Department of Earth System Science

We report a new method for methane oxidation rate measurements that uses 10³-10⁵times less ¹⁴C-CH₄ than existing measurements by taking advantage of the high sensitivity of accelerator mass spectrometry. Methane oxidation in the marine environment is a microbial process of global importance because it prevents methane released from underlying reservoirs from reaching the ocean and atmosphere. Rate measurements provide a crucial tool for assessing the efficacy of this process across a range of environments, but the current methods use high amounts of radioactive elements (³H- or ¹⁴C-CH₄), tend to increase methane concentrations in a sample markedly over in situ levels, and are limited by strict health and safety regulations. The low-level method presented here uses levels of ¹⁴C-CH₄ that are below transportation regulations, produce samples that do not require treatment as radioactive waste, and allow for tracer level rate measurements in low methane environments. Moreover, the low-level method lays the analytical foundation for a below-regulation rate measurement that could be used broadly and in-situ. Parallel rate measurements with the low-level ¹⁴C-CH₄ and existing ³H-CH₄ methods are generally consistent with a correlation coefficient of 0.77. However, the low-level method in most cases yields slower rates than the ³H method possibly due to temperature, priming, and detection limit effects.