FY 1981

Processes affecting the distribution of low-molecular-weight aliphatic hydrocarbons in Cook Inlet, Alaska

Katz, C.N., and J.D. Cline

NOAA Tech. Memo. ERL PMEL-26, NTIS: PB82-250663, 84 pp (1981)

Measurements of low-molecular-weight (LMW) aliphatic hydrocarbons, C to C, were made in Cook Inlet, Alaska, on five cruises between 1977 and 1979. The distributions of these compounds were variable in both space and time. Concentration levels and composition of these gases in the lower Inlet appeared to be typical of those found in other coastal marine environments. The upper Inlet, however, had significantly elevated concentrations of methane and the C+ paraffins. On the basis of the contrasting hydrographic conditions of the two regions and the differing compositional characteristics of the hydrocarbon assemblages, the source of hydrocarbons was qualitatively determined to be predominantly biogenic in the lower Inlet and thermogenic in the upper Inlet. Hydrocarbon analyses made in concert with biological measurements in the lower Inlet suggested that the major source of methane was from the sediments, whereas the C+ hydrocarbons were produced in the water column. The conclusions of these field studies were supported by in vitro experiments. A methane budget calculated on the basis of a two-box model of the Inlet showed air-sea exchange and tidal mixing to be the major sinks of hydrocarbons in the Inlet. The budget quantitatively supported the evidence of a thermogenic source of hydrocarbons in the upper Inlet. This source may be the result of submarine seepage or leakage from existing wells.