Arctic hydrate dissociation as a consequence of climate change: determining the vulnerable methane reservoir and gas escape mechanisms

2008年，在斯皮茨卑尔根西部边缘，墨西哥湾流系统的北部延伸将温暖的大西洋水输送到北冰洋，发现了数百股甲烷气泡，这些气泡从海床上升起，深度接近甲烷水合物稳定带向陆地的界限。甲烷水合物是一种看起来像冰的固体，水在其中形成一个笼状结构，包裹着甲烷分子。甲烷水合物在低温和高压条件下是稳定的，例如在永久冻土区或海洋下300-600米以下的水中，取决于水温。在过去的30年里，海床上的海洋温度上升了1摄氏度，导致水合物稳定的区域沿着大陆斜坡收缩，明显的后果是水合物分解并释放出甲烷，甲烷已经迁移到海床和海洋中。目前，除了极少量的甲烷外，甲烷的释放速度通常太慢，无法克服海洋中的溶解和氧化作用，无法到达大气。然而，已知发生在其他地方的灾难性气体泄漏可能会在短时间内释放大量甲烷。这种泄气的强度取决于海床下当地储存了多少天然气，以及将天然气输送到海床的途径的种类。这项拟议的研究试图定义这些途径，并量化气体的数量。一支海洋研究考察队将使用一种深拖曳、非常高分辨率的地震系统来拍摄将天然气输送到海床的小型结构的图像，并检测海床下沉积物中是否存在天然气。这项工作将与一个电磁勘探系统一起进行，该系统在海底使用一个深拖曳发射器和接收器，以求出海床下沉积物中的电阻率变化。高于正常水平的电阻率是由天然气和水合物共同引起的，而天然气的存在降低了地震速度，而水合物增加了地震速度。结合起来，这两种技术可以区分不同数量的水合物和天然气。与安装在船体上的类似系统相比，深海拖曳地震系统SYSIF使用了压电线性调频信号源，提供了非常高分辨率的图像和更深的海底渗透，并将辅之以海底地震仪，以精确测量地震速度随深度的变化，并使用小型气枪(迷你GI炮)提供更深层次的高分辨率地震成像。多波束声纳将用于提高海床形状的清晰度，高频找鱼声纳将拍摄气泡羽流的图像并确定其位置，在许多情况下可与2008年首次发现气泡时获得的图像进行比较。将调查两个区域，甲烷水合物稳定区的陆地界限区域，在那里许多气泡羽状物出现在400米以下的浅水中，作为比较，在1200米深的维斯特内萨海脊有一个麻坑，天然气从那里逃逸，下面是通过水合物稳定区将天然气输送到海床的“烟囱”，天然气通常会在那里形成水合物。地质和地球物理数据，包括在2008年一次研究巡航期间在这两个地区获得的96道地震反射剖面，将补充新数据。该项目将为海底观测站(MASOX监测北极海底--海洋交易所)提供海底下的情况，该观测站将由一个欧洲科学联盟于2010年夏天在浅层羽状区建立，以监测羽流的活动以及通过海底的物理和化学通量。

Along the western margin of Spitsbergen, where the northern extension of Gulf Stream system conveys warm Atlantic water into the Arctic Ocean, hundreds of plumes of bubbles of methane gas were discovered in 2008, rising from the seabed at a depth close to that of the landward limit of the methane hydrate stability zone. Methane hydrate is a solid with the appearance of ice, in which water forms a cage-like structure enclosing molecules of methane. Methane hydrate is stable under conditions of low temperature and high pressure such as those found in regions of permafrost or under the ocean in water deeper than 300-600 metres, depending on the water temperature. Over the past thirty years, the ocean's temperature at the seabed has increased by 1 degree C, causing the zone in which hydrate is stable to contract down the continental slope, with the apparent consequence that hydrate has broken down and released methane, which has migrated to the seabed and into the ocean. At present, the rate of release of methane is generally too slow to overcome dissolution and oxidation in the ocean to reach the atmosphere, except in very small quantities. However, catastrophic gas venting, which is known to occur elsewhere, could release large amounts of methane over a short period of time. The strength of such venting depends upon the how much gas is stored locally beneath the seabed and the kinds of pathways that bring gas to the seabed. The proposed research seeks to define these pathways and to quantify the amount of gas. A marine research expedition will use a deep-towed, very high-resolution seismic system to image the small-scale structures that convey gas to the seabed and to detect the presence of gas in the sediments beneath the seabed. This will be done in conjunction with an electromagnetic exploration system that uses a deep-towed transmitter and receivers on the seabed to derive the variations in electrical resistivity in the sediments beneath the seabed. Higher-than-normal resistivity is caused by both gas and hydrate, whereas the presence of gas reduces seismic velocity and hydrate increases it. In combination, the two techniques can distinguish the separate amounts of hydrate and gas. The deep-towed seismic system, SYSIF, which uses a piezo-electric chirp source that gives very-high-resolution images and deeper sub-seabed penetration than similar systems mounted on a ship's hull, will be supplemented by the use of ocean-bottom seismometers to provide precise measurements of the variation of seismic velocity with depth, and seismic profiles with small airgun (mini-GI gun) to provide deeper high-resolution seismic imaging. Multibeam sonar will be used to improve definition of the shape of the seabed and high-frequency, fish-finding sonar will image plumes of gas bubbles and define their positions, providing, in many cases, comparisons with the images obtained in 2008 when they were first discovered. Two areas will be investigated, the region of the landward limit of the methane hydrate stability zone, where many bubble plumes occur in water shallower than 400 metres, and, for comparison, a pockmark in the Vestnesa Ridge, at a depth 1200 metres, from which gas is escaping and is underlain by 'chimneys' that convey gas to the seabed through the hydrate stability zone, where the gas would normally form hydrate. Geological and geophysical data, including 96-channel seismic reflection profiles, acquired in both areas during a research cruise in 2008, will complement the new data. The project will provide the sub-seabed context for a seabed observatory (MASOX Monitoring Arctic Seafloor - Ocean Exchange), which will be established in the shallow plume area in summer 2010 by a European scientific consortium to monitor the activity of the plumes and the physical and chemical fluxes through the seabed.