Historical changes in trace metals and hydrocarbons in nearshore sediments, Alaskan Beaufort Sea, prior and subsequent to petroleum-related industrial development: Part II. Hydrocarbons

Marine Pollution Bulletin 77 (2013) 147–164

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Historical changes in trace metals and hydrocarbons in nearshore sediments, Alaskan Beaufort Sea, prior and subsequent to petroleum-related industrial development: Part II. Hydrocarbons M. Indira Venkatesan a,⇑, A. Sathy Naidu b, Arny L. Blanchard b, Debasmita Misra c, John J. Kelley b a b c

Institute of Geophysics and Planetary Physics, University of California at Los Angeles, 5863 Slichter Hall, Los Angeles, CA 90095, USA Institute of Marine Science, PO Box 757220, University of Alaska Fairbanks, Fairbanks, AK 99775, USA Department of Mining and Geological Engineering, PO Box 755800, College of Engineering and Mines, University of Alaska Fairbanks, Fairbanks, AK 99775, USA

a r t i c l e

i n f o

Keywords: Sediment hydrocarbons Contaminants Beaufort Sea Alaska

a b s t r a c t Composition and concentration of hydrocarbons (normal and isoprenoid alkanes, triterpenoids, steranes, and PAHs) in nearshore surface sediments from Elson Lagoon (EL), Colville Delta–Prudhoe Bay (CDPB) and Beaufort Lagoon (BL), Alaskan Beaufort Sea, were assessed for spatio-temporal variability. Principal component analysis of the molecules/biomarkers concentrations delineated CDPB and BL samples into two groups, and cluster analysis identified three station groups in CDPB. Overall there was no geographic distribution pattern in the groups. The diversities between groups and individual samples are attributed to differences in n-alkanes and PAHs contents, which are influenced predominantly by sediment granulometry and sitespecific fluvial input. The predominant hydrocarbon source is biogenic, mainly terrigenous, with hardly any contribution from natural oil seeps, oil drill effluents and/or refined crude. The terrigenous source is corroborated by d13C, d15N, and OC/N of sediment organic matter. Time interval (1976–1977, 1984 and 1997) changes in hydrocarbon compositions and concentrations in CDPB are not significant. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The Alaskan Arctic has been historically considered a relatively pristine environment free of metal and hydrocarbon contaminants. However, industrial growth has increased in the coastal region of the North Slope of Alaska during the past four decades as a result of exploration and development of onshore and offshore petroleum reserves (NAS, 2003). Concurrently urbanization of the villages along the coast (e.g., Barrow, Kaktovik) has increased. Long-range transport of anthropogenic contaminants [i.e., trace metals, organochlorines, polycyclic aromatic hydrocarbons (PAHs)] from Eurasia over the arctic region including Arctic Ocean via vapor phase or aerosols has been reported in recent decades (Welch et al., 1991; Barrie et al., 1992; Gubala et al., 1995; Chernyak et al., 1996; AMAP, 1997; Valette-Silver et al., 1999; Macdonald et al., 2000). Additional environmental changes are expected in the near future. For example, in the 1.5 million acres of coastal plain (titled the 1002 area) of the Arctic National Wildlife Refuge (ANWR)

DOI of original article: http://dx.doi.org/10.1016/j.marpolbul.2012.07.037

⇑ Corresponding author. Tel.: +1 310 473 9441; fax: +1 310 206 3051.

E-mail addresses: [email protected], [email protected] (M.I. Venkatesan), [email protected] (A.S. Naidu), [email protected] (A.L. Blanchard), debu. [email protected] (D. Misra), [email protected] (J.J. Kelley). 0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.10.012

which has potential petroleum reserves, future oil drilling activities could contaminate the adjacent nearshore region with industrial trace metals and hydrocarbons. A related concern especially is that the lipid-rich food webs in the fragile arctic ecosystem could efficiently biomagnify contaminants in the region (Johansen et al., 2000; Chapman and Riddle, 2005) with deleterious implication on subsistence food source (Douglas et al., 2002). Several investigations were initiated in Arctic Alaska to provide baselines on the composition and concentration of hydrocarbons in the nearshore (Shaw et al., 1979; Venkatesan et al., 1981; Venkatesan and Kaplan, 1982; Boehm et al., 1987; Steinhauer and Boehm, 1992; Naidu et al., 2001, 2003, 2006; Macdonald et al., 2004; Brown et al., 2005, 2010). The emphasis was on sediments because they serve as potential sink and/or source of particle-reactive contaminants including hydrocarbons (Valette-Silver et al., 1999; Lee and Wiberg, 2002; Currie and Isaacs, 2005). The ability of Alaskan arctic nearshore sediments to sequester hydrocarbons, by adsorption, was demonstrated empirically by Terschalk et al. (2004). However, all of the above studies were limited to site-specific areas and little effort was made to draw correlations between the content of hydrocarbons and associated sediment parameters [(i.e., grain size, organic matter and their C/N ratios, and carbon and nitrogen isotopes (d13C and d15N respectively)]. An integration and statistical analysis of the available data will improve our

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understanding of the factors that control the regional differences in sediment hydrocarbons and their potential sources (natural and anthropogenic, terrigenous versus marine). This paper presents a synthesis of the hydrocarbons data [normal and isoprenoid alkanes, triterpenoids, steranes, and polycyclic aromatic hydrocarbons (PAHs)] on nearshore surface sediment samples from the North Slope of Arctic Alaska for 1977–2010. An emphasis is placed here on unpublished data sets (Naidu et al., 2001, 2003, 2006) with particular reference to the Colville Delta–Prudhoe Bay (CDPB) region where longer time-interval information is available. Statistical analysis was directed to identify regional differences in the hydrocarbon distribution pattern(s) and to deduce the possible sources of hydrocarbons in the nearshore sediments. The composition of d13C and d15N in organic matter of selected sediment samples are discussed to supplement the inference on the hydrocarbon sources. This paper constitutes part II of a two-part series of studies; a detailed treatise of sources and dynamics of trace metals in the three study areas from the same samples considered in the current report was published as part I (Naidu et al., 2012). These syntheses will provide regional database to the ongoing studies in nearshore region by the Alaska Monitoring and Assessment Program (AKMAP, 2005) on current status, trends, and changes in chemical contaminants. 1.1. Area of study and environmental setting The study area extends from the North Slope shore of the Alaskan Beaufort Sea to about 30 km seaward including the US Federal Outer Continental Shelf (OCS) zone. The study was focused on three disjointed nearshore regions in the North Slope (Fig. 1). These regions were selected as they have been exposed to different ongoing or past anthropogenic activities. For example, the Colville Delta–Prudhoe Bay (CDPB, Fig. 2a) is currently exposed to petroleum related industrialization (currently operating, future oil prospects

and slated oil lease sale sites) and Elson Lagoon (EL, Fig. 2b) to municipal and local community recreational and subsistence activities. The Beaufort Lagoon (BL, Fig. 2c) was exposed to now defunct Distant Early Warning (DEW) line military station’s operations. Natural oil seeps adjacent to Beaufort Lagoon may be additional source of hydrocarbons to that area. The regions studied are sheltered by the mainland coast and chains of barrier islands and are thus, located in an environment generally devoid of intense wave-current and tidal (amplitude 16 cm) turbulence (Barnes et al., 1984; Naidu et al., 1984). Wide seasonal disparity occurs in the cryogenic-dominated environment. Extreme frigid winter conditions (40 to 46 °C) extend for eight to nine months (mid October–mid June), with sea-ice or shore-fast ice covering the Beaufort Sea. At this time sea floor reworking by ice gouging is pervasive. In spring (mid June), the nearshore region is exposed to the unique phenomenon of turbid fluvial overflow on sea ice extending up to 10 km offshore. Spring primary production (<10 g C m2 y1) is dominated by sea ice algae (Gradinger, 2009). In the summer (July–August) open water season, water column primary production increases to 10–20 g C m2 y1. Intense wave action during occasional summer storms results in sediment resuspension and coastal erosion 1–10 m y1. Presence of mixed sediment types (poorly sorted gravelly sand to sandy mud) is typical and organic matter is predominantly terrigenous (Naidu et al., 2000; Macdonald et al., 2004).

2. Material and methods 2.1. Samples and database Sediments were collected by van Veen grab sampler or by Haps corer with a stainless steel barrel. The upper 2-cm oxidized surficial sediment layer was subsampled and placed in a pre-baked

Fig. 1. Area of study: Elson Lagoon, Colville Delta–Prudhoe Bay, and Beaufort Lagoon from North Slope of Arctic Alaska.

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Fig. 2. Locations of sediment samples collected from (a) Colville Delta–Prudhoe Bay (CDPB in 1997), (b) Elson Lagoon (EL) in 1999, and (c) Beaufort Lagoon (BL) in 2003.

glass jar, closed by a cap lined by aluminum foil and stored frozen until analysis. Figs. 1 and 2 show station locations. The individual regions with their acronym, year of sampling in 1990s suffixed to the acronym, the number (n) of samples collected in each region, and the sources of the database for hydrocarbons analyzed in gross sediment are as follows: Colville Delta–Prudhoe Bay (CDPB97, n = 39, including a duplicate sample from one station and triplicate samples from 9 stations (Naidu et al., 2001); Elson Lagoon (EL99, n = 3, Naidu et al., 2003) and Beaufort Lagoon (BL03, n = 18, Naidu et al., 2006). Replicate samples were collected approximately within 5 m of each other to test the sampling precision. The natural oil seep sample (OS) was located on the bank of a small unnamed creek opening into south BL (Fig. 2c). The molecular composition

and concentrations of individual hydrocarbon biomarkers, coordinates of sample locations, water depths, and selected sediment parameters (granulometry, organic carbon and organic matter, carbon and nitrogen isotopes ratios, OC/N) for individual samples are posted online in Microsoft Access database (cf. naidu Online database, ftp://ftp.sfos.uaf.edu/naidu/cmistat2007/) at the University of Alaska Fairbanks. Henceforth this database will be referred to as NODA. 2.2. Analytical methods and QA/QC The molecular composition of hydrocarbons that were analyzed consisted of individual alkanes from n-C10 to n-C36, pristane, phytane, PAHs from naphthalene to coronene and some methyl

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homologs, and triterpenoids such as 17a(H), 21b(H)- and 17b(H), 21b(H)-hopanes and diploptene (Table 1). The hydrocarbon analysis on sediment samples was performed by well-established methods (Venkatesan, 1998; Venkatesan and Kaplan, 1982; Venkatesan et al., 1987). Briefly, thawed wet sediments were spiked with appropriate surrogates, extracted with methanol followed by methylene chloride. The combined extract after exchanging into hexane was subjected to silica gel chromatography to separate it into aliphatic, aromatic, and polar fractions. Aliphatic and aromatic fractions were analyzed by gas chromatography (GC) and/or gas chromatography/mass spectrometry (GC/ MS). A natural oil seep sample from Beaufort Lagoon (BL03-OS in Fig. 2c) was subjected to similar analysis as sediments to compare its hydrocarbon content and molecular fingerprint with those of the lagoon sediments. A crude oil sample, ‘C’, transported by the Trans Alaska Pipeline Service (TAPS) from Prudhoe Bay, was qualitatively analyzed by GC and GC/MS to obtain alkanes, triterpanes and steranes profiles, to compare with those of CDPB sediments. Due to community recreational activities and possible sewage contamination resulting in Elson Lagoon, aromatic and polar fractions Table 1 List of hydrocarbon variables and their abbreviations used in text and in PCA for sediments in the study area. Compound/parameter P Sum n-alkanes ( C10–C36) P Lower molecular weight n-alkanes ( C12–C19) P Sum n-alkanes ( C15–C23) P Odd carbon n-alkanes ( C15–C33)/even carbon n-alkanes P ( C16–C34) 24 n-alkane 25 n-alkane 26 n-alkane 27 n-alkane 28 n-alkane 29 n-alkane 30 n-alkane 31 n-alkane 32 n-alkane 33 n-alkane Pristane Phytane 27a hopane 27b hopane 29ab hopane 30ab hopane 30bb hopane Diploptene Sum PAHs: parent and methylated PAHs from naphthalene to coronene including methylated homologs from 2- to 4,5-ring PAH Naphthalene 1 Methylnaphthalene 2 Methylnaphthalene C1(or monomethyl) naphthalenes C2(or dimethyl) naphthalenes Fluorene Phenanthrene Anthracene C1(monomethyl) phenanthrene/anthracenes C2(or dimethyl) phenanthrene/anthracenes Na + C1–C4 homologs/Pn +C1-C4 homologs Fluoranthene Pyrene C1(monomethyl) fluoranthenes/pyrenes Chrysene/triphenylene Benzanthracene Benzofluoranthenes Benzo(e)pyrene Benzo(a)pyrene Perylene Benzo(ghi)perylene

Abbreviation TALK LALK T1523 Odd/even C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 pr ph 27a 27b 29ab 30ab 30bb dip P PAH

Na 1Na 2Na N1 N2 F1 Pn An P1 P2 N/P Fl Py Fl1 Ch BaA BF BeP BaP Per Bghi

from EL sediments were also analyzed for linear alkylbenzene and fecal sterols after the methods of Venkatesan et al. (2010). Concentrations of hydrocarbons were normalized to sediment organic matter for cluster analysis, perylene vs n-alkane and PCA plots and correlation. Table, whereas only concentrations of total alkanes or PAHs in lg/g are presented in other Tables wherever appropriate and in the plot against mud %. Data from duplicate and triplicate samples were comparable and within acceptable, 20%, range and therefore were averaged to reflect hydrocarbon content from a given station. Quality assurance/quality control (QA/QC) of the analytical methods were validated by round-robin inter-laboratory calibration exercises, NOAA/11, conducted in 1997 by the National Research Council of Canada (NRC) for the NOAA Status and Trends (NOAA/NS&T) and in 1999 by NOAA programs. Concentrations of all the target analytes reported from UCLA were well within the consensus values generated from over 40 participating laboratories (except only for acenaphthylene which was slightly lower than the accepted value) (NOAA, 2000). Stringent protocols of QA/QC requirements were followed by analyzing certified NIST reference sediment standard SRM 1941a for ensuring analytical accuracy, running duplicate analysis of selected samples for determining analytical precision, and including matrix spike and procedure blanks with every batch of field samples. To relate the sources (terrigenous versus marine) of total organic matter (OM) to the hydrocarbon analytes, the concentrations of organic carbon (OC) and nitrogen (N) and their stable isotope ratios (d13C and d15N respectively) in OM were determined on acid-treated carbonate-free sediments, following the methods outlined in Naidu et al. (2000) and using a Thermo-Finnigan Model Delta Plus XP isotope ratio mass spectrometer (IRMS). The d13C (‰) values are referenced to the V-PDB standard, and the d15N (‰), to air standard. The standard error of the isotope analysis is ±0.2‰. The content of OM in the nearshore sediments was computed by OC  1.8 (Trask, 1939), and the OC/N ratio is based on the elements0 weight percent. 2.3. Statistical analyses The statistical analysis [univariate and multivariate statistics, i.e., principal component analysis (PCA) and cluster analysis] was restricted to the concentration of major hydrocarbon homologs/ biomarkers and selected ratios (Table 1). PCA was conducted on the concentration data from the sediments in CDPB, El and BL to determine their net regional differences and spatial trends among stations. PCA was also used to distinguish the extent of natural and anthropogenic sources of hydrocarbon input (Yunker et al., 1991; Yunker and Macdonald, 2003). Cluster analysis on hydrocarbon concentrations was performed to discriminate possible station groups among the CDPB stations. Concentration data of hydrocarbon homologs were ln(X + 1) transformed following normalization to percent organic matter prior to multivariate analyses. The value ‘‘1’’ was added due to some zero values in the data set. Statistical analyses were performed using the statistical programs R (www.R-project.org) and Statistica (http://www.statsoft.com/#). Similar statistical approaches have been applied to environmental monitoring studies in Arctic and Subarctic Alaska (Naidu et al., 1997; Blanchard et al., 2002) and in hydrocarbon studies (i.e., Yunker et al., 1991; Yunker and Macdonald, 2003). Pearson’s correlation coefficient analysis was used for determining linear relationships between the various parameters (Townend, 2002). 3. Results Abbreviations of hydrocarbon homologs/biomarkers and other parameters discussed in this paper are qualified in Table 1. Table 2

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Table 2 Range and mean ± standard error of concentrations (lg/g) and ratios of selected hydrocarbons in sediments of Elson Lagoon (EL), Colville Delta–Prudhoe Bay (CDPB), Beaufort Lagoon (BL) and a natural oil seep (BLO3-OS) from SW Beaufort Lagoon. Source: Naidu OnlineData (NODA). N

Year sampled

Elson Lagoon (EL) 3 1999

TALK

LALK

Pristane

Phytane

LALK/TALK

Odd/even

P

14.93 ± 20.17 (0.80–39.00)

1.08 ± 1.40 (0.07–2.78)

0.06 ± 0.07 (<0.01–0.14)

0.03 ± 0.03 (<0.01–0.07)

0.07 ± 0.07 (0.07–0.10)

2.63 ± 0.61 (1.90–3.00)

0.35 ± 0.36 (0.01–1.22)

0.03 ± 0.03 (0.001–0.099)

0.02 ± 0.10 (0.001–0.043)

0.20 ± 0.09 (0.02–0.43)

0.53 ± 0.53 (<0.01–2.50)

0.01 ± 0.01 (nd-0.05)

nd (nd-0.02)

956.60

nd

nd

Colville Delta–Prudhoe Bay (CDPB) 21 1997 2.37 ± 2.83 (0.20–11.80) Beaufort Lagoon (BL) 18 2003

7.23 ± 7.21 (0.14–33.80)

NATURAL OIL SEEP (BLO3-OS) 1 2003 6819.40

PAH

N/P

Mud %

1.30 ± 1.70 (0.02–3.20)

1.03 ± 0.97 (0.00–1.93)

30.17 ± 20.14

3.10 ± 0.70 (1.17–4.25)

0.64 ± 0.66 (0.02–1.96)

0.87 ± 0.35 (0.25–1.64)

41.43 ± 34.59

0.07 ± 0.07 (0.05–0.11)

5.14 ± 1.04 (3.50–8.10)

0.35 ± 0.21 (0.03–0.70)

0.44 ± 0.47 (0.03–0.90)

47.49 ± 36.00

0.14

1.30

207.90

7.62

–

For explanation of abbreviations, refer to Table 1; nd: not detected.

Table 3 Comparison of time-interval means ± standard error of concentrations (lg/g) and selected ratios of hydrocarbons in sediments from different CDPB sectors. Samples collected in 1984–1986 (Boehm et al., 1987), in 1997 (this study); in 2000 & 2002 (Brown et al., 2005; and in 2004–2006 (Brown et al., 2010). P Region (stations) TALK LALK Pristane Phytane LALK/ PAH N/P Mud % Year TALK sampled

a

Camden Bay area: 1D, 2E, 2F

2.20 ± 1.11 1.20 ± 0.97 –

0.35 ± 0.13 0.21 ± 0.21 –

0.02 ± 0.01 0.02 ± 0.02 –

0.01 ± 0.00 0.007 ± 0.006 –

0.16 0.18 –

0.03 ± 0.02 0.36 ± 0.28 0.13 ± 0.13

1.00 1.00 –

23.33 ± 7.31 14.13 ± 14.35 12.60 ± 7.10

1984–86 1997 2005

Foggy Island Bay area: 3A, 3B, 4A, 5G

3.53 ± 1.57 2.58 ± 1.25 – – –

0.58 ± 0.20 0.58 ± 0.33 – – –

0.04 ± 0.01 0.05 ± 0.02 – – –

0.02 ± 0.00 0.02 ± 0.01 – – –

0.16 0.21 – – –

0.05 ± 0.02 0.74 ± 0.43 0.43 ± 0.17 0.43 ± 0.07 0.58 ± 0.25

1.00 0.80 – – –

38.00 ± 22.70 55.74 ± 28.91 76.00 ± 7.94 55.60 ± 41.64 60.30 ± 20.90

1984–86 1997 2000 2002 2004–2006

Prudhoe Bay: WPBa, 5(1), 5(2), 5(5), 5(10)

1.99 ± 1.06 0.62 ± 0.45 – – –

0.37 ± 0.14 0.10 ± 0.08 – – –

0.02 ± 0.01 0.01 ± 0.01 – – –

0.01 ± 0.01 0.005 ± 0.003 – – –

0.19 0.16 – – –

0.05 ± 0.01 0.11 ± 0.09 0.41 ± 0.12 0.14 ± 0.14 0.23 ± 0.15

0.90 0.50 – – –

15.33 ± 10.69 10.45 ± 7.97 31.00 ± 2.00 21.67 ± 19.00 23.90 ± 14.00

1984–86 1997 2000 2002 2004–2006

Kuparuk River Bay area: 5A, 5F

6.00 ± 1.70 4.70 ± 3.40 – – –

0.87 ± 0.13 0.68 ± 0.49 – – –

0.06 ± 0.01 0.04 ± 0.03 – – –

0.03 ± 0.00 0.02 ± 0.01 – – –

0.15 0.14 – – –

0.13 ± 0.00 1.16 ± 1.12 1.10 ± 1.00 0.59 ± 0.31 0.57 ± 0.13

1.40 0.90 – – –

52.50 ± 0.71 71.05 ± 28.88 67.00 ± 25.45 81.50 ± 14.85 60.7 ± 21.4

1984–86 1997 2000 2002 2004, 2006

East Harrison Bay area: SLa, 6A, 6B, 6C, 6D, 6G

10.30 ± 6.40 4.74 ± 4.58 –

1.38 ± 0.72 0.55 ± 0.47 –

0.11 ± 0.07 0.05 ± 0.04 –

0.05 ± 0.03 0.02 ± 0.02 –

0.13 0.15 –

0.25 ± 0.22 1.04 ± 0.73 1.24 ± 0.27

1.90 1.5 –

56.40 ± 30.66 53,51 ± 34.49 61.3 ± 21.2

1984–86 1997 2006

Not sampled by Boehm et al. (1987); For explanation of abbreviations, refer to Table 1.

Table 4 Correlation coefficients for alkanes, PAHs, total organic matter (OM) and grain size parameters of sediments from Colville Delta–Prudhoe Bay (CDPB) (N = 21 except for OM where N = 19; only significant correlations (P < 0.005 are shown). P P P Depth (m) TALK C12–C19 C20–C33 Pr/Ph Odd/even 4,5 PAH PAH OM % Gr % Sd % St % Cl % Mud % Depth (m) TALK P C12–C19 P C20–C33 Pr/Ph Odd/even 4,5 PAH P PAH OM % Gr % Sd % St % Cl % Mud %

1.00 1.00 0.93 0.96 0.49 0.49 0.92 0.88

1.00 0.87 0.57 0.47 0.91 0.90

1.00 0.47 0.87 0.76

1.00 0.18 0.54 0.50

1.00 1.00 0.95

1.00 1.00 1.00

0.75 0.71 0.53 0.77

0.90 0.69 0.68 0.84

For explanation of abbreviations, refer to Table 1.

0.66 0.65 0.45 0.68

0.56 0.45 0.47

0.47 0.64 0.54

0.80 0.62 0.74 0.86

0.84 0.66 0.74 0.86

0.48

1.00 0.77 0.74 0.93

1.00 0.83

1.00 0.80

1.00

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Table 5 Comparison of time interval mean ± standard error and range of concentrations (lg/g) and selected ratios of hydrocarbons in sediments from the CDPB region. Year of sampling and data source are referred to respectively as follows, 1976: Venkatesan and Kaplan, 1982; 1977: Shaw et al., 1979; 1984–1986: Boehm et al., 1987; 1997: NODA; Brown et al., 2005; 2010. P N Year sampled TALK LALK Pristane Phytane LALK/TALK Odd/even PAH N/P Mud % Colville Delta–Prudhoe Bay (CDPB) 11 1976 (1.40–5.00) 20 1977 (0.10–11.60) 17 1984–1986 5.19 ± 4.99 (0.78–14.00) 21 1997 2.37 ± 2.83 (0.20–11.80) 8 2000 – 8 2002 – 15 2004–2006 –

– – 0.77 ± 0.59 (0.57–2.50) 0.35 ± 0.36 (0.01–1.22) – – –

– – 0.06 ± 0.05 (0.01–0.23) 0.03 ± 0.03 (0.001–0.10) – – –

– – 0.03 ± 0.02 (0.01–0.06) 0.02 ± 0.10 (0.001–0.043) – – –

– – 0.15 ± 0.12 – 0.20 ± 0.09 (0.02–0.43) – – –

(1.80–5.00) – – – 3.10 ± 0.70 (1.17–4.25) – – –

(0.20–0.30) – 0.13 ± 0.15 (0.01–0.64) 0.64 ± 0.66 (0.02–1.96) 0.59 ± 0.50 0.40 ± 0.24 0.53 ± 0.41

– – 1.55 ± 2.29 – 0.87 ± 0.35 (0.25–1.64) – – –

– – 38.18 ± 25.18 – 41.43 ± 34.59 – 56.88 ± 24.18 48.48 ± 34.52 43.00 ± 27.50

For explanation of abbreviations, refer to Table 1.

compares mean concentrations of selected hydrocarbons and ratios for sediments from EL, BL and CDPB and for the natural oil seep sample, BL03-OS, from BL. Detailed molecular compositions and concentrations of individual hydrocarbon biomarkers in sediments from this study are available at NODA. The concentration of TALK (total n-alkanes, sum: n-C10 to n-C36) ranges from 0.14 to 39 lg/ g in the sediments (Table 2). The EL sediments have the highest mean value (14.93 lg/g) between the three regions. The levels of RPAH (total polycyclic aromatic hydrocarbons) in sediments of the study area range from 0.02 to 3.20 lg/g, with EL stations again containing the maximum mean value (1.3 lg/g) compared to those of sediments in CDPB and BL. Table 3 shows the time-interval variations in the concentrations of hydrocarbons and their ratios in sediments from five sectors in CDPB for two sampling time-intervals [1984–1986, after Boehm et al. (1987) and 1997 (this study)]. Data on RPAH which were available for the region for additional three time-periods [2000, 2002 and 2004–2006, after Brown et al. (2005, 2010)] expanded the above comparison. The CDPB sectors originally delineated by Boehm et al. (1987) presumably represent sub-regions that are exposed to different intensities of petroleum-related industrial activities and/or fluvial sediment input. Correlation coefficients between TALK, RPAH, selected hydrocarbon variables and their ratios, total organic matter (OM) and granulometry (Gr: gravel, Sd: sand, St: silt, Cl: clay, Mud: silt + clay) in sediments of CDPB region are presented in Table 4. The data show that concentrations of the hydrocarbon homologs co-vary with the silt, clay or mud contents. The time-interval mean concentrations of hydrocarbons and their ratios in sediments from the entire CDPB are presented in Table 5. The earliest data reported by Shaw et al. (1979) and Venkatesan and Kaplan (1982) on sediments collected in 1976 and 1977 respectively from the nearshore adjacent to the west margin of CDPB are also included to identify any historical changes in hydrocarbons in recent decades. The comparison shows that the hydrocarbon data have not significantly varied over the decades. In Table 6 are included the ranges and means of d13C, d15N and OC/N of OM in sediments of the CDPB, as well as the ratios of potential end-members sources for OM for the study area. The mean values in the CDPB and BL, respectively, of OC/N are 10.7 ± 2.6 and 11.0 ± 0.9, d13C are 25.8 ± 0.5‰ and 26.7 ± 0.6‰, and d15N are 3.8 ± 1.7‰ and 2.1 ± 0.7‰. Gas chromatographic traces of n-alkanes from two selected samples from CDPB are presented in Fig. 3 illustrating as typical examples of the alkane profiles. The alkane profiles of the sediments from all three regions exhibit nearly a baseline separation of the components, characteristic of pristine environment. In Fig. 4 are displayed the triterpenoid distribution profiles from two CDPB sediment samples [one typical of pristine sediments (Fig. 4a) and one of the two samples showing weathered petroleum

Table 6 Ranges and means of OC/N, d13C and d15N in Sediment OM of Colville Delta–Prudhoe Bay (CDPB) and Beaufort Lagoon (BL). The ratios of primary end member sources of OM applicable to Study area are included for comparison. Study region a

CDPB

N

10.7 ± 2.6 (7.3–17.7) BLa 20 11.0 ± 0.9 (8.6–12.3) End member primary sources of OM Terrestrial (C3 Plants) 14b (13–17) Marine phytoplankton 6–7c Sea-ice algae – Tundra peat 19e a b c d e

18

OC/N

d13C (‰)

d15N (‰)

25.8 ± 0.5 (26.9 to 25.3) -26.7 ± 0.6 (27.5 to 25.0)

3.8 ± 1.7 (1.8–8.4) 2.1 ± 0.7 (0.8–3.4)

27.7 to 26.9a 24.0a 15.0 to 8.0e 28.4e

1.4–2.1a 6.0d – –

NODA. Naidu, (1985). Stein and Macdonald (2004). Schell et al. (1998). Naidu, unpublished.

characteristics (Fig. 4b)], and from Prudhoe Bay crude oil ‘C’ (Fig. 4c) for comparison. The distribution was based on single ion monitoring at m/z 191 by GC/MS. Consistent with the n-alkanes profiles, the distributions of triterpanes (i.e., 27(17b)-, 29bb-, 29ba-, 30bb-hopanes), and also diploptene (Table 1) which were identified in sediments from all the three regions reflect their origin in biogenic precursors. The relative abundances of parent naphthalene (NAP) and phenanthrene/anthracene (P/A) and their methylated (C1–C4) homologs measured from GC/MS analysis are illustrated by representative CDPB sediments in Fig. 5. Clearly there is a dominance of C2-naphthalenes and C1- or C2-phenanthrenes. Plots of perylene against C29 n-alkane (a proxy for typical terrigenous vascular plant source) in Fig. 6 demonstrate a strong correlation (r = 0.75). The dendrogram displayed in Fig. 7, from the cluster analysis of ln-transformed concentrations of all major hydrocarbon homologs, shows that there are three station clusters in CDPB (Fig. 7a); however, no coherent distributional pattern in geographic clustering of stations is obvious within the CDPB (Fig. 7b). Plots between total nalkanes or PAHs versus mud % for sediment from all the three regions combined (Fig. 8a and d), and individually for CDPB (Fig. 8b and e) or BL (Fig. 8c and F) show statistically significant (P < 0.005) covariances. However, relative differences exist in the correlation levels between the different binary plots. Box (whisker) plots (Fig. 9) highlight the relative differences in the concentrations of representative hydrocarbons between the three study regions (Cf: Table 2). Hydrocarbon variables (Table 1) from all three regions (CDPB, BL, and EL) were used in PCA for source correlation and to assess

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Fig. 3. Representative gas chromatograms of the saturated hydrocarbon fraction from pristine sediments, 6D1 and 5(5)/1 from CDPB. GC response in mVolts plotted against carbon number of n-alkanes. Arabic numeral adjacent to the GC peak represents carbon number of n-alkane.

temporal changes in the study area. As hopanes were only qualitatively identified in EL sediments because of limited number of samples, two sets of PCAs for the loading of hydrocarbon variables and scores plots were constructed. One including EL, but excluding hopane values (Fig. 10a and b); the other (Fig. 11a and b), including hopanes but restricted to BL and CDPB samples. The PCA loadings of PAHs for all three regions are shown in Fig. 12a and b.

4. Discussion 4.1. Composition and sources of sediment hydrocarbons The n-alkane distribution in the sediments is bimodal, with a major maximum centered at n-C27, n-C29, or n-C31 and a secondary maximum at n-C17 (Fig. 3) with no measurable unresolved complex mixture (UCM, characteristic of weathered petroleum, Yunker et al., 2011, 2012). Two GC traces of the sediments (Fig. 3) represent typical alkane profiles of pristine sediments in the three study areas. The predominance of normal alkanes >C25 in all the sediment samples and the ratio of C20 to C33 odd/even carbon alkanes spanning the range 1.2–8.1 (Table 2) reflect major inputs of hydrocarbons in the sediments from higher (vascular) plants (Wakeham and Carpenter, 1976; Simoneit, 1985; Prahl and Muehlhausen,

Fig. 4. Representative triterpenoid distribution as determined by SIM of m/z 191 from pristine sediments (i.e., 5(10)/3) and one of the two sediments exhibiting weathered petroleum input (5(10)/1) from CDPB. Oil ‘C’ is from Beaufort Sea region from undisclosed location.

1989). A similar dominance of terrigenous n-alkanes has also been reported in the sediments of the entire Arctic ocean (Yunker et al., 2011). However, Macdonald et al. (2004) earlier found that the Alaskan shelf contains much lower concentrations of the high molecular weight homologs than Mackenzie shelf. Further, the secondary maximum recorded at n-C17 (Fig. 3) is characteristic of aquatic algae (e.g., Meyers and Ishiwatari, 1993). The C20 and C21 olefins detected are presumably derived from plankton and bacteria, and a third minor maximum observed at n-C23 in some samples (i.e., EL sediments, Naidu et al., 2003) reflects alkanes typically derived from microbial degradation (Han and Calvin, 1969). A relatively less odd/even predominance in the LALK composition was noticed in almost all the samples. The alkanes data in this study (Tables 2, 3 and 5) are comparable to those reported earlier on pristine sediments collected from the same general region in 1976 (Venkatesan and Kaplan, 1982), in 1977 (Shaw et al., 1979) and in 1984–1986 (Steinhauer and Boehm, 1992) from the same

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Fig. 5. Relative abundance of parent naphthalene (NAP) and parent phenanthrene/anthracene (P/A) and their C1–C4 methylated homologs in CDPB sediments.

Fig. 6. Scatter plot of perylene concentrations against C29 n-alkane. Data were ln-transformed following normalization to percent organic matter.

stations occupied subsequently by us. However, there was at least one sample, 1D/2 from the Camden Bay within CDPB (Fig. 2) which did not show odd/even predominance in the entire carbon range and the GC of its aliphatic fraction implied possible fresh petroleum input (Peters and Moldowan, 1993). Two other samples, 5(1)/1 and 5(10)/1 from Prudhoe Bay in CDPB (Fig. 2) exhibited an UCM in the GC spectrum suggesting input from weathered crude oil (Simoneit and Kaplan, 1980). The predominant hopanoid in our samples is diploptene followed by 29bb-hopane which are biogenic triterpanes (Simoneit and Kaplan, 1980). Thermally mature 29ab- and 30ab-hopanes which are present in the Prudhoe Bay crude oil ‘C’ (Fig. 4) which have also been identified in other Alaskan crude oils by Lillis et al. (1999) are found in a few sediment samples in trace amounts relative to the homologous bb-biogenic hopanes. Further, unlike in the oil sample, extended hopanes with >C31 are either not detected or only their R isomer is detected in trace quantities in the sediments. These compositions confirm that the hydrocarbons in the sediment samples are thermally immature (Simoneit and Kaplan, 1980; Philp, 1985; Peters and Moldowan, 1993) and, therefore, signify a hydrocarbon source other than petroleum. Steranes which

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Fig. 7. Dendrogram showing station groups formed by group averaging cluster analysis of sediment hydrocarbon concentrations in CDPB samples. Figure (b) shows the distribution of the three cluster groups.

are either absent or occur only in trace amounts and about 10 times lower than triterpanes in all sediments we investigated corroborate the absence of petroleum input (Philp, 1985). Clearly the cyclic compounds in the sediments are biogenic and very different from those of Prudhoe Bay crude or the natural oil seep, BL03-OS (Fig. 2C) which will be described later. Sample 1D/2 from CDPB (Fig. 2) exhibited only a weak m/z 191 and 217 spectra in the GC/mass chromatogram despite the trace presence of n-alkanes typical of fresh petroleum. However, thermally mature triterpanes and steranes were detected in two samples, 5(1)/1 and 5(10)/1, from CDPB (Fig. 2) which also exhibited UCM in the alkanes profile (5(10)/1, Fig. 4) suggesting possibly trace contamination of petroleum from local oil drilling operations. Neither of the duplicate samples from the same station nor all other samples including from Prudhoe Bay (the area with intense drilling) showed

petroleum characteristics. Unfortunately, onshore samples of potential source components such as indigenous coal or shale were not available from CDPB hinterland region to compare the triterpane fingerprints vis a vis our sediment samples and clarify their precise origin. The PAH composition in the sediments is invariably dominated by monomethyl or dimethyl phenanthrenes (Fig. 5). The general elevated presence of parent and monomethylated PAHs like phenanthrenes, flouranthenes and pyrenes over higher methylated homologs found in the sediment samples suggest products derived from diagenetic inputs of hydrocarbons (Youngblood and Blumer, 1975; Wakeham et al., 1980). The PAH homolog profiles with especially dominant C2-naphthalenes and C1- and C2-phenanthrenes as illustrated in Fig. 5 resemble those of peat samples from the Alaska region (Steinhauer and Boehm, 1992). The likely source for such

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hydrocarbons in our sediment samples is onshore peat, coal, and/or immature shales. Further, the low thermal maturity of the triterpenoids and the absence of extended triterpanes >C33 in almost all samples investigated from the study area exclude a petroleum derivative of PAHs. Although samples 5(1)/1 and 5(10)/1 exhibited UCM in alkane GC and trace amounts of mature triterpanes in their GC/MS spectrum as discussed earlier, their PAH composition is very similar to the PAH profiles of sediments from other stations in this study, implying the possible presence of again only trace/ insignificant quantities of petroleum in these two samples. Compared to lower molecular weight PAHs, four- and five-ring PAHs are minor contributors to the overall PAH composition, while perylene is the most dominant parent PAH in sediments from all the regions investigated here. Large terrigenous particulate inputs in the sediments is also demonstrated by the strong correlation between perylene (PER) and C29 n-alkane, a typical land plant biomarker (Fig. 6). Perylene is naturally formed during early diagenetic processes in the sediments from biologic precursors, while it is found only at trace levels in crude oils (Wakeham et al., 1980; Venkatesan, 1988). Further, Perylene consistently has been found in great abundance in sediments in the nearshore contiguous to our study area (Venkatesan and Kaplan, 1982; Venkatesan et al., 1981; Steinhauer and Boehm, 1992), and in fluvial sediments and peat of the north Alaskan Arctic region (Steinhauer and Boehm, 1992; Brown et al., 2005). For example, Kuparak and Colville river sediments from the North Slope contain PAHs and n-alkanes as high as 2–25 lg/g (Tables 3–2 in Brown et al., 2005). Thus, it would seem organic output from these rivers,

entrained with eroded peat particles, is the most likely main source of terrigenous alkanes and PAHs in the nearshore sediments in our samples and those of others before (i.e., Venkatesan and Kaplan, 1982; Macdonald et al., 2004). Relatively higher levels of TALK, LALK, RPAH and N/P were consistently found in several nearshore sediments from Foggy Island, Kuparak and east Harrison bay areas which lie off Shaviovik, Kuparak and Colville rivers respectively within the CDPB region (Table 3, Fig. 2). Presumably intense (1–10 m y1) thermo-erosion of the permafrost and peat overlain coast results in large depositional flux of peat to the nearshore regions (Naidu et al., 1984; Yunker et al., 1991, 1993; Steinhauer and Boehm, 1992; Macdonald et al., 2004; Jones et al., 2009). The d13C, d15N and OC/N ratios of sediment OM from CDPB and BL (Table 6) also corroborate the predominant terrigenous source of hydrocarbons which constitute an integral part of the total OM. This is consistent with the dominanace of vascular plant n-alkanes in the Arctic sediments in the adjacent regions (Yunker et al., 2003) and the mass balance in the region of particulate organic carbon/OM derived from various sources (Macdonald et al., 2004). Comparing the isotope ratios with those of potential endmembers sources (Table 6) it is asserted that the predominant source of OM is land-derived with relatively less of the marine component. This conclusion is supported by the net strong covariance between n-alkane C29 (a well-known terrestrial biomarker) and OC/N, and the negative correlations between the biomarker and d13C and d15N in CDPB and BL sediments. (Figure not shown). CDPB and BL samples exhibit comparable OC% but 3–4‰ lighter carbon isotope ratios than the shelf sediments in the western edge

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Fig. 9. Box (whisker) plots of selected hydrocarbons from CDPB, EL, and BL. Numbers preceding n-alkane(s) refer to their carbon number. Data were normalized to percent organic matter. The horizontal lines represent the mean, the boxes represent ±1 standard error, and the whiskers represent ±1 standard deviation.

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Fig. 10. Variable projections from PCA of selected hydrocarbons (excluding hopanes) data from CDPB, BL, and EL for 1997–2003 period. Data were ln-transformed following normalization to percent organic matter. Circles or ellipses represent clusters of variables with high correlations. (b) Score plots from PCA. For a list of variables and abbreviations used in PCA refer to Table 1.

of the Alaskan Beaufort Sea studied by Belicka et al. (2009) The total N measured is presumably entirely organic, as insignificant amount of inorganic (adsorbed, nitrate) N is reported for Beaufort Sea nearshore sediments (Stein and Macdonald, 2004). The underlying reasoning in the above inferences is that higher OC/N and lower isotope ratios reflect larger terrigenous particulate input of OM. Results of this study, focused on CDPB and BL sediments, are consistent with the findings of Naidu et al. (2000) on samples from the north Arctic Alaska nearshore. The natural oil seep sample, BL03-OS, contains 6819 lg/g of TALK which on average is 2900 and 900 times higher than in CDPB and BL sediments respectively (Table 2). Additionally, the odd/even ratio of the seep sample is close to 1 which is considerably lower than in CDPB and BL respectively (Table 2). High amounts of n-C10 through n-C15 and significant levels of alkanes from n-C20 to n-C32 over a pronounced UCM hump (spanning nC21 to n-C34) are identified in the seep sample and the most dominant alkane in the sample is n-C11. Further, the ratio of LALK/high molecular weight (C20–C33) n-alkanes is 0.6 (calculated from Table 10, Naidu et al., 2006) which is much higher than the range of ratios in the nearshore sediments (0.05–0.13). This n-alkanes profile and odd/even ratio of 1.3 at the high molecular weight end are typical of hydrocarbons of petroleum derivative (Simoneit and Kaplan, 1980). Total petrogenic triterpanes are also at much elevated concentrations (at lg/g level) in the seep sample; about 1000-fold greater than in the sediments (which have only trace

Fig. 11. Variable projections from PCA of selected hydrocarbons (including hopanes) data from CDPB, BL, and EL for 1997–2003 period. Data were ln-transformed following normalization to percent organic matter. Hopanes data not available for EL. Circles or ellipses represent clusters of variables with high correlations. (b) Score plots from PCA. For a list of variables and abbreviations used in PCA refer to Table 1.

levels). Additionally, the oil seep sample is composed of a wide suite of thermally mature ab-hopanes with only trace amounts of biogenic bb- and ba-hopanes or hopenes. In the seep sample both S and R diastereomers of the extended hopanes (i.e., >C31-hopanes) are also present and are analogous to the Prudhoe Bay crude oil sample ‘C’ (GC/MS shown in Fig. 4). Similar to triterpanes, all of the target steranes in the seep sample are present at 1000 fold greater than in sediments of our study (Tables 12 and 13 on triterpanes and steranes respectively, Naidu et al., 2006). The hopanes and steranes profiles of the oil seep sample (BL03-OS) are thermally mature (Peters and Moldowan, 1993) and most likely reflect degraded petroleum residue. The natural oil seep sample also contains elevated concentration (208 lg/g) of RPAHs, about 1000 folds greater than the nearshore sediments, (Table 2). Unlike in the latter, naphthalenes are the most dominant PAHs in the seep sample. This is also consistent with the dominance of low molecular weight C10-C15 n-alkanes over high molecular weight analogs in this sample. Further, the seep sample has a high naphthalenes/phenanthrenes (N/P) ratio (7.62) in contrast to the very low N/P range in the sediments (0.44–1.03) (Table 2). Additionally, in the seep sample C2-naphthalenes, C2 and C3-phenanthrenes and C3-chrysenes/triphenylenes were also the most dominant methyl homologs, with no PAH beyond C4-chrysenes/triphenylenes or perylene. The sum of parent PAH and methyl homologs follow the order: naphthalenes o phe-

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Fig. 12. Variable projections from PCA of PAHs data from CDPB, BL, and EL. Data were ln transformed following normalization to percent organic matter. (b) Score plots from PCA. For a list of variables and abbreviations used in PCA refer to Table 1.

nanthrenes  chrysenes/triphenylenes > fluoranthenes/pyrenes (Naidu et al., 2006). The PAH composition of the seep sample is characteristic of weathered petroleum (Youngblood and Blumer, 1975; Peters and Moldowan, 1993). The fingerprints of the alkanes and triterpanes and the molecular composition of PAHs in the BL sediments contrast to those of the natural oil seep sample. Overall petrogenic hydrocarbons from oil seep were undetectable in the lagoon sediments. The similarity of the hydrocarbon profiles from EL, CDPB and BL also suggests that none of the sediments investigated here have detectable influx of hydrocarbons from the area’s natural seeps. We also scrutinized the hydrocarbon composition of the sediments to assess potential anthropogenic contamination from local petroleum drilling activities. The study area has been subjected to oil exploration, with more focused drilling in 1970–1980 (Naidu et al., 2012). Currently water-based mud (WBM) is the only fluid mixture used offshore in the drilling operations in the North Slope region of Beaufort Sea. The WBM contains formulated mixtures of natural clays, organic polymers, weighing agents, and other additives suspended in fresh or salt water (Neff, 2010). Discharge of WBM and associated cuttings and synthetic based drilling mud (SBM) is regulated by the current Beaufort Sea general NPDES permit which forbids discharge of effluents entrained with free oil (Table 4–1 in Neff, 2010). However, cuttings resulting during WBM-based drilling may contain minor amounts of petroleum hydrocarbons originating from spotting fluids or lubricants added to the mud or from geologic strata penetrated by the drill. Current

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Effluent Limitation Guidelines (ELG) for discharge of drilling muds and cuttings to State and Federal waters of USA from offshore oil and gas exploration and development platforms limit their PAH content to be < mg/kg based on phenanthrene/wt of stock base fluid (Neff, 2005). As suggested by the RPAH concentrations which are at or close to baseline values in all the sediments analyzed here (Tables 2, 3 and 5), especially of the CDPB region where intense industrial activities have occurred during the past 40 years, it is believed that little contamination of the sediments has resulted from the drilling effluents. In August 1944 one of the cargo ships (‘‘Liberty’’) carrying fuel oil for the Barrow community got grounded off the Doctor Island at north Elson Lagoon. The gasoline and heating oil in drums were offloaded onshore without any spill. But to lighten the ship 25,000 (82 tons) to 100,000 gallons of Bunker C oil, a heavy oil also known as ‘‘black oil’’, was deliberately discharged into the Beaufort Sea nearshore with a proportion dispersed southward into the Elson Lagoon (Kiley, 1944). Subsequent fate of the spilled oil has remained unresolved, as it is unknown as to what proportion of the original oil has remained buried deep below, biodegraded or was exported out of the lagoon by currents and/or evaporation. Bunker C oil contains hydrocarbons (i.e., alkanes) in the carbon range approximately from C9 to C36. In addition, it also contains highly resistant mono- and triaromatic steranes which have been found to be the dominant PAH even in biodegraded Bunker C fuel in soil extracts (Kaplan et al., 1996). Our analysis did not show even traces of the above resistant aromatic steranes in the three surface samples analyzed from EL which confirms the absence of fuel oil in the these sediments. It is to be expected that any oil deposited in the lagoon will be entombed by a blanket of recent sediments. At the sediment accumulation rate of 0.27 cm y1 at EL (Naidu et al., 2003) the estimated overlying deposit, if undisturbed since the 1944 spill, would be 15 cm, implying that none of the oiled deposits would likely be sampled by the van Veen sampler used in this study. Minor mixing within the oxidized top 1-cm of selected cores was noted in our trace metals study (Naidu et al., 2012), which would entail slight over estimation of the sediment accumulation rate as discussed by Naidu et al. (1999). So the top 1-cm mixed layer could possibly represent a composite sediment deposit spanning 4–5 years time frame rather than an age for the core top assumed to be contemporary to the sampling year but would still imply the absence of oil in the EL surface sediment. This caveat is to be also considered in the discussion on temporal changes in HC in the study region included in Section 4.3. The results of the Principal component analysis (PCA), as discussed in the following, has bolstered the inference of the source of hydrocarbons and helped assess regional differences in hydrocarbon composition in our study area as previously applied to Beaufort Sea shelf and other coastal environments (i.e., Yunker and Macdonald, 2003; Yunker et al., 1991). The loading and score plots on hydrocarbon homologs/biomarkers are illustrated in Fig. 10a and b, Fig. 11a and b). The first two PCs account for a total of > 80% of the original data variance. The higher plant wax n-alkanes, that is, odd carbon from 25 to 33 maximizing at n-C29 or n- C31, are highly correlated with the x-axis in a single cluster and project the strongest positive contribution to the first PC (factor 1); they thus contribute most to sample separation in the xaxis. Ellipses or circles in Figs. 10a and 11a represent clusters of variables with high correlations. Alkanes with even carbon from 24 to 32 contribute less to the first PC than the odd carbon alkanes, as is evident from the factor loadings (circled). Pristane and phytane were present in all the sediments from CDPB, but only in 4 samples out of 20 from BL and 2 out of 3 samples from EL. The ratio of pristane/phytane is uniformly around 2.0 and is delineated in a separate group, contributing only weakly to the second PC.

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Hopanes and diploptene (Fig. 11a) are in the same cluster along with plant wax alkanes, contributing strongly to factor 1, which reflects their biogenic source rather than petrogenic (thermogenic) origin. The PCA data thus corroborates the molecular compositions summarized earlier. These results are also consistent with the observations of a previous study on biomarkers in sediments of the Beaufort Sea, where mostly biogenic (thermally immature) hopanes were detected with a notable absence of petrogenic (thermally mature) hopanes (Venkatesan and Kaplan, 1982). Most of the PAHs form a loose cluster and correlate highly with factor 2 (circled), while BeP, Per, Pn, and BF correlate to a lesser extent, yet still show a relatively stronger contribution to factor 2 (Figs. 10a and 11a). In many samples, anthracene (An) is either absent or detected at trace levels; its remote placement in the PCA does not contribute to the real separation of samples. The first two PCs, amounting to a total of 88%, account for the largest percentage of the total variance in the data set as shown in Fig. 12a and b in the PCA loadings of PAHs and score plots. Monomethylphenanthrenes/anthracenes (P1 or C1-phenanthrenes) are highly correlated with the x-axis and project the strongest positive contribution to the first PC (factor 1); they thus contribute most to sample separation in the x-axis. Monomethylphenanthrenes always dominate among all PAHs except for perylene in some cases. The strong positive loading of monomethylphenanthrenes clearly confirms its major source in the coastal peats (Venkatesan and Kaplan, 1982; Steinhauer and Boehm, 1992). The strong contribution of Per, Pn, and P2 (dimethyl or C2-phenanthrenes) to factor 2 (Fig. 12a) can also be attributed to significant carbon inputs from peat to sediments of the study regions (Venkatesan, 1988) and corroborates the high correlation of perylene content with the terrestrial marker, n-C29 (cf: Fig. 9), as discussed earlier. The remaining components are not well differentiated, although BeP, Na, N1, N2, F, Py, Fl, Fl1, and Ch show moderately strong correlation to factor 1, while F and F1 show a stronger correlation to factor 2. Again, a weak contribution of anthracene to factor 2 and its remote placement do not influence the real separation of samples. A strong loading of BaA, Bghi, and BaP and a moderately strong loading of BF at factor 2 and the general dominance of parent PAHs are characteristic of hydrocarbons derived from combustion (Gshwend and Hites, 1981; Sporstol et al., 1983; Simoneit, 1985; Galperin and Camp, 2002). PAH and organochlorine pollutants could be transported to the North slope coast from Eurasia via long distance atmospheric transport entrained in Arctic haze, snow and ‘brown snow’. Such a transport has been reported for a variety of trace metals (Welch et al., 1991; Barrie et al., 1992; Garbarino et al., 2002). Consistent with this is the documentation by Welch et al. (1991), based on air mass trajectories and soot particle composition, that airborne contaminants are indeed transported from Eurasia to the Canadian Arctic. However, it is not clear from the current hydrocarbon data if there is a decreasing trend in the hydrocarbon levels from west to east in the North Slope nearshore to match with such a transport trajectory. PAH combustion products (perhaps derived from Eurasia and/or from the gas combustion plant at Deadhorse, south of Prudhoe Bay) are at such low concentration levels in the study regions that they do not contribute to significant sample separation in the factor plots. High molecular weight pyrolytic PAHs in the sediments could also derive from local combustion relicts accumulated in the peat (Yunker et al., 1993). Apparently, the overall PAH loadings suggest only a concentration gradient in the PAH input in general. In addition to hydrocarbons, fecal sterols (i.e., coprostanol) and linear alkylbenzenes (LABs) were investigated in EL to look for signatures of anthropogenic wastes. Coprostanol ranged in concentration from 26 to 264 ng/g in the three samples while biogenic sterols like cholesterol and stigmasterol occur at the level of

10–60 times that of coprostanol. The very low relative proportion of coprostanol, which is a well-known sewage marker (Takada and Eganhouse, 1988; Venkatesan and Kaplan, 1990), in the EL samples indicates that sewage input to this region is negligible. This finding is also in accord with LABs which are considered reliable sewage tracers (Eganhouse et al., 1983, 1988). LABs were found below quantifiable limit in the EL sediments and only a qualitative examination by GC/MS confirmed their presence. Elson Lagoon used to be the dumping site for solid human waste for Barrow, the Naval Arctic Research Laboratory (NARL) and the Distance Early Warning (DEW, now defunct) line station up to 1950s. As expected, after >60 years no strong signature of fecal sterols from these anthropogenic inputs are evident in the surface sediments of EL. Presumably, the wastes have been biodegraded and/ or any lingering residues buried below recent sediments. Considering the sediment accumulation rate at EL we assume the overlying deposit since 1950 would be 13 cm, implying that any remnants of the past sewage-laced deposits would likely escape being sampled by us. Consequently, it is reasonable to assume that the more likely source of traces of coprostanol found in EL is from human waste from local recreational and subsistence users and/or marine mammalian feces and from in situ processes from marine precursors such as those reported in prior studies (Venkatesan and Santiago, 1989; Sherblom et al., 1997). In summary, the hydrocarbon molecular markers in surface sediments in all the three regions (EL, CDPB and BL) are compositionally similar and indicate origin from predominantly natural terrestrial source. Generally, the three regions are relatively pristine with no inputs of petroleum hydrocarbons (i.e., natural crude, oil seep and/or refined oil) and other anthropogenic contaminants. 4.2. Regional distribution of hydrocarbons and factors controlling their differences Several of the hydrocarbon components/parameters (TALK, LALK, pristine, RPAH and N/P) in EL sediments are relatively higher than those in CDPB or BL (Table 2, Fig. 9). Abundances of n-alkanes and PAHs follow the order, EL > CDPB > BL, and that of hopanes is BL > CDPB. Since the number of samples analyzed from EL was limited to 3, a comparison of data from EL against those from CDPB and BL seems statistically not meaningful. However, comparison between CDPB and BL samples shows some wide differences. In CDPB the LALK/TALK, RPAH and N/P values are nearly twofold higher than in BL. The abundances of rest of the molecules in CDPB and BL are comparable (Table 2). Further, binary plot between the concentrations of C29 alkane and perylene (Fig. 6) in sediments manifests distinct clusters of plots separated by the three regions. Within the five sectors of the CDPB region there is an apparent geographic dichotomy in the pattern of distribution of several hydrocarbons. The two western-most sectors (Kuparak River and East Harrison Bay) are marked by significantly higher mean concentrations of TALK, LALK and RPAH than in rest of the three eastern sectors (Table 3). This trend is similar to that reported by Boehm et al. (1987) on samples analyzed in 1984–1986. The latter samples were collected from the same locations as ours. Results of cluster analysis (based on the concentrations of major hydrocarbon homologs) have further demonstrated presence of three station clusters in CDPB (Fig. 7a). A few stations off major river mouths fall under Group II (Fig. 7b). However, no coherent geographic segregations of the clusters are obvious probably due to the significant lateral variation in the sediment lithology, a typical feature of the study area. Several possible factors, singly or combined, may have controlled the above geographic variations in sediment hydrocarbons. However, from the discussion in Section 4.1 it is clear that hydrocarbon contamination to CDPB sediments is insignificant

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from natural oil seeps, sewage waste, discharge from oil drilling activities, raw crude or refined oil, and local and long-distance airborne emissions. This is particularly true from the lack of specific enrichment of LALK and RPAH within the CDPB sectors that have been subjected to intense petroleum-related activities recently (e.g., Prudhoe Bay, Oliktok Point, Fig. 7b). The absence of petroleum hydrocarbons in these sediments is consistent with a recent report from Camden Bay where no significant differences were found in concentrations of total PAH and individual PAH (excluding naturally occurring perylene) in the sediments at reference versus several drilling-site stations (Trefry et al., 2013). The spatial differences in hydrocarbons in CDPB, therefore, may be attributed to the regional variations in sediment granulometry and/or sitespecific inputs of hydrocarbons from natural terrestrial sources (i.e., fluvial and coastal erosion). The influence of granulometry on hydrocarbon contents in CDPB is, indeed, demonstrated by the significant correlations (P < 0.005) between the concentrations of LALK, RPAH and other hydrocarbon homologs and the contents of sediment of finer grain size classes (mud, silt and clay) (Table 4). The granulometry, PAHs, and n-alkane covariances are further illustrated in Fig. 8. A relatively moderate correlation (r > 0.44, Fig. 8a and d) of n-alkanes and PAHs with mud percent is exhibited in all three (CDPB, BL and EL) regions combined or BL samples plotted alone (Fig. 8c and f), while a strong association is indicated by the high correlation values (r > 0.72) when only CDPB sediments are plotted (Fig. 8b and e). Site-specific examples substantiating the above is the Harrison Bay–Kuparak Bay from western sector sediment samples collected by us in 1997, which have the highest mud content (mean 62%) and concentration of hydrocarbon (n-alkanes + PAHs) (mean: 5.8 lg/g, recalculated from Table 3.). In contrast, samples from Camden Bay-Foggy Is-Prudhoe Bay sector which have a mean mud content of 27% contain substantially lower hydrocarbon levels (mean = 1.87 lg/g) which is consistent with the results of Boehm et al. (1987) for the same sectors. Sediment granulometry evidently appears to be the predominant factor deciding the concentration of hydrocarbons. The correlations documented by data in Table 4 and Fig. 8 are not surprising given that fine-grained particles (silt, clay), relative to coarser ones (gravel and sand) are invariably enriched in clay minerals. These minerals, by adsorption and organic-clay complex formation, are known to sequester and bind hydrocarbons in their crystal structure (Terschalk et al., 2004, and references therein). This interaction was also substantiated in our study area from the empirical investigations conducted by Terschalk et al. (2004) on representative sediment samples (2F, 5A, 6B, 6D, 6G, and WPB, Fig. 2a) from our sample suite. The other possible explanation for the hydrocarbon-mud relationship is the co-deposition of fine grained sediments and hydrocarbons bound to organic particulate matter because of their similar depositional behavior (or hydraulic equivalence). The three EL samples contain uniformly much higher hydrocarbons than the samples from CDPB and BL as seen in Fig. 9. The scores plot based on the concentrations of all the biomarker components (Fig. 10b) essentially enhances the difference between the EL and other samples. All but one sample from the CDPB cluster in the left half and all but 4 of the BL samples cluster about the right half in the scores plot (Fig. 10b), projecting only a small variation in the second PC (y-axis). However, the three EL samples are clearly differentiated from CDPB and BL, generally exhibiting a narrow range in their alkane contents, especially in the plant wax component, which is dominant in this PC. The large separation reflects the relatively elevated abundance of the higher alkanes in the EL samples. Two of the EL samples are further differentiated, reflecting also their high PAH levels (topmost right quadrant). Fig. 11b (EL excluded for lack of hopanes data) illustrates similar sample

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separation, between CDPB and BL sediments as in Fig. 10b (note the expanded scale of factor 2 in Fig. 11b). In the scores plot of the PAHs, there is little separation between locations except for one sample in CDPB and two samples from EL along the y-axis, reflecting the high concentration of PAHs (Fig. 12b). The third EL sample (EL99-1) has PAHs at a level comparable to those of the CDPB and BL samples (cf. Fig. 12b). The outlier station, 6G, in CDPB has the maximum PAH content of all the samples collected from CDPB and BL. Thus, the PC2 score essentially enhances the difference between the three samples with higher PAH content and all other samples that exhibit only a small variation in PAH in the second PC (y-axis). However, the overall composition of hydrocarbons in the three regions is remarkably similar, despite the fact that each of the three regions has been exposed to different anthropogenic and urban activities of varying intensities concurrent with industrial growth. This confirms our earlier findings that generally hydrocarbon contamination to these sediments are insignificant from natural oil seeps, sewage waste, discharge from oil drilling activities, raw crude or refined oil, and local and long distance airborne emissions. Regional differences in hydrocarbons could, therefore, likely be explained by the site-specific inputs of hydrocarbons from natural terrestrial sources (i.e., fluvial and coastal erosion). As also discussed in Section 4.1 fluvial sources for input of terrestrial molecules (n-alkanes and RPAHs) are highly possible to the stations located off major rivers in the study area (Fig. 7a and b). However, on a regional basis the relative importance of site-specific contribution of coastal erosion and output of hydrocarbons associated with eroded peat to the nearshore is difficult to quantitatively assess. It is because linear rate and volume of coastal erosion varies randomly along the North Slope (Naidu et al., 1984; Macdonald et al., 2004). Also the subsequent lateral dispersal pathway of peat debris by currents and their specific depositional sites are unknown. Nonetheless, tundra peat mats are found randomly deposited throughout the lagoons and strewn along shoreline, suggesting possible intercalation of the peaty debris into sediments. Further, the available data cannot distinguish between the possibly much older terrestrial signal contributed from the erosion of peat beds submerged on the continental shelf 10,000 years ago, subsequent to the sea level rise, and the modern riverine input or coastal erosion. A highlight of our investigation is that there is no net regional gradient in lateral variations in any of the hydrocarbon molecules, though between regions some differences in hydrocarbon concentrations are discerned. 4.3. Comparison of hydrocarbon concentrations in sediments collected over time Historical changes in hydrocarbon concentrations are focused here on sediments from the Colville Delta–Prudhoe Bay (CDPB) region because long-term database is available only for this region within the study area. Time-interval data collected by different investigators for the various sectors within CDPB and also the entire CDPB region are summarized respectively in Tables 3 and 5. It is assumed that these data are intercomparable based on the results of roundrobin inter-laboratory calibration exercises, participated by the investigators, which demonstrated that the hydrocarbon measurements on the same reference sediment samples were within the variations acceptable between different laboratories. The n-alkanes and PAH homologs used for summation as TALK, LALK and RPAH concentrations from our measurements (this study; cf: Table 1) are slightly different from those reported by Boehm et al.(1987) which are compared in Table 3 and 5. In their study, TALK and LALK refer to the sum of the concentrations of C10-C34 and C10–C20 respectively. Further, the concentration of

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PAH in their report included only sum of parent and methylated PAHs from 2- and 3-ring and parent 4,5-ring PAH, whereas the RPAH values from Brown et al. (2005), also included in Table 3 and 5, represent the sum of 2–6 ring parent and methylated PAHs. We consider the differences between the above summations minor and have no consequence in the discussion that follows. A cursory examination of the data in Table 3 and 5 would tentatively suggest some temporal variations in the mean concentrations and ratios of selected hydrocarbons (TALK, LALK, RPAH, N/P). However, on further scrutiny it appears that the temporal differences are presumably statistically insignificant, based on the high standard errors associated with the means of all the components/parameters. The high errors obviously are expressions of the large inter-sample variability in the concentrations of hydrocarbons in the study area, controlled by corresponding wide disparities in sediment granulometry. A proximate deduction, thus, is that the mean concentrations of the suite of hydrocarbons in the five sectors from CDPB (Table 3) shows no major difference over at least a time-interval of 13 years (1984–1997). Also considering the earlier reports to OCSEAP on selected hydrocarbons by Shaw et al. (1979) and Venkatesan and Kaplan (1982) and on recent studies by Brown et al. (2005, 2010), the entire CDPB region (Table 5), during a time span of three decades (1976–2006) appears to have remained pristine, not impacted by natural or anthropogenic petroleum. The outcome of this study was surprising as it ran counter to our expectation that in CDPB increasing hydrocarbon contamination would have progressively occurred since 1976 concomitant with the accelerated levels of petroleum-related activities. Further, we found the sediment hydrocarbon contents in all the three regions (EL, BL and CDPB), are at comparable levels to those found in various other pristine nearshore marine sediments (i.e., Venkatesan et al., 1981; Venkatesan et al., 1987). The findings from the current study run parallel to the results of our investigations on trace metals on sediment samples that were taken from the same stations at the same time as for hydrocarbons analysis, where no major temporal changes in the concentrations of metals were noted (Naidu et al., 2012). Since the sample base is small, constrained by logistics in sampling in remote locations, the conclusions are to be considered with some caution.

5. Conclusions Overall hydrocarbon composition is uniformly similar in the surface sediments of the three nearshore regions (Colville–Delta Prudhoe Bay, Beaufort and Elson Lagoons) from Arctic Alaska, West Beaufort Sea. In the different CDPB sectors there is no significant temporal difference in the mean concentrations of selected n-alkanes, TALK and RPAH in sediments over a duration of recent 13 years (1984–1997) and, in TALK and RPAH in the entire CDPB region for 30 years (1976–2006). Hydrocarbon in the sediments is mainly biogenic and predominantly from natural terrestrial sources. Coastal peat and fluvial sediments appear to be the major sources of the alkanes and PAHs. The PAH profiles possibly reflect only trace inputs, at best, from coal residues and pyrolytic components presumably derived either from Eurasia, local gas combustion plants via airborne transport or those historically concentrated in peat. Anthropogenic as well as natural petroleum components are either absent or present only in trace amounts in the region. That the associated gross organic matter (OM) including intercalated hydrocarbons is largely terrigenous is corroborated by the OC/N, d13C and d15N of sediments. The major conclusion is that the nearshore marine sediments off the North Slope of Alaska have remained relatively pristine, free of petroleum components over the last three decades, in spite of the recent accelerated petroleum-related activities in the region.

We believe that this synthesis of current compositions and concentrations of alkanes and PAHs will provide regional database to the Alaska Monitoring and Assessment Program on current status, trends, and changes in chemical contaminants. Acknowledgments This research was funded to Sathy Naidu (Project PI) by the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE), US Department of the Interior through Cooperative Agreement 1435-01-98-CA-30909 (Task Order 39921) between the BOEMRE Alaska OCS Region and the Coastal Marine Institute (CMI), University of Alaska Fairbanks. Matching funds were provided by the University of Alaska Fairbanks, Institute of Marine Science, and the former School of Mineral Engineering (currently named College of Engineering and Mining), and the Institute of Geophysics and Planetary Physics, University of California, Los Angeles. Thanks are due to Vera Alexander and Mike Castellini, past and present Directors respectively, CMI, and to Ruth Post, Kathy Carter and Sharice Walker of CMI for excellent cooperation throughout the project. Zygmunt Kowalik and John Goering assisted in sample collection, which were conducted onboard the R/V Anna Marie under the Captainship of Bill Koplin. Jawed Hameedi kindly provided the KynarÒ-coated van Veen sampler. Thanks are due to Kate Wedemeyer, Cleve Cowles, Richard Prentki and Heather Crowley of BOEMRE, Anchorage for coordinating and guiding this project. R.A. Perkins is acknowledged for communicating the unpublished data of F.M. Kiley. Laboratory technical assistance of Olivia Merino, Timothy Lin and Joo-Yeul Baek and GC/MS analysis by Edward Ruth are appreciated. This is publication no. 6531 from IGPP, UCLA. California. References Alaska Department of Environmental Monitoring and Assessment Program (AKMAP), 2005. . AMAP (Arctic Monitoring and Assessment Programme), 1997. Arctic Pollution Issues: A State of the Arctic Environment Report. Oslo, Norway, 188 pp. Barnes, P.W., Reimnitz, E., Schell, D.M. (Eds.), 1984. The Alaskan Beaufort Sea: Ecosystem and Environments. Academic Press, New York, p. 466. Barrie, L.A., Gregor, D., Hargrave, B., Lake, R., Muir, D., Shearer, R., Tracey, B., Bidleman, T., 1992. Arctic contaminants: sources, occurrence and pathways. Sci. Total Environ. 122, 1–74. Belicka, L.L., Macdonald, R.W., Harvey, H.R., 2009. Trace elements and molecular markers of organic carbon dynamics along a shelf-basin continuum in sediments of the western Arctic Ocean. Mar. Chem. 115, 72–85. Blanchard, A.L., Feder, H.M., Shaw, D.G., 2002. Long-term investigation of benthic fauna and the influence of treated ballast water disposal in Port Valdez, Alaska. Mar. Pollut. Bull. 44, 367–382. Boehm, P.D., Steinhauer, M., Crecelius, E., Neff, J., Tuckfield, C., 1987. Beaufort Sea Monitoring Program: Analysis of trace metals and hydrocarbons from outer continental shelf (OCS) activities. OCS Study MMS 87-0072. Final Report to MMS, Alaska OCS Region, Anchorage, 264 pp. and appendices. Brown, J., Boehm, P., Cook, L., Trefry, J., Smith, W., 2005. ANIMIDA Task 2: Hydrocarbon and Metal Characterization of Sediments, Bivalves and Amphipods in the ANIMIDA Study Area. OCS Study MMS 2004-024. Final Report to MMS, Alaska OCS Region, Anchorage, 48 pp. Brown, J., Boehm, P., Cook, L., Trefry, J., Smith, W., Durell, G., 2010. ANIMIDA Task 2: Hydrocarbon and Metal Characterization of Sediments in the ANIMIDA Study Area. OCS Study MMS 2010-004. Final Report to MMS, Alaska OCS Region, Anchorage, 235 pp. Chapman, P.M., Riddle, M.J., 2005. Polar marine toxicology – future research needs. Mar. Pollut. Bull. 50, 905–908. Chernyak, S.M., Rice, C.P., McConnell, L.L., 1996. Evidence of currently-used pesticides in air, ice, fog, seawater and surface microlayer in the Bering and Chukchi Seas. Mar. Pollut. Bull. 32, 410–419. Currie, D.R., Isaacs, L.R., 2005. Impact of exploratory offshore drilling on benthic communities in the Minerva gas field, Port Campbell, Australia. Mar. Environ. Res. 59, 217–233. Douglas, D.C., Reynolds, P.E., Rhode, E.B. (Eds.), 2002. Arctic Refuge Coastal Plain Terrestrial Wildlife Research Summaries Rept., USGS/BRD/BSR-2002-0001. US Geol. Survey, Reston, VA, 75 pp.

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