Dissolved methane in deep-sea cold seeps can vary from background levels to highly enriched seep water over short spatial scales, which requires in situ measurements with both high sensitivity and a wide dynamic range. Single-transition laser absorption measurements often face a trade-off: strong absorption lines improve low-concentration detection but can saturate at high methane concentrations, whereas weak lines extend the measurable range but reduce the sensitivity. We developed a dual-wavelength off-axis integrated cavity output spectroscopy analyzer for the continuous in situ measurement of dissolved methane in deep-sea environments. The system uses methane transitions near 1651 and 1658 nm to combine trace-level detection with high-concentration quantification. Gas-phase calibration showed excellent linearity for both channels (R2 > 0.999), with the 1651 nm channel calibrated over 2-20 ppm and the 1658 nm channel calibrated over 10-2000 ppm. During continuous measurement of a 10.06 ppm of CH4 standard gas, the concentration precision improved from 0.016 ppm at 20 s to 1.95 ppb at 400 s for the 1651 nm channel and from 0.69 ppm to 52 ppb for the 1658 nm channel. A membrane-based gas extraction module was then calibrated to convert optical measurements to dissolved methane concentrations. Field deployment on a manned submersible in the northern South China Sea resolved near-bottom methane enrichment, rapid vertical attenuation, and localized hotspots exceeding 250 ppm. These results show that the dual-wavelength OA-ICOS strategy can support continuous, quantitative methane measurements across strongly heterogeneous deep-sea seepage environments.
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