶解态样品的Mg(OH)2-阳离子交换树脂的预处理方法,对共沉淀引发剂加入量、沉淀离心条件和样品体积等条件进行优化选择。结果表明通过该预处理方法可以将样品中存在大量杂质离子与硅元素分离开来,并且对硅进行有效富集,富集倍数可达50倍。不同基体水样加标回收率均在99～105%之间,方法检出限为0.16μmol/L(3σ,n=8)。实验结果表明,本方法中的各个步骤不会引起硅同位素分馏的发生。2006.08至2007.03月份间徐六泾定点站溶解态硅δ30Si值的月季变化范围在+1.25‰～+3.17%。,平均值为+2.17±0.73‰,高于世界其他淡水河流的溶解态δ30Si值。δ30Si与DSi含量之间具有显著正相关性,可能与长江流域岩石化学风化条件以及支流对干流的贡献程度有一定的关系。在2005年8月长江口航次调查范围内,浮游植物生长吸收所需要的溶解态硅酸盐主要来自于长江冲淡水的输送。浮游植物生长吸收对硅同位素的分馏作用,导致表层溶解态硅同位素δ30Si值高于底层。由于影响硅同位素组成δ30Si的因素较为复杂,溶解态δ30Si与DSi之间未显示具有相关性。悬浮颗粒物样品δ30Si值与DSi之间显著的正相关性表明浮游植物对硅酸盐的吸收利用水平越高,硅同位素在生源硅体内的分馏程度越大。根据理想条件下的开阔稳态模型计算,长江口区域生源硅的分馏因子ε=1.95±0.73‰。2006年8月长江口及邻近东海区域调查结果发现,表层溶解态δ30Si与硅藻类浮游植物的生长水平有关,同时表明,在整个区域的表层存在不同来源水团对当地溶解态硅的持续输送。在低氧区由于生源颗粒物在底层的降解程度较高,引起的硅同位素δ30Si值升高的程度也越明显。在浮游植物生长水平较低的区域,溶解态硅同位素组成可能能够用于指示水团特征。总体来水,外海水的溶解态硅同位素组成δ30Si值较低,为+0.6‰～+1.5%。;沿岸淡水系的溶解态硅同位素δ30Si较高,约在为±2.89‰～+3.21‰。更多还原

【Abstract】 In the present thesis, a method based on Mg（OH）₂ coprecipitation followed by cation-exchange separation for the determination of stable silicon isotopic compositions in natural waters by single-collector inductively coupled plasma mass spectrometry （SC-ICPMS, Finnigan Element 2） was developed. The dissolved and particulate silicon isotope samples were collected from sample locations in Xuliujing, Yangtze River estuary and its adjacent East China Sea. The main purposes of this study are to elucidate the distribution of the silicon isotopes and their relationships with the other biogeochemical factors in the study area. Hoping to provide some basic helps in the further studies of chemical weathering, primary production and water mass transportation in the Yangtze River estuary and its adjacent East China Sea.The use of medium resolution （R=4000） of single-collector inductively coupled plasma mass spectrometry allows polyatomic ion interference-free measurements of silicon isotopes using conventional nebulization sample introduction without aerosol desolvation. Studies on the parameters of sample introduction, signal detection setting and date acquisition indicate that the variation caused by instant fluctuating ion currents could be compensated by increasing the scan numbers, and a large mass window scale setting might minimize the potential effect from mass shift. Three kinds of Si isotopic standard reference materials, NBS28, GBW04421 and GBW04422, were used to evaluate the potential availabilities of determination of silicon isotopic ratios by SC-ICPMS. Optimization of scanning condition led to a relative standard deviation for 19 consecutive ratio measurements of 0.012～0.045%. The δ^29,30 Si of GBW04421 and GBW04422 were -0.04±0.04 ‰, -0.06±0.06 ‰ and -1.42±0.12 ‰, -2.71±0.1 ‰, respectively. The long-term reproducibilities would reach the value of 0.1 ‰ for δ^29,30Si analysis. These results showed a good consistency with previous study （Ding et al., 2006）.A pretreatment method based on Mg（OH）₂ coprecipitation followed by cation exchange separation for natural water was also established. Studies on the procedure parameters of coprecipitation procedure indicate that dissolved Si could be quantitatively removed from solution by the formation of brucite Mg（OH）₂, initiated by the addition of NaOH, and the separation of Si from Mg²⁺ was achieved by using cation-exchange resin with 98% yields. The optimized procedure provides Si recovery between 96%～105% among various natural water samples. The detection limit （3 a, n=8） of dissolved Si is 0.16μmol/L. The reproducibility （n=8） of the present study is less than 3%. This procedure is effective for quantitative removal of Si from samples as large as 200ml, and a concentration factor of 50 can be reached. Furthermore, no isotopic fractionation was observed at each step of this sample pretreatment method.The seasonal variation of dissolved silicon isotope δ³⁰Si value of Xuliujing is from +1.25 ‰ to +3.17 ‰, the average value is 2.17±0.73 ‰. The δ³⁰Si value is much higher than that of other riverine water in the world. It was found that a positive relationship between the concentration （DSi） and isotopic composition of dissolved silicon, and the discharge is inversely related to the δ³⁰Si composition. These trend mainly was controlled by the extent of chemical weathering of crustal rocks along the Yangtze River mainstream and the amount of water and δ³⁰Si value of DSi from tributaries.Dissolved and particulate silicon isotope samples collected in the Yangtze River estuary in 2005 were analyzed. The dissolved silicate uptaken by phytoplankton growth was mainly contributed by Changjiang River diluted water. Due to the silicon isotopic fractionation during opal biomineralizaion, the dissolved δ³⁰Si value in surface water were found higher than that in bottom water. However, there was no correlation between silicon isotope composition and DSi, for the processes controlling the dissolved Si isotope composition within this area were complex. The δ³⁰Si value of particulate matters showed a positive linear relationship with the AOU in surface water. It might show a potential correlation between the fractionation of silicon isotope in biogenic matter and the nutrient utilization. The silicon biological fractionation factor ε=1.95±0.73 ‰ was calculated on the assumption of open-system model in this study area.The variations of the dissolved silicon isotopic composition of oxygen-deficient zone in the Yangtze River estuary and its adjacent East China Sea in 2006 were also examined. The dissolved silicate δ³⁰Si value in surface water vary from +0.94 ‰ to +3.21 ‰.The distribution δ³⁰Si in surface water of was consistent with DSi and fucoxanthin, respectively, which showed that high level of phytoplankton growth might lead the increasing of dissolved δ³⁰Si value, and the study area was undergone the consecutive transportation of dissolved silicate from different water masses. The high value of δ³⁰Si at bottom water in the oxygen-deficient zone was due to the high decomposition and dissolution of biogenic particles. In the low primary production area, the characters of δ³⁰Si value of different water mass were more distinguishable. Based on this study, the δ³⁰Si values of sea water were between +0.6 to +1.5 ‰, which are comparable with the value of open ocean water, and the fresh water source showed higher δ³⁰Si values relative to the ocean.更多还原