Here we measured the concentrations and carbon isotopic composition of methane and ethane in sediment cores from several gas-rich sites in the Gulf of Cadiz. The aim of this sampling was to reveal the fate of both gases within the sediment in relation to the consumption of sulfate (Anaerobic oxidation of methane) and to measure isotope effects of this process on the carbon isotopic compositions. Therefore, sediment samples were collected during Meteor expedition M167 in October 2020 by gravity coring. After recovery, the core was cut into 1 m sections that were vertically split into an archive and a working half. Directly after this, defined volumes of 2 ml sediment were samples with cut-off syringes and transferred into 20 ml vials filled with 5 ml NaOH (1M) solution. Vials were immediately sealed with butyl septa and crimps. Methane and ethane concentrations were measured by gas chromatography coupled to flame ionization detection. Carbon isotopic composition of methane and ethane were measured as isotopomers of CO2 produced from these gases by in-line combustion coupled to isotope ratio mass spectrometry. Both methane and ethane were consumed with the consumption of methane. Methane isotopic composition was strongly altered in these zones. By contrast, isotope values of ethane remained rather stable.

Methane and ethane concentration measurements: 2 ml sediment sampled into 5 ml NaOH in 20 ml crimp vials, Methane concentration was measured using gas chromatography coupled to flame ionization detection (Agilent7820A GC with a Carboxen 1010 PLOT column, oven temperature 60°C for 2 min; gradient 25°C min-1 to 160°C for 3 min).Methane and ethane carbon isotopic composition was determined using in line combustion to CO2 with a Thermo Finnigan Trace GC and Alltech AT-Q column (30m x 0.32mm length internal diameter, 0.32 mm, carrier gas He); column temperature setting: 2min 40°C for 2 min, gradient 40°C min to 240°C, hold for 3 min) coupled via GC Combustion III interface (940°C) to Thermo Delta plus X ratio mass spectrometer.