Development and application of a method to quantify methane oxidation rates using low-levels of C14-CH4 and accelerator mass spectrometry

Microbial methane oxidation in the marine environment is a process of global importance because it prevents methane released from underlying reservoirs from reaching the atmosphere. However, due to limited rate measurements and logistical hurdles associated with the existing rate measurements, the process is not fully characterized or quantified. Previously published rate measurements introduce radioactive labeled methane (14C-CH4 or 3H-CH4) to environmental samples, incubate, and quantify the amount of label incorporated in the oxidation products by decay counting. These methods use kilobecquerel levels (above regulation) of labeled methane and encounter a number of logistical hurdles related to regulations for radioactive applications.;A new low-level 14C-CH4 rate measurement (LL 14C) that uses 103-107 times less radioactivity than the existing methods was developed by taking advantage of the high sensitivity of accelerator mass spectrometry. The LL 14C method minimizes many hurdles associated with radioactive labeled material, but more importantly, it lays the analytic foundation for a below regulation rate measurement that may be applied broadly.;Following analytical development, the LL 14C method was tested against the existing 3H method using parallel rate measurements in two environments: [1] coastal waters with elevated methane, and [2] open ocean low-methane waters in the eastern tropical north Pacific (ETNP). Depth profiles of parallel oxidation rates in both environments were generally consistent. However, the LL 14C rates were mostly slower than 3H rates and the greatest difference between methods was seen at slower rates. Two separate trends in the results indicate that the LL 14C method is more sensitive than the existing 3H method in low-methane environments with slow oxidation rates: [1] the LL 14C rates were generally slower than the parallel 3H rates, and [2] the ETNP LL 14C data show a correlation between oxidation rate and the product of methane and oxygen concentrations that is consistent with expected kinetics and a similar trend is not seen in the 3H data. However, the source of the difference between methods is unclear and carefully controlled experiments are needed to eliminate experimental artifacts and identify the analytical advantages of the LL 14C method.;The methane oxidation rates measured in the ETNP are slow at depths within the region's characteristic low oxygen core and support the existing hypothesis that methane from organic rich coastal sediments can build up and advect within the core because methane oxidation is substrate limited by the low oxygen conditions. The data also shows that oxidation rates in the region are controlled by both methane and oxygen concentrations and that methane oxidation in ETNP surface waters is an important methane sink equal to or greater than sea-air exchange.