Stability of the Ocean Clathrate Reservoir to Anthropogenic Climate Perturbation

0403862 Archer大量的甲烷被一种称为甲烷笼形物（或甲烷水合物）的冰状固体隔离在沿着深大陆边缘的海洋沉积物中。古新世的地质数据记录了当时的甲烷释放量，与目前传统化石燃料的存量相当（古新世始新世热极大事件，或PETM）。甲烷释放的时间与深海变暖相吻合，这与长期全球变暖预测没有根本不同。 因此，今后为应对人为气候变化而从笼形物中最终释放甲烷的前景值得认真考虑。 笼形碳库包含的碳比大气、生物圈、土壤和化石燃料的总和还多，但这种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。 包合物在水中有浮力，就像普通的水冰一样，并通过覆盖沉积物的重量保持在深海中。 在现代海洋中，与笼形物平衡的溶解甲烷在化学上对氧气和硫酸盐不稳定。 在这个提议中，我们展示了一个初步的模型结果，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非常敏感。 该模型结合了两个先前存在的组成部分：（1）沉积物中有机质早期成岩作用的Muds模型，以及（2）甲烷包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形物的估计，但预测随着海洋温度升高6摄氏度（PETM之前的条件），几乎没有笼形物。 该模型能够存储足够的包合物在古新世海洋占PETM，如果我们假设一个凉爽和缺氧的北冰洋。 然而，这种分析没有告诉我们从一个稳态条件到另一个稳态条件的过渡，这是我们提出要解决的一个缺点。 我们将特别注意甲烷相变条件下的沉积柱的结构稳定性，通过将孔隙压力和亚稳包合物行为的包合物模型。 智力优势：总之，我们建议的长期和短期的稳定性的笼形物水库在海洋中的气候和地球的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如何？(2)笼形水库对强迫变化的反应有多快？(3)在什么条件下我们可能预期灾难性的甲烷释放，如在PETM？(4)在温度和包合物储层之间是否存在正反馈？ 更广泛的影响：毫无疑问，全球变暖将在未来破坏包合物的稳定性，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.