Distribution Of Silicon Isotope Composition Of Particulate And Dissolved Matter In The Yangtze River Estuary And Its Adjacent Sea By Single-Collector Inductively Coupled Plasma Mass Spectrometry

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.