台灣西南海域增積岩體成岩作用與深部流體遷移

Abstract

流體遷移和早期成岩作用控制著增生楔地球化學的訊號與循環。來自陸地、海水及深部流體的化合物之間的交互作用促進許多微生物反應，並使大量的有機物降解；不易分解的有機物則進而被掩埋於沉積物深處。有機物分解的最終產物—甲烷，其透過有機物的熱分解和微生物分解作用而形成，經由遷移而排放到海水甚至大氣中，對生物地球化學循環和溫室效應產生深遠影響。隱沒的含水礦物隨著溫度和壓力的增加從而被釋放，在被擠壓的構造環境中，隨著流體向上遷移，將深部流體帶入淺層沉積物中，亦使得淺層沉積物帶有深部流體的訊號。然而，對於隱沒帶甲烷、流體通量以及微生物作用尚未有系統性探討。因此，為了探索流體過程和微生物活動，這項研究結合了流體地球化學分析和數值模擬，旨在系統性了解有機碳相關的微生物活動以及增生楔中甲烷和水的循環。 本研究總共分成三個部分。本研究的第一部分在台灣西南海域的燦堯泥火山群（Tsanyao Mud Volcano Group）中的一座泥火山—TY1採了一系列的岩心，用以探討歐亞大陸與菲律賓海板塊之間的匯聚所產生的流體的特徵。流體地球化學表明，蒙脫石在平衡溫度為100至150 oC時因脫水而產生了礦物解離水。此向上遷移的流體速度影響厭氧甲烷氧化的速率和效率。利用TY1的水排放量外推至西南海域其他12座泥火山，約有1.1–28.6％的蒙脫石結合水在隱沒後，從西南海域的海底泥火山排出，這表明海底泥火山可以作為將深層/高溫下產生的流體引導到隱沒系統中的海底環境的管道。 在第二部分的研究進行台灣西南沿海近海大陸邊緣的甲烷源與匯系統性量化與探討。結合前人研究的發表數據，結果指出高甲烷通量和甲烷碳同位素值與地質構造相關，也就是說甲烷的遷移主要受到構造的控制。大部分的站位皆顯示厭氧氧化作用消耗了大量上升遷移的甲烷，顯示其為有效甲烷生物濾網。甲烷源與匯的通量整合計算顯示多餘的深層微生物和熱成熟產生的甲烷可能被儲存於天然氣水合物中抑或是以吸附於粘土上的形式存在。 在第三部分中，著重於深部流體與淺層沉積物的相互作用，以探討有機碳相關的微生物活動與環境之間的相互作用。 燦堯泥火山群中的一座海底泥火山（TY1）被選為此研究的模型系統，由於海底泥火山流體為海水與深部流體的混合，因此該系統可以反應出海水和深層流體混合中的微生物活動。無機地球化學和模擬結果顯示深層流體對孔隙水有明顯影響，而有機地球化學卻指出溶解有機碳和短鏈化合物主要被微生物活動控制。 綜合三個部分的研究成果，在增生楔中，構造控制的流體強烈影響了地下環境中的微生物活動。對甲烷和水的源與匯進行系統調查可以更深入了解影響循環系統下通量大小的主要機制。

The fluid migration and early diagenesis impose an important control on the geochemical cycling in accretionary prisms. The interaction between compounds supplied from land, seawater, and deeply-rooted fluids facilitate many microbial processes and degrade considerable organic matter, leaving refractory organic matter buried into great depths. The final product, methane, can be formed through thermogenic and microbial processes in subsurface sedimentary environments and then discharge into seawater and even the atmosphere, exerting profound impacts on biogeochemical cycles and greenhouse effects. Mineral-bound water would be also released as temperature and pressure increase with depths, bearing deeply-rooted signals as fluids migrating upward. However, the methane and fluids budgets along with microbial processes remain rarely explored, qualified and quantified in subduction systems. This study combined fluid gechemical analysis and numerical modeling to explore fluid processes and microbial activities, aiming at better systematically understanding of organic-carbon related microbial activities as well as cycling of methane and water in accretionary prisms. In the first part, sediment cores distributed across a submarine mud volcano (SMV), TY1 (one of the Tsangyao Mud Volcano Group), were investigated to determine the characteristics of fluids generated through the convergence between the Eurasian and Phillippine Sea Plates. The fluid geochemistry indicated that fresh water derived from smectite dehydration at an equilibrium temperature of 100 to 150 oC. The upward fluid velocities affected the rate and efficiency of anaerobic methanotrophy. About 1.1–28.6% of the smectite-bound water originally stored in the incoming sediments was exploded from SMVs, suggesting that SMVs could act as a conduit to channel the fluids produced from great depth/temperature into seafloor environments in a subduction system. In the second part, the production, consumption, and migration of methane were systematically quantified along a continental margin in offshore southwestern Taiwan. Combined with published data, the results showed that high methane fluxes and the methane carbon isotopic values tend to be associated with structural features, suggesting a strong structural control on the methane transport. Anaerobic oxidation of methane played an effective biological filtration as a significant portion of ascending methane was consumed. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption. In the third part, TY1 was chosen to denote a model system that could witness how microbial activities react under mixing of seawater and deeply-sourced fluids in subsurface environment. Inorganic geochemical and simulation results indicated the porewater profiles were obviously affected by deep fluids while the organic geochemistry showed that the in situ microbial activities controlled the distribution of dissolved organic carbon and short-chain compounds. In conclusion, tectonic-controlled fluids migration in accretionary prisms strongly influenced microbial activities in subsurface environment. Systematically investigation into methane and water could better understand the predominant mechanism in their cycling.