水合物生成过程中碳同位素组成变化的实验研究
Experimental Study on Carbon Isotopic Composition Changes During the Formation of Gas Hydrates

摘要： 天然气水合物是一种新型的洁净能源。甲烷天然气水合物是储量最丰富的一种类型，常出现在深海中或极地大陆上，其生成的过程中会发生同位素的分馏效应。通过实验室模拟水合物生成的过程，利用天然海水与甲烷或二氧化碳气体反应，以及更接近实际生成环境的甲烷-海水-沉积物动态聚散实验，对甲烷水合物和二氧化碳水合物生成前后δ13C值进行测定，研究水合物生成过程中δ13C的变化情况。实验证明，水合物反应中碳同位素分馏是存在的，其变化程度明显小于氧同位素和氢同位素。甲烷水合物碳同位素的分馏系数αC的值为1000 3~1000 9。二氧化碳水合物生成反应后气相的碳、氧同位素变轻，重同位素趋向于进入水合物中，二氧化碳水合物碳同位素的分馏系数αC的值为1000 7~1001 2。海水中溶解的CO2气体在甲烷水合物形成过程中会被水合物捕获，从而使得δ13CDIC值变小，重的碳同位素趋于进入水合物中，而较轻的碳同位素留在海水中。但由于海水中含有的溶解CO2气体有限，经过多轮水合物动态聚散后δ13CDIC值的变化幅度会越来越小。
Abstract: Gas hydrates are a new type of clean energy, and they often appear in the deep sea or in the polar continent. Isotopic fractionation occurs in the gas hydrate formation. In this paper, experiments of methaneseawater hydrates, carbon dioxideseawater hydrates, and methaneseawater hydrates formed repeatedly in sediments were carried out. Changes of carbon isotopic composition during the formation of gas hydrates were researched. The results prove that the carbon isotope fractionation exists during the gas hydrate formation, and the change of carbon isotope composition is significantly smaller than that of oxygen and hydrogen. The fractionation factor of carbon isotope is 1000,3 to 1000,9 in methaneseawater gas hydrate. Carbon and oxygen isotopic composition lightened in gas phase after carbon dioxide hydrate formation, and heavy isotopes tend to enter hydrates. The fractionation factor of carbon isotope is 1000,7 to 1001,2 in carbon dioxideseawater gas hydrates. CO2 gas dissolved in seawater will be captured in hydrate cage during methane hydrate formation and as a result,δ13CDIC value becomes small and heavy carbon isotope tend to enter hydrates, and lighter carbon isotope is left in the seawater. Because the dissolved CO2 gas content is limited in seawater, the variation of δ13CDIC value would be smaller with time in the process of methaneseawater hydrates forming repeatedly in sediments