Preparation of natural methane samples for stable isotope and radiocarbon analysis

We describe procedures for preparing methane samples from natural waters or sediments for radiocarbon (C-14-CH4) analysis. These procedures also allow the determination of H-2/H-1 and C-13/C-12 ratios of this methane. Methane is extracted from natural samples in the field using Lamont radon stripping boards (Mathieu et al. 1988) and trapped using liquid nitrogen-cooled U-traps filled with HiSiv 3000. The field procedures eliminate the need to return large samples to the laboratory. The U-traps are returned to the laboratory where a new two-stage purification-oxidation vacuum line is used to extract, purify, and oxidize the methane. The methane oxidation products ( carbon dioxide and water) are analyzed for the natural content of C-14, H-2, and C-13 with standard procedures. These procedures were evaluated under the conditions that methane was extracted from between 19 to 114 L water or 0.07 to 0.35 L sediment. No isotope fractionation was observed and the blanks are small (0.52(8) +/- 0.39 mu mol methane with radiocarbon content C-14/C = 96.1 +/- 0.3 pMC [percent Modern Carbon] [Stuiver and Polach 1977]). Using C-14-accelerator mass spectrometry (AMS), these procedures can produce a successful radiocarbon measurement when at least 1.81 mu mol methane are collected. Thus, C-14-CH4 measurements can be made when the dissolved methane concentration is between saturation (mM) and 15.9 nM for the water column or 5.2 mu M for the sediment.