| 冷泉系统甲烷气体渗漏的声学特征及潜在环境效应 |
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| 引用本文： | 尉建功,吴婷婷,邓希光,于宗泽,王力峰.冷泉系统甲烷气体渗漏的声学特征及潜在环境效应[J].海洋学报(英文版),2020,42(5):133-144. |
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| 作者姓名： | 尉建功 吴婷婷 邓希光 于宗泽 王力峰 |
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| 作者单位： | 天然气水合物工程技术中心, 广州海洋地质调查局, 广州, 510760;自然资源部海底矿产资源重点实验室, 广州海洋地质调查局, 广州, 510760 |
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| 摘 要： | The amount of methane leaked from deep sea cold seeps is enormous and potentially affects the global warming,ocean acidification and global carbon cycle. It is of great significance to study the methane bubble movement and dissolution process in the water column and its output to the atmosphere. Methane bubbles produce strong acoustic impedance in water bodies, and bubble strings released from deep sea cold seeps are called "gas flares"which expressed as flame-like strong backscatter in the water column. We characterized the morphology and movement of methane bubbles released into the water using multibeam water column data at two cold seeps. The result shows that methane at site I reached 920 m water depth without passing through the top of the gas hydrate stability zone(GHSZ, 850 m), while methane bubbles at site II passed through the top of the GHSZ(597 m) and entered the non-GHSZ(above 550 m). By applying two methods on the multibeam data, the bubble rising velocity in the water column at sites I and II were estimated to be 9.6 cm/s and 24 cm/s, respectively. Bubble velocity is positively associated with water depth which is inferred to be resulted from decrease of bubble size during methane ascending in the water. Combined with numerical simulation, we concluded that formation of gas hydrate shells plays an important role in helping methane bubbles entering the upper water bodies, while other factors, including water depth, bubble velocity, initial kinetic energy and bubble size, also influence the bubble residence time in the water and the possibility of methane entering the atmosphere. We estimate that methane gas flux at these two sites is 0.4×10~6–87.6×10~6 mol/a which is extremely small compared to the total amount of methane in the ocean body, however, methane leakage might exert significant impact on the ocean acidification considering the widespread distributed cold seeps. In addition, although methane entering the atmosphere is not observed, further research is still needed to understand its potential impact on increasing methane concentration in the surface seawater and gas-water interface methane exchange rate, which consequently increase the greenhouse effect.
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| 关 键 词： | 气泡 甲烷 冷泉 尼日利亚陆坡 温室效应 马克兰增生楔 |
| 收稿时间： | 2019/4/28 0:00:00 |
Acoustic characteristics of cold-seep methane bubble behavior in the water column and its potential environmental impact |
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| Wei Jiangong,Wu Tingting,Deng Xiguang,Yu Zongze,Wang Lifeng.Acoustic characteristics of cold-seep methane bubble behavior in the water column and its potential environmental impact[J].Acta Oceanologica Sinica,2020,42(5):133-144. |
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| Authors: | Wei Jiangong Wu Tingting Deng Xiguang Yu Zongze Wang Lifeng |
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| Institution: | 1.Gas Hydrate Engineering and Technology Center, Guangzhou Marine Geological Survey, Guangzhou 510075, China2.MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China |
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| Abstract: | The amount of methane leaked from deep sea cold seeps is enormous and potentially affects the global warming, ocean acidification and global carbon cycle. It is of great significance to study the methane bubble movement and dissolution process in the water column and its output to the atmosphere. Methane bubbles produce strong acoustic impedance in water bodies, and bubble strings released from deep sea cold seeps are called “gas flares” which expressed as flame-like strong backscatter in the water column. We characterized the morphology and movement of methane bubbles released into the water using multibeam water column data at two cold seeps. The result shows that methane at site I reached 920 m water depth without passing through the top of the gas hydrate stability zone (GHSZ, 850 m), while methane bubbles at site II passed through the top of the GHSZ (597 m) and entered the non-GHSZ (above 550 m). By applying two methods on the multibeam data, the bubble rising velocity in the water column at sites I and II were estimated to be 9.6 cm/s and 24 cm/s, respectively. Bubble velocity is positively associated with water depth which is inferred to be resulted from decrease of bubble size during methane ascending in the water. Combined with numerical simulation, we concluded that formation of gas hydrate shells plays an important role in helping methane bubbles entering the upper water bodies, while other factors, including water depth, bubble velocity, initial kinetic energy and bubble size, also influence the bubble residence time in the water and the possibility of methane entering the atmosphere. We estimate that methane gas flux at these two sites is 0.4×106–87.6×106 mol/a which is extremely small compared to the total amount of methane in the ocean body, however, methane leakage might exert significant impact on the ocean acidification considering the widespread distributed cold seeps. In addition, although methane entering the atmosphere is not observed, further research is still needed to understand its potential impact on increasing methane concentration in the surface seawater and gas-water interface methane exchange rate, which consequently increase the greenhouse effect. |
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| Keywords: | gas bubble methane cold seep Nigerian Continental Margin Greenhouse effect Makran accretion wedge |
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