| Wei Jiangong,Wu Tingting,Deng Xiguang,Yu Zongze,Wang Lifeng. 2020. Acoustic characteristics of cold-seep methane bubble behavior in the water column and its potential environmental impact. Acta Oceanologica Sinica, 42(5):133－144 |
| Acoustic characteristics of cold-seep methane bubble behavior in the water column and its potential environmental impact |
| 冷泉系统甲烷气体渗漏的声学特征及潜在环境效应 |
| Received:April 28, 2019 |
| DOI：10.1007/s13131-019-1489-0 |
| Key words:gas bubble methane cold seep Nigerian Continental Margin Greenhouse effect Makran accretion wedge |
| 中文关键词: 气泡 甲烷 冷泉 尼日利亚陆坡 温室效应 马克兰增生楔 |
| 基金项目: |
| Author Name | Affiliation | E-mail | | Wei Jiangong | Gas Hydrate Engineering and Technology Center, Guangzhou Marine Geological Survey, Guangzhou 510075, China
MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China | weijiangong007@163.com | | Wu Tingting | MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China | wutingtingqd@163.com | | Deng Xiguang | MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China | | | Yu Zongze | MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China | | | Wang Lifeng | Gas Hydrate Engineering and Technology Center, Guangzhou Marine Geological Survey, Guangzhou 510075, China
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. |
| 中文摘要: |
| 冷泉渗漏的甲烷会对全球气候变化、海水酸化及全球碳循环产生影响。因此研究甲烷气泡的运动、在水中的分解过程以及甲烷进入大气的量，是有重要意义的。甲烷气体在水体中产生强烈的声阻抗，气泡从深海冷泉中释放出来被称为气体羽状流。甲烷气泡串在水体中显示出像火焰一样的形状，因此也称为气体火焰。我们用多波束水体数据，对两个冷泉站位（马克兰增生楔和尼日利亚陆坡）甲烷渗漏的形态以及运动特征进行了描述。结果显示，第I个站位渗漏的甲烷到达920 m水深处，没有穿过水合物稳定域（850 m水深）。而第II个站位的甲烷穿过了水合物稳定域（597 m水深），到达550 m水深。应用两种方法对多波束数据进行处理，获得了甲烷气泡在水体中向上的运动速度分别为9.6 cm/s和24 cm/s。气泡运动速度与水深成正相关，我们推断这种情况是由于甲烷气体在向上运动过程中气泡尺寸发生变化而造成的，同时我们结合数值模拟发现，在气泡表层形成的天然气水合物壳，对帮助甲烷气体进入上层水体有重要帮助，同时水深、气泡速度、初始动能以及气泡尺寸都影响甲烷气泡在水体中的逗留时间，并进一步影响甲烷进入大气中的情况。我们推算，从这两个站位中释放的甲烷气体大约有0.4~87.6×106 mol/yr，虽然与海洋中巨大的甲烷含量相比是较小的，但是考虑到全球广泛分布的冷泉，这种甲烷渗漏可能会对全球海洋酸化产生重要影响。另外，尽管没有发现甲烷气体直接进入大气的现象，但是仍然需要进一步研究气-水界面中的甲烷交换，来验证甲烷气体从冷泉中释放可能产生的温室效应。 |
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