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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水柱中冷深甲烷气泡行为的声学特征及其对环境的潜在影响

深海冷渗漏泄漏的甲烷数量巨大，并可能影响全球变暖，海洋酸化和全球碳循环。研究甲烷在水柱中的运动和溶解过程及其向大气中的输出具有重要意义。甲烷气泡在水体中产生很强的声阻抗，而从深海冷渗漏中释放出的气泡串称为“火炬”，在水柱中表现为类似火焰的强反向散射。我们使用在两个冷渗流处的多束水柱数据来表征释放到水中的甲烷气泡的形态和运动。结果表明，位置I处的甲烷未经过天然气水合物稳定区（GHSZ，850 m）的顶部，就达到了920 m的水深，站点II的甲烷气泡穿过GHSZ的顶部（597 m）并进入非GHSZ（550 m以上）。通过对多波束数据应用两种方法，估计站点I和站点II处水柱中的气泡上升速度分别为9.6 cm / s和24 cm / s。气泡速度与水深呈正相关，据推测是由于甲烷在水中上升期间气泡尺寸减小而引起的。结合数值模拟，我们得出结论，天然气水合物壳的形成在帮助甲烷气泡进入上层水体中起着重要作用，而其他因素（包括水深，气泡速度，初始动能和气泡大小）也会影响气泡的停留时间在水中停留的时间以及甲烷进入大气的可能性。⁶ -87.6×10 ⁶ mol / a与海洋中甲烷的总量相比非常小，但是考虑到广泛分布的冷渗漏，甲烷泄漏可能会对海洋酸化产生重大影响。此外，尽管没有观察到甲烷进入大气，但仍需进一步研究以了解其对增加地表海水中甲烷浓度和气水界面甲烷交换速率的潜在影响，从而增加温室效应。