Abstract
We report a new method for methane oxidation rate measurements that uses 103–105 times 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.
Highlights
The increase in the 14C-content of the DIC (14CDIC ) increase and the cell biomass (14CCell ) increase during sample incubation with 14C-CH4 tracer was calculated as shown in Eqs. 3 and 5, respectively, using data obtained from the analyses described in “Step 4: 14C-DIC analysis” and “Step 5: 14C-Cell biomass analysis.”
Methane oxidation rate measurements are essential to our understanding of ocean CH4 geochemistry; existing RT rate measurements are limited by strict health and safety regulations for radioactive applications (Table 1)
Samples inoculated with the low-level 14C-CH4 tracer do not require handling as radioactive waste because they contain 0.12 Bq 14C mL–1, which is three orders of magnitude below the strictest regulation for liquids (296 Bq 14C mL–1, Table 1)
Summary
2005; Girguis et al 2003; Nauhaus et al 2002; Sansone and Martens 1978), comparing water mass age with methane saturation and modeling methane turnover (water column only; e.g., Heeschen et al 2004; Rehder et al 1999; Scranton and Brewer 1978), and one-dimensional numerical models with sediment CH4 and sulfate profiles (sediment AOM only; e.g., Jørgensen et al 2001). Radiotracer methods measure the incorporation of 3H-CH4 or 14C-CH4 tracers in the oxidation products during a timed incubation by decay-counting. These methods are the most sensitive and direct of the available methods and are the most commonly used (Heintz 2011). Incubating a water sample (120 mL, 100 nM CH4, 0.6 nM CH4 d–1 oxidation rate) with the low-level 14C-CH4 for 1 d will raise the 14C-concentration in the oxidation products by a factor of 120-140 This increase in 14C is below the minimum detection limit for standard decaycounting techniques, but is detected by AMS. The LLRT method introduced here and below-regulation RT methods to follow will be useful for routine methane oxidation rate measurements in low-methane environments and when application of the existing RT methods for oxidation rate measurements is not practical
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