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Low-molecular-weight hydrocarbons in the C(l) to C(4) range have been detected and measured in sea water. Methane shows by far the highest concentration, followed in order by the C(2), C(3), and C(4) fractions. In general, the concentration tends to decrease with depth, but there is a strong indication that a maximum may exist about 30 to 50 meters below the surface.
Swinnerton et al. (Fri,) studied this question.