天然氣水合物 (gas hydrate)，是極具開發價值的潔淨能源，近期研究發現可利用海底沉積物中的次生重晶石 (Diagenetic Barite)，推測地層中甲烷通量的變化情形，做為天然氣水合物蘊藏可能指標。本研究以台灣西南海域之海底泥火山沉積區為研究目標，分析位處於同一個海底泥火山沉積區有異常高甲烷通量之G23-1、G23-2、GT1站位岩心，探討其次生重晶石分佈情形及地化意義。海底地層甲烷氣體向上逸散導致甲烷厭氧氧化作用(anaerobic oxidation of methane, AOM)，加速地層孔隙水硫酸根的消耗。在硫酸根耗盡與甲烷氣體交會帶(Sulfate- Hydrocarbon transition, SHT)之下，重晶石會被分解成硫酸根與鋇離子。當鋇離子向上擴散到SHT之上，則再結晶沉積形成次生重晶石富集層，稱為「barite front」，可利用barite front來判斷地層甲烷氣體通量及紀錄通量的變化情形。
本研究採用前人所提出的BASEX重晶石萃取法，並經過改良及測試，確定改良後的「modi-BASEX」萃取法可有效的萃取出沉積物中的重晶石，並配合HR-ICP-MS 對萃取試液進行鋇離子濃度與其它元素濃度的分析。分析得到的沈積物鋇離子濃度變化曲線，與孔隙水中硫酸根濃度及鋇離子濃度比較，發現兩站位沉積物中，在SHT深度上方有barite front的存在，且深度都很淺，代表所研究的泥火山區甲烷通量非常大。相對於GT1而言，G23-1現今的barite front深度較淺，在僅50公分處，代表甲烷通量較大，可推測G23-1站位較靠近海底泥火山口。
本研究分析的岩心中都可發現多個barite front的存在，且每個barite front的濃度都不高，這個現象與國外在天然氣水合物埋藏區常發現單一、高濃度的barite front不同，可能代表海底泥火山區甲烷通量變化頻繁，地質作用活躍的特性。最後，由於 barite front可紀錄地層中甲烷通量及氧化速率變化，因此用數值模型計算其累積形成時間，推測一個地區氧化深度的擺動情形。所計算出的barite front形成時間約在數百~數千年尺度，累積時間短，同樣證實此地區甲烷通量變化大的現象。

Gas hydrate is an ice-like crystalline mineral in marine environment and has a potential as the future energy. Recent studies indicated that it’s possible to estimate the gas hydrate storage in continental margin using the diagenetic barite. On the other hand, previous studies observed methane gas diffused upward, subsequently labile Ba re-precipitated as barite front involved with Anaerobic Oxidation Methane (AOM) and induced to barite enriched at the depth of Sulfate Hydrocarbon Transition (SHT). Therefore, the barite front can be used to assess changes in upward methane flux over time. Consequently, the depth variation of barite front could be related to the evolution of the upward methane flux in sediments.
Three sediment cores, G23-1、G23-2 and GT1, were sampled from submarine mud volcano area, offshore southwestern Taiwan. In order to obtain the actual sedimentary barite, barite sequential extraction procedure (BASEX) was adopted from Rutten et al. (2002), and then analyzed accurately by HR-ICP-MS. Based on preliminary results, we can effectively extract the barites from ambient sediments, and the peaks of sedimentary barium can be found at depth around SHT. This is consistent with published studies, and further confirms the observation of Ba profile in the surrounding pore waters.
We found the modern barite fronts are lie on very shallow depth, especially on core G23-1 (~50 cm), indicating a strong methane flux occurred in these sites. Furthermore, spiky but relatively low content of barite fronts can be observed in the three sites. This may demonstrate the geological activity and methane flux change frequently in submarine mud volcano area around Taiwan. Finally, we calculated the barite concentration of barite front by integral, and estimate the formation time of barite front, to try to reestablish the oxidation depth changes. The calculated results are about few hundreds to thousands years, it also indicate the methane flux change frequently.

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