台灣西南部地區增積岩楔之甲烷貢獻量評估

Estimation of Methane Contribution from the Accretionary Prism in SW Taiwan

洪瑋立(Wei-Li Hong)

指導教授 ： 楊燦堯

國立臺灣大學/理學院/地質科學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.01618

摘要

甲烷為一近年來逐漸受到重視的溫室效應氣體；尤其是地質來源的甲烷，因為可能是導致過去許多異常氣候事件的主因，所以更是受到關注。在所有的地質來源中，和增積岩楔此一構造背景有關之來源又占了大多數。因為有大量的沉積物累積於增積岩楔中，再加上其擠壓應力為主的構造環境，故全世界泥火山、噴氣口、以及天然氣水合物之分佈和增積岩楔關係密切。因此，了解增積岩楔中甲烷循環的機制，對於評估地質來源的甲烷如何影響全球氣候以及碳儲庫之大小有非常大的幫助。台灣位處於兩個板塊的交界，弧陸碰撞的結果將部分的增積岩楔抬升到陸地上。因為這樣的地質條件，台灣西南部的陸上和海上遍佈著泥火山以及噴氣口，甚至還可能有為數可觀的天然氣水合物存在於西南外海之主動及被動大陸邊緣。這樣的條件提供了我們一個絕佳的機會了解甲烷如何在這樣一個隱沒、增積、以及碰撞的環境下循環。

本研究以三種不同的方式分別測量以及估算甲烷氣在陸上泥火山、海域環境以及由增積岩楔累積的厚層沉積物中所產生的甲烷量；藉由比較這些量的大小，可以幫助我們了解整個增積隱沒系統中甲烷的宿命為何。本研究測量結果顯示，每年有約130噸的甲烷會由台灣西南部的陸上泥火山逸散出來；而西南海域的海底泥火山地區則每年約有196-317噸的甲烷由沉積物深部遷移到淺部，但是大部分的甲烷都會在沉積物淺部被生物活動所消耗掉而不會逸散到海水或是大氣中。至於我們以油氣系統的方法所估算之甲烷由增積岩楔之沉積物中的產生量則為14.4-226 Gt；如果將此產生量除以現今陸域和海域之甲烷逸散量則可發現，增積岩楔中產生的甲烷可供應泥火山以現今的通量逸散至少32個百萬年，這個數字是遠大於現今台灣增積岩楔之年紀的。可能的解釋是，台灣地區泥火山過去可能貢獻了比現今還要多的甲烷；這樣的結論可以由台灣西南部陸上和海域所觀察到廣佈的13C缺乏的碳酸鈣來加以證實。另外一個解釋不平衡的甲烷量的方式為，有絕大多數的甲烷都隨著板塊一起隱沒了；但是這個可能性在目前的研究中還沒有辦法驗證，需要更進一步的研究來探討。

另外，本研究結果對於地質來源甲烷如何影響氣候及碳儲庫系統可以有進一步評估與討論。由天然氣水合物解離所造成的海洋環境之碳儲庫變化的確是解釋浮游有孔蟲碳同位素變化的最可能原因，但是這樣的解釋有些侷限存在，海洋中其他生物或是熱分解來源甲烷之逸散之通道應該同時被考慮，尤其是淺層沉積物中二氧化碳還原反應所產生之甲烷，這部分的甲烷有非常大的潛力會對海洋碳儲庫產生鉅大的影響。另外，陸上泥火山貢獻了許多的甲烷到大氣中，這部分的甲烷在過去是被嚴重低估的，而他們對於氣候系統的影響又是最直接的；從我們的測量結果結合世界上其他國家之結果所觀察出來的關係，未來可以對於全世界陸上泥火山之甲烷貢獻有一更為客觀之估算。

關鍵字

甲烷 ； 泥火山 ； 天然氣水合物 ； 增積岩楔 ； 甲烷生成反應

並列摘要

Methane from geological sources draws a lot of attention recently due to its possibility being responsible for some abrupt climate events in the past. Among all kinds of geological sources (volcanic eruption, thermal degradation of organic matter, microbial degradation of organic matter), sources related to accretionary prisms are particular important. Such importance could be observed from the world-wide distribution of mud volcanoes, seepages, and gas hydrate. These manifests are closely related to the distribution of accretionary prisms due to the huge amount of sediments accumulated and compressional stress in the accretionary prisms. As a result, in order to know better about how geological sources of methane affect world climate and carbon budget, it is essential to understand methane cycling in the accretionary prisms. Taiwan is located at the boundary of two plates; arc-continental collision results in the uplifting of Taiwan accretionary prism. Such geological background of Taiwan serves as an excellent example for the study of methane cycling in the subduction-accretion- collision system.

In this study, three different kinds of methods were applied to estimate methane emission and generation in Taiwan accretionary prism including methane emission from on-land and off-shore mud volcanoes in SW Taiwan and methane generation from sediments in accretionary prism. By comparing these numbers, the fate of methane in accretionary prism could be better understand. Our results show that, there is about 130 tons of methane emitted from on-land mud volcanoes to atmosphere; another 196-317 tons of methane is emitted through off-shore mud volcanoes and seeps to shallow sediments; the amount of methane generated in accretionary prism is ~14.4-226 Gt. The amount of methane generated from these sediments could afford gas emission from mud volcanoes for 32 Ma under current flux. This is an unrealistic number which is significant larger than the age of Taiwan accretionary prism. This disequilibrium could be explained by the higher methane flux of these mud volcanoes in the past than today. Such explanation could be supported by the widely distributed 13C deplete carbonate in SW Taiwan. The other possible explanation is that some methane generated from the sediments subducted along with the slab during subduction. However, the validity of this explanation could not be justified so far.

This study could possibly provide some evaluation and discussion to the current debate on the relationship between geological methane emission and abrupt climate change in the past. Gas hydrate might be a suitable candidate to explain the variation of marine carbon budget observed from benthic foraminifera records. However, gas hydrate should not be the only explanation. Methane from marine seeps or mud volcanoes should also be taken into consideration. Especially, methane generated from microbial activities (methanogenesis) should have great impact to the marine carbon budget. On the other hand, methane emitted from on-land mud volcanoes, which is previously underestimated, might provide great influence to the climate. The relationship observed from the data including mud volcanoes in Taiwan and European is useful in objectively estimating world-wide methane budget from on-land mud volcanoes. The importance of methane cycling in accretionary prism to the world carbon budget and climate is revealed by our study; more works are required to better understand this system.

並列關鍵字

methane ； mud volcano ； gas hydrate ； accretionary prism ； methanogenesis

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