8.2.1 Distribution and Sources of Sedimentary 

 Hydrocarbons 



MAHLON C. KENNICUTT II. JAMES M. BROOKS, and THOMAS J. McDONALD 



Geochemical and Environmental Research Group, Department of Oceanography. Texas A&M University, 



College Station, Texas, USA 



Introduction 



The distribution, sources, and fate of natural and 

 anthropogenic hydrocarbons in polar regions are poorly 

 understood. As increased exploration and development of oil 

 and gas reserves in polar regions continue, an understanding of 

 the hydrocarbon geochemistry in these areas is essential. A 

 preliminary study to assess baseline sediment hydrocarbon 

 levels and distributions, sources (including biogenic, 

 anthropogenic, and natural seepage), and potential pathways of 

 hydrocarbon transport was undertaken by Venkatesan and 

 Kaplan (1982) on the outer continental shelf of Alaska. 

 Sediments from the Beaufort Sea, southeastern Bering Sea, 

 Norton Sound, Navarin basin. Gulf of Alaska, Kodiak Shelf, 

 and lower Cook Inlet were collected and analyzed. This study 

 concluded that these areas exhibit little evidence of petroleum 

 hydrocarbons except at a few isolated locations. The sediments 

 contain mixed marine autochthonous and terrestrial 

 allochthonous hydrocarbons. A complex mixture of polynuclear 

 aromatic compounds (PAH), attributed to pyrolytic sources, 

 were detected at all sites. One station in lower Cook Inlet and 

 one in the southeastern Bering Sea had indications of the 

 presence of weathered petroleum. 



The present study is a continuation of a program designed 

 to determine the concentrations, distributions, and sources of 

 hydrocarbons in the Bering Sea. Sediment samples were 

 obtained from grab sampler and gravity cores during the Third 

 US-USSR Joint Bering & Chukchi Seas Expedition aboard the 

 Soviet Research Ves.sel AA:fl(/t7/»A' A'()/()/e\' August 1988. 



During a previous joint cruise in July 1984, aliphatic and 

 aromatic hydrocarbons were detected in sediments at all 

 locations sampled in the Bering Sea (Kennicutt et ai. 1990). 

 The hydrocarbons were determined to be a mixture of marine 

 biological debris (bacteria, algae, zooplankton, phytoplankton ), 

 terrestrial plant biowax( normal alkanes), "recycled"' or exposed 

 immature sediments, petroleum ( natural seepage ), and pyrolytic 

 sources. The levels of hydrocarbons detected were similar to 

 those reported for sediments on the Alaskan outer continental 

 shelf. Unique biological olefins, previously identified in the 

 area, were distributed over a wide area. The relative amounts 

 and composition of hydrocarbons varied widely over the area 

 sampled. The presence of a complete suite of normal alkanes 

 and isoprenoids, an unresolved complex mixture, petroleum 

 related PAH, mature biological markers (hopanes), and vertical 

 distributions of hydrocarbons confirmed the presence of 



petroleum at stations in the western and southern Bering Sea. 

 This petroleum is most likely derived from natural seepage 

 from much deeper source rocks and/or reservoired fluids. The 

 petroleum seepage at one southern Bering Sea location is 

 typical of a condensate and is significantly different from that 

 observed at western Bering Sea locations. 



Experiment 



The analytical methods utilized in this study are described 

 in detail elsewhere ( Brooks f/rt/., 1986; Kennicutt e/ a/., 1988). 

 Briefly, sediment samples werefreeze-dried, Soxhlet extracted 

 for 12 hours (hexane), and analyzed. Polynuclear aromatic 

 hydrocarbons were detected by spectrofluorescence on a Perkin- 

 Elmer 650-40 fluorometer. Extracts were then analyzed for 

 aliphatic hydrocarbons by gas chromatography with flame 

 ionization detection. Component separations were attained 

 using fused silica capillary columns (DB-5) and temperature 

 programming. Selected sediments were further analyzed by 

 gas chromatography/mass spectrometry (HP 5890/HP5790 

 MSD) in both scanning and selected ion monitoring modes. 



Results and Discussion 



One hundred eighty-eight samples from 132 surficial 

 sediment grab samples and gravity cores were taken over a 

 large portion of the Bering and Chukchi Seas (Fig. I). 



Fluorescence 



Total scanning fluorescence analysis of sediment extracts 

 shows that varying amounts of petroleum-related hydrocarbons 

 are present throughout the study area (Fig. 2). The majority of 

 fluorescence intensities were less than 400 and represent low 

 level background values. Fluorescence R ratios were highly 

 variable and spanned the range for both condensates and oil 

 (Fig. 2). The average fluorescence maximum excitation and 

 emission wavelengths for the entire study were 340 nm and 

 380 nm, respectively, typical of petroleum (Fig. 2) . The 

 geographic distribution of fluorescence intensities is presented 

 in Fig. 3. These maps, and all subsequent maps, show the 

 highest 25% of the values for the parameter mapped as the dark, 

 shaded areas. Significant geographic variations in both 

 fluorescence intensity and fluorescence R ratios are apparent 

 ( Figs. 3,4). The southern portion of the study area had relatively 

 high fluorescence intensities at several locations with a 



293 



