Correcting for Methane Interferences on δ2H and δ18O Measurements in Pore Water Using H2O(liquid)-H2O(vapor) Equilibration Laser Spectroscopy

Cavity ring-down spectroscopy (CRDS) is a new and evolving technology that shows great promise for isotopic delta O-18 and delta H-2 analyses of pore water from equilibrated head-space H2O vapor from environmental and geologic cores. We show that naturally occurring levels of CH4 can seriously interfere with CRDS spectra, leading to erroneous delta O-18 and delta H-2 results for water. We created a new CRDS correction algorithm to account for CH4 concentrations typically observed in subsurface and anaerobic environments, such as ground waters or lake bottom sediments. We subsequently applied the correction method to a series of geologic cores that contain CH4. The correction overcomes the spectral interference and provides accurate pore water delta O-18 and delta H-2 values with acceptable precision levels as well as accurate concentrations of CH4.