Carbon isotopic composition of dissolved methane and carbon dioxide in sediments

Interstitial water samples for carbon isotopic analysis of methane and carbon dioxide were collected in 2010 — 2011 through three courses at study site. Sampling was performed by set of minipiezometers placed in a depth of 20 to 60 cm randomly in a sediment. After sampling, refrigerated samples were transported (within 72 hours) in 250 mL bottles to laboratory at the Department of Plant Physiology, Faculty of Science University of South Bohemia in Ceske Budejovice, which are equipped with mass spectrometry for carbon isotopes measurements. Firstly both water samples, for methane and for carbon dioxide, were extracted to helium headspace. After relaxation time isotopic equilibrium was achieved and four subsamples of gas were determined by GasBanch (ThermoScientific) and IRMS DeltaplusXL equiped by TC/EA (ThermoFinnigan) for analysis of S13CO2. Afterwards S13CO2 of water samples were calculated from gaseous S13CO2 by fractionation factor from a linear equation (Szaran 1997):

sJ3C = — (0.0954 + 0.0027) T[°C] + (10.41 + 0.12) (6)

Stable isotope analysis of 13C/12C in gas samples was performed using preconcentration, kryoseparation of CO2 and gas chromatograph combustion of CH4 in PreCon (ThermoFinnigan) coupled to isotope ratio mass spectrometer (IRMS, Delta Plus XL, ThermoFinnigan, Brehmen, Germany). After conversion of CH4 to CO2 in the Finningan standard GC Combustion interface CO2 will be tranfered into IRMS. The obtained 13C/12C ratios (R) will be referenced to 13C/12C of standard V-PDB (Vienna-Pee-Dee Belemnite)(Rs), and expressed as S13C = (Rsample/Rstandard — 1) x 1000 in %. The standard deviation of S13C determination in standard samples is lower than 0.1% with our instrumentation. From our data, we also calculated an apparent fractionation factor ае that is defined by the measured SCHt and SCO2 (Whiticar et al. 1986):

aC = (dCO2 + 103) / (dCH4 + 103) (7)

This fractionation factor gives rough idea of magnitude of acetoclastic and hydrogenotrophic methanogenesis.