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

Techniques have been developed that allow on-line simultaneous analysi s of concentration and carbon isotopic composition of dissolved methan e in seawater and porewater using isotope-ratio monitoring gas chromat ography/mass spectrometry. The method uses either headspace equilibrat ion or He-sparging of water, followed by sample drying, cryofocusing, subambient GC separation using a PoraPLOT-Q analytical capillary colum n, on-line 1000 or 1150 degrees C combustion, and measurement on a MAT 252 isotope-ratio-monitoring mass spectrometer. Analyses of porewater s using headspace equilibration take less than 15 min per sample where as the He-sparging technique requires similar to 30 min/sample. The de tection limit with an isotopic ratio standard deviation of 0.5 parts p er thousand is 10 nM using 10 mL samples. Analytical blanks associated with these methods are negligible. The procedures were evaluated thro ugh analyses of porewater CH4 ina core taken in muddy sediments of Tom ales Bay, CA, and of interstitial and surface seawater from Checker Re ef in Kaneohe Bay, Oahu, HI. Carbon isotopic analyses of low concentra tions of dissolved CH4 in porewaters should prove to be a useful monit or of anaerobic diagenesis of sedimentary organic matter and the origi ns of CH4, especially in systems where inputs of organic matter are lo w.