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標題: | 臺灣西南海域四方圈合海脊底棲性有孔蟲殼體碳同位素與甲烷逸漏之關聯 Carbon isotopes of benthic foraminifera associated with methane seeps in the Four-Way Closure Ridge, offshore southwestern Taiwan |
作者: | Wei-Ru Wang 王緯茹 |
指導教授: | 魏國彥 |
關鍵字: | 天然氣水合物,甲烷,底棲性有孔蟲,碳同位素, gas hydrate,methane,benthic foraminifera,carbon isotopes, |
出版年 : | 2016 |
學位: | 碩士 |
摘要: | 當貯藏於海底地層中的天然氣水合物解離,可以釋放出大量甲烷進入海洋與大氣,進而影響全球氣候。生存於海底沉積物的底棲性有孔蟲,其殼體穩定碳同位素,可以記錄海底地層的甲烷逸氣。以往的研究顯示,在現代全球多個天然氣水合物甲烷逸漏環境中,底棲性有孔蟲穩定碳同位素受到周遭天然氣水合物甲烷逸漏產生偏輕變化。然而,不同的區域由於有不同的生地化沉積環境,底棲性有孔蟲穩定碳同位素的偏輕變化各異。
從地球物理震測發現臺灣西南海域海底地層中有大量的仿擬反射層 ( BSR ),代表著海底可能蘊藏有大量的天然氣水合物。海底表層沉積物的地球化學資料,也證實許多地區有高通量的甲烷逸漏,而被推測其下為天然氣水合物賦存區。若能究明本區域的底棲性有孔蟲穩定碳同位素偏輕變化與甲烷逸漏之關聯,將可以此關聯性解讀臺灣西南海域岩芯中底棲性有孔蟲殼體穩定碳同位素變化,重建該區表層沉積物中之過去甲烷逸漏歷史。本研究主要探討,臺灣西南海域天然氣水合物赋存區底棲性有孔蟲是否會受到天然氣水合物解離所致甲烷逸漏,而導致殼體穩定碳同位素數值呈現偏輕的變化。 本研究分析了採集於臺灣西南海域四方圈合海脊 (水深1330-1580公尺) 5 根岩芯 (OR1-1092-WFWC-1, OR1-1092-WFWC-4, OR1-1092-WFWC-6, OR3-1806-C5-2和OR3-1806-C10) 上部15公分沉積物中的底棲性有孔蟲Uvigerina proboscidea ( 150-250 μm ) 殼體穩定碳同位素,發現U. proboscidea殼體同位素數值在甲烷逸漏站位OR3-1806-C5-2介於-0.98‰到-6.21‰ ( VPDB ) 之間,在背景站位的碳同位素數值介於-0.40‰到-0.86‰之間,受到甲烷逸漏影響的站位比背景站位碳同位素數值偏輕約0.12‰到5.81‰。暗示產甲烷作用所產生的偏輕碳同位素紀錄於底棲性有孔蟲殼體 。 Release of large amounts of methane from marine gas hydrate reservoirs has been considered as a possible trigger of climate change. The degassing of methane could be recorded by stable carbon isotopes (d13C) of benthic foraminifera in the sediments. Many previous studies have shown that foraminiferal d13C become more negative when influenced by methane seeps. However, values of d13C of benthic foraminifera might vary with different species and sedimentary settings in different regions. Seismic profiles in offshore southwestern Taiwan have shown a wide distribution of the Bottom Simulating Reflector (BSR), indicative of gas hydrate reservoirs. Various methane seepages have been found, and they are suspected to be related to the gas hydrates buried underneath. A better understanding of the d13C signals of benthic foraminifera near the methane seepages can further clarify the origin of the methane and to evaluate it as a proxy of methane release for the geologic past. We analyzed d13C of benthic foraminifera Uvigerina proboscidea (150-250 μm) in the topmost 15 cm sediments in five marine cores (OR1-1092-WFWC-1, OR1-1092-WFWC-4, OR1-1092-WFWC-6, OR3-1806-C5-2 and OR3-1806-C10) collected from the Four-Way Closure Ridge in offshore southwestern Taiwan (water depth from 1330 to 1580 m). Our results show that d13C values of U. proboscidea range from -0.98‰ to -6.21‰ (VPDB) for core OR3-1806-C5-2, which is considered as a seeps-influenced site. On the other hand, d13C values of U. proboscidea from the background sites range from -0.40‰ to -0.86‰. The difference between the methane seep-affected and the background sites is in the range of 0.12‰ to 5.81‰, comparable to those documented in previous studies in other areas. The significant negative excursion in carbon isotopes in the seep site foraminifera. Suggests an incorporation of light biogenic carbon generated by methanogenesis in the system. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/51372 |
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