台灣西南海域上部斜坡海底泥火山氣體來源及甲烷通量估算

Abstract

台灣西南海域上部斜坡區域已確定有 13 座泥火山存在活躍地噴氣中，這些 泥火山分別被命名為 MV1 ~ MV13。透過海洋沉積物之孔隙氣體及海水溶解氣體 分析，得知本區域海底泥火山噴發氣體主要為甲烷，與少量其他碳氫化合物與二 氧化碳等，這些碳氫化合物可能逸出海表形成重要的天然溫室氣體來源。本研究 利用海研三號及海研五號研究船於 2011-2012 年航次，有系統地採集本地區海底 泥火山逸氣正上方及周圍區域之海水剖面（water column）及海底沉積物岩心樣 品，以探討其甲烷逸氣來源並估算其通量。 海底沉積物岩心樣品所含甲烷氣體之碳同位素分析結果約為 -30 ~ -50 ‰， 指示該區域之甲烷氣體主要為熱分解來源，而後可能有不同比例生物性甲烷來源 之混染。由海水樣品中溶解氣體分析結果顯示，其甲烷濃度高於正常海水背景值 2~20 倍，暗示甲烷氣由泥火山逸散至海水中。氣泡大小分佈、泥火山噴發的初 始條件造成的上升及側向海流等因素影響了甲烷在海水中的移棲型態，許多證據 都指向本研究區域深水泥火山噴發之甲烷可以被傳送到淺層海水之中。 透過氣體擴散方程式（Gas Diffusion Equation）計算，本區域海洋對大氣甲 烷通量為 0.15 ~ 127 μmol m^-2 d^-1 ；透過 Fick’s Law 計算沉積物對海洋底水的甲烷擴散通量為 1.14 ~ 157 μmol m^-2 d^-1 ，此結果顯示本區域海底泥火山對海洋及大氣扮演提供甲烷的角色。進一步考慮噴氣影響的面積，初步估算西南海域上部斜坡海底泥火山區貢獻到大氣中的甲烷量為每年 4.1 噸。

Thirteen submarine active mud volcanoes, which named MV1~ MV13, have been recognized in the area between Kaoping Canyon and Fangliao Ridge, offshore southwest Taiwan. It has been considered that these mud volcanoes are closely related to the intrusion of mud diapirs. The predominant composition of those gas seeps is methane, which may escape to the atmosphere and become an important natural source of greenhouse gas. To estimate the methane flux emission via those mud olcanoes in this area, we have conducted three-cruise surveys during the period of 2011-2012. In this study, we traced the location of gas plume for each mud volcano by echo sonar (EK60) survey first. And then, we can collect the water column samples right above the venting mud volcanoes, and also the sediment samples by gravity corer. The carbon isotopic data of methane gas from cored sediments range from -30 to -50 ‰. It indicates that the methane gas is mostly thermogenic in origin, and may mix with different proportions of biogenic gas source. Meanwhile, the dissolved methane concentrations of sea water above the seepages are 2-20 times higher than those in the background area. Many factors may affect the migration of dissolved ethane, including the distribution of methane gas bubble sizes and upwelling / lateral current resulted by initial condition of venting. All the evidences point out the issolved methane which supplied by deep water submarine mud volcanoes can be transferred to the shallow depth in study area. Based on the diffusive exchange equation and Fick’s First Law, the methane flux of sediment-to-bottom water and ocean-to-air can be estimated ca. 1.14 ~ 157 and 0.15 ~ 127 μmol m^-2 d^-1, respectively. The result shows that the sea water would play an important role in methane source to atmosphere, at least, in the studied region. Furthermore, we can have an approximate estimation of the total methane flux of ca. 4,100 kg yr^-1 in this region.