台灣西南海域永安海脊沉積物中厭氧甲烷氧化對於硫同位素及黃鐵礦物生成的影響

Abstract

海洋環境中藉由微生物所進行的硫酸鹽還原作用是沉積物中氧化有機碳的主要機制。地層深部的甲烷水合物汽化所提供的甲烷提供了硫酸鹽還原菌及厭氧甲烷氧化菌額外的碳及能量來源，加速硫酸鹽的還原作用。活動大陸邊緣（active margin）地殼運動可能是造成地層深部的甲烷往淺層移棲的主要機制。本研究擬探討台灣西南海域活動性大陸邊緣永安海脊中甲烷通量的改變對於硫酸鹽還原作用及厭氧甲烷氧化作用變化的影響，並評估活動大陸邊緣地殼運動對厭氧甲烷反應與硫同位素變化的重要性。為探討甲烷通量在空間上與時序上的變化，本研究於ｒ/v Marion Dufresne 178航次及海研一號OR1-961期間，分別利用Calypso巨型活塞岩心、CASQ巨型箱型岩心與12m活塞岩心採樣。 本研究測站均含高甲烷通量，導致快速的硫酸鹽還原作用及厭氧甲烷氧化作用，控制碳與硫的生地化循環。高甲烷通量造成小的硫同位素分化係數、高濃度的溶解硫化氫以及在SMTZ有高黃鐵硫含量。硫同位素及黃鐵硫含量呈現空間與區域性的變化，硫同位素分化係數介於α = 1.009 to 1.035。研究結果顯示甲烷通量是控制AOM及硫同位素分化的主要因素。黃鐵硫以及C/S比值在垂直剖面上的變化，紀錄了過去甲烷通量的改變。透過黃鐵硫在沉積物富集峰的數量以及垂直上分布的特性，顯示永安海脊區域中甲烷通量的變化可能是受到研究區域地殼構造活動頻率所影響導致氣體遷移與其後之數次硫酸鹽還原作用及厭氧甲烷氧化作用。

Microbial sulfate reduction is a major pathway for organic matter oxidation in marine sediment. Upward diffusion of methane from gas hydrate in deep sediment could provide extra source of carbon and energy for sulfate reducing bacteria and anaerobic methanotroph with subsequently induced higher sulfate reduction rate in sediment. Tectonic activity and glaciation/de-glaciation have been hypothesized to explain dissociation of gas hydrate and subsequent gas migration, with glaciation/de-glaciation a more common mechanism. This study investigated the effect of methane migration on the sulfate reduction and anaerobic methane oxidation process in Yung-An Ridge offshore southwestern Taiwan, in particular, degree of methane migration as a result of tectonic activities in the active margin offshore southwestern Taiwan. Samples were taken on r/v Marion Dufresne,cruise 178 using Calypso giant piston core, CASQ box core and OR1-961 cruise with a 12 m piston core in order to evaluate spatial and vertical variation of the study biogeochemcical process. High methane flux induced rapid sulfate reduction and anaerobic methane oxidation in the study sites. The anaerobic methane oxidation play a major role in altering the carbon and sulfur cycle in the region. Smaller sulfur isotopic fractionation factor, high concentrations of dissolved sulfide and high pyrite content near SMTZ were results of rapid sulfate reduction induced by higher methane flux. Pyrite concentrations and sulfur isotopic values showed strong spatial and vertical variations. Sulfur isotopic fractionation factor range from α = 1.009 to 1.035. Vertical variations of pyrite content and C/S ratio were found at the study sites. The results showed that methane venting strongly control AOM process and also altering sulfur isotopic fractionation. Appearances of multiple pyrite peaks representing past and present SMTZ in the study area indicated that methane gas venting may by relate to tectonic activities of the region.