Stability of the Ocean Clathrate Reservoir to Anthropogenic Climate Perturbation

大量的甲烷被一种被称为甲烷包合物（或甲烷水合物）的冰冷固体封存在大陆边缘深处的海洋沉积物中。古新世的地质数据记录了当时甲烷的释放，其规模与当前传统化石燃料的存量相当（古新世始新世最大热事件，简称PETM）。甲烷释放的时间恰逢深海变暖，这与长期全球变暖的预测没有根本的不同。因此，未来人为气候变化导致笼形物最终释放甲烷的前景值得认真考虑。笼形碳库所含的碳比大气、生物圈、土壤和化石燃料的总和还要多，但这个库的稳定性似乎不稳定。包合物的物理稳定性依赖于深海的低温和高压。笼状物在水中是浮力的，就像普通的水冰一样，并被上覆沉积物的重量固定在深海中。在现代海洋中，包合物平衡的溶解甲烷在化学上对氧和硫酸盐是不稳定的。在这个提议中，我们展示了一个初步的模型结果，即在地质时间尺度上，稳态包合物储层的大小可能对深海的温度和氧合极为敏感。该模型结合了两个已有的组成部分：(1)研究沉积物中有机质早期成岩作用的mudds模型，以及(2)研究甲烷包合物形成的Davies和Buffett物理化学模型。该模型再现了目前海洋中包合物的估计，但预测随着海洋温度升高6摄氏度，即新千年新世之前的情况，几乎没有包合物。这个模型能够在古新世的海洋中储存足够的水合物，如果我们假设北冰洋是一个凉爽缺氧的海洋的话，就可以解释PETM。然而，这个分析没有告诉我们从一个稳态状态到另一个稳态状态的过渡，这是我们建议解决的一个缺点。通过将孔隙压力和亚稳包合物行为纳入包合物模型，我们将特别关注甲烷相变条件下沉积物柱的结构稳定性。知识价值：总之，我们建议将海洋中包合物储存库的长期和短期稳定性描述为地球气候和碳循环的一个组成部分。(1)稳态包合物储层对深海温度和氧合的敏感性是什么？(2)包合物储层对强迫变化的响应速度有多快？(3)在什么条件下，我们可以预期甲烷的灾难性释放，如在第三纪新世？(4)温度与笼形物储层之间是否存在正反馈关系？更广泛的影响：全球变暖无疑将在未来破坏水合物的稳定，但任何可能的甲烷释放的程度和后果尚不清楚。

0403862ArcherLarge volumes of methane are sequestered in marine sediments along deep continental margins by an icy solid known as methane clathrate (or methane hydrate). Geologic data from the Paleocene document a methane release at that time which is comparable to the size of the current inventory of traditional fossil fuels (the Paleocene Eocene thermal maximum event, or PETM). The timing of the methane release coincides with deep sea warming which is not radically different from the long-term global warming forecast. Thus the prospect of an eventual release of methane from clathrate in response to anthropogenic climate change in the future warrants serious consideration. The clathrate carbon reservoir contains more carbon than the atmosphere, the biosphere, soils, and fossil fuels combined, but the stability of this reservoir appears precarious. The physical stability of the clathrates relies on the cold temperatures and high pressures of the deep sea. Clathrate is buoyant in water, like regular water ice, and is held in the deep sea by the weight of overlying sediment. The dissolved methane that the clathrate equilibrates with is chemically unstable against oxygen and sulfate in the modern ocean. In this proposal we show a preliminary model result that on geologic time scales the size of the steady-state clathrate reservoir may be extremely sensitive to the temperature and oxygenation of the deep ocean. This model combines two previously existing components: (1) the Muds model for early diagenesis of organic matter in sediments, and (2) the Davies and Buffett model of the physics and chemistry of methane clathrate formation. The model reproduces the present-day estimates of clathrate in the ocean, but predicts almost no clathrates as the ocean temperature is raised by 6C, the condition before the PETM. The model is able to store enough clathrate in the Paleocene ocean to account for the PETM if we assume a cool and anoxic Arctic ocean. This analysis however tells us nothing about the transition from one steady state condition to another, a shortcoming we propose to address. We will devote particular attention to the structural stability of the sediment column under conditions of methane phase change, by incorporating pore pressure and metastable clathrate behavior into the clathrate model. Intellectual Merit: In summary, we propose to characterize the long- and short-term stability of the clathrate reservoir in the ocean as a component of the climate and carbon cycle of the earth. (1) What is the sensitivity of the steady-state clathrate reservoir to temperature and oxygenation of the deep ocean?(2) How quickly does the clathrate reservoir respond to changes in forcing?(3) Under what conditions might we expect catastrophic release of methane, as in the PETM?(4) Is there a positive feedback between temperature and the clathrate reservoir? Broader Implications: Global warming will undoubtedly destabilize clathrate in the future, but the extent and consequences of any possible methane release are not known.