臺灣東部雷公火泥火山中厭氧型甲烷氧化作用與含鐵礦物之關係

Abstract

本研究旨在瞭解陸域泥火山系統中含鐵礦物的變化與 AOM 作用的關係。研究區域為台灣東部雷公火泥火山，前人研究已初步證實AOM 作用與鐵還原作用的關聯。本研究於不同的噴發泥池邊採取6根岩芯，長度自30 cm ~ 60 cm不等，萃取沈積物中3種不同型式的鐵含量，分別為可反應總鐵、碳酸鐵與易還原氧化鐵，並進行沈積物中總碳、有機碳與無機碳的含量分析。對比沈積物孔隙水中甲烷、鐵離子和溶解無機碳濃度，發現各岩芯分別在不同深度都有AOM的存在，並且鐵還原菌消耗的氧化鐵主要為易還原的氧化鐵。在岩芯中各深度的碳酸鐵含量大部分都較噴泥中的含量高，顯示在沈積物累積由淺層深埋至底層的過程中，曾經發生或正在進行鐵還原作用，總還原態鐵含量的證據也支持了岩芯中碳酸鐵的含量是經由長時間累積的結果，不過 AOM 與鐵還原作用合作或是鐵還原菌代謝有機碳的產物，均可促進碳酸鐵的沈澱。影響有機碳與無機碳含量的生地化作用很多，與鐵還原作用合作的AOM作用可能增加有機碳含量，但代謝有機碳鐵還原作用則會消耗有機碳，岩芯中有機碳與無機碳含量變化與AOM或其他生地化反應的關係仍有待進一步釐清。

Anaerobic oxidation of methane (AOM) is widely performed by the cooperation between anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria in marine sediments. Due to the lack of sulfate, AOM may carry out by ANMEs and iron reducers (IRs) in terrestrial mud volcanoes. Both mechanisms could enhance carbonate precipitates and also regulate iron cycling. The objective of this study is to understand the relationship between AOM and the iron-bearing minerals in the Lei-Gong-Huo mud volcano in eastern Taiwan. Six cores were retrieved beside different eruptive pools. The content of three different types of iron (total reactive iron, easily reducible iron and carbonate iron), total organic carbon (TOC) and total inorganic carbon (TIC) were analyzed. The iron-methane transition zone (IMTZ) can be reconginzed in all cores, inferring the cooperation between ANMEs and IRs. Iron carbonate contents and/or fractions in the IMTZ are generally higher than that of adjacent bubbling fluids. Apparently, iron-dependent AOM is channeled into siderite (FeCO3) precipitates. Siderite was widely accumulated in whole cores, suggesting iron reduction does not only perform in certain depth recently. However, both iron-dependent AOM and organotrophic iron reduction could enhance siderite precipitatation. Many biogeochemical processes could change the TOC and TIC contents. It seems that iron-dependent AOM could increase the TOC concent, but organotrophic iron reduction may consume TOC. The correlation between TOC/TIC content and AOM or other biogeochemical processes need to be further evaluated.