Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet

Archive Data

Author(s)

Getachew Agmuas Adnew, Thomas Ro¨ckmann, Thomas Blunier, Christian Juncher Jørgensen, Sarah Elise Sapper, Carina van der Veen,

Malavika Sivan, Maria Elena Popa, Jesper Riis Christiansen

Title

Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet

Description

Data collection related to the manuscript/paper " Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet".

The data are the basis for figure 4, 5, 6,7 and 9, table 1 and supplementary figure 2 in manuscript/paper. The data is provided as an excel file as described below.

Sheet 1: Mass scan for clumped methane measurement cup configuration. This data set is used to produce figure 4.

Sheet 2:   contains all data for the concentration and isotope composition of methane  (bulk and clumped isotope) and the 𝛅13C of carbon dioixed . These data is provided in Table 1 and used to produce Figure 5  to 7 and supplemetary figure 2

Sheet 3: Water isotope data of the subglacial melt water

Sheet 4: Data for end pont members used to produce Figure 9

2.1 Site description

The study site is located at an elevation of 450 m above sea level at a lateral subglacial meltwater outlet on the southern flank at the terminus of the Isunnguata Sermia Glacier at the western margin of the GrIS (67°09’16.40’’N 50°04’08.48’’W).

The area in front of the meltwater outlet consists of abraded granodioritic gneiss bedrock with large boulders and patches of gravel, sand and silt deposited by meltwater. The glacier front contained highly irregular cracks and air-filled cavities, which changed over the season as the ice melted and deformed.

2.2. Collection of air samples

Air samples were collected in june 2022 from the ice cave using a small compressor that compressed ambient air into pre-evacuated 6 and 15 L volume canisters to a final pressure of 3 bar (Entech Instruments,USA) (Figure 2 and 3). The mole fraction of CH4 and CO2 in the air sampled was monitored with a microportable Greenhouse Gas Analyzer (MGGA) (GLA131-GGA, ABB-Los Gatos Research, San Jose, CA, USA). Background air was collected at the top of the glacier in 2 L volume stainless steel canisters.

2.2. Collection of water samples

We collected water samples from the subglacial meltwater at the margin of ISG for water isotope analysis in June 2022 between 12:00 and 15:00 (local time). The water samples were filtered using 0.45 µm pore size syringe filters and stored in an amber glass with a small headspace in a refrigerator prior to analysis.

2.7 Isotopic analyses of gas and water samples

The bulk isotope composition (δ13C(CH4) and δD(CH4)) and clumped isotopes (∆13CH3D and ∆12CH2D2) of CH4 were measured using a high-resolution gas source isotope ratio mass spectrometer (HR-IRMS) (Thermo Scientific, Germany) in dual inlet mode at Utrecht University. CH4 from air samples was extracted using cryogenic separation (see Figure 3). The extraction process employed two traps to remove CO2 and other condensable gases, followed by a charcoal trap to collect CH4, all operated at a temperature of liquid nitrogen (-196°C). Further purification of CH4 was achieved using a custom-made gas chromatography setup with two columns: a stainless steel column packed with molecular sieve to separate gases like H2, Ar, O2, and N2 and a second column packed with HayeSep-D porous polymer to separate CH4 from higher-order hydrocarbons such as C2H6 and C3H8.

The isotopic composition of CO2 (δ13C) was analyzed with the CF-IRMS system at Utrecht University. In short, the CO2 is cryogenically separated from the air, further purified chromatographically, and then injected into the IRMS via an open split inlet. The results are related to the VPDB and VSMOW scales via a reference air cylinder with known isotopic composition.

Water isotopes were measured at Physics of Ice and Climate (PIC), Niels Bohr Institute, University of Copenhagen, Denmark with a cavity ring-down spectroscopy (Picarro L2130, PICARRO, USA).