Distributed natural gas venting offshore along the Cascadia margin.

Area of Science:

• Geosciences
• Oceanography
• Environmental Science

Background:

• Widespread gas venting occurs along the Cascadia margin.
• Previous investigations have mapped over 1100 acoustic flares indicating gas release.
• The distribution and flux rates of this methane require detailed quantification.

Purpose of the Study:

• To investigate the spatial distribution and quantify methane gas venting along the Cascadia margin.
• To estimate the total methane flux from the Cascadia margin to the ocean.
• To analyze the relationship between venting and regional oceanographic features.

Main Methods:

• Acoustic water column data analysis to map and count gas flares.
• Determination of flow rates at individual flare locations.
• Application of a tidal-modulation model and depth-dependent methane density for flux calculations.
• Extrapolation of results across the margin using normalization techniques.

Main Results:

• Over 1100 acoustic flares were mapped, showing a nonuniform distribution.
• The highest number of flares were observed on the shelf, with highest density near the Juan de Fuca Strait outflow.
• Instantaneous flow rates ranged from 6 mL min⁻¹ to 18 L min⁻¹.
• An average in situ flow-rate of approximately 88 × 10⁶ kg y⁻¹ was calculated for the margin.
• The estimated average methane flux-rate is approximately 0.9 g y⁻¹ m⁻².

Conclusions:

• Gas venting is a significant process along the Cascadia margin, particularly influenced by shelf dynamics and localized outflows.
• The estimated methane flux provides crucial data for understanding carbon cycling in this region.
• Further research is needed to refine flux estimates and understand the broader implications of this methane release.