Stable carbon isotope ratio analyses on trace methane from ice samples

Measurement of the concentration and stable isotope ratios of methane from air enclosures in ancient ice can aid in understanding the temporal dynamics of methane sources and sinks. These measurements can reconstruct past atmospheric compositions on scales of tens-to-hundreds of years with adequate precision and accuracy. We present an improved micro-extraction gas-chromatography continuous-flow isotope ratio mass spectrometer (CF-IRMS) method to extract and measure the methane 13C/12C ratio (δ13CH4) of air occluded in glacial ice. The technique described uses a unique post-combustion trapping of the methane signal to produce a high amplitude sample peak increasing the signal to noise ratio of the sample. The technique requires only ca. 165 pmol of CH4 (ca. 30–150 g of ice depending on methane concentration). The small sample requirement permits very fine temporal sampling resolution and the possibility of routine duplicate measurements from multi-parameter ice core samples. The technique was applied to atmospheric air, artificial ice samples, and samples from the GISP2 ice core. The precision of our technique was evaluated using both atmospheric air and artificial ice samples and is less than ± 0.3‰ (1σ). Our test measurements of shallow, poorer quality GISP2 ice from 225 ± 20 yr BP give a δ13CH4 value of − 47.37 ± 0.6‰ (corrected for gravitational fractionation and referenced to VPDB), which is in excellent agreement with other published data.