Surface CO2 emission and rising bubble plumes from degassing of crater lakes in Sao Miguel Island, Azores
Surface CO2 emission and rising bubble plumes from degassing of crater lakes in Sao Miguel Island, Azores
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DOI：
10.1144/sp437.14
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发表时间：
2017-01-01
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影响因子：
--
通讯作者：
Franca, Zilda
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作者：
Melian, Gladys;Somoza, Luis;Franca, Zilda
我们首次详细研究了葡萄牙亚速尔群岛圣米格尔岛三个主要火山湖(Sete Cidade、Fogo和Furns)的水下活动气孔和地表弥漫性CO2脱气的类型和分布。2011年5月使用浮动蓄积室和双波束50和200千赫回声测深仪进行的调查结果显示，三个湖泊的地表二氧化碳脱气很低，范围为32-08 kg d(-1)。然而，在Furns陨石坑湖北部发现了密集的水下脱气羽流(每100 m(2)羽流7.5-9)，在Fogo(每100 m(2))和Sete Cidade陨石坑湖泊(每100 m(2))发现了中等密度的脱气羽流(1-1.5羽流(2))。回声探测仪探测到了被解释为声学耀斑的水声信号，吹起的气泡羽流或与许多水下喷气孔相关的气泡壁。记录的声像图显示，气泡在底部以19~30 cm S 21的平均速度上升，释放频率从1~2次到31次S，大多数水下气孔在到达湖面之前由于二氧化碳的溶解而消失。在三个火山湖中观察到的pH值范围(约7-9)加强了这些溶解过程。观测到的溶解二氧化碳数值表明，这三个湖泊中这种气体的压力仍然远远低于流体静压，因此发生湖湖喷发的风险可以忽略不计。我们认为，来自水下气泡的不断上升的二氧化碳水平可能构成了地下浮游植物层的关键燃料，这些层被解释为具有高后向散射值的水平声层。水下起泡的最高密度与圣米格尔岛三个晚第四纪层状火山杂岩的破火山口边缘形成的最近淹没的次生火山口有关。我们的结果强调，有必要定期进行地表除气研究，作为亚速尔群岛重要的火山监测工具。
We report the first detailed study on the types and distributions of active subaqueous fumaroles and surface diffuse CO2 degassing in the three main volcanic lakes of Sao Miguel Island (Sete Cidades, Fogo and Furnas), Azores archipelago, Portugal. The results of the surveys, carried out in May 2011 using a floating accumulation chamber and a dual beam 50 and 200 kHz echo sounder, revealed a very low surface CO2 degassing at the three lakes, in the range of 32-08 kg d(-1). However, dense subaqueous degassing plumes were found in the north of the Furnas crater lake (7.5-9 plumes per 100 m(2)), and moderate-density degassing in the Fogo (1.5-2 plumes per 100 m(2)) and Sete Cidades crater lakes (1-1.5 plumes per 100 m(2)). The echo sounder detected hydroacoustic signatures interpreted as acoustic flares, 'puffing' bubble plumes or walls of bubbles associated with numerous subaqueous fumaroles. The recorded echograms show that the bubbles rise at average speeds of between 19 and 30 cm s 21 at the bottom, with frequencies of release from 1-2 to 31 s. Most subaqueous fumaroles disappear due to the dissolution of CO2 before reaching the lake surface. These dissolution processes are enhanced by the pH range observed in the three volcanic lakes (c. 7-9). Observed dissolved CO2 values indicate that the pressure of this gas in the three lakes remained much lower than the hydrostatic pressure and the risk of a limnic eruption is therefore negligible. We suggest that the rising levels of CO2 from the subaqueous bubbles could constitute a critical fuel for subsurface phytoplankton layers, interpreted as horizontal acoustic layers with high backscattering values. The highest density of subaqueous bubbling correlates with recent submerged secondary craters formed around the caldera rims of the three Late Quaternary stratovolcano complexes of Sao Miguel Island. Our results emphasize the need to perform regular surface degassing studies as an important volcanic surveillance tool in the Azores archipelago.