Estimation of Methane Contribution rom the Accretionary Prism in SW Taiwan
Date Issued
2009
Date
2009
Author(s)
Hong, Wei-Li
Abstract
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. n 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.his 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.
Subjects
methane
mud volcano
gas hydrate
accretionary prism
methanogenesis
Type
thesis
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