Resolving the Origin of the Petrogenic Hydrocarbon Background in

Jan 3, 2001 - Sediments in stream or beach strand lines that appeared to contain ...... Paul D. Boehm, John S. Brown, David S. Page, and Keith R. Park...
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Environ. Sci. Technol. 2001, 35, 471-479

Resolving the Origin of the Petrogenic Hydrocarbon Background in Prince William Sound, Alaska P A U L D . B O E H M , * ,† D A V I D S . P A G E , ‡ W I L L I A M A . B U R N S , §,| A. EDWARD BENCE,§ PAUL J. MANKIEWICZ,§ AND JOHN S. BROWN† Arthur D. Little, Acorn Park, Cambridge, Massachusetts 02140, Bowdoin College, Brunswick, Maine 04011, and ExxonMobil Upstream Research Co., Houston, Texas 77252-2189

The dominant sources of the petrogenic hydrocarbon background in benthic sediments of Prince William Sound, AK (PWS), site of the 1989 Exxon Valdez oil spill, are eroding Tertiary shales and residues of natural oil seepage. Mass balance considerations and statistical analyses of hydrocarbon fingerprints independently indicate that coal contributes generally less than 1% of the polycyclic aromatic hydrocarbons (PAH) and chemical biomarkers in this background. This is environmentally significant because of presumed differences in the bioavailability of PAH in coal, seep oil residues, and shales. Coal particles are present in PWS sediments, but their PAH and chemical biomarker contributions are overwhelmed by those of seep oil residues and organic particles from shales of low-tohigh thermally maturity. In the late Tertiary or early Quaternary, the currently exposed and eroding shale formations were heated into the oil-generation window and, consequently, are now relatively rich in extractable PAH and chemical biomarkers. The exposed and eroding coals in the area, in contrast, experienced long hot burial and are now thermally overmature with respect to oil generation. The concentrations of thermally sensitive PAH and biomarker compounds in PWS sediments are not consistent with a mature coal origin but are consistent with the low-tohigh maturity shales and seep oils in the area.

Introduction The presence of a natural background of petrogenic hydrocarbons, and specifically the polycyclic aromatic hydrocarbons (PAH), in the benthic sediments of Prince William Sound, AK (PWS), is well-established (1-4). However, a controversy has recently arisen (1997-1999) over the origin of this background. The environmental significance is whether the PAH compounds in the background come from coal and thus may be of limited bioavailability or from oil seepage and eroded shales and, therefore, may be more bioavailable than coal. Some of these shales are classified as petroleum source rocks by virtue of their organic carbon (OC) contents (5). * Corresponding author e-mail: [email protected]; phone: (617)513-1351; fax: (978)369-5122. † Arthur D. Little. ‡ Bowdoin College. § ExxonMobil Upstream Research Co. | Present address: 758 West Forrest, Houston, TX 77079. 10.1021/es001421j CCC: $20.00 Published on Web 01/03/2001

 2001 American Chemical Society

Our earlier studies identified the pre-spill, natural petrogenic hydrocarbon background in PWS using age-dated cores (1). These cores showed a continuous and ongoing flux of hydrocarbons into this area for at least 160 yr. It was determined that these hydrocarbons came from a large petroliferous region east of PWS (Figure 1) and appeared to be associated with suspended particulate matter (SPM) brought into PWS by the Alaska Coastal Current (1-3). It was hypothesized that extractable hydrocarbons associated with SPM might not be as bioavailable as those comprising bulk (free phase) oil (1). We ultimately concluded that the source of the background was “oil seepage and source rock erosion” (2) and that coal was not a dominant source (2, 3). Recently, however, others contend that the background comes from coal rather than oil (4, 6-8). These other studies considered Bering River coals and oil from the Katalla area as possible sources but not the shales and other seep oils we previously identified (1-3). Arguments put forth in support of coal include the following: (i) high PAH concentrations in offshore sediments adjacent to a coastal region containing extensive coal deposits; (ii) PAH composition patterns (i.e., fingerprints) consistent with coal but not with oil; (iii) low ratios of triaromatic steroids to methylchrysenes (called a refractory index, RI) found in PWS sediments and coals but not in oil from the Katalla area east of PWS; (iv) the presence of the biomarker 28,30-bisnorhopane in sediments and coals but not in Katalla oil; and (v) bioaccumulation in salmon collected near the Katalla seeps but not in mussels 9 km away. To resolve this controversy, in 1999 we undertook a comprehensive field sampling and chemical analysis study to supplement our earlier work. An understanding of the geology and physical geography of the area guided this effort.

Methods Hydrocarbon fingerprinting techniques, organic petrographic analysis, mass balance considerations, thermal history indicators, and statistical analysis of fingerprints were used to identify and quantify the contributions of the sources to the natural background in PWS. Sampling Strategy. A sequential sampling strategy, conducted over 8 yr, was used to track the hydrocarbon background and to confirm and quantify its sources. Initially, cores of benthic sediments were taken to confirm the presence of a pre-spill background in PWS. Next, benthic sediments were collected between PWS and potential hydrocarbon sources east of PWS along the Gulf of Alaska (GOA) coast. Finally, sediments along the eastern GOA coast, stream sediments, and the hydrocarbon sources contributing to them were sampled. Each year’s sampling program was based on the results of the previous efforts. In 1999, seep oils and Tertiary shales from the Katalla and Yakataga areas (Figure 1) were sampled. The seep oil was collected by skimming it from the surfaces of streams and pools where it had accumulated. The shales were generally sampled from outcrops, but a few stream boulders were also collected to determine the composition of inaccessible shales from the steep, eroding mountainsides or from further upstream. Bering River coalfield samples were provided by R. Sanders (29). Sediments from major rivers and streams from the Copper River to the Malaspina Glacier (Figure 1) were sampled. Offshore sediment samples were collected from as far east as Yakutat. Sediment samples generally came from the top 2 cm of sediment, and cores were occasionally collected. Sediments in stream or beach strand lines that appeared to VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Location of coal, oil seepage areas, exposed Tertiary shales, and offshore sediment samples along the eastern Gulf of Alaska (GOA) coast. contain coal-like particles (e.g., Katalla Beach) were collected by spoon to maximize any coal content. Replicate samples were collected at these sites including 6 in. deep cores to determine both the lateral and the vertical distribution of fossil organic matter. The four PWS sediment samples used in this study were chosen to minimize possible interference with residue from the Exxon Valdez spill. They came from the pre-spill portions of archived age-dated cores from our 1991 program and from the top 2 cm of a PWS sample collected outside of the Exxon Valdez spill path. For some PAH and chemical biomarker ratio comparisons, we were able to take advantage of previous PWS sediment analyses increasing the PWS sample count to 19. A few GOA offshore sediment samples were collected in 1999, and a subset of archived GOA offshore samples was reanalyzed along with the four archived PWS samples. Analysis Methods. PAH and chemical biomarker concentrations were quantified using GC/MS-SIM techniques previously reported (1, 2, 9-11). These techniques were extended to include quantification of 53 two-six-ring PAH analytes, including alkyl isomers of phenanthrene, dibenzothiophene, and chrysene. Similarly, we quantified 44 C19C35 terpanes (m/z 191), 14 C21-C29 regular steranes (RRR and Rββ) (m/z 217 and 218), 8 C27-C29 diasteranes (m/z 259), and 7 C20-C28 triaromatic steroids (m/z 231). The saturate biomarkers were quantified using 5β(H)-cholane. The PAH and aromatic biomarkers were quantified using phenanthrene-d10. Sample size ranged from 30 to 50 g dry weight. Before solvent extraction, shales and coal samples were pulverized to a grain size approximating that of PWS benthic sediments. This was done to minimize potential grain size effects on extraction efficiency. Total organic carbon (TOC) was measured on 1-g subsamples using combustion techniques (12). We measured, under immersion oils, the reflectance to incident light of small (>4-5 µm), polished, organic particles 472

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in rock, sediment, and coal samples using an optical microscope (12, 13). This reflectance technique, when applied to vitrinite, is a standard method for characterizing the thermal maturity of coal and shale formations. The reflectance of vitrinite particles under immersion oils (Ro %) in coals and those dispersed in shales is recognized to increase with increasing thermal maturity (see, for example, ref 5). Because the benthic sediments are very fine-grained and the distinction between fusinite/inertinite and high-maturity vitrinite must be made on textural relationships that are no longer preserved, reflectivities of all organic particles were measured in the sediments, coals, and shales. Source Allocation. Benthic sediments in the study area contain mixtures of hydrocarbon inputs from contributing streams and ocean currents. Each stream integrates contributions from its local sources, including eroding shales and coals, seep oil residues, etc. Thus, it is not likely that any one source will explain all aspects of benthic sediment chemical fingerprints in such a diverse petroliferous area. A source allocation method (14) involving principal component analysis (PCA) of all analyte data sets was used to identify potential hydrocarbon sources. The PCA analysis determined which combination of analytes best defined the potential sources of hydrocarbons. On the basis of the PCA results, 38 PAH analytes and biomarker compounds were selected and up to 18 sources were identified. A least-squares iterative method was then used to find the linear combination of analyte distributions from the 18 different sources that best matched the analyte distribution of the sample. This method, which was recently applied to the identification of sites of past human activity based on sediment PAH distributions (15), is independent of TOC mass balance constraints. To account for the fact that PAH and biomarker concentrations were often orders of magnitude different in a given sample, the logarithms of the individual PAH and

FIGURE 2. Organic particle reflectance in sediment, coal, and shale samples. (A) Composite of PWS sediment measurements (n ) 4). (B) Queen Vein coal from the Bering River coalfield. (C) Natural coke from Carbon Mountain, Bering River coalfield. (D) Coaly sediment from Bering River sandbar near mouth. (E) Coaly sediment from among rocks at Katalla Beach. (F) Tertiary shale from Katalla Formation, Mt. Hazelet. (G) Tertiary shale from Yakataga Formation, Yakataga Reef. (H) Tertiary Shale from Katalla Formation, Pt. Hey. biomarker concentrations, normatlized to the sum of the analytes in the least-squares analysis, were used. We minimized the following expression by adjusting the xj term: 38

18

∑ ∑ (ln(s

i,j xj/SumAnalj)

- ln(di/SumAnalsamp))2

(1)

i)1 j)1

Here i refers to the selected PAH and biomarker analytes (1-38), j refers to the hydrocarbon sources (1-18), si,j is the concentration of the ith analyte of source j, SumAnalj is the sum of the 38 PAH and biomarker analytes from source j, di is the concentration of the ith analyte of the sample, and SumAnalsamp is the sum of the 38 PAH and biomarkers from the sample. The sum of all 18 xj ) 1.

Results and Discussion Organic Petrography. The reflectance distribution of all organic particles in PWS sediment is bimodal over a wide range of values (0.3-∼6%) (Figure 2A), indicating two major types of sources. Tertiary shales from the Katalla (Oligocene), Yakataga (Plio-Pleistocene), and Poul Creek (Oligocene) Formations, which are exposed near the coast, are major sources of organic particles. These particles are dominated by vitrinites with minor inertinites (Figure 2F-H). The vitrinites in these shales generally have reflectance values in the 0.4-1.2% range, indicating low-to-high thermal maturity with respect to oil generation (13). Vitrinite reflectance values for the shales roughly match those of the lower reflectance bimodal hump observed in PWS sediments. These shales, however, cannot be significant contributors of organic particles to the high reflectance bimodal hump. Other major sources of organic particles are the older, coal-bearing Kushtaka and Kulthieth (both early Tertiary) Formations (5, 16). Reported ranks (16) of the Kulthieth Formation coals range from low volatile bituminous (i.e., Ro ∼ 1.5-2.0%) to semi-anthracites (i.e., Ro ∼ 2.0-2.5%). Bering River coals and coke, which occur in the Kushtaka Formation, have a range in rank that matches the upper bimodal hump (1.6-6.0% reflectance) observed in PWS sediments. Coal beds in the Kulthieth and Kushtaka Formations east of the Bering River were not directly accessible to us. However, sediments from the Duktoth River, whose drainage area includes these older formations, are dominated (83 vol %) by vitrinites having

Ro >1.5% (range 0.42-3.57%). This indicates that the coal beds in the Duktoth River drainage area have high thermal maturities similar to those of the Bering River coals. Strandline sediments from Katalla Beach have a similar bimodal distribution to that observed in PWS, indicating contributions from both major types of sources. Particles eroded from the shales and coals do not contribute equally to the PAH and biomarker content of PWS sediment, in part because of the large differences in their thermal maturities. Because, on a total organic carbon (TOC) basis, they are far richer than the coals in PAH and chemical biomarkers, the low-to-high maturity sources (i.e., shales and seep oil residues, see below) dominate the benthic sediments fingerprints. Most coastal GOA Tertiary shales were heated into the oil-generation window. Consequently, they contain oil sorbed to dispersed organic and clay particles and as unexpelled bitumen in pores (16-18). Solid bitumen particles, identified petrographically, occur in stream and offshore sediments. These bitumen particles may represent either the unexpelled liquid phase or the degradation products of the seep oils. Mass Balance Constraints. A number of potential sources contribute organic carbon to PWS sediments. Mass balance assessment of source input based on TOC, PAH, and chemical biomarkers is useful because sources having different thermal maturity levels and different organic facies can exhibit large differences in PAH and biomarker concentrations normalized to TOC. The shales have relatively high concentrations of PAH and chemical biomarkers on a total organic carbon (TOC) basis. Bering River coals, in contrast, have been heated past the oil-generation window and have lost much of their PAH and biomarkers due to expulsion and thermal degradation, consistent with the observations of Dzou et al. (20). Consequently, their TOC-normalized PAH and biomarker concentrations are low. Recent organic matter (e.g., terrestrial plant debris, algae, and the remains of marine organisms) contributes organic carbon but not those PAH or chemical biomarkers studied here. Remnants of asphalt spilled during the 1964 earthquake (21), organic particles from shales, seep oil residues, coal, products of incomplete fuel combustion, soot, forest fire fallout, atmospheric fallout, and boat fuel/oil residues are potential sources of the PAH studied here. Many of them also contribute chemical biomarkers. VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Background hydrocarbons in PWS benthic sediments and in possible sources of the background shown on a TOC-normalized basis. (A) TPAH. (B) C30-hopane. (C) C26,27-triaromatic steroids. Error bars represent the total range for all samples analyzed for that source, and N is the number of samples. QVC, Bering River Queen Vein coal; BR, Bering River sandbar; PH, Point Hey shoreline sediment; KB, Katalla Beach. TOC levels in our PWS sediment samples average 0.64 ( 0.18 wt % (including recent organic matter), total PAH (TPAH) averages 1145 ( 470 ng/g, and TPAH/OC averages 180 000 ( 24 000 ng/g OC (Figure 3A). Our richest Bering River coal 474

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(QVC) (TOC ∼85%, TPAH ) 44 000 ng/g, and TPAH/OC ) 48 000 ng/g OC) cannot contribute more than ∼300 ng/g TPAH in PWS sediment without exceeding the average measured TOC level. Coaly beach sediments can supply only

FIGURE 4. PAH and triaromatic steroid fingerprints. (A) Pre-spill sediment from PWS core. (B) Queen Vein coal. (C) Katalla Beach sediment. (D) Malaspina glacial flour. (E) Poul Creek seep oil. (F) Johnston Creek seep oil. (G) Katalla Formation shale, Mt. Hazelet area. (H) Yakataga Formation shale, Cape Yakataga. (I) Yakataga Formation shale, Munday Creek. (J) Yakataga Formation shale, Johnston Creek. Np, naphthalenes; Biph, biphenyl; Fl, fluorenes; Ph, phenanthrenes; Db, dibenzothiophenes; Ch, chrysenes; Per, perylene; TAS, triaromatic steroids; N0, naphthalene; N1, C1-naphthalene; P0, phenanthrene; P1, C1-phenanthrene; P2, C2-phenenathrene; D2, C2-dibenzothiophene. 20-150 ng/g TPAH. Using the same argument for chemical biomarker input, area coals are negligible (,1%) contributors, and “coaly” beach sediments cannot contribute more than ∼5% of measured levels of any of the chemical biomarkers studied here. Figure 3B,C illustrates this point for TOCnormalized C30-hopane (representing the saturate biom-

arkers) and C26,27-TAS (representing the triaromatic steroids). Area seep oils (elemental C ≈ 80-85 wt %, TPAH levels between 5 × 106 and 55 × 106 ng/g, and TPAH/OC from 9 × 106 to 70 × 106 ng/g OC) can satisfy the PAH and biomarker levels observed in PWS sediment with less than a 0.1 wt % VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Triterpane and sterane fingerprints. Samples as for Figure 4. Ts, 17r(H)-22,29,30-trisnorhopane; Tm, 18r(H)-22,29,30-trisnorhopane; 28B, 28,30-bisnorhopane; 29Nor, C29-norhopane; Hop, C30-hopane; Dpl, diploptene from recent organic matter (ROM) (the Copper River is a major source); Diaster, diasteranes. TOC addition to the sediment. Even the most weathered seep oils we sampled are enriched in PAH and biomarkers as compared to the coal (Figure 3). A kilogram of seep oil contains about the same amount of TPAH as a metric ton of Bering River coal. Shales sampled from the Katalla, Yakataga, and Poul Creek Formations have ranges in TOC of 0.3-4.81 wt %, in TPAH 476

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of 75-45 000, and in TPAH/OC of 26 000 to 3 × 106 ng/g OC (Figure 3A). These shales can provide the observed TPAH and chemical biomarker levels in PWS sediments without violating the TOC constraint. Similarities in the TPAH and chemical biomarker concentrations in the shales and stream sediments (Figure 3A-C) reflect the dominance of the shale component in the stream sediments.

TABLE 1. Average Analyte Ratios and Standard Deviations for PWS Marine Sediment and Potential Hydrocarbon Sourcesa analyte ratios samples sedimentb

PWS marine Queen Vein coal and bate Katalla oils Yakataga oils Tertiary shales Malaspina glacial flour

DPI 0.14c (

0.015 0.35 f ( 0.01 0.13 f ( 0.003 0.11d ( 0.024 0.16i ( 0.10 0.13 f ( 0.001

OI 0.25c (

0.024 0.22g 0.16 f ( 0.07 0.29d ( 0.046 0.17i ( 0.10 0.29 f ( 0.001

4-mDBT/1-mDBT 6.2d (

0.8 37 f ( 2.8 3.4 f ( 0.85 5.8d( 3.5 6.3i ( 5.6 6.7g

MPI-3 1.4d (

0.045 5.4 f ( 0.22 1.2 f ( 0.14 1.4d ( 0.41 0.90i ( 0.22 1.3 f ( 0.15

Ts/(Ts + Tm) 0.28c (

0.023 0.51 f ( 0.07 0.53 f ( 0.07 0.19d ( 0.017 0.27i ( 0.17 0.24 f ( 0.14

RI (or T/C) 0.067d ( 0.011 0f 6.2 f ( 7.5 0.022h ( 0.013 1.1i ( 2.4 0.063 f ( 0.035

a DPI, C2-dibenzothiophene/C2-phenanthrene; OI, oleanane/C30-hopane; 4-mDBT, 4-methyl-dibenzothiophene; 1-mDBT, 1-methyl-dibenzothiophene; MPI-3, (2-methyl-phenanthrene + 3-methyl-phenanthrene)/(9-methyl-phenanthrene + 1-methyl-phenanthrene); Ts, 18R(H)-22,29,30trisnorhopane (more stable); Tm, 17R(H)-22,29,30-trisnorhopane; RI ≡ T/C (C26 + C27 triaromatic steroids)/1-methylchrysene). b Mostly core bottoms. c n ) 19. d n ) 4. e A black carbonaceous shale interstratified with coal beds. f n ) 2. g n ) 1. h n ) 3. i n ) 10.

FIGURE 6. Johnston Creek sediment and seep oil samples. (A) Stream sediment upstream of Johnston Creek oil seep. (B) Stream sediment ∼1 mi downstream of Johnston Creek oil seep. (C) Difference between panels B and A. (D) Seep oil sample taken at Johnston Creek seep. Fingerprints. A number of the PAH and triaromatic steroids (TAS) in the coal differ substantially from those in PWS sediment (e.g., coal has higher parent/alkylated PAH, high biphenyl concentration, and no TAS) (Figure 4A,B). Furthermore, Bering River coal has negligible biomarker concentrations as compared to PWS marine sediment (Figure 5A,B). Consequently, Bering River coals cannot account for the observed distribution and concentrations of both PAH and chemical biomarkers in PWS sediment. Additionally, the biomarker 28,30-bisnorhopane (28B) found in PWS sediments does not indicate a coal origin because its concentration in area coals is negligible. It is found in Yakataga seep oils and area shales (Figure 5). Fingerprints of Katalla Beach sediment more closely resemble those of Katalla shale than Bering River coals (Figures 4 and 5). This beach sediment contains a mix of hydrocarbons from various sources of which coal is a relatively minor component of each of the five samples collected. The dark appearance of Katalla Beach strand lines results primarily from a high mica content and not coal particles. Typical strand-line sediment scraped from the beach identified by Figure 2 in Short et al. (6) has the following modal mineralogical abundances as determined by petrographic analysis and scanning electron microscopy-energydispersive X-ray (SEM-EDX) particle counts: organic particles ∼1-12%, micas ∼45-50% (dominantly biotite, chlorite, and muscovite), feldspars ∼25-30%, quartz ∼12-15%, and shell fragments ∼1%. These sediments contain 0.12-11.62% TOC and 10-5000 ng/g TPAH. Seep oils (Figure 4E,F) differ from the background, particularly in the lighter, more degradable PAH. However, the heavier PAH and biomarker compounds are relatively resistant to weathering and remain in the environment mixed with hydrocarbons from other sources. Malaspina glacial flour (Figures 4D and 5D), which contains organic particles having a wide range of reflectance values (0.5-6%), provides the closest fingerprint match to PWS sediment. PAH and chemical biomarker fingerprints (e.g., C29-RRR(20R) > C27-RRR(C20R) ≈ C28-RRR(20R)

steranes) of Yakataga Formation shales (Figure 5G-J) are consistent with their being important contributors to the PWS background (Figure 5A). The seep oils have similar C27: C28:C29 sterane distributions (Figure 5E,F); however, these distributions are partially obscured by the dominance of the thermally more stable Rββ steranes, reflecting the higher thermal maturity of the oils with respect to the sampled shales. Analyte Ratios. Source- and maturity-sensitive analyte ratios suggest that the PAH and biomarkers in PWS marine sediment come predominantly from low-to-moderate maturity seep oil residues and eroding shales (Table 1). The PAH ratios 4-mDBT/1-mDBT (20) and MPI-3 (22) and the biomarker ratio Ts/(Ts + Tm) (12) are consistent with lowto-moderate maturity sources. Bering River coals are thermally overmature and consequently exhibit very different values for these diagnostic analyte ratios. Low triaromatic steroids/1-methylchrysene (RI) ratios in PWS benthic sediments are cited as evidence that the hydrocarbon source is coal and not seep oil (6). As the values in Table 1 indicate, coal is only one possible explanation for the low RI values. A number of potential sources from a wide area not sampled by Short et al. (6) have low RI values, including Yakataga oils, some Tertiary shales, and Malaspina glacial flour. Other analyte ratios argue strongly against coal as the source of the background (Table 1). Weathering and Transport. Exposure to wind, water, and microbes changes the composition of bulk oil with time (1, 4, 5). The lighter, more water-soluble components of the oils gradually disappear while heavier, less water-soluble components remain. Hydrocarbon components in kerogen, bitumen, and coal particles are less susceptible to weathering processes, as are hydrocarbons fixed to clay surfaces (23). With some exceptions, the seep oils are already extensively degraded at the surface. Characteristically, the normal alkanes have been completely biodegraded, and the water-soluble PAH (the naphthalenes and parent compounds of fluorene, phenanthrene, and dibenzothiophene) are depleted. Oil VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Sources of petrogenic hydrocarbons in offshore, beach, and stream sediments based on least-squares fit of sediment PAH and biomarker fingerprints. Four categories of source are recognized: coal, seep oil, eroding Tertiary shales, and Copper River sediment. KB, Katalla Beach; MGF, Malaspina Glacial Flour. seeping from wells at Katalla Field is relatively fresh; however, the light ends (C1-C12) have been removed. Comparison of PAH fingerprints of sediments from a seep stream and the seep oil show that the PAH are incorporated into the fine-grained sediment downstream from the seep (Figure 6). When the PAH profile of stream sediment above the Johnston Creek oil seep is subtracted from the PAH profile of sediment deposited 1 mi downstream, the chemical residual closely matches weathered Johnston Creek seep oil. These seep hydrocarbons are subsequently transported to the GOA and then to PWS sorbed to fine SPM rather than as oil droplets or as sheens on top of the water. In PWS benthic sediments, PAH and biomarker concentrations are positively 478

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correlated with the clay-sized fraction (24). That fraction also includes low-to-moderate thermal-maturity organic particles and high-maturity coal particles. The bulk of the PAH and biomarkers are contained in bitumen sorbed to dispersed organic particles and clays and, consequently, do not exhibit solubility distributions. Source Allocation. Some of the results of the least-squares source allocation are shown in Figure 7. The results support the fingerprinting and analyte ratio results discussed above. Noise in the data and a possible incomplete set of source candidates can introduce errors (14). Offshore sediments tend to reflect contributions of nearby streams. Sediment plumes fan out from stream mouths and

are swept in a generally westerly direction near the coast by the Alaska Coastal Current. Stream sediment plumes also appear to extend short distances to the east of the river mouths. Thermally overmature coals, like that from the Bering River, contribute 2-6% of the PAH and biomarkers found in nearshore sediments near the mouths of the Bering and Duktoth Rivers and in and near Icy Bay (Figure 7). In contrast, more than 90% of the PAH and biomarkers in coaly sediment from a Bering River sandbar were found to be from the Bering River coalfield. This is consistent with reflectance measurements that show this particular sample to be mostly mature coal. Less than 15% of Katalla Beach sediment’s petrogenic hydrocarbons were found to be from the Bering River coalfield. Only one of the four PWS offshore sediment samples was determined to contain coal by least squares, and in that one, only about 1% of its PAH and chemical biomarkers came from coal. Contributions of seep oil residues to offshore sediments are greatest near seep streams (Figure 7). Some 10-20% of the PAH and biomarkers in PWS sediments appear to come from the seep oils. These seep oil residues must be in the form of small particles or sorbed to SPM because they are transported hundreds of kilometers before settling to the bottom. The remaining background hydrocarbons appear to be from low-to-high maturity sources such as the Tertiary shales. Combustion products are also expected to be small contributors to the background (15) but were not considered in the present study. Copper River sediment contains low levels of petrogenic hydrocarbons (TPAH = 30 ng/g; TPAH/OC ) 2000-14 000 ng/g OC). As a consequence, this river, although a large supplier of inorganic sediment to PWS (25), contributes only 5-10% of the PAH and biomarkers found in PWS benthic sediments (Figure 7). Winnowing of fine hydrocarbon-bearing particles must occur for the Copper River contribution to PWS to be this large. Long-distance transport is exhibited by glacial flour from Malaspina Glacier outwash sediments. This fine material settles to the seafloor in greatest proportions in Icy Bay, in protected waters west of Kayak Island, and in the deep waters of PWS, where it may contribute some 60-70% of the hydrocarbon background (Figure 7). The near-coast seafloor from Icy Bay to Kayak Island appears to be an erosional area scoured by the coastal current rather than a depositional area for glacial flour. This area is dominated by output from local streams rather than Malaspina glacial flour. Implications for Bioavailability of the PAH. The findings reported here support the hypothesis that the vast majority of petrogenic hydrocarbons that comprise the background in the benthic sediments of PWS, including the PAH and chemical biomarkers, come from seep oil residues and associated organic shales. These PAH are associated with the sediment clay-sized mineral fraction (24) and, consequently, may have limited bioavailability. However, recent studies suggest that sediment-bound PAH can enter the food chain via deposit feeders that solubilize them in their digestive fluids (26). Furthermore, PAH-metabolizing enzyme systems in fish may be activated even when background hydrocarbons appear to be tightly bound in the sediments (27). Consequently, assertions (28) that PAH from eroded organic shales and seep oil residues are not bioavailable are unsubstantiated in the literature.

Acknowledgments We thank R. Sanders for providing Bering River coalfield samples.

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Received for review June 27, 2000. Revised manuscript received November 29, 2000. Accepted November 30, 2000. ES001421J VOL. 35, NO. 3, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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