DETERMINATION OF CONCENTRATION AND CARBON ISOTOPIC COMPOSITION OF DISSOLVED METHANE IN SEDIMENTS AND NEARSHORE WATERS

Techniques have been developed that allow on-line simultaneous analysis of concentration and carbon isotopic composition of dissolved methane in seawater and porewater using isotope-ratio monitoring gas chromatography/mass spectrometry. The method uses either headspace equilibration or He-sparging of water, followed by sample drying, cryofocusing, subambient GC separation using a PoraPLOT-Q analytical capillary column, on-line 1000 or 1150 degrees C combustion, and measurement on a MAT 252 isotope-ratio-monitoring mass spectrometer. Analyses of porewaters using headspace equilibration take less than 15 min per sample whereas the He-sparging technique requires similar to 30 min/sample. The detection limit with an isotopic ratio standard deviation of 0.5 parts per thousand is 10 nM using 10 mL samples. Analytical blanks associated with these methods are negligible. The procedures were evaluated through analyses of porewater CH4 ina core taken in muddy sediments of Tomales Bay, CA, and of interstitial and surface seawater from Checker Reef in Kaneohe Bay, Oahu, HI. Carbon isotopic analyses of low concentrations of dissolved CH4 in porewaters should prove to be a useful monitor of anaerobic diagenesis of sedimentary organic matter and the origins of CH4, especially in systems where inputs of organic matter are low.