Biogenic And Thermogenic Gas In Gas-Charged Sediment Of Norton Sound, Alaska

Abstract

ABSTRACT Chemical and isotopic compositions of sediment gas from Norton Sound have been determined for near-surface, gas-charged sediments at two sites identified in acoustic profiles and bottom observations. At one site our air-driven vibracorer penetrated sediment saturated with methane that has a carbon isotopic composition (?13 CPDB) of -80%. This isotopic value suggests that the methane originated from active biological processes operating on peat in the top 4 m of sediment. At the other site, characterized by a large subsurface acoustic anomaly, smaller near-surface acoustic anomalies and active seepage of gas, our vibracorer obtained sediment saturated with gas composed of 98% CO2 which had a ?13 CPDB value of -2.7%. Associated with the CO2 are minor concentrations of petroleum-like 1ight hydrocarbons. Methane in this mixture has a ?13 CPDBvalue of -36%. The carbon isotopic compositions of CO2 and methane along with the chemical distribution of gaseous hydrocarbons indicate that at this site these gases are derived from thermal processes operating at depth in Norton Basin. Apparently CO2 from the decarbonation of marine limestone acts as a carrier for hydrocarbon gases that have been generated from organic matter buried in the basin. The gases reach the surface by faults and escape at the seafloor as a submarine seep. The presence of near-surface gas charged sediment in Norton Sound reduces the stability of the seafloor. Areas where sediments are charged with gas may pose potential hazards for engineering developments. INTRODUCTION Previous investigations in Norton Sound describe acoustic anomalies that are attributed to the presence of near-surface gas-charged sediment. 1-4 One anomaly in particular has been studied in detail because of the discovery in 1976 of submarine seepage of petroleum-like gaseous hydrocarbons into the water column. The gas seep comes from sediment that is inferred to be saturated with hydrocarbons. 1- 4 In the summer of 1977 a geochemical investigation of the seep site showed that near-surface sediment contains petroleum-like gas and gasoline-range hydrocarbons; However, the measured concentrations of hydrocarbons in the sediment, although unusually high, were well below saturation. The contradiction between geophysical evidence suggesting gas-saturated sediment and the geochemical analyses showing concentrations of hydrocarbons greatly below saturation led to the work described in this paper. A major objective was to determine the chemical composition and concentration of the gas in the sediment and to indicate possible sources for this gas. We focused on two areas (Fig. 1) where geophysical, geological and geochemical information indicates that the sediment is charged with gas. One area, approximately 50 km south of Nome, is designated Site 3 from the core number of our 1978 survey. This area corresponds in location to the seep mentioned above. 1- 4 About 12 km northwest of site 3 we investigated a second area, designated Site 4. At Site 4 there is evidence of gas-charged sediment, but there is no indication of surface seepage of gas. At both sites the depth of water is 19 m. The geophysical, geologic and geochemical results obtained at these two contrasting sites have explained the earlier contradictory evidence from the seep area.