New technique for high-precision, simultaneous measurements of CH<sub>4</sub>, N<sub>2</sub>O and CO<sub>2</sub> concentrations, isotopic and elemental ratios of N<sub>2</sub>, O<sub>2</sub> and Ar, and total air content in ice cores by wet extraction

Abstract

Abstract. Air in polar ice cores provides various information on past climatic and atmospheric changes. We developed a new method combining wet extraction, gas chromatography and mass spectrometry, for high-precision, simultaneous measurements of eight air components (CH4, N2O and CO2 concentrations, δ15N, δ18O, δO2/N2, δAr/N2 and total air content) from an ice core sample of ~60 g. The ice sample is evacuated for ~2 hours and melted under vacuum, and the released air is continuously transferred into a sample tube at 10 K within 10 minutes. The air is homogenized in the sample tube overnight at room temperature, and split into two aliquots for mass spectrometric and gas chromatographic measurements. Cares are taken to minimize contamination of greenhouse gases with long evacuation time, consumption of oxygen during sample storage by passivation treatment on sample tubes, and fractionation of isotopic ratios with long homogenization time for splitting. Precisions are assessed by analysing standard gases with artificial ice, and by duplicate measurements of the Dome Fuji and NEEM ice cores. The overall reproducibility (one standard deviation) from duplicate ice-core analyses are 3.2 ppb, 2.2 ppb and 3.1 ppm for CH4, N2O and CO2 concentrations, 0.006, 0.010, 0.09 and 0.12 ‰ for δ15N, δ18O, δO2/N2 and δAr/N2, and 0.67 mlSTP kg-1 for total air content, respectively. Our new method successfully combines the high-precision, small-sample and multiple-species measurements, with a wide range of applications for ice-core paleoenvironmental studies.

Fractionation of O<sub>2</sub>∕N<sub>2</sub> and Ar∕N<sub>2</sub> in the Antarctic ice sheet during bubble formation and bubble–clathrate hydrate transition from precise gas measurements of the Dome Fuji ice core

“…We note that δ 15 N is insensitive to the removal thickness (Fig. B1c), confirming its immunity from gas loss (Severinghaus et al, 2009;Oyabu et al, 2020).…”

“…Bereiter et al (2009) predicted that the original gas composition is preserved in the inner part of the cores for a few decades at low temperature (e.g., −50 • C), based on a numerical simulation of gas diffusion. The prediction was verified by analyzing the DF core, which has been stored at −50 • C for ∼ 20 years, after sufficiently removing its fractionated outer part (Oyabu et al, 2020). In this work, we employ the method by Oyabu et al (2020) and measure δO 2 /N 2 and δAr/N 2 from the bubbly ice to clathrate hydrate ice in the DF core.…”

“…The prediction was verified by analyzing the DF core, which has been stored at −50 • C for ∼ 20 years, after sufficiently removing its fractionated outer part (Oyabu et al, 2020). In this work, we employ the method by Oyabu et al (2020) and measure δO 2 /N 2 and δAr/N 2 from the bubbly ice to clathrate hydrate ice in the DF core. In addition to the discrete sampling, we obtain high-resolution, continuous gas and chemical data at selected depth intervals to understand the smoothing process of δO 2 /N 2 and δAr/N 2 below the BCTZ.…”

“…1a), which is cut out from a bulk section (∼ 500 mm long). Because δO 2 /N 2 and δAr/N 2 near the sample surface are depleted by gas loss during storage, we removed the surface by more than 8 mm for clathrate ice (Oyabu et al, 2020) and 5 mm for bubbly ice (see below). For some samples in the BCTZ (1000-1200 m), we removed the surface by 20 mm (Fig.…”

“…The experimental procedures at NIPR are described elsewhere (Oyabu et al, 2020). Briefly, ice sample was evacuated for 2 h and melted, and the released air was cryogenically collected into a sample tube.…”