Methane Oxidation Rates by AMS: Use of low-level 14C-CH4 and Accelerator Mass Spectrometry to Determine Rates of Water Column Aerobic and Anaerobic Methane Oxidation

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ABSTRACTOCE-0622759Our understanding of methane geochemistry in the ocean depends to a large extent on measurements of methane oxidation rates in oxic and anoxic waters and sediments. Relatively few oxidation rate measurements are available for seawater, which has low methane concentrations (~10 nM to 10 uM). Because of the specific activity of 14C-CH4, rate measurements involving addition of this tracer require additions at concentrations higher than ambient and extrapolation of rates measured at several concentrations back to in situ concentrations. A research team at the University of California at Irvine is setting out to overcome these analytical limitations. With support from this grant, they will exploit the high sensitivity of Accelerator Mass Spectrometry (AMS) to measure methane oxidation rates with only small changes in the CH4 pool size. This approach also lowers the activity level of tracer 14C-CH4 concentrations to levels that are close to being exempt from regulations covering transportation and handling of radioisotopes. They expect to make two advances in the use of 14CCH4 in determination of methane oxidation rates: 1) conducting parallel measurements of methane oxidation rate in the Cariaco Basin involving low-level 14C-CH4 and established 3H-CH4 tracer techniques, and 2) developing methods that will allow the use of 14C-CH4 at activity levels that are below those regulated as hazardous waste. The use of these low tracer activities in future measurements will simplify shipping, relax the requirement that radioisotope tracer experiments be performed on large ships with isotope vans, and should permit wider application of these measurements in remote locations and time series measurements. This project will have a number of broader impacts. The research will involve one graduate student one undergraduate student. The senior principal investigator is active as a board member of the Long Beach Aquarium of the Pacific's Marine Conservation Research Institute. MCRI's commitment to outreach and education in the ethnically-diverse Long Beach community should facilitate contact with community groups and students. The junior principal investigator and her other colleagues have developed a short course to introduce young scientists to the use of radiocarbon in carbon cycle science. The goals of the course are (1) to educate a new group of young scientists in the science of the global carbon (C) cycle and (2) to introduce them to the technology of AMS radiocarbon measurements and the applications of radiocarbon to C cycle studies.