Stability of the Ocean Clathrate Reservoir to Anthropogenic Climate Perturbation

大量甲烷被一种被称为甲烷笼合物(或甲烷水合物)的冰冻固体隔离在大陆深部边缘的海洋沉积物中。来自古新世的地质数据记录了当时甲烷的释放，这与目前传统化石燃料库存的规模相当(古新世始新世最热事件，或PETM)。甲烷释放的时间与深海变暖的时间重合，这与长期的全球变暖预测没有根本不同。因此，未来因应人为气候变化而最终从笼状化合物中释放甲烷的前景值得认真考虑。笼状碳库包含的碳比大气、生物圈、土壤和化石燃料的总和还要多，但这个碳库的稳定性似乎不稳定。笼状物的物理稳定性依赖于深海的低温和高压。笼状物在水中漂浮，就像普通的水冰一样，并被覆盖的沉积物的重量固定在深海中。在现代海洋中，笼合物与之相平衡的溶解甲烷对氧气和硫酸盐是化学不稳定的。在这个方案中，我们展示了一个初步的模型结果，即在地质时间尺度上，稳态笼状储集层的大小可能对深海的温度和氧气非常敏感。该模型结合了先前存在的两个组成部分：(1)沉积物中有机质早期成岩作用的MUDS模型，以及(2)甲烷笼状化合物形成的物理和化学的Davies和Buffett模型。该模型再现了目前对海洋中笼状物的估计，但预测几乎不会出现笼状物，因为海洋温度上升了6摄氏度，这是PETM之前的条件。该模型能够在古新世海洋中储存足够的笼状物，如果我们假设一个凉爽和缺氧的北冰洋，就可以解释PETM。然而，这一分析没有告诉我们从一个稳态条件到另一个稳态条件的转变，这是我们建议解决的一个缺点。我们将特别关注甲烷相变条件下沉淀柱的结构稳定性，将孔压和亚稳态包合物行为纳入包合物模型。智力价值：总而言之，我们建议将海洋中笼状水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)稳态笼状储集层对深海温度和氧合作用的敏感度是多少？(2)笼状储集层对压力变化的反应有多快？(3)在什么条件下，我们可能会看到甲烷的灾难性释放，就像在PETM中？(4)温度和笼状储集层之间是否存在正反馈？更广泛的影响：全球变暖无疑将在未来破坏笼状结构的稳定，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.