台灣西南海域天然氣水合物潛藏區之孔隙水中鹵素濃度變化

Abstract

前人研究顯示孔隙水中的溶解碘 (iodide) 是一種親生物性元素，它的成因與有機物分解有很密切的關係。本研究針對台灣西南海域天然氣水合物潛藏區採取代表性沈積物岩芯樣品，藉由觀察岩芯樣品孔隙水中溶解鹵素（氯、溴、碘）和銨根濃度來探討流體之來源與硫酸鹽還原反應之途徑。 本研究中在天然氣水合物潛藏區，根據碘、硫酸鹽、銨根及甲烷氣濃度隨深度之變化，可以將觀察結果區分成四大類。第一類：碘、硫酸鹽、銨根、及甲烷氣濃度隨深度沒有明顯變化，此為背景值站。第二類：碘、銨根濃度隨深度增加有明顯增加趨勢，而硫酸鹽濃度隨深度增加有明顯減少趨勢，其中再依甲烷濃度是否有隨深度變化而增加分成兩小類；2A：甲烷濃度隨深度增加有明顯增加、2B：甲烷濃度隨深度增加沒有增加，此類代表有大量深部來源加入之站位。第三類：碘、銨根、及甲烷氣濃度隨深度增加而增加，而硫酸鹽濃度隨深度增加有明顯減少趨勢，但是銨根濃度增加斜率大於第二類。第四類：甲烷濃度隨深度有明顯增加，而硫酸鹽濃度隨深度增加有明顯減少趨勢，但是碘和銨根濃度隨深度增加沒有增加趨勢。 利用溶解無機碳（DIC）和硫酸鹽消耗之比值我們可以推得以下結論：第二類樣品之硫酸鹽消耗的途徑，以甲烷厭氧氧化反應（AMO）為主、混合一些有機物氧化反應；第三類樣品之硫酸鹽消耗的途徑以一些有機物氧化反應為主；而第四類樣品之硫酸鹽消耗的途徑以AMO為主。

Recent studies showed that iodide is a biophilic species. When the organic matters decomposed, methane and I- will be generated and released. Variations of halogen concentrations (Cl-, Br- and I-) and ammonium (NH4+) in pore water of marine sediments of selected sites were studied to help understanding the fluid source and the pathway of sulfate reduction in the potential gas hydrate area of offshore SW Taiwan. According to the concentration profiles (I-, Br-, CH4, SO4-2, NH4+), we can classify those studied sites into four groups. In group-1, which represents background group, concentrations of CH4, I-, Br- and NH4+ show no clear variations with depth. In group-2, rapid reduction of sulfate and significant increase in NH4+ and/or CH4 are observed. In sub-group 2A, the concentrations of CH4, I-, Br- and NH4+ all increase with increasing depth; in 2B, except methane concentration all other species (I-, Br- , NH4+) increase with increasing depth. In group-3, CH4, I-, Br- and NH4+ all increase with increasing depth. Unlike group-2, NH4+ concentration increases faster than those in group-2. In group-4, the CH4 concentration increases at depth, while there are only very low I- , Br- and NH4+ concentrations. The category except group-1 indicated the mixing of in situ and deep source fluids. Based on the stoichiometry ratio between sulfate consumption and DIC increasing from the profiles of the studied coring sites, we can summary the pathway of sulfate reduction for group-2 to -4 as followings: For group 2, which could represent the majority sites in offshore SW Taiwan, sulfate is mainly consumed by AMO process with methane from the depth. For group-3, sulfate is consumed by decomposition of local organic matters with very few fluid input from depth. For group-4, almost all sulfates are consumed by AMO process.