/ Flat-bottomed depression 50-1 50 m in diameter and 60-80 cm deep occur in the floor of Norton Sound, Bering Sea. These large erosional bedforms and associated current ripples are found in areas where sediment grain size is 0.063-0.044 mm (4-4.5 ~), speeds of bottom currents are greatest (20-30 cm/s mean speeds under nonstorm conditions, 70 cm/s during typical storms), circulation of water is constricted by major topographic shoals (kilometers in scale), and small-scale topographic disruptions, such as ice gouges, occur locally on slopes of shoals. These local obstructions on shoals appear to disrupt currents, causing separation of flow and generating eddies that produce targe scale scour. Offshore artificial structures also may disrupt bottom currents in these same areas and have the potential to generate turbulence and induce extensive scour in the area of disrupted flow. The size and character of natural scour depressions in areas of ice gouging suggest that large-scale regions of scour may develop from enlargement of local scour sites around pilings, platforms, or pipelines. Consequently, loss of substrate support for pipelines and gravity structures is possible during frequent autumn storms.
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.
A dynamic environment of moderate te.ctonism, sediment instability, and active erosional and depositional processes on the shallow sea floor of northern Bering Sea creates several potential geologic hazards. Active faulting, thermogenic gas seeps, sea-floor gas cratering, sediment liquefaction, ice gouging, scourdepression formation, storm-sand deposition, and large-scale bedform movement all occur in the study area. Geologic processes may interact to cause such potential hazards as nearsurface faulting south of Nome, Alaska that provides pathways for seeps of thermogenic gas leaking from a large subsurface g~s accumulation. Numerous faults also cut the sea floor in the area west of Port Clarencef fault activity is difficult to determine, however, because current scour may be preserving or exhuming old fault scarps.Interaction between the processes of liquefaction and the formation of gas craters, scour depressions, storm-sand deposits, and slumps results in potential sediment instability problems. Liquefaction of the upper 1-2 m of sediment can be caused by cyclic stormwave loading of the Holocene coarse-grained silt and very fine-grained sand covering Norton Sound. The widespread occurrence of gas-charged sediment with small surficial craters (3-8 m in diameter and less than 1 m deep) in central Norton Sound indicates that the Ejea-floor sediment is periodically disrupted by sudden venting of biogenic gas from the underlying peaty mu'd. During major storms, liquefaction not only may help trigger crater formation, but also may enhance erosional and depositional processes that create large-scale scour areas and transport sand in the Yukon prodelta area. Surficial, small-scale slumps on the west. flank of the Yukon prodelta may also be triggered by liquefaction.Interaction of erosional and depositional processes may result in hazards_in the shallower parts of northern Bering sea. Ice gouges are numerous and ubiquitous in the area of the~ukon prodelta, and the sediment is scoured to depths of 1 m. Although much less common, ice gouges are present throughout the remainder of northern Bering sea where water depths are less than 20 m. In the Yukon prodelta area and iñ eferences and illustrations at end of paper central Norton Sound, where currents are constricted by shoal areas and flow is made turbulent by local topographic irregu+arities (such as ice gouges), 'storm-induced currents have scoured large (50-150 m diameter), shallow «1 m deep) depressions. Abundant storm-sand layers in Yukon mud show that storm-sur.ge activity has a significant effect on the bottom, particularly around the Yukon prodelta, where storm surge and waves have generated bottom transport currents that deposit layers 9f saud as far as 75 km from land. Large sand waves (10-200 m wavelength and 0.5-5 m wave height) west of Port Clarence do not continually migrate, but may move only intermittently when storm surge run-off reinforces the strong geostrophic currents that continually flow north over the Bering Shelf.
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