Investigating the CH4 isotopic signature of debris rich basal ice: insight from Camp Century ice core.

Runoff waters at the margin of the Greenland Ice Sheet export CH₄-supersaturated waters originating from the ice-sheet bed, contributing to the global atmospheric CH₄ budget [1, 2]. This methane is of microbial origin, likely produced from a mixture of inorganic and ancient organic carbon buried beneath the ice sheet [1, 2, 3].

Debris-rich basal ice layers provide a unique opportunity to investigate the sources and sink of methane at the ice/bedrock interface. Previous studies have shown that Greenland debris-rich basal ice preserves large methane accumulations [4, 5, 6] and may represent a potential endmember contributing to CH₄-rich meltwaters released during the melting season. At Camp Century, CH₄ mixing ratios increase sharply from ~200 ppm to up to 30 000 ppm within 1 m above the ice/bed material transition [6]. Prokaryotic DNA analyses support the microbial origin, and indicate the presence of in situ methanotrophic communities, suggesting active CH₄ consumption and oxidation to CO₂ within the debris-rich ice [6].

Here, we present methane stable isotope measurements (δ¹³C-CH₄ and δD-CH₄) from 7 samples spanning this transition zone. Despite the large methane accumulation, debris-rich ice is strongly gas-depleted, and the limited sample size combined with high CH₄ variability makes isotopic analyses technically challenging. CH₄ was extracted using a melting-freeze extraction coupled to a cold-trap finger filled with HayeSep Q at Université Libre de Buxelles (ULB, Belgium) laboratory, allowing gases to be sealed in glass tubes to prevent atmospheric contamination. CH₄ isotope analyses were performed at the Institute for Marine and Atmospheric Research Utrecht (IMAU, Netherlands) using a Thermo Delta Plus XP (δ¹³C and δD) [7].

The overall isotopic signature supports a microbial origin of CH₄ via methanogenesis, consistent with GRIP values [4]. Despite substantial scatter in δ¹³C-CH₄, a negative correlation is reported between CH₄ concentration and both δ¹³C-CH4 and δD-CH₄, consistent with preferential oxidation of lighter isotopes during methanotrophy. However, the observed relationship suggests a relatively low apparent fractionation factor compared to literature estimates. This could result from under-expression of the true isotope effect due to superimposed processes such as mixing or diffusion, or because the fractionation is intrinsically smaller under low-temperature conditions.

 

[1] Lamarche-Gagnon et al., 2019, Nature, 565(7737), 73-77. [2] Christiansen et al., 2021, Journal of Geophysical Research: Biogeosciences, 126(11), e2021JG006308. [3] Adnew et al., 2023, Geochimica et Cosmochimica Acta, 389, 249-264. [4] Souchez et al., 2006, Geophys. Res. Lett., 33, L24503. [5] Verbeke et al., 2002, Annals of Glaciology 35, 231-236. [6] Ardoin et al., submitted, The Cryosphere. [7] Menoud et al., 2020, Tellus B: Chemical and Physical Meteorology, 72(1), 1-20.