Collaborative Research: Towards a mechanistic prediction of methane ebullition fluxes from northern peatlands.

泥炭地只占地球陆地面积的3%左右，但在产生甲烷（一种强温室气体）方面却有着不成比例的重要作用。泥炭地向大气排放的甲烷约占全部甲烷的5-10%，泥炭地也被认为是全球碳循环中的一个重要储存库，约占全球土壤碳的33%。该项目通过促进对调节这种强效温室气体向大气释放的机制的科学理解的进展，服务于国家利益和美国国家科学基金会的使命。本项目解决的基本问题涉及调节气态甲烷突然（间歇性）释放到大气中的机制和水文因素。这些释放是由水位和大气压力的变化驱动的，这些变化促进了甲烷向上的气泡运输/释放。这个项目将对水位、大气压力和泥炭结构的变化是如何导致甲烷的突然释放产生新的认识，这远远超过了之前关于甲烷释放的理论。尽管该项目的重点是北方泥炭地，但它将影响我们对其他气候条件下甲烷动态的理解，包括沼泽地等亚热带系统和北极系统。该项目包括少数民族学生参加实地地球科学研究的暑期研究经历。本项目将有两名全日制研究生和一名博士后科学家参与。这项工作的结果将通过学生在国家/国际会议上的发言和向国际期刊提交的文章来传播。泥炭地对大气CH4负荷的贡献仍不清楚，这在很大程度上是由于对沸腾途径的不完全了解。溶解甲烷的氧化减少了甲烷的扩散释放，但气泡通过沸腾释放的传递时间太短，无法发生广泛的氧化，即沸腾释放增加了泥炭地的温室气体潜力。该项目将通过将新的、创新的测量策略与物理模型开发相结合来推进对沸腾的理解。这种测量和建模的结合将使我们朝着更加定量地了解泥炭地CH4沸腾的方向迈出根本性的一步。本文将检验两个假设：H1：当与泥炭结构（和强度）相关的可测量特性被纳入模型拟合参数时，泥炭地沸腾事件的频率和大小可以通过泥炭柱内CH4产量和压力变化来预测；H2：来自泥炭地的沸腾受到与累积气体体积相关的阈值、与泥炭强度相关的可测量物理特性以及泥炭柱内气体凝聚的方式（例如，分散的气泡与几个大的“气泡”）的调节。测量将在位于缅因州的多单元泥炭地Caribou Bog进行。体积气体含量将使用探地雷达进行监测，而沸腾通量将使用声波沸腾传感器、气体圈闭延时成像、压力计中的液压头和使用快速甲烷分析仪的室内测量相结合进行监测。孔隙水CH4样品将使用微型压力计巢获得。将现有的描述气泡膨胀的沸腾模型与入侵渗流方法相结合，通过孔隙水中的扩散和层间的沸腾来描述CH4在多个泥炭层之间的输运。尽管所提出的模型没有明确地纳入泥炭的地质力学特性，但对最大气体含量的模型预测将与可能控制沸腾的关键可测量的地质力学特性进行比较。

Peatlands cover only about 3% of the Earth's land area, but are disproportionately important in producing methane, a strong greenhouse gas. Peatlands yield an estimated 5-10% of all methane to the atmosphere and are also recognized as an important reservoir in the global carbon cycle, accounting for about 33% of the global soil carbon. This project serves the national interest and NSF's mission by promoting the progress of scientific understanding of the mechanisms that regulate the release of this potent greenhouse gas to the atmosphere. The fundamental issue addressed by this project relates to the mechanisms and hydrological factors that regulate the sudden (episodic) release of gaseous methane to the atmosphere. These releases are driven by changes in water level and atmospheric pressure that encourages upward bubble transport/release of methane. This project will generate new understanding of how changes in water level, atmospheric pressure and peat fabric lead to sudden releases of methane that far exceed previously held theories on methane release. Although the project is focused on a boreal peatland it will impact our understanding of methane dynamics in other climates, including sub-tropical systems such as the Everglades, and Artic systems. The project includes summer research experiences for the participation of minority students in field geoscience research. Two full-time graduate students and one postdoctoral scientist will be involved this project. Results of the work will be disseminated through student led presentations at national/international meetings and articles submitted to international journals.The contribution of peatlands to the atmospheric CH4 burden remains unclear in large part due to incomplete understanding of the ebullition pathway. Oxidation of dissolved methane reduces the release of methane by diffusion, but the transit time of bubbles released via ebullition is too short for extensive oxidation to occur, i.e. ebullition releases increase the greenhouse gas potential of peatlands. This project will advance understanding of ebullition by coupling new, innovative measurement strategies to physical model development. This integration of measurement and modeling will permit a fundamental step forward towards a more quantitative understanding of CH4 ebullition from peatlands. Two hypotheses will be tested: H1: The frequency and size of ebullition events from peatlands can be predicted from CH4 production rates and pressure changes within the peat column when measurable properties related to the peat structure (and strength) are incorporated into model fitting parameters; H2: Ebullition from the peatland is regulated by a threshold related to the accumulated gas volume, measurable physical properties related to the peat strength, and how gas coalesces within the peat column (e.g. dispersed bubbles versus a few large 'bubbles'). Measurements will be performed in Caribou Bog, a multi-unit peatland located in Maine. Volumetric gas content will be monitored using ground penetrating radar, whereas ebullition fluxes will be monitored using a combination of acoustic ebullition sensors, time-lapse imaging of gas traps, hydraulic heads in piezometers and chamber measurements using a fast methane analyzer. Pore water CH4 samples will be acquired using mini piezometer nests. An existing ebullition model describing gas bubble expansion will be combined with an invasion percolation approach to describe the transport of CH4 between multiple peat layers by both diffusion in the pore water and ebullition between layers. Although the proposed model does not explicitly incorporate the geomechanical properties of peat, model predictions for maximum gas contents will be compared with key measurable geomechanical properties that may control ebullition.