台灣西南海域沉積物之自生重晶石富集與甲烷移棲之關係

Abstract

準確量測天然氣水合物上層甲烷遷移之時間與尺度，是了解天然氣水合物遷移所產生相關變化之重要依據。傳統測量甲烷通量的方法，例如甲烷濃度隨深度之變化、硫酸鹽還原速率等，只能得知現今甲烷的通量，無法探究過去甲烷通量大小與改變。一般古海洋常用之方法如碳-14和浮游生物殼體氧同位素等量測方法，則因為老碳與硫污染影響，不適用於天然氣水合物甲烷通量之研究。自生重晶石在硫酸鹽-甲烷反應帶的富集，則可作為現今與過去甲烷通量變化的良好指標，並藉以推估甲烷通量穩定持續的時間長短，更且藉由自生重晶石多次富集的出現與否，探討甲烷通量的時序變化，以充分了解甲烷移棲的歷史，進而了解甲烷通量變化與天然氣水合物賦存及氣候變遷的關係。 本研究於台灣西南海域不同甲烷通量區域採集沉積物岩心樣品，分析其間隙水與沉積物中鋇離子含量，探討自生重晶石的垂直分佈與富集。藉由鋇離子擴散通量模式計算，估算自生重晶石富集時間，以探討研究區域內甲烷通量的時序變化。本研究亦進一步探討間隙水中鋇離子與甲烷和硫酸鹽之間的相互關係。研究結果顯示，台灣西南海域沉積物中發現多種不同類型的自生重晶石富集，包括單一峰值、多個峰值以及沒有峰值等數種。同一岩心中，多個自生重晶石的富集，顯示了本研究區域甲烷通量的大小在過去與現今不同時期有明顯的改變，且自生重晶石富集時間約為一至八千年，顯示甲烷通量曾改變且呈現多次的變化，且部份測站在硫酸鹽-甲烷反應帶並未發現自生重晶石富集峰，故冰期/間冰期海平面上升下降可能並非唯一會造成天然氣水合物穩定帶與甲烷通量改變之控制因素。在活動大陸邊緣地層活動頻繁的海域，活躍的板塊運動產生之活動可能也是造成甲烷通量改變的重要因素。

Accurate determination the duration and time of methane migration above the gas hydrate layer is important in better understanding mechanism(s) driving methane migration and its association with climatic change. Difficulty arises from interference by old carbon in methane and sulfur in proper analyzing carbon and oxygen isotopic value rendering the traditional methods such as carbon-14 or planktonic oxygen records untrustworthy. Diffusion model through determination peak concentration of barium sulfate precipitation has been successfully employed to calculate time and length of methane migration in this study. This thesis study barium sulfate peak(s) accumulated in the continental margin sediments in order to understand mechanism(s) controlling methane migration in sediments offshore southwestern Taiwan. Pore water and sediment barium concentrations were analyzed. Barium fluxes were calculated in order to estimate duration of barite accumulation in sediment. Downward sulfate and upward methane flux as well as barium in sediment were also evaluated to better understand authigenic barite precipitation. Results showed that different types of barite peaks existed in our study area, a single barite peak, multiple peaks and no visible barium sulfate peak. The appearances of more than one barite peaks demonstrated that multiple stages of methane vents may have occurred at present and/or in the past at different geological time at sites in our study region. Diffusion model calculation demonstrated that duration of barite accumulation may require about one to eight thousand years. The appearances of both multiple barite peaks as well as no barite peak associated with the present SMTZ in the study area indicated that methane gas venting may not controlled by a single general process such as sea level changes but by the local active tectonic activities.