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

Ocean sediment dissolved CH4 concentrations are of interest for possible climate‐driven venting from sea floor hydrate decomposition, for supporting the large‐scale microbial anaerobic oxidation of CH4 that holds the oceanic CH4 budget in balance, and for environmental issues of the oil and gas industry. Analyses of CH4 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 CH4 occur rising to 30 mM (pCH4 = 3 MPa) or an excess pressure ∼3× greater than CO2 in a bottle of champagne. Normalization of the CH4 Raman ν1 peak to the ubiquitous water ν2 bending peak provides a fundamental internal calibration. Very large losses of CH4 and fractions of other gases (CO2, H2S) must typically occur from recovered cores at gas rich sites. The new data are consistent with observations of microbial biomass and observed CH4 oxidation rates at hydrate rich sites and support estimates of a greatly expanded near surface oceanic pore water CH4 reservoir. Key Points In situ measurement gives a 30× increase in concentration Pore water methane is a massively underestimated reservoir In situ measurement yields a fundamentally calibrated accurate measurement