Award: OCE-1318102

During the Deepwater Horizon disaster in the Gulf of Mexico, approximately eight million moles of methane, a potent greenhouse gas, were released. Interestingly, all of that methane appeared to have dissolved in the deep Gulf waters and was not emitted to the atmosphere. In addition, numerous lines of evidence suggest that a significant portion of this methane was microbially oxidized within four months after the start of this disaster. However, this methane oxidation event occurred over an area of approximately 73,000 square kilometers and a time period of roughly four months, thus a thorough understanding of (1) the kinetics of methane oxidation, (2) what may have enhanced or limited this reaction, and (3) how these properties changed over space and time were not obtained. Since natural releases of methane from the seafloor have been observed to increase with increasing ocean temperature, a more thorough understanding of methane oxidation kinetics has impacts beyond anthropogenic disasters like the Deepwater Horizon oil spill. The overarching goal of this project was to determine the efficiency of methane oxidation and how it may be different in different regions with different chemical, biological, and physical conditions. The results of this investigation are enabling a better understanding of the ultimate fate of methane released from the seafloor, be those releases natural or anthropogenic. This project began by developing a new technique enabling the real-time and automated analyses of chemical and isotopic changes in seawater during a methane oxidation event. This technique developed a reservoir to house large volumes of seawater cleanly and to maintain that level of cleanliness over a period of several months. Also, a system was engineered for the automated, user-defined analysis and collection of numerous parameters from these large seawater samples such as the concentrations of methane, carbon dioxide, dissolved oxygen, nutrients, and trace metals, the isotopes of methane and carbon dioxide, and biological properties such as cell counts and microbial community structure. The application of these techniques was the central focus of two oceanographic expeditions, one to the northern Gulf of Mexico, adjacent to the location of the former Deepwater Horizon well, and the other in the Hudson Canyon, US Atlantic Margin. The Hudson Canyon served to as a contrasting environment and helped to assess how this process is different under different conditions. The results of these experiments are leading to eight scientific publications: five are fully published in peer-reviewed scientific journals, two have been submitted for publication and are currently in peer-review, and one is in the final stages of preparation. This project supported several Ph.D. students at two different universities and enabled unique interactions with elementary and high school scientific education programs. The knowledge gained from these experiments has uncovered numerous unknowns about how methane is aerobically oxidized. First, after methane is released into seawater, our results suggest that it takes between one and four weeks before methane oxidation rates become aggressive. Second, once aggressive methane oxidation begins, the oxidation reaction continues for only a few days until it is limited by a reactant. The specific reactant that ultimately limits this reaction depends on the initial concentration of oxygen or a specific nutrient or trace metal as well as the size of the methane perturbation. Third, aggressive methane oxidation was observed in waters that were both directly influenced by and not directly influenced by methane bubbles released from the seafloor. However, the start of aggressive methane oxidation was delayed in waters not directly influenced by natural methane emissions. Fourth, when methane is oxidized, the lighter natural isotopes of methane react faster than the heavier isotopes leading to a c...