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

沿着斯匹次卑尔根群岛的西缘，墨西哥湾流系统的北方延伸将温暖的大西洋水输送到北冰洋，2008年发现了数百个甲烷气泡柱，从海底升起，深度接近甲烷水合物稳定区向陆地的界限。甲烷水合物是一种具有冰外观的固体，其中水形成笼状结构包围甲烷分子。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米深的水中发现的，这取决于水温。在过去的三十年里，海底的海洋温度上升了1摄氏度，导致水合物稳定的区域沿着大陆坡向下收缩，其明显后果是水合物分解并释放出甲烷，甲烷迁移到海底并进入海洋。目前，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而到达大气层，除非数量非常少。然而，已知在其他地方发生的灾难性气体排放可能会在短时间内释放大量甲烷。这种排放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带到海床的途径的种类。拟议的研究旨在定义这些途径并量化气体的数量。一次海洋研究考察将使用深拖式高分辨率地震系统，对向海底输送气体的小型结构进行成像，并探测海底沉积物中是否存在气体。这项工作将与电磁勘探系统一起进行，该系统使用海底深拖发射器和接收器，以得出海底沉积物电阻率的变化。高电阻率是由天然气和水合物引起的，而天然气的存在会降低地震速度，水合物则会增加地震速度。深海拖曳地震系统SYSIF使用压电线性调频脉冲源，比安装在船体上的类似系统提供非常高分辨率的图像和更深的海底穿透力，将辅之以使用海底地震仪，以精确测量地震速度随深度的变化，并使用小型气枪（微型GI枪）进行地震剖面，以提供更深的高分辨率地震图像。将使用多波束声纳来提高对海底形状的清晰度，高频寻鱼声纳将对气泡羽流进行成像并确定其位置，在许多情况下，将其与2008年首次发现时获得的图像进行比较。将调查两个区域，一个是甲烷水合物稳定区向陆地的界限区域，在那里，许多气泡羽流出现在400米以下的浅水中，另一个是作为比较的Vestnesa海脊的一个凹坑，深度为1 200米，天然气正在从那里逸出，下面有“烟囱”，通过水合物稳定区将天然气输送到海底，在那里气体通常会形成水合物。2008年研究航行期间在这两个地区获得的地质和地球物理数据，包括96道地震反射剖面图，将补充新的数据。该项目将为一个海底观测站（MASOX监测北极海底-海洋交换站）提供海底环境，该观测站将于2010年夏季由一个欧洲科学财团在浅羽流区域建立，以监测羽流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.