Testing the 'Clathrate Gun Hypothesis' with Atmospheric Methane from the Greenland Ice Sheet Project Two (GISP II) Ice Core

这项研究将记录格陵兰冰盖计划二(GISPII)冰芯中大气甲烷的同位素组成，这一时间间隔与过去40,000年来的“笼形炮假说”所涉及的时间间隔有关，这些时间间隔跨越了冰川消融序列、博林暖期和年轻的仙女木冷期。“笼形枪假说”表明，大气中甲烷突然增加的主要因素是海洋沉积物中海洋笼状物的不稳定，而不是陆地湿地甲烷排放量的增加。来自地质记录的证据表明，轻微的海洋温度上升或海洋下沉事件(由大陆架的不稳定引起)可能导致过去笼状沉积物的灾难性不稳定，并随之而来的是向大气中释放甲烷。例如，在过去的4万年里，加利福尼亚州海岸外的圣巴巴拉盆地发生的笼状结构不稳定事件的记录，与冰芯记录的大气甲烷突然增加的记录非常相似。这两个记录之间的强烈相似性促使科学家们从它们对气候的影响来考虑甲烷突然增加的原因。由于甲烷是一种强有力的温室气体，笼状结构的不稳定事件可能是过去全球气温突然上升的原因之一。这是一个重要的想法，因为它确定了一种以前不为人知的机制，导致大气在地质时期内突然变暖。在许多大陆边缘和北极永久冻土深处，产甲烷细菌产生大量甲烷作为代谢副产品。释放出的甲烷向表面扩散，直到达到临界温度和压力，气体被浓缩成一种冰冷的物质，称为笼状物质或天然气水合物。笼状沉积物对环境压力和温度的微小变化非常敏感，因为较高的温度和较低的压力往往会熔化笼状沉积物，导致甲烷的释放。幸运的是，海洋笼状甲烷与来自湿地的陆地甲烷在同位素上是不同的，因此应该可以确定大气中甲烷快速浓缩的原因是否源于海洋笼状甲烷的脱气。

This research will develop a record of the isotopic composition of atmospheric methane from the Greenland Ice Sheet Project Two (GISP II) ice core for time intervals implicated in the "clathrate gun hypothesis" over the past 40,000 years spanning the deglaciation sequence, the Bolling warm period, and the Younger Dryas cool phase.The "clathrate gun hypothesis" suggests that the primary factor governing abrupt methane increases in the atmosphere is the destabilization of marine clathrate in ocean sediments as opposed to increased methane emissions from terrestrial wetlands. Evidence from the geologic records suggest that slight ocean temperature increases or oceanic slumping events (caused by instabilities in the continental shelf) may have caused catastrophic destabilization of clathrate deposits in the past and a consequent release of methane into the atmosphere. For example, throughout the last 40,000 years, a record of clathrate destabilization events in the Santa Barbara basin off the California coast closely resembles the record of abrupt increases in atmospheric methane recorded in ice cores. The strong similarity between these two records has prompted scientists to consider the origin of the abrupt methane increases in terms of their impact on climate. Because methane is a potent greenhouse gas, clathrate destabilization events may have contributed to abrupt global temperature increases in the past. This is an important idea because it identifies a previously unknown mechanism for abrupt warming of the atmosphere over geologic time.Along many continental margins and deep below the Arctic permafrost, methanogenic bacteria produce large quantities of methane as a metabolic byproduct. The liberated methane diffuses up towards the surface until it reaches a critical temperature and pressure where the gas is concentrated into an icy substance know as a clathrate or gas hydrate. Clathrate deposits are very susceptible to small changes in environmental pressure and temperature in that higher temperatures and lower pressures tend to melt the clathrate deposits leading to the release of methane. Fortuitously, marine clathrate methane is isotopically distinct from terrestrially derived methane from wetlands, so it should be possible to determine whether or not the cause of rapid methane concentration in the atmosphere was due to degassing from marine clathrates.