Dissolved
Methane
Natural
marine seepage near Coal Oil Point, Santa Barbara Channel, California,
injects large quantities of hydrocarbons into the coastal ocean. The dispersal
and source strength of the injected methane, ethane, and propane from
this seep field was determined using a variety of oceanographic and geochemical
techniques. The results show that hydrocarbons seep into stratified coastal
waters creating plumes that extend for at least 12 km.
The
plume structure is complex because of the large geographical distribution
of seep vents and because of the chaotic nature of advection and mixing
near the seeps. At the time of the survey, hydrocarbons were injected
onto density surfaces between sigma = 24.5 - 26.0 kg/m3.
Earlier
work has shown that subsurface methane maxima in the upper waters of the
southern California Bight are typically found on these density surfaces.
We estimate that the total flux of methane into the water column above
the Coal Oil Point seeps is 2 x 10^10 g/yr and is approximately equal
to the total flux of dissolved methane to the atmosphere estimated for
the entire southern California Bight.
These
observations strongly support the inference of others that coastal sources,
which includes some of the world's largest marine hydrocarbon seeps, maintain
the methane maximum observed offshore California. Estimates of the global
methane flux from coastal waters derived by extrapolating the flux from
coastal California may be too large because of the anomalous amount of
marine hydrocarbon seepage in these waters.
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![](../images/pictures/dissolved_hc/ch4_c3h8_offshore_COP_web_t.jpg)
dissolved methane samples offshore of the Santa Barbara coastline
![](../images/pictures/dissolved_hc/ch4_c3h8_sections_web_t.jpg)
dissolved methane and propane concentrations along transects
A-A' and C-C'
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