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The distribution of C1 to C4 hydrocarbon gases in surficial sediments from the Alaskan continental shelf was determined as part of a regional environmental survey sponsored by National Oceanic and Atmospheric Administration and Bureau of Land Management (NOAA/BLM). Methane, ethane, ethene, propane, propene, n-butane and iso-butane were observed in low concentrations (0.1-10 ppb, as weight gas to dry weight sediment) in nearly all of the 190 sediment samples examined, with methane exceeding C2 to C4 hydrocarbons in abundance by about a factor of 10. No gasoline range (C5 to C8) hydrocarbons were detected above blank levels. Ethene was found generally to be more abundant than ethane, while propane was slightl more abundant than propene--the correlation between these hydrocarbons suggests a common, possibly biogenic, origin. Sediments from Norton Sound have anomalously high methane concentrations, probably as a result of biological methanogenesis in subsurface organic-rich peat layers. Most of the regions sampled showed median values of the C1/(C2 + C3) ratio of < 20. The generally low C1 concentrations probably resulted from the selective loss of C1 by biologic oxidation or ebullition from the surface sediments. No major C2 to C4 hydrocarbon anomalies were discovered; however, at some stations in Kachemak Bay, Kodiak shelf, and Norton Sound, anomalously high C3/C3:1 ratios indicated the poss ble presence of thermogenic gases at depth.
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