Distributed natural gas venting offshore along the Cascadia margin

摘要

Widespread gas venting along the Cascadia margin is investigated from acoustic water column data and reveals a nonuniform regional distribution of over 1100 mapped acoustic flares. The highest number of flares occurs on the shelf, and the highest flare density is seen around the nutrition-rich outflow of the Juan de Fuca Strait. We determine ∼430 flow-rates at ∼340 individual flare locations along the margin with instantaneous in situ values ranging from ∼6 mL min⁻¹ to ∼18 L min⁻¹. Applying a tidal-modulation model, a depth-dependent methane density, and extrapolating these results across the margin using two normalization techniques yields a combined average in situ flow-rate of ∼88 × 10⁶ kg y⁻¹. The average methane flux-rate for the Cascadia margin is thus estimated to ∼0.9 g y⁻¹m⁻². Combined uncertainties result in a range of these values between 4.5 and 1800% of the estimated mean values.