Correcting for methane interferences on δ2H and δ18O measurements in pore water using H2Oliquid-H2Ovapor equilibration laser spectroscopy.

Cavity ring-down spectroscopy (CRDS) is a new and evolving technology that shows great promise for isotopic δ(18)O and δ(2)H analyses of pore water from equilibrated headspace H(2)O vapor from environmental and geologic cores. We show that naturally occurring levels of CH(4) can seriously interfere with CRDS spectra, leading to erroneous δ(18)O and δ(2)H results for water. We created a new CRDS correction algorithm to account for CH(4) 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 CH(4). The correction overcomes the spectral interference and provides accurate pore water δ(18)O and δ(2)H values with acceptable precision levels as well as accurate concentrations of CH(4).