Low-temperature formation of hydrocarbon gases in San Francisco Bay sediment (California, U.S.A.)

Abstract

To understand the processes responsible for the presence of low-molecular-weight hydrocarbons (C₁-C₄) in anoxic environments, we studied sediments collected from an anaerobic estuarine mudflat. In these sediments methane (C₁) was several orders of magnitude more abundant than all other C₂-C₄ hydrocarbons; the ) ratio was ∼ 13,000. Mean ethane/ethene and propane/propene ratios were 0.4 and 0.7, respectively. Production of C₁-C₄ hydrocarbons was monitored during prolonged incubation (7 months) of sediments at 27° and 4°C. Samples stored at 27°C generated significant quantities of C₁-C₄ hydrocarbon gases. Incubation at 4°C inhibited production of these gases. Several bactericides were tested with respect to their ability to inhibit formation of gaseous hydrocarbons. Sodium azide, chloroform, and 2-bromoethanesulfonic acid effectively inhibited methane formation, but not ethene formation in dilute continuously-shaken sediment slurries. Zephiran chloride only caused partial inhibition of methanogenesis (46%) and ethene generation (34%) in these slurries. In experiments with more concentrated unshaken sediment slurries, however, zephiran chloride and sodium azide did not block formation of methane, ethane, or propane. Only storage at −10°C prevented production of these gases. These results indicate that C₁-C₄ hydrocarbons can be formed by low-temperature reactions, possibly mediated by microorganisms.