Project: Collaborative Research: Methane efflux from river influenced coastal marine sediments

NSF Award Abstract: 
Methane is a strong greenhouse gas and the amount of methane in the atmosphere is increasing. Large amounts of methane are present in sediments. Thus, understanding how much methane is released from sediments is a key first step for improving estimates of methane emissions. Coastal sediments are thought to be large sources of methane to the overlying water and potentially the atmosphere. Yet, fluxes of methane from coastal environments are poorly known. This project will measure methane fluxes from coastal sediments into the waters of the Gulf of Mexico. Methane is produced in sediments when microorganisms break down organic matter and from geological sources. However, the amount of methane released from sediments can be reduced by microorganisms that oxidize methane near the sediment surface where oxygen is present (aerobic oxidation) and deep in the sediment where oxygen is not present (anaerobic oxidation). This study focuses on environments where the aerobic oxidation of methane is the key process that prevents its release to the water column. Results from this project will help identify areas of the coastal ocean that are significant sources of methane release. This work is timely, as river-influenced marine sediments are sensitive to anthropogenic activities and global climate change. Scientists involved in this study will translate the findings into useful information for carbon cycle scientists, policy makers, students, teachers, and the public. They will work with education and outreach programs at Louisiana Universities Marine Consortium, University of Georgia, and Louisiana State University, and provide training to scientists from undergraduate to postgraduate levels. High sedimentation rates, like those found near rivers, can drive methane efflux from sediments, as evidenced by elevated methane concentrations in bottom waters. Observational data suggest that sediments from river influenced shallow shelf regions with low dissolved oxygen concentrations in the water column emit more methane to the water column than typical continental shelf sediments. This project combines benthic flux measurements, geochemistry and rate analysis into a reaction-transport model that will evaluate environmental influences on benthic processes. The team of scientists will measure benthic fluxes of oxygen and methane using an in situ lander, and sediment micro-profiles of oxygen and sulfide. Sedimentation and mixing rates will be determined using naturally occurring radioisotopes. Sulfate reduction and methane oxidation rates will be determined using radiotracers and a broad suite of contextual geochemical analysis will be performed. During two cruises off the southeastern coast of Louisiana, a total of 5 sites will be targeted. The data synthesis will help refine our understanding of the interactions between rivers, marine sediments, and methane releases from marine sediments. The project supports the development of an early career investigator and will provide training for one postdoctoral associate, two graduate students, three research assistants. This effort will leverage the LUMCON REU site program to support at least one student per year. The education goals emphasize informal education through outreach, formal education in the classroom, and authentic research experiences for community college students. Many of the activities emphasize the importance of communicating information to the scientific community, students, policy makers, and the general public.