Pelagic tar balls collected in the North Atlantic Ocean and Caribbean Sea from 1988 to 2016 have natural and anthropogenic origins

Marine Pollution Bulletin 137 (2018) 352–359

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Pelagic tar balls collected in the North Atlantic Ocean and Caribbean Sea from 1988 to 2016 have natural and anthropogenic origins

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Hilary S. Greena, Sarah A. Fullerb, Audrey W. Meyerb, Paul S. Joyceb, Christoph Aepplic, Robert K. Nelsona, Robert F. Swarthoutd, David L. Valentinee, Helen K. Whitef, ⁎ Christopher M. Reddya, a

Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Sea Education Association, Woods Hole, MA 02543, USA c Bigelow Laboratory for Ocean Sciences, East Boothbay, ME 04544, USA d Department of Chemistry, Appalachian State University, Boone, NC, USA e Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA f Department of Chemistry, Haverford College, Haverford, PA 19041, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Oil Petroleum Hydrocarbon Gas Chromatography

Tar balls are prevalent in oceans and the coastal environment; however, their origins are not well constrained on a global scale. To address this, we used gas chromatography to analyze the molecular composition of a unique set of 100 pelagic tar balls collected in the Western North Atlantic and Caribbean Sea between 1988 and 2016. Hierarchal cluster analysis (HCA) was employed to classify the samples into groups based on the relative proportions of resolved and unresolved hydrocarbon distributions. Additional analysis of polycyclic aromatic hydrocarbons revealed that 28% of samples originated from heavy fuel oils and therefore had anthropogenic origins consistent with the classifications based on HCA. Other samples examined could originate from anthropogenic or natural origins, such as natural seeps. This study provides a preliminary record of 100 classified pelagic tar ball samples and demonstrates an approach to determine their origin to the environment.

1. Introduction Pelagic tar balls and those found stranded on beaches have been linked to multiple sources including natural seeps (Hostettler et al., 2004; Geyer and Giammona, 1980), accidental releases from the extraction of petroleum (Ekanim et al., 2015), operational discharges from tankers (Butler et al., 1973; Hegazi et al., 2004; Suneel et al., 2013; Lucas and MacGregor, 2006; Golik et al., 1988; Joyce, 1998), tanker spills (NRC, 2003), pipeline releases (United States Coast Guard, 2016; Nelson et al., 2016), coastal facility spills (NRC, 2003), and sunken vessels (Monfils et al., 2006; Hampton et al., 2003). Operational discharges were once considered to be the main origin of tar balls to the ocean (see aforementioned studies), but their overall abundance in the Western North Atlantic, for example, has decreased over the last 30 years (Peters and Siuda, 2014), indicating that the origins of tar balls (and their hydrocarbon composition) should be reexamined. This decrease in anthropogenic tar balls has been attributed to the MARPOL 73/78 Annex I legislation (Joyce, 1998; Peters and Siuda, 2014; Carpenter, 2018). MARPOL 73/78 was created in 1973 and

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amended in 1978 to respond to several tanker accidents and other oilpollution related inputs. This legislation became effective in 1983 with Annex I specifying when and where ships can discharge oil at sea. For example, a tanker cannot have a discharge rate > 30 L per nautical mile or be within 50 nautical miles of land when it discharges oil. Marine environments determined to be at high risk to oil pollution are governed with stricter guidelines and prohibit ships over 400 t from discharging oil mixtures that have oil concentrations > 15 parts per million (ppm). The positive influence MARPOL 73/78 Annex I legislation has had on reducing releases of oil have been documented by several studies and reports (Carpenter, 2018; Peters and Siuda, 2014; Joyce, 1998; Golik et al., 1988). The National Research Council (NRC) estimated that operational discharges were reduced to 36,000 metric tonnes/year in 2003, compared to an estimate of 1,080,000 metric tonnes/year in 1975 (NRC, 2003). Oil inputs into the North Sea have decreased since MARPOL implementation, particularly since it was designated as a special area (Carpenter, 2018). In addition, the decline in the quantity of tar balls released to the oceans is supported by a longitudinal study of

Corresponding author. E-mail address: [email protected] (C.M. Reddy).

https://doi.org/10.1016/j.marpolbul.2018.10.030 Received 23 June 2018; Received in revised form 12 September 2018; Accepted 13 October 2018 0025-326X/ © 2018 Elsevier Ltd. All rights reserved.

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pelagic tar collected in neuston net tows (335 μm mesh net) in the Mediterranean Sea, a MARPOL-designated special area; in this study between 1969 and 1987, pelagic tar decreased from 37 to 1.2 mg m−2 (Golik et al., 1988). In a separate study, pelagic tar balls from 1 to > 10 cm in diameter collected in neuston net tows by Sea Education Association (SEA; Woods Hole, MA) from the western North Atlantic and Caribbean Sea, decreased between 1982 and 1996 from an averaged concentration of > 1 mg m−2 to about 0.25 mg m−2 (Joyce, 1998). A similar deceasing trend in tar was found in a different SEA study in the same geographic region with a decrease in the collection of tar balls from an average of 29 tar balls per tow during 1977 to 1996, to an average of less than three tar balls per tow from 2003 to 2012 (Peters and Siuda, 2014). The decrease in the abundance of pelagic tar balls collected in the SEA studies is also thought to be due to the increased regulations of tanker discharges. The SEA studies, however, do not include any organic geochemical measurements of the oil residues present in the tar balls, limiting the ability to identify and apportion the origins of the tar balls. Geochemical analyses of oil residues in tar balls have great utility for source identification and been performed on samples from localized regions (Ekanim et al., 2015; Suneel et al., 2013; Hegazi et al., 2004). For example, a study of pelagic tar balls collected in the 1970s from Bermuda and the Sargasso Sea characterized the physical properties of the samples to be consistent with a waxy, crude-oil sludge, citing tanker discharges as a likely origin (Butler et al., 1973). The tar balls collected in this study also contained elevated abundances of normal alkanes with 30 to 40 carbons and high amounts of iron oxide, which likely originated from metal tankers. Tar samples collected from other geographic regions have also been analyzed for hydrocarbon indicators that are specific for oil biodegradation, such as heptadecane/pristane and octadecane/phytane ratios, which were used to link tar from the Niger Delta and Goa coast to crude oils present in the surrounding area (Ekanim et al., 2015 and Suneel et al., 2013 respectively). Polycyclic aromatic hydrocarbons (PAHs), biomarkers as well as the carbon preference index of n-alkanes have also been used to determine that tar collected off the coast of the city Alexandria in the Mediterranean originated from tanker ballast discharges of oil as well as heavy fuel oil (Hegazi et al., 2004). Despite the advances in the chemical fingerprinting of oil that have occurred since the 1970s (Stout and Wang, 2016), the geochemical composition of oil residues in tar balls has not been investigated in a longitudinal context. As such, there has been no test of whether contributions of tar balls from different origins has changed over time. Improved understanding of the origin(s) of tar balls will provide insights into the effectiveness of the MARPOL regulations, or the presence or absence of geographical or historical trends regarding their distribution. In this study, we report on the organic geochemical composition of oil residues from pelagic tar balls and stranded oil samples collected in the North Atlantic Ocean and Caribbean Sea from 1988 to 2016 and, using their chemical compositions, describe the different origins of oil in relation to their geographic and historic context.

Fig. 1. Map of sample collection locations organized by group number assigned by hierarchical cluster analysis (see Fig. 3). Zeros indicate the nine samples that contained no solvent-extractable material and were not included in the cluster analysis.

beneath the surface. The net is deployed for approximately one nautical mile at a speed of two knots, resulting in an approximate 1850 m2 tow area subject to current strength and direction relative to ship's heading. The contents of the net are then strained over a 335 μm mesh sieve, visually, and hand-sorted. This is the same protocol that has contributed to SEA's historical plastic record (Lavender Law et al., 2010; Morét-Ferguson et al., 2010). Tar samples were visually identified as solid and/or sticky black masses via this method and were stored in sealed glass containers, either dry or submerged in seawater, and archived in darkness at room temperature. Beginning in 2013, samples were stored at −18 °C in sealed, pre-cleaned aluminum foil packets. We selected samples based on collection date and location and size (at least 0.5 cm diameter), which allowed the inner portion of all specimens to be isolated for analysis. This operationally defined approach minimized any weathering-induced changes, shipboard, or thereafter contamination, as well as the effects of storage and preservation that would affect the initial composition of the released petroleum hydrocarbons, and hinder the identification of the origin(s) of the tar. These selection criteria also sought to provide ample material for numerous analytical methods. Remaining material was archived for future study. Collectively, selected samples, with minimal risk of contamination, were analyzed for size and geographic location and comprise a catalog of large pelagic tars (> 1 cm) at an ocean-basin spatial (43° latitude × 30° longitude) scale within a multi-decadal temporal scale. In addition, a limited number of stranded tar samples (5) collected in January 2016 from beaches in the northern Caribbean, specifically Tulum, Mexico (20.20° N, 87.44° W) and Akumal, Mexico (20.39° N, −87.32° W) were also analyzed as part of this study. For reference, two National Institute of Standards and Technology (NIST) standard reference materials (SRMs), “SRM 1642d Sulfur in diesel fuel oil”, and “SRM 1582 Petroleum Crude Oil”, as well as a Southern Louisiana crude oil and a sample of heavy fuel oil from the 2014 Texas City oil spill in Galveston Bay (Nelson et al., 2016) were also analyzed. Furthermore, the tar sample extracts were compared to synthetic motor oil, transmission fluid, and to oil originating from natural oil seeps collected from the sea surface in the northern Gulf of Mexico in 2015.

2. Materials and methods 2.1. Sampling information 100 pelagic tar samples were chosen for geochemical analysis, spanning a temporal (1988 to 2014) and geographic (11 to 44° N and 57 to 87° W) range that were accessible from 900 archived neuston tows collected by SEA aboard the R/V Westward and SSV Corwith Cramer (Fig. 1). The pelagic samples were all collected using a surface net-tow protocol that has been uniformly employed on SEA vessels for the past four decades: a neuston net (335 μm mesh and 0.5 m × 1.0 m opening) is deployed off the center beam of the ship and towed along the sea surface with approximately 0.25 m2 of the net submerged

2.2. Oil-residue extraction Samples were dried for 24 h at room temperature in a hood and then 353

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total PAH concentrations are the sum of the following parent PAHs and their alkylated homologs: naphthalene, fluorene, dibenzothiophene, phenanthrene/anthracene, and chrysene/benz[a]anthracene.

photographed (see Figs. S1, S2, and S3 in the supporting information for several images of tar balls from this study). Each sample was then cut in half in order to obtain 10 mg from the center of the tar ball, excluding at least the outer two millimeters of the sample. The 10-mg sample was added to a pre-combusted glass centrifuge tube (450 °C, 8 h) and spiked with an internal standard, perdeuterated hexadecane-d34 (Sigma Aldrich). The sample was extracted three times with 2 mL 99/1 dichloromethane/methanol by vigorously shaking for 1 min. The liquid phase was collected after centrifuging the tube for 5 min at 1600 revolutions per minute (RPM). The extracts were then combined, dried over sodium sulfate, and brought to a final volume of 500 μL with a rotary evaporator. The centrifuge tubes were dried and weighed to determine the mass remaining and the total solvent extractable material. Ninety-one out of the 100 tar ball samples had > 95% total solvent extractable material. The remaining nine samples had no measurable solvent-extractable material and notably lacked any color in the extract. For these nine samples, elemental analysis (C and H) was performed by Midwest Microlab in Indianapolis, Indiana.

2.5. Comprehensive two-dimensional gas chromatography (GC × GC) A Leco Pegasus IV two-dimensional gas chromatograph coupled to a time of flight mass spectrometer (GC × GC-TOF-MS) system was used in this study. The instrument was equipped with an Agilent 6890 GC and configured with a split/splitless auto-injector (7683B series) and a dual stage cryogenic modulator (Leco, Saint Joseph, MI). Samples were injected in splitless mode. The thermal modulator operates with a cold and hot jet. The cold jet gas was dry nitrogen (N2) gas and chilled with liquid N2. The hot jet temperature offset was 20 °C above the temperature of the main GC oven and the inlet temperature was isothermal at 185 °C. Two capillary GC columns were utilized in this GC × GC experiment. The first-dimension column was a Restek Rxi-1 ms, (60 m length, 0.25 mm I.D., 0.25 μm film thickness) and the second-dimension separations were performed on a 50% phenyl polysilphenylene-siloxane column (SGE BPX50, 1.2 m length, 0.10 mm I.D., 0.1 μm film thickness). The temperature program of the main oven started isothermal at 50 °C (15 min) and was then ramped from 50 to 335 °C at 1.5 °C min−1. The hot jet pulse width was 0.75 s and the modulation period was 6.00 s with a 2.25 s cooling period between stages. The second-dimension oven was programmed to remain isothermal at 60 °C (15 min) and then ramped from 60 to 340 °C at 1.5 °C min−1. TOF-MS data was sampled at an acquisition rate of 50 spectra per second in the mass range from 40 to 500 amu. The TOF detector voltage was 1470 V and the source temperature 240 °C. The mass spectrometer employs −70 eV electron ionization and operates at a push pulse rate of 5 kHz allowing sufficient signal averaging time to ensure good signal-to-noise ratios while still operating at a high enough data acquisition rate to accurately process (signal average) spectra from the peaks eluting from the second-dimension column in this high-resolution separation technique (GC × GC-TOF second dimension peak widths range between 50 and 150 milliseconds).

2.3. Gas chromatography-flame ionization detection (GC-FID) Samples extracts were injected onto an Agilent 6890 series gas chromatograph (GC) interfaced to a flame ionization detector (FID). Compounds were separated on a J&W DB-XLB capillary column (30 m, 0.25 mm internal diameter (I.D.), 0.25 μm film thickness) with the carrier gas (He; 99.999%) at a constant flow of 2 mL/min. The GC oven had an initial temperature of 70 °C (2 min hold) and was ramped at 3 °C min−1 to 120 °C, and then 6 °C min−1 until 320 °C (15 min hold). Total GC-amenable petroleum hydrocarbons were quantified by integrating the (blank-corrected) total area of the FID signal and response factors determined from the internal standard. The relative amount of unresolved hydrocarbons in a sample, known as the unresolved complex mixture (UCM), was determined by subtracting the sum of the area of the resolved compounds in the FID signal (i.e., the identifiable nalkane peaks) from the total area of the FID signal (as in White et al., 2016). Laboratory blanks (20 mg of combusted sand; Fisher Scientific) were run with every 10 samples and were free of petroleum hydrocarbons. Three oil residue samples were taken from one tar ball, used a test of precision, and analyzed GC-FID. Their chromatograms appeared similar with quantitative measures of the res/UCM (dimensionless) of 0.82, 0.80, and 0.78 for each sample.

2.6. Hierarchal cluster analysis To compare and contrast the GC-FID chromatograms, they were classified into groups using hierarchical cluster analysis (R statistical computing software; R Core Team, 2013). The groups were created based on the presence and distribution of n-alkanes and the relative size and shape of the UCM. To objectively assess chromatogram shapes, each chromatogram was subdivided into eight sections (Fig. 2): 20–25 min (n-C15-n-C16), 25–30.5 min (n-C18-n-C20), 30.5–35 min (nC21-n-C23), 35–40 min (n-C24-n-C27), 40–45 min (n-C28-n-C32), 45–49.5 min (n-C33-n-C37), 49.5–55 min (n-C38-n-C43), and 55–60 min (n-C44-n-C45). For each section, the resolved area and unresolved area were determined and the proportion of the total resolved signal and the resolved area/unresolved area ratios (res/UCM) were calculated. These values were centered around the mean and scaled to a variance of one prior to calculation of a Euclidian distance matrix. Hierarchical clustering was then performed on the resulting distance matrix using the complete linkage method to visualize groups of similar chromatograms (Fig. 3). The triplicate oil residues analyzed from one field sample were also all sorted into the same group using this analysis. A Moran's I test was performed using the “ape” and “spdep” packages in R to determine if there were any geographical autocorrelations within any of the groups produced from hierarchical clustering.

2.4. Gas chromatography-mass spectrometry (GC–MS) Parent and alkylated PAHs were quantified on a gas chromatograph coupled to a quadrupole mass spectrometer (GC–MS; Agilent 7890B GC and Agilent 5977 MS; Agilent Technologies, Santa Clara, CA). Volumes of 1 μL were injected through a split/splitless injector held at 320 °C using auto-injector. The GC was equipped either with a J&W XLB capillary column (60 m length, 0.25 mm i.d., 0.25 μm film thickness; Agilent Technologies). The following temperature program was used: 10 min at 40 °C, and ramped to 320 °C at 5 °C min−1 (held 30 min). The carrier gas flow (He 99.999%) was 1.2 mL/min. The MS was operated in selected ion monitoring (SIM) mode with a dwell time of 10 ms of each mass trace. Three retention windows were defined in order to have a maximum of 18 mass traces per retention window. Target analytes within the samples were quantified using a four-point calibration curve, using responses relative to the perdeuterated standard naphthalene-d8. Calibration standard were used that contained n-alkanes (n-C7 to n-C40 as well as hexadecane-d34), pristane, phytane, hopane, and the following PAHs: anthracene, biphenyl, chrysene, 6-methylchrysene, 6ethylchrysene, dibenzothiophene, 4,6-dimethyldibenzothiophene, fluorene, 1-methylfluorene, naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 1,2-dimethylnaphthalene, 1,6-dimethylnaphthalene, 2,3,5-trimethylnaphthalene, phenanthrene, 1-methylphenanthrene, 3,6-dimethylphenanthrene, and 9-ethyl-10-methylphenanthrene. The

3. Results and discussion Substantial differences in GC-FID chromatograms were observed between tar ball samples, varying in the amount and range of the nalkanes present in addition to having different UCM shapes, if a UCM 354

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ID: C186_017_NT

Group 2

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Fig. 2. Example GC-FID chromatogram of a tar-ball sample. The subdivisions by carbon range of the chromatogram (represented by grey-dashed lines) indicate the eight sections that were compared for hierarchical cluster analysis. The ratio of the resolved signal to the unresolved area (Res/UCM) is shown for each section as well as the whole chromatogram (top right), revealing the resolved signal can vary substantially across the different sections.

Res/UCM: 0.66

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groups, with carbon numbers ranging from 16 to 45 and an average alkane chain length of 26. The samples in this group are thus likely the freshest and least weathered of all the samples. For example, chromatograms comparable to the samples in Group 1 have been found from tar balls collected off the coast of Louisiana that were thought to originate from unweathered heavy fuel oil (Henry et al., 2003). The oil residues in Group 2 have a similar average n-alkane chain length (25) as Group 1 but the average res/UCM is lower (0.53 ± 0.14; Fig. 3b). Unlike the Gaussian distribution of n-alkanes seen in the chromatograms of Group 1, many of the Group 2 chromatograms have a bimodal shape, which indicates a blend of oils or, alternatively, oil from paraffin waxes, which can precipitate out of crude oil (Peters et al., 2004; Hegazi et al., 2004). Tar balls collected from the Mediterranean Sea in a published study have GC-FID chromatograms with a high abundance of alkanes, similar to our Group 2, and were concluded to be crude oil released from tanker ballast discharges of oil (Hegazi et al., 2004). Paraffin wax from crude oil can stick the walls of the tank, which then can be released upon washing (Hegazi et al., 2004 and Butler and Harris, 1975). Therefore, it is possible these bimodal chromatograms are from an unweathered crude oil or a mixture of oil-residues (Butler and Harris, 1975). Groups 3 and 4 (Fig. 3c and d) have larger contributions from UCMs compared to Groups 1 and 2, with res/UCM ratios of 0.27 ± 0.09, and 0.34 ± 0.08 respectively. Both of these groups are missing n-alkanes with chain lengths shorter than 25 and have higher average carbon chain lengths of 31 and 30 for Group 3 and 4, respectively. Pelagic tar balls with similar chromatograms as our Groups 3 were previously reported for samples collected in the Niger Delta Basin of Nigeria that were concluded to likely be weathered crude oil originating from oil wells in the Niger Delta and the neighboring countries Brazzaville, Republic of Congo and Gabon (Ekanim et al., 2015). A different study, Hegazi et al., 2004, also analyzed samples similar to Group 4 and concluded they were likely formed from a heavy fuel oil (Hegazi et al., 2004). The presence of long chain n-alkanes (30–45) in Groups 3 and 4 also suggest that this may be paraffin wax precipitated from crude oil and these samples may originate from tanker washings as previously described for Group 2. Group 5 samples, shown in Fig. 3e, have a shorter carbon range (20 to 30) than Groups 1 through 4 (15–40). With an average carbon chain length of 25, Group 5 also has a higher abundance of lighter n-alkanes compared to Groups 1 through 4. Our Group 5 sample chromatograms are comparable to samples taken from oil-contaminated sea birds, which authors predicted to be a crude oil originating from tanker

was present at all. To determine the differences and similarities between tar balls collected for this study, we first examined the res/UCM over different carbon ranges in each of the GC-FID chromatograms, shown in Fig. 2, and used hierarchical cluster analysis to group the samples. Further information regarding origin and weathering of oil residues in the tar balls was determined by examining specific PAH ratios quantified by GC–MS. The ultimate reservoir origin of the hydrocarbons in the tar balls was examined for select samples via GC × GC, to compare the presence of thermal maturity indicators, hopane biomarkers, and oleanane, a biomarker that indicates if the initial organic matter was < 50 million years, in the samples to ascertain relationships between hierarchical cluster groups and the reservoir from which the petroleum hydrocarbons originated. 3.1. Hierarchical cluster analysis of GC-FID chromatograms Pelagic tar balls collected in the Western North Atlantic over the past two decades contain oil residues with variable compositions as observed in GC-FID chromatograms (examples shown in Fig. 3 and details provided in Table S2). The res/UCM ratio ranges from 0.1 to 1.6 across the samples, some are nearly baseline resolved (e.g. Fig. 3a, b) while other samples contain a large UCM (e.g. Fig. 3e, f) that is comparable to highly weathered crude (NRC, 2003). The carbon chain length of alkanes present ranged from 14 to 45 carbons with the average chain length at 27 carbons considering all samples. Some samples contained alkanes spanning this entire carbon range (e.g. C108-005-NTA, Fig. 3b) while others, such as C131-025-NTB, only had long-chain alkanes remaining with a total range of 35–45 carbons (shown in Fig. 3c). In order to organize the chemical variations of the oil residues evidenced in the chromatograms, hierarchical cluster analysis was used. Chromatograms from all samples were compared and sorted into six different groups based on their chromatographic similarity. Seven percent of the samples were in Group 1, 34% in Group 2, 5% in Group 3, 6% in Group 4, 4% in Group 5, and 36% in Group 6 (see Table S2). A representative sample chromatogram from each group is shown in Fig. 3. 3.2. Characteristics of the groups created in hierarchal cluster analysis Oil residues from tar balls in Group 1 are characterized by the near absence of a UCM hump and therefore the largest average res/UCM ratio (0.96 ± 0.36; see Fig. 3a). The chromatograms of oil residues in this group have the greatest range of n-alkanes relative to the other 355

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(a) Group 1

(b) Group 2

(c) Group 3

(d) Group 4

(e) Group 5

(f) Group 6

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n-alkane carbon number Fig. 3. Hierarchical cluster analysis of chromatograms of tar ball extracts with example chromatograms corresponding to each group: (a) Group 1, (b) Group 2, (c) Group 3, (d) Group 4, (e) Group 5, (f) Group 6. Grey shaded boxes (on right) indicate the six different groups of samples identified by the hierarchical cluster analysis.

Interestingly, Butler et al. (1973) also saw a range of chromatograms, although their chromatograms did not afford the same chromatographic resolution, similar to each of the six groups described in this analysis. They also noted that chromatograms with a bimodal appearance and, particularly, those with a high abundance of n-alkane chains > 30 (our Groups 2 and 3) tended to correspond to samples with a waxy appearing exterior, which could have been formed from heavily weathered crude oil released from tanker washings. To assist in the origin determination of our samples, four reference oils were analyzed and classified using the hierarchal cluster analysis. A Southern Louisiana crude oil that had not been environmentally weathered and a sample of heavy fuel oil spilled from cargo or fuel

discharge (Lucas and MacGregor, 2006). Samples from this group, however, were also comparable to tar ball samples found off the coast of California and British Columbia, which were confirmed to originate from heavy fuel oil (Wang et al., 1998). Group 6 had the greatest number of samples and also the greatest amount of variability in n-alkane distribution (Fig. 3f). The average chain length for individual samples in this group ranged from 20 to 31. The samples in this group are related by their small value for res/UCM, which ranged from 0.11 to 0.36 with a group average of only 0.25 ± 0.07. Group 6 samples were comparable to heavily weathered crude oil seen in Butler et al. (1973), the only previous study that has performed extensive analysis of tar found in the Western North Atlantic. 356

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11% of the samples are within this range and 17% are above. Based on this sample set and our interpretation of Uhler et al., 2016, we have determined that 28% of all the tar ball samples in this study are likely heavy fuel oil. In order to better understand if our contentions of the origin of the tar from each group as correct, we determined the percent of samples in each group that were formed from heavy fuel oil. These results, on a whole, are consistent with our classification for the oil composition of the tar balls using hierarchal cluster analysis. Group 1 was classified to either be unweathered heavy fuel oil or crude oil and 43% of samples in this group are from refined oil. The small percentage of heavy fuel oil derived samples in Groups 2 (6%) and 3 (16%) supports our classification that these groups were most representative of unweathered crude oil and weathered crude oil, respectively. The PAH data also confirms that Group 4 is largely composed of heavy fuel oil (86%). Half of the samples in Group 5 were of heavy fuel oil origin, which is expected as GC-FID chromatograms from this group were comparable to both weathered crude oil samples and to weathered heavy fuel oil. Group 6 was predicted to be heavily weathered crude oil and fuel oil and like Group 5, about half of the samples (41%) were determined to be of heavy fuel oil origin. The consistency in the hierarchal cluster analysis predictions with the 2-methyl anthracene data is promising for this method's use in predicting oil origin without extensive PAH data.

tanks from the Kirby 27706 that spilled in Texas City in 2014 were both sorted into Group 3 (Nelson et al., 2016). While a Southern Louisiana crude oil might not be expected to be categorized into Group 3, a closer examination suggests that its UCM content is responsible for this classification. The res/UCM for this sample was only 0.14, as compared to the samples in Group 1, which have res/UCM values between 0.67 and 1.69. Two samples from natural seeps (0602615TN_24 and 0602615TN_23) collected with Teflon nets in the Gulf of Mexico in 2015 were sorted into two different Groups (5 and 6). Weathering through biodegradation and evaporation leads to the preferential depletion of shorter n-alkanes to those with longer carbon chains. The extent of weathering as unweathered, weathered, and heavily weathered is measured by the res/UCM ratio in combination with the number of alkanes remaining in the sample. Unweathered samples show a near absent UCM and contain a wide range of n-alkanes. Weathered samples show a decrease in the res/UCM and a decrease in the n-alkanes observed compared to the unweathered samples. Heavily weathered samples have a res/UCM < 0.3 and a significant decrease in the n-alkane concentration. Based on the characteristics of our GC-FID chromatograms compared to reference oils and their similarities to other studies, we contend that Group 1 (containing 8% of samples) is either unweathered fuel oil or crude oil, Group 2 (36%) is most characteristic of unweathered crude oil, Group 3 (7%) heavily weathered paraffinic crude oil, Group 4 (4%) weathered heavy fuel oil, Group 5 (4%) heavily weathered heavy fuel oil or crude oil, and Group 6 (41%) heavily weathered crude oil or heavy fuel oil. At a first pass, hierarchical cluster analysis of GC-FID seems to be a useful tool to sort and compare many samples based on similarities in alkane distribution and UCM although determining the origin of these samples requires additional analysis such as PAH analysis.

3.4. Oil reservoir origin determination from petroleum biomarkers Unlike the chemical indicators analyzed thus far, the GC × GC-TOFMS elucidates hopanoid biomarkers that provide information about the petroleum reservoir rather than the refining process. If the biomarker composition for several oil residues is similar, this provides evidence that they may have originated from the same reservoir (e.g. Peters et al., 2007 and Nelson et al., 2016). Thermal maturity indicators, hopane biomarkers and the presence of oleanane, a biomarker derived from angiosperms, which indicates that the oil is 65 million years old or younger (Peters et al., 2007), were examined via GC × GC-TOF-MS in a representative set of eleven tar ball samples. Samples were chosen based on different defining characteristics of their chromatograms, such as a resolved baseline, bimodal appearance, or average alkane chain length, etc. Of the eleven samples analyzed, Ts/Tm ratios, a thermal maturity indicator, ranged widely from 0.18 to 1.59, shown in Table S3. Although these indicators can be used to predict the origin of oil in the samples, they cannot differentiate between natural seeps and anthropogenic emissions. Overall, there is variability in the biomarkers observed in the subset of eleven samples that were measured, indicating that they did not arise from a single type of crude oil and there are likely many oil reservoirs contributing to tar ball formation. It is likely that with the oil originating from different reservoirs, the tar balls were likely formed from many different incidents. Additional biomarker data from all samples in this study could reveal additional information regarding spatial or temporal trends in the origin of tar balls in the Western North Atlantic. This, however, is a topic for future research.

3.3. Examining origin and weathering of tar balls using PAHs For a more detailed determination of the oil present in the tar balls, we performed PAHs analysis. Specifically, we investigated the distribution of parent PAH compounds and their alkylated homologs to determine if the samples had been weathered, and we used the ratio 2methylanthracene/C1-phenanthrenes + C1-anthracenes to determine tar balls derived from heavy fuel oil. The PAH content of the tar balls was highly variable between and within groups, with total PAHs ranging from 0 to 21.0 μg g−1 dry weight of tar ball (see Table S2). Group 3 had the smallest average PAHs at only 1.98 ± 1.90 μg g−1, followed by Group 1 with a total PAH average of 2.57 ± 1.22 μg g−1, Group 2 and 4 had similar average values of was 3.91 ± 3.01 μg g−1 and 3.87 ± 4.64, respectively. Groups 5 and 6 had the highest average total PAHs per samples at 4.19 ± 4.70 and 5.21 ± 4.74 μg g−1, respectively. The alkylated PAH profiles of the oil residues from the tar balls provide additional data regarding the extent to which they have been weathered. Regardless of group, 92% of the samples have a distribution of parent PAHs and alkylated homologs of naphthalene, fluorene and phenanthrene (shown in Table S4) in the order C0 < C1 < C2 < C3, which is typical of weathered oil (Wang et al., 1998). This finding indicates even though the center of each tar ball was used for analysis, we were unable to avoid the effects of weathering on the PAH composition for most of the samples. As an indicator for crude oils that have been refined, the ratio 2methylanthracene/C1-phenanthrenes + C1-anthracenes was used to apportion natural seep oils and crude oils from refined products like heavy fuel oil (Uhler et al., 2016). Uhler et al., 2016 reported that heavy fuel oils typically have a percent 2-methyl anthracene content within the range of 4.2 to 8.9% of the total C1-anthracenes/phenanthrenes concentration (Uhler et al., 2016). Based on this method, we found 72% of the samples have 2-methylanythracene values below this range and thus are unlikely to be derived solely from heavy fuel oil.

3.5. Analysis of samples that were not GC-amenable Nine samples had no measurable solvent-extractable material and could not be analyzed with gas chromatography and be sorted into a group. To determine if these samples were hydrocarbon based, they were analyzed for their bulk C and H composition. The C:H molar ratios ranged from 0.5 to 3.5 and seven of the nine samples had C:H > 1.0, which are comparable to values of engine soot, coal, or other highly condensed material (Clague et al., 1999; Vassilev et al., 2003). 3.6. Geographic and temporal relationships To explore whether there are any geographic trends within the groups assigned to the tar balls, the samples were plotted according to 357

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impact of weathering on origin determination. Furthermore, we do not have any pre-MARPOL samples. In order to conclusively determine any effects of these regulations on tar ball pollution, we recommend that, at least, additional samples be analyzed, especially those < 0.5 cm in diameter, which were excluded from this study. The Sea Education Association has been a part of a long-term study of plastic analyzing samples found in the Western North Atlantic (Lavender Law et al., 2010). This study analyzed plastic debris collected by SEA over 22 years, from 1986 to 2008. Similar to our study, there were no strong temporal trends found for the samples, however, these long-term studies are valuable to understand the origin and fate of different chemical contaminants (and/or natural seeps in the case of oil) and the efforts to reduce ocean pollutants. Both the plastic and this tar ball study provide a baseline for each of these pollutants found in the Western North Atlantic, providing important information to which future studies can compare.

location and group number, shown in Fig. 1. No significant correlations were observed for Groups 1, 3, 4 or 5 although Group 1 samples appear to be clustered just North of the Caribbean and the samples that were not GC amenable were concentrated in the Caribbean. Groups 2 and 6, which had the largest number of samples and were spread over the widest geographical area were examined using the Moran's I test to quantitatively examine the spatial relationship between the samples. Fig. S5 shows the autocorrelation grid used to separate Groups 2 and 6 samples into boxes in a 5° × 5° grid. Both Group 2 and 6 had p-values < 0.05 (0.0001 and 0.03, respectively), which indicates that for the samples in these groups there is a correlation between the collection site and group number. Because these samples are similar in composition and were also collected in the same area there is the potential that the samples in Groups 2 and 6 were released from related or similar incidents. Temporal trends were also assessed, however, the trends between group number and the date the samples were collected are more ambiguous (Fig. S6). The entire range for all samples was only 28 years and most groups contained samples with a date range spanning at least a decade, thus there are few trends to draw from temporal analysis alone. However, using both temporal and geographic relationships together can be valuable in elucidating a common origin, or lack thereof, for the tar ball samples in a group. Group 5 was the only group that did not contain samples collected in the 1990s, however, there was no corresponding relationship observed in where these samples were collected and thus no conclusions could be made about a common origin for the samples. Group 4 samples had the smallest date range from 1990 to 1997 and, interestingly, most samples in this group were found along the Bahamas, which could indicate they were formed from related incidents or commonly used products.

Acknowledgements The authors wish to acknowledge Z. Geng, A.E. Morrison for assisting with sample collection and analysis. We thank the hundreds of SEA Semester students and the scientific staff at Sea Education Association who collected and sorted tar ball samples from many decades of neuston net tows on the R/V Westward and SSV Corwith Cramer. Funding: This work was supported by a Gulf Research Program Early-Career Research Fellowship to HKW. CMR and DLV were funded by NSF grant OCE-1333162. Declarations of interest None.

3.7. Conclusions and implications Appendix A. Supplementary data Tar balls collected in the Western North Atlantic over the past 28 years vary in chemical composition and origin. Hierarchal cluster analysis organized GC-FID chromatogram data into six preliminary groups. PAH data, specifically the relative abundance of 2-methyl anthracene, revealed that 28% of the samples were from heavy fuel oil and thus are of anthropogenic origin. Although the center of each tar ball was collected, additional PAH data indicated that 92% of the samples are likely weathered as they have a distribution of parent PAHs and alkylated homologs in the order C0 < C1 < C2 < C3. Statistical analysis showed that the samples in Groups 2 and 6 had geographic correlations, therefore, it is possible the samples in these groups were from related or similar events. In total, 28% of the samples in this study have a clear anthropogenic origin from heavy fuel oil. For the remaining 72%, it cannot be conclusively determined if they are from an anthropogenic release or natural seep. Although methane seeps have been confirmed in the Western North Atlantic, little is known about natural oil seeps in this region (USGS, 2017). A 2010 NASA study used MODIS imagery to detect oil seeps off of the Virginia and North Carolina coasts, however, no conclusive evidence was found (Reahard et al., 2010). It is possible seeps exist along other coasts, which could explain the origin for some of these samples. However, there is likely no natural oil production in the Deep Atlantic due to lack of sediment, thus many of the samples collected in this region are likely from anthropogenic origins such as a tanker ballast discharge that also released crude oil (Parson and Edwards, 2011; Kvenvolden and Cooper, 2003). Further work is needed on both the sedimentation and currents in the Western North Atlantic to determine if natural seeps are a likely potential origin of the tar balls in this region. The longitudinal aspect of this study would offer the potential to analyze the effectiveness of MARPOL, however, this was challenging to do for this study because we did not analyze the total size range of tar balls and instead picked only one size class (> 0.5 cm) to reduce the

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