In situ Raman‐based measurements of high dissolved methane concentrations in hydrate‐rich ocean sediments

Ocean sediment dissolved CH 4 concentrations are of interest for possible climate‐driven venting from sea floor hydrate decomposition, for supporting the large‐scale microbial anaerobic oxidation of CH 4 that holds the oceanic CH 4 budget in balance, and for environmental issues of the oil and gas industry. Analyses of CH 4 from recovered cores near vent locations typically show a maximum of ∼1 mM, close to the 1 atmosphere equilibrium value. We show from novel in situ measurement with a Raman‐based probe that geochemically coherent profiles of dissolved CH 4 occur rising to 30 mM ( p CH 4 = 3 MPa) or an excess pressure ∼3× greater than CO 2 in a bottle of champagne. Normalization of the CH 4 Raman ν 1 peak to the ubiquitous water ν 2 bending peak provides a fundamental internal calibration. Very large losses of CH 4 and fractions of other gases (CO 2 , H 2 S) must typically occur from recovered cores at gas rich sites. The new data are consistent with observations of microbial biomass and observed CH 4 oxidation rates at hydrate rich sites and support estimates of a greatly expanded near surface oceanic pore water CH 4 reservoir.