Observations and modeling of submarine hydrocarbon seeps within the hydrate stability zone

Submarine hydrocarbon seeps are ubiquitous around the world on the continental margins. These seeps distribute dissolved hydrocarbon through the continuous mass transfer process between hydrocarbon particles (i.e. bubbles/drops) and sea water. However, this mass transfer process is not fully understood due to current constraints in observations and modeling, which hinders the quantification of the seep’s contribution to the ocean interior’s biogeochemistry. Current observations (e.g. shipboard sonar mapping and in-situ survey using ROV and AUV) only provide the trajectory of hydrocarbon particles but do not provide quantitative information on the evolving particle contents. Previous numerical simulations do not provide satisfactory results that agree with observations for a wide range of seeps. Here, we present an integrated effort using comprehensive observations and model validation to understand and predict the entire transport process of hydrocarbon bubbles emanated within the hydrate stability zone. With detailed measurements at the seep source, we obtained the bubble size distribution, gas chemical composition, and the total volume flux at a 1200 m deep seep site in GC600, Gulf of Mexico. These data were used to initiate a particle dissolution model with consideration of hydrate formation and the impact of hydrate on the dissolution rate of hydrocarbons. With the application of the model to the measured seep, we predicted the evolving bubble sizes and composition, stable isotopic ratios of methane dissolved in water, trajectories of the seep bubbles, horizontal footprint of the bubble flare, total height-of-rise, and the acoustic response from bubbles to different sonar frequencies. These model results were then validated with mid-water information measured through ROV-tracking of seep flares through 400 m height-of-rise, and two ROV-board and shipboard multi-beam sonars. Through this integrated observation and modeling approach and through data analysis of the hydrocarbon particles from their source to fate constitutes, a broad picture of the transport processes of hydrocarbon seeps through the water column was achieved.