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A Not-So-Bubbly Fate for Sea Floor Methane
17 Jan, 2008 10:38 am
Good news about methane bubbling up from the ocean floor near Santa Barbara, California
One of the world’s largest and best studied seep regions, the Coal Oil Point seep field, is located along the northern side of the Santa Barbara Channel. The seep field emits gas, oil, and tar in water 5 m to 70 m deep. Most gas bubbles are composed of ~90% methane at the seafloor and ~60 – 70% methane at the sea-surface. The gaseous methane emission is in the range of two million cubic feet per day. Colleagues at the University of California, Santa Barbara, investigated the amount of methane dissolved in the ocean originating from Coil Oil Point seep field in an earlier study (published in Journal of Geophysical Research). They estimated another two million cubic feet per day of methane are dissolved into the water above the seafloor vents.
The fate of that dissolved methane, however, remained uncertain. Usually dissolved methane is transported with the ocean currents and forms so called methane ‘plumes’ (water with higher methane concentrations than background), which can degas to the atmosphere. As methane is a potent greenhouse gas that warms the Earth 23 times more than carbon dioxide when averaged over a century, quantification of the amount of methane entering the atmosphere is critical.
In order to estimate how much methane reaches the atmosphere, we went out by boat and sampled the surface water at 79 locations encompassing a 280 square kilometer area. The area is located down-current from the seep field, i.e., several kilometers from the actual site of gas release and in direction of the ocean flow. In addition, water was also sampled at 14 locations on a monthly basis to observe if methane concentrations change with time.
We found that methane concentrations vary with time corresponding to changes in surface currents. Methane concentrations were similar when ocean currents moved counterclockwise in their typical circular pattern. The northern limb of this current transports water westward from the Coal Oil Point seeps into the study area in about a day. Lower methane concentrations were observed when a small clockwise current was present in the study area disrupting the normal westward flow. Higher methane concentrations were observed when currents were weak.
We also investigated if our coarse grid sampling was sufficient to calculate the quantity of methane transferred to the air. For this, a mass spectrometer was towed behind the boat collecting data every 5 seconds. Results from the mass spectrometer showed no significant difference in the numbers.
Based on the time series we concluded that ‘normal’ ocean conditions were present when we sampled the 280 square kilometer area. On that day the methane plume spread over 70 square kilometers. Together with data of wind speed, the amount of methane transferred into the air was estimated.
Significantly, our results indicate that only one percent of the dissolved methane from the Coal Oil Point seep field escapes into the air. Good news for earth’s atmosphere. But what happens to the rest of the dissolved methane?
This methane could enter the atmosphere farther away from the seep field, become oxidized by microbes in shallow water, or mix deeper into the water column where it is subsequently oxidized. Assuming background concentrations and taking into account the area of the Santa Barbara Channel, we find that maybe another percent of the dissolved methane can enter the atmosphere outside of our study area. Therefore, we hypothesize that most of the dissolved methane is transported below the surface and oxidized by microbial activity. Measurements to verify our hypothesis are currently underway.
Our results have been published as the cover story in volume 34, number 22 of Geophysical Research Letters .
Mau S., D. L. Valentine, J. F. Clark, J. Reed, R. Camilli, L. Washburn (2007), Dissolved methane distributions and air-sea flux in the plume of a massive seep field, Coal Oil Point, California, Geophys. Res. Lett. , 34, L22603, doi:10.1029/2007GL031344.
Clark, J. F., L. Washburn, J. S. Hornafius, B. P. Luyendyk (2000) Dissolved hydrocarbon flux from natural marine seeps to the southern California Bight, J. Geophys. Res. , 105(C5), 11509-11522, 10.1029/2000JC000259.