Carbon storage in an orogenic system – An example from Tai-Yuan Basin, Taiwan

• Main Authors: Nai-Wun Shih, 石乃文
• Other Authors: Jiun-Yee Yen
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/2hv9qv

碩士 === 國立東華大學 === 自然資源與環境學系 === 107 === The concentration of carbon dioxide affects the atmospheric temperature of Earth, and scientists have been attempting to link the changes of the global temperature to the carbon cycle related to plate tectonics. During orogenesis, an enhanced rate of erosion and denudation could potentially increase the burial efficiency of organic carbon of terrestrial origin in the ocean, and, over a carbon cycle of extended time scale, decrease the concentration of carbon dioxide in the atmosphere. This study will attempt to investigate the burial efficiency of organic carbon in an active orogenic system. The island of Taiwan is located at the edge of an active continental plate; it is one of the most active sites of orogenesis in the globe, as well as a zone with frequent typhoon activity. Since 1 Ma ago, the exhumation rate of the Central Range has been 3 – 10 mm yr-1, while the rate of erosion 3 – 6 mm yr-1, and even as much as 60mm yr-1 in the subduction zone. The rivers in Taiwan are characterized with high slope, short length and strong stream power, leading to the quick delivery of terrestrial sediments from the source to the adjacent ocean. While the landmass of Taiwan is only 0.02% in relation to that of the entire globe, it’s annual export of suspended sediments is roughly 1.9% of the world’s. In addition, the yearly yield of particulate organic carbon (POC) is about 25 to 85 times more than that of other major rivers such as the Amazon and the Congo; therefore, it is worthwhile to examine the relationship between the orogenic belt and the organic carbon burial in Taiwan. The Tai-Yuan Basin, located in the Southern Coastal Range, is a part of the collision basin during the collision period of the ancient Taiwanese island and the Luzon Arc. Its strata are dated at 4.24 – 1.15 Ma and records the exhumation history of the south-eastern ancient Taiwanese island during the entire impact; the amount of carbon found in the strata is therefore indicative of whatever that is eventually preserved as part of the lithosphere. The samples of this study are sand-shale collected from the outcrops of Madagida-chi, and the ratios of sand, shale and carbon are recorded. The total organic carbon (TOC), total inorganic carbon (TIC), total carbon (TC), total nitrogen (TN) and stable isotopes 13C (δ13C) are determined with Elemental Analyzer and Elemental Analyzer-Isotope Ratio Mass Spectrometer (EA – IRMS). The results of the analysis indicate that in the same sandstone, the smaller the the grain size is the higher the TOC is, and that the sources of the organic carbon in the basin’s sedimentary rocks are a mixture of land and marine. Along with the analysis and field record, this study estimates that in the time span of 2.3 My, 3.82 × 1015g of organic carbon has been preserved in the sedimentary rocks; furthermore, changes in sedimentation rate and carbon burial yield of different time periods inferred from previous data matches those in exhumation rate of the ancient Southern Central Range determined by other studies. It is also estimated that the percentage of preservation of organic carbon from land into the basin sedimentary rocks is less than 10%, and that only during a cataclysmic event would it reach 40%. Thus, even though the island of Taiwan is similar to an area with high organic carbon burial, perhaps the ability of which organic carbon will end up preserved in the lithosphere is not as high as expected.