| Geochemical methods are the most effective ways to indicate the occurrence of submarine gas hydrate and get the information about the composition and structure, occurrence state and environment of submarine gas hydrates. Marine sediment pore water of gas hydrate occurrence area is of great importance for it carries large quantities of geology and biogeochemistry information. It is difficult for traditional analysis methods to obtain high-fidelity pore water geochemical parameter data from retrieved samples. Laser Raman spectroscopy is capable of in situ molecular identification of solids, liquids, and gases and is well suited to extreme environments. The development of deep ocean Raman in situ spectrometer provide us new technical reference for the study of marine sediment pore water in gas hydrate occurrence regions. We conducted a feasibility study of quantitative analysis of sediment pore water parameters using laser Raman spectroscopy.It is reasonable to regard water as the internal standard for quantitative analysis of sediment pore water. Raman spectroscopy of liquid water can be divided into OH stretching vibration bands and OH bending vibration bands. The former is asymmetric and multi-mode superimposed, and is susceptible to salt and temperature. Skewing parameter was introduced to describe Raman spectroscopy distortion of water molecule stretching variation. The results show that the skewing parameter and distortion of OH stretching vibration bands increase with increasing salinity adding to the solutions. Effects of some common cations and anions in seawater on Raman spectra of water were analyzed, and effects of ions are in the following sequence: SO42- > CO32-, I- > Br- > Cl- > HCO3- > F-, Sr2+ > Ca2+ > Mg2+, K+ >Na+. On the contrary, the OH bending vibration bands of liquid water is symmetric and stable. Therefore, OH bending band of water(1300-2000 cm-1) is treated as the internal standard in this study for its stability.We study the quantitative analysis of the two important pore water geochemical composition SO42- and CH4 with Raman spectroscopy. The results show that the Raman spectroscopy quantitative techniques based on internal standard method can be used for quantitative analysis of SO42- and CH4 in aqueous solution with good accuracy.We propose here a sample alkalization aided Raman spectroscopy quantitative analysis method of DIC in sediment pore water. HCO3- is chosen as the study object for it takes the largest share and has the weakest Raman activity in all DIC species. The solution is alkalized and HCO3- is converted into CO32- which is much moor Raman active. The Raman spectroscopy analysis detecting limit of HCO3- is decreased, and the result of quantitative analysis shows good accuracy(relative error<6.5%).Sulfide species in water have a relatively strong Raman signal, which often appears in the form of a characteristic overlapping peak between 2550~2620 cm-1 and can be decomposed into HS- at 2572cm-1and H2 S at 2592cm-1. In the present paper, quantitative analysis of H2 S and HS- with Raman spectroscopy is proved practicable and the accuracy is good. The p H of pore water is an important influencing factor of the diagenetic processes. As H2 S and HS- are conjugate acid-base pairs, sulfide species within pore water exist as a function of p H and their concentration ratio depend on p H. This relationship is also shown in the Raman spectrum. To formulate the pore water p H calculation, sulfide solutions with p H range from 6.11 to 13.05 were prepared and their Raman spectra were observed. It is verified that the morphology of overlapping peaks change regularly with p H values. This phenomenon provides us the possibility of measuring the p H of pore water in situ via Raman spectroscopy. Based on peaks decomposition and correlativity analysis, we propose here a novel in situ p H measuring method for sediment pore water containing sulfide. This method can be used to measure the p H of pore water when the overlapping peak of sulfide is resolvable. The application scope of this p H measuring method in this study is 6.11~8.32, which covers almost all p H value of marine sediment pore water already known. |
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