HR: 16:30h
AN: OS42C-03    [PDF]
TI: Compositional changes in natural marine bubble plumes: Observations from Shane Seep, Coal Oil Point hydrocarbon seep field, CA
AU: * Clark, J F
EM: jfclark@geol.ucsb.edu
AF: Dept. of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Leifer, I
EM: ira.leifer@bubbleology.com
AF: Chemical Engineering, University of California, Santa Barbara, CA 93106 United States
AU: Washburn, L
EM: washburn@icess.ucsb.edu
AF: Dept. of Geography, University of California, Santa Barbara, CA 93106
AU: Luyendyk, B P
EM: luyendyk@geol.ucsb.edu
AF: Dept. of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AB: Detailed measurements of bubble composition, dissolved gas concentrations, and plume dynamics were conducted during a 9-month period at Shane Seep, a very large (surface gas flux = 1900 m$^{3}$ day$^{-1}$), shallow (22 m water depth) marine hydrocarbon seep in the Santa Barbara Channel, California. The field study illustrates the important chemical and physical processes that occur in the modified environment of large bubble plumes. Despite the very short travel time of bubbles from the seafloor to the surface (about 40 s), a significant amount of gas exchange between the bubbles and plume water occurs, creating distinctly different seafloor and surface gas compositions. Methane, carbon dioxide, and heavier hydrocarbons were lost from rising seep bubbles, while nitrogen and oxygen were gained. Enough methane was lost from the bubbles to increase the dissolved methane concentration within the plume by more than four orders of magnitude above atmospheric equilibrium values. However, there was insufficient time (rise distance) to bring the plume water into equilibrium with the bubbles (methane concentrations $<$10% saturation). Leifer et al. (2000) reported that surface methane concentrations were slightly supersaturated within the bubble plumes of deeper (70 m water depth), large (surface gas flux $>$ 600 m$^{3}$ day$^{-1}$) Coal Oil Point seeps. This observation demonstrates that given enough time, large bubble plumes can saturate the plume water with methane and, thus, the loss of this gas from the bubbles will cease. Once saturated, the methane remains in the seep bubbles and is transported through the water column directly to the atmosphere. The depth of methane saturation in the Coal Oil Point seep field is relatively shallow, between 20 m and 70 m, suggesting that large bubble plumes will transfer methane to the atmosphere regardless of depth.
DE: 0312 Air/sea constituent fluxes (3339, 4504)
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 4807 Chemical speciation and complexation
DE: 4820 Gases
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting