INTEGRATING BUBBLE FLUX SPECTRA AND DIRECT FLUX MEASUREMENTS OF MARINE HYDROCARBON SEEPAGE

Marine hydrocarbon seep gas often escapes the seabed as bubbles that rise towards the sea surface. The fate of this gas (transport to the atmosphere or dissolution followed by bacterial oxidation or exchange across the air-sea interface) depends upon several factors, most significantly, seep depth. Numerical simulations show that for a given seep depth there is a strong sensitivity to bubble size. As a result, size-dependent bubble flux spectra are critical to predicting the fate of escaping gas. Currently, this data can be provide solely by bubble measurement systems (BMS); however, due to video's large data storage requirements, it is not amenable to long term deployment. Direct flux measurements by capture devices or turbine seep tents provide time data at reasonable data storage size.

To study the fate of marine seep gas, a BMS was developed and deployed to measure the size dependent bubble flux spectra from the Coal Oil Point (COP) Seep Field, in the Santa Barbara Channel, California. A detailed survey of seep bubbles at the seabed and sea surface was conducted at an active seep area, Shane Seep, in the seep field. For direct measurements, three turbine seep tents were deployed that measured gas flux by relating the turbine rotation rate to the flux. The rising bubbles and advected water drive the turbine. Also deployed was a direct capture flux buoy that allowed determination of spatial variability and flux rate estimation.

Besides bubble size, simulations show ambient conditions such as the upwelling flow and dissolved gas concentrations are important to seep gas's fate. The local upwelling flow was measured by the BMS while dye studies measured water column values. Dissolved concentrations were obtained from water samples. SCUBA divers collected seabed and sea surface samples. Bubble gas samples were collected and analyzed for subsequent model validation.

Data from the approaches was synthesized and used to predict the fates of the seep gas (transport to the atmosphere versus dissolution in the water). Data was used to initialize and validate a numerical bubble model that predicted the gases' fates. Then, sensitivity studies were used to illustrate the importance of both flux spectra and long-term flux measurements to understanding marine seepage.