Collaborative Research: In Situ Oxidation Rates of Methane Injected from Seafloor Gas Seeps

合作研究：海底气体渗漏注入甲烷的原位氧化速率

• 批准号: 1948720
• 负责人: Karen Lloyd
• 金额: $ 32.12万
• 依托单位: University of Tennessee Knoxville
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-02-01 至 2024-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1948720&HistoricalAwards=false
• 关键词: Collaborative; Research; Situ; Oxidation; Rates

Thousands of natural gas seeps have been discovered as streams of bubbles rising up from the seafloor just offshore from coastlines around the world. Extensive fields of seeps, largely releasing the same methane gas that we use to heat our homes, have recently been found about a hundred miles east of North Carolina’s Cape Hatteras at water depths as shallow as 300 feet. It is estimated that at least tens of thousands of these hydrocarbon-rich seeps occur on continental margins around the world. The seeps, usually seen as large rising plumes by sonar systems on research ships, inject huge, but poorly quantified, amounts of methane into overlying waters as they rise through the water column. A primary question is how much of the methane, a potent greenhouse gas, actually reaches the atmosphere. This question is the subject of much current research funded by several Federal research agencies including the National Science Foundation and the Department of Energy. Oceanographers believe that the methane (and other gases included in the bubble streams such as ethane and propane) are either transported away by ocean currents or consumed by microorganisms specially adapted to live with hydrocarbons as their main carbon source. This proposal seeks to determine the importance of microbial consumption in controlling methane distributions in the deep ocean and how the consumption rates depend on concentrations of methane, oxygen and other chemicals, as well as in situ pressures and temperatures. Previous studies in laboratories aboard research ship using samples returned to the surface suggest that microbial consumption of methane from the seeps may lag for a week after its injection into the water column and thus that physical dispersion by currents may dominate deepwater methane dynamics through dilution. However, other measurements suggest that there is no lag before aggressive microbial oxidation of injected methane begins. We have proposed to conduct in situ measurements of microbial methane consumption and related microbial community structure in bottom waters at several coastal and continental margin sites off the North Carolina coast and the northern Gulf of Mexico, where numerous natural seeps have also been observed. We will use new technologies developed after the release of massive quantities of methane during the Deepwater Horizon disaster in the Gulf. The sites offer varying concentrations of methane and other chemical and physical conditions such as oxygen concentrations and temperature that will allow us to test specific hypotheses about the role of microbial processes. Through performing the experiments with instruments right on the seafloor next to the seeps we can remove much of the uncertainty surrounding previous shipboard measurements. We will use newly developed seafloor landers equipped with advanced laser methane sensors that are capable of multi-week measurements while assaying dissolved oxygen, dissolved inorganic nitrogen and the microbial community for the presence of methane-consuming methanotrophs and their activity using advanced genomics techniques that can reveal the nature of methanotrophic responses to ambient methane concentrations. Successful collection of in situ methane consumption rate data and associated microbial community changes should prove important for modeling ocean methane dynamics over a range of oceanographic conditions including seep-enriched bottom waters and bottom waters impacted by accidental hydrocarbon releases.Graduate and undergraduate students supported by the project will gain critical skills in laboratory and field settings and will also benefit from frequent interactions with established researchers from diverse fields. Team members will participate in hands-on undergraduate education and training through developing individualized research projects leading to honors theses, presentations at national meetings and excellent graduate school placements. Graduate and undergraduates will also participate in K-12 science outreach efforts that help to attract and inform the next generation of oceanographers. The team will work with the University of North Carolina Morehead Planetarium and Science Center and participate directly in the North Carolina Science Festival. Through media contacts made from TED talks, exciting results will be broadly disseminated to the public. The project will have immediate relevance for understanding microbially mediated responses to hydrocarbon inputs from accidental releases along the Southeast Atlantic margin where oil and gas exploration are a constant topic of state and national policy discussions. Hundreds of recently discovered gas seeps along the continental margin offshore of Cape Hatteras, North Carolina plus thousands in the northern Gulf of Mexico, inject huge amounts of dissolved methane into overlying shelf and slope waters through dissolution of rising bubble plumes. The fate of the methane is largely controlled by a balance between microbial oxidation and advective transport away from seep sources. The efficacy of microbial oxidation likely depends on concentrations of methane, oxygen and ambient dissolved inorganic nitrogen (DIN), as well as in situ pressures and temperatures. Recent shipboard aerobic methane oxidation rate (AMOR) measurements suggest that microbial consumption of seep methane may lag for a week and thus physical dispersion could dominate deepwater methane dynamics through dilution. However, methane stable carbon isotopic measurements in bottom waters suggest that there is no lag before aggressive oxidation of injected methane begins. We propose to conduct in situ measurements of AMOR while simultaneously investigating its microbial drivers at representative North Carolina and Gulf of Mexico continental margin sites featuring numerous active bubble seeps. These sites offer varying concentrations of methane and DIN; performing the experiments in situ will remove much of the uncertainty of shipboard rates. We will conduct the measurements utilizing newly developed benthic lander systems equipped with advanced laser methane sensors that are capable of multi-week AMOR measurements while assaying dissolved oxygen, DIN and the microbial community for the presence of methanotrophs (through metagenomes) and their activity (through metatranscriptomes) that can reveal the nature of methanotrophic responses to ambient methane concentrations.The project will test key hypotheses about deep-sea methane dynamics including determining if there are significant lags in microbial responses after exposure to elevated methane concentrations, determining the relationships of AMOR to methane and DIN concentrations and investigating the response times and magnitude of methanotrophs to spatial and temporal variability in methane concentration. Successful collection of in situ AMOR and associated microbial community data will prove important for modeling ocean methane dynamics over a range of oceanographic conditions including seep-enriched bottom waters and accidental hydrocarbon releases.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

成千上万的天然气泄漏被发现，就在世界各地海岸线附近的海底，气泡流从海底升起。最近，在北卡罗来纳州哈特拉斯角以东约100英里处，在水深300英尺的地方发现了大面积的渗水，其中大部分释放的甲烷气体与我们用来取暖的甲烷气体相同。据估计，在世界各地的大陆边缘，至少有数万处富含碳氢化合物的渗漏。这些渗漏，通常被科考船上的声纳系统视为巨大的上升羽状物，当它们通过水柱上升时，向上面的水域注入了大量的甲烷，但数量却很难量化。一个主要的问题是，有多少甲烷（一种强效温室气体）真正到达了大气中。这个问题是目前由包括国家科学基金会和能源部在内的几个联邦研究机构资助的许多研究的主题。海洋学家认为，甲烷（以及气泡流中包含的其他气体，如乙烷和丙烷）要么被洋流带走，要么被特别适应以碳氢化合物为主要碳源的微生物消耗掉。该提案旨在确定微生物消耗在控制深海甲烷分布中的重要性，以及消耗速率如何取决于甲烷、氧气和其他化学物质的浓度，以及原位压力和温度。先前在科考船上的实验室使用返回水面的样本进行的研究表明，微生物对渗漏的甲烷的消耗在注入水柱后可能会滞后一周，因此，水流的物理分散可能会通过稀释来主导深水甲烷动力学。然而，其他测量结果表明，在微生物对注入的甲烷进行积极氧化之前并没有滞后。我们建议在北卡罗来纳州海岸和墨西哥湾北部的几个沿海和大陆边缘地点的底部水域进行微生物甲烷消耗和相关微生物群落结构的原位测量，在那里也观察到许多自然渗漏。我们将使用在墨西哥湾深水地平线（Deepwater Horizon）灾难中释放大量甲烷后开发的新技术。这些地点提供了不同浓度的甲烷和其他化学和物理条件，如氧气浓度和温度，这将使我们能够测试关于微生物过程作用的特定假设。通过在靠近渗漏点的海底用仪器进行实验，我们可以消除以前船上测量的许多不确定性。我们将使用新开发的海底着陆器，配备先进的激光甲烷传感器，能够进行多周的测量，同时分析溶解氧、溶解无机氮和微生物群落中消耗甲烷的甲烷氧化菌的存在及其活动，使用先进的基因组技术，可以揭示甲烷氧化对环境甲烷浓度的响应性质。成功地收集原位甲烷消耗率数据和相关的微生物群落变化对于模拟一系列海洋条件下的海洋甲烷动力学非常重要，这些条件包括深水富集的底部水域和受意外碳氢化合物释放影响的底部水域。该项目支持的研究生和本科生将获得实验室和现场设置的关键技能，并将从与来自不同领域的知名研究人员的频繁互动中受益。团队成员将通过开发个性化的研究项目参与动手本科教育和培训，从而获得荣誉论文，在国家会议上发表演讲和优秀的研究生院安置。研究生和本科生也将参与K-12科学推广工作，以帮助吸引和告知下一代海洋学家。该团队将与北卡罗来纳大学莫尔黑德天文馆和科学中心合作，并直接参加北卡罗来纳科学节。通过TED演讲的媒体接触，令人兴奋的成果将广泛传播给公众。该项目将与了解微生物介导的对东南大西洋边缘意外释放的碳氢化合物的反应直接相关，在东南大西洋边缘，石油和天然气勘探一直是州和国家政策讨论的主题。最近在北卡罗莱纳哈特拉斯角近海的大陆边缘发现了数百处天然气渗漏，在墨西哥湾北部发现了数千处天然气渗漏，通过溶解上升的气泡羽状物，将大量溶解的甲烷注入了上盖的大陆架和斜坡水域。甲烷的命运在很大程度上是由微生物氧化和远离渗漏源的平流运输之间的平衡控制的。微生物氧化的效果可能取决于甲烷、氧气和环境溶解无机氮（DIN）的浓度，以及原位压力和温度。最近船上的好氧甲烷氧化率（AMOR）测量表明，微生物对渗漏甲烷的消耗可能滞后一周，因此物理分散可能通过稀释来主导深水甲烷动力学。然而，海底甲烷稳定碳同位素测量表明，在注入的甲烷开始剧烈氧化之前没有滞后。我们建议进行AMOR的原位测量，同时在具有代表性的北卡罗莱纳州和墨西哥湾大陆边缘站点调查其微生物驱动因素，这些站点具有许多活跃的气泡渗漏。这些地点提供不同浓度的甲烷和DIN；在原地进行实验将消除船上速率的许多不确定性。我们将利用新开发的底栖着陆器系统进行测量，该系统配备了先进的激光甲烷传感器，能够进行多周的AMOR测量，同时分析溶解氧、DIN和微生物群落中甲烷氧化菌的存在（通过宏基因组）及其活性（通过元转录组），这些微生物群落可以揭示甲烷氧化对环境甲烷浓度的响应性质。该项目将测试有关深海甲烷动力学的关键假设，包括确定暴露于高甲烷浓度后微生物反应是否存在显著滞后，确定AMOR与甲烷和DIN浓度的关系，以及调查甲烷氧化菌对甲烷浓度时空变化的反应时间和幅度。成功地就地收集AMOR和相关的微生物群落数据对于在一系列海洋学条件下模拟海洋甲烷动力学非常重要，这些条件包括富含渗漏的海底水域和意外的碳氢化合物释放。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。