Accelerated Burial of Petroleum Hydrocarbons in Arabian Gulf Blue Carbon Repositories

Accelerated burial of petroleum hydrocarbons in Arabian Gulf blue carbon repositories

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Authors Ashok, Ananya; Cusack, Michael; Saderne, Vincent; Krishnakumar, Periyadan K.; Rabaoui, Lotfi; Qurban, Mohammed A.; Duarte, Carlos M.; Agusti, Susana

Citation Ashok, A. et al., 2019. Accelerated burial of petroleum hydrocarbons in Arabian Gulf blue carbon repositories. Science of The Total Environment, 669, pp.205–212. Available at: http:// dx.doi.org/10.1016/j.scitotenv.2019.01.437.

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Link to Item http://hdl.handle.net/10754/631741 Science of the Total Environment 669 (2019) 205–212

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Accelerated burial of petroleum hydrocarbons in Arabian Gulf blue carbon repositories

Ananya Ashok a,⁎, Michael Cusack a, Vincent Saderne a,PeriyadanK.Krishnakumarb,c,Lotﬁ Rabaoui b,c, Mohammed A. Qurban b,c, Carlos M. Duarte a, Susana Agustí a a King Abdullah University of Science and Technology, Red Sea Research Centre, Thuwal 23955-6900, Saudi Arabia b Marine Studies Section, Center for Environment and Water, Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), P. B. No. 391, Dhahran 31261, Saudi Arabia c Geosciences Department, The College of Petroleum Engineering & Geosciences, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia

HIGHLIGHTS GRAPHICAL ABSTRACT

• Vegetated, blue carbon habitats are sites of major burial of carbon and organic matter. • Arabian Gulf blue carbon habitats have also sequestered massive amounts of petroleum. • Sediment records revealed periods of elevated petroleum concentrations since 1900. • Burial rate of petroleum in blue carbon habitats has accelerated over the 20th century. • Conserving blue carbon habitats will avoid remobilization of sequestered. article info abstract

Article history: Massive consumption of petroleum since the past century has led to considerable emissions into marine ecosys- Received 11 November 2018 tems. Marine sediments may accumulate substantial quantities of petroleum and associated contaminants in oil- Received in revised form 16 January 2019 producing areas. Here, we report accelerated accumulation of total petroleum hydrocarbons (TPH) in ‘blue car- Accepted 17 January 2019 bon’ vegetated ecosystems of the Arabian Gulf – the world's most important region for oil production. In addition Available online 06 March 2019 to increased accumulation with the onset of oil exploitation, sediment records reﬂect a large depositional event Editor: Damia Barcelo associated with the 1991 Gulf War, with the magnitude of these maxima varying across habitats, depending on their elevation along the shoreline. Blue carbon ecosystems of the Arabian Gulf currently bury about 2300 Keywords: megagrams (Mg) of TPHs annually and have accumulated TPH stocks of 59,799 Mg over the past 25 years Blue carbon alone. Massive burial and sequestration of TPH by blue carbon ecosystems is an important, but thus far unrecog- Oil pollution nized, removal mechanism in the Arabian Gulf. Conserving these ecosystems is important to avoid possible remo- Petroleum hydrocarbons bilization of sequestered TPH into the surrounding environment. Arabian Gulf © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// Vegetated coastal habitats creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction environmental history of the biosphere. The current global consumption of oil is estimated at 4418.2 million tonnes per annum (British Discovery of the world's second largest and most productive oil de- Petroleum, 2017). This consumption is mainly supported by the Middle- posits in the Arabian Gulf in the early 1900s is a major milestone in the East region and the Arabian Gulf, which, by containing 65.5% of world's proven crude oil reserves (OPEC, 2017), contributes 34.5% of global pro- ⁎ Corresponding author. duction (British Petroleum, 2017). Excessive use of oil over the past cen- E-mail address: [email protected] (A. Ashok). tury has led to global climatic transformations triggered mainly by a rise

https://doi.org/10.1016/j.scitotenv.2019.01.437 0048-9697/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 206 A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212

in CO2 emissions from the burning of oil, leading to an increase in ambient interior diameter) were collected in 2016 from four sampling locations air and water temperatures, sea level rise and ocean acidiﬁcation. A signif- along a 400 km stretch of the Saudi Arabian coast of the Arabian Gulf, ex- icant fraction of the oil extracted from below ground is accidentally re- tending from 27° 58′ N (Safaniya) on the NW Gulf coast to 25° 43′ N leased to the environment (GESAMP (Joint Group of Experts on the (Uqair) on the SW Gulf coast (Fig. 1). Three coastal blue-carbon habitats Scientiﬁc Aspects of Marine Environmental Protection), 2007). While of salt-marshes, mangrove forests of Avicennia marina and seagrass

CO2 emissions from oil combustion are distributed globally, the accidental meadows from high to low tide zone were sampled (Fig. 1). Due to the release of uncombusted oil products produced during exploration and ex- presence of abundant fossil fuel reserves, the studied region hosts an in- traction occurs largely at the sites of extraction (Croquer et al., 2016; Li tense oil industry with infrastructure, such as the Ras Tanura refinery et al., 2012). Hence, the Arabian Gulf is likely to rank among the most which is among the world's largest. Industrial development in the region heavily impacted regions, receiving the highest inputs of petroleum hy- supports densely populated urban areas such as Dammam near Ras drocarbons on the planet (El Samra and El Deeb, 1988; Johansson et al., Tanura, and overall, the Gulf has some of the highest shipping trafficany- 2017; Sheppard et al., 2010). where in the world (Aboukorin and Al-shihri, 2015; Johansson et al., Shallow sediments along coastlines have been identified to accumu- 2017). Also, the studied region is part of the coastal stretch identified as late petroleum hydrocarbons (Gustitus and Clement, 2017), within the most impacted post the Gulf war oil-spill of 1991 (Readman et al., 1992; Arabian Gulf (Ehrhardt and Burns, 1993; Fowler et al., 1993; Joydas Sadiq and McCain, 1993). Sediment cores collected were distributed as et al., 2017; Massoud et al., 1998) and in other regions across the globe 3 cores at Safaniya (only seagrasses), 9 cores at Abu-Ali (3 mangrove, 3 which receive high inputs of oil and oil-combustion products (Burns seagrass, 3 saltmarshes), 10 cores at Ras Tanura (4 mangroves, 3 et al., 2010; Croquer et al., 2016; Li et al., 2012; Sammarco et al., 2013). seagrasses and 3 saltmarshes) and 3 cores at Uqair (3 seagrasses only) While most of those studies refer to surface sediments, where recent (Fig. 1; Table 1). emissions accumulate, a number of studies have demonstrated that compounds present in oil, primarily petroleum hydrocarbons, do not fully de- 2.2. Collection of cores and establishment of sediment chronology with grade over time. Furthermore, petroleum hydrocarbons preferentially 210Pb partition into the organic carbon component of sediments and soils. Thus, the persistent nature of certain oil compounds means they are pre- Sediments were collected by manual percussion of the plastic-cores served in sediments, and therefore oil residues can be found at depths in (PVC) gently into the sediment. Compression of sediments during coring the sediments corresponding to historical release events (Bigus et al., was corrected by measuring the difference in depth between the sedi- 2014; Boonyatumanond et al., 2007; Vane et al., 2011). ment surface within the collected core and the actual sediment depth in Indeed, coastal sediments, particularly vegetated coastal sediments, the habitat. This difference was distributed as compression throughout are considered to be major sinks for organic carbon globally, earning the total length of the core uniformly as described in Saderne et al., them the title ‘Blue Carbon’ habitats (Fourqurean et al., 2012; McLeod 2018. The collected sediment cores were sliced into 1 cm thick segments et al., 2011), and their capacity to store organic carbon could possibly ex- at KAUST laboratories. Each slice was then dried at 60 °C until constant tend to the sequestration of persistent oil contaminants in their sedi- weight to determine bulk density (g cm−3). The dried sediment slices ments. Blue carbon habitats occur where the overlying vegetation (salt- were then ground by pestle and mortar and sieved into different size frac- marshes, mangroves and seagrass) promote accumulation and preserva- tions. Concentrations of 210Pb were determined from the dried sediment tion of organic materials in the sediment due to the prevalence of anoxic fraction sieved for b125 μm size, in each slice up to 19 cm of individual conditions (Duarte et al., 2013) Moreover, blue carbon habitats occupy core depth. 210Pb analysis was done at Edith Cowen University shallow depths, from the supratidal (salt-marshes) to the intertidal (man- (Australia) by alpha spectrometry through the measurement of its grand- groves) and shallow subtidal (seagrass meadows), where petroleum hy- daughter 210Po, assuming radioactive equilibrium between both radionu- drocarbons, which are largely buoyant, intersect the shoreline and are clides. The concentrations of excess 210Pb used to obtain the age models known to preferentially accumulate (Burns et al., 2010; Gustitus and were determined as the difference between total 210Pb and 226Ra (sup- Clement, 2017) Here, we test our hypotheses that blue carbon habitats ported 210Pb). Concentrations of 226Ra were determined for selected sam- act as a sink for petroleum hydrocarbons in regions subjected to recurrent ples along each core by low-background liquid scintillation counting inputs, such as the Arabian Gulf, and that the position of particular habi- method (Wallac 1220 Quantulus) adapted from Masqué et al. (2002). tats along the water-land interface influences accumulation rates. We ex- These concentrations were found to be comparable with the concentra- amine concentrations, stocks and burial rates of total petroleum tions of total 210Pb at depth below the excess 210Pb horizons. Analyses hydrocarbons (TPH) in sediments from salt-marsh, mangrove and of reagent blanks, replicates and a reference material (IAEA - 315, marine seagrass habitats along the Saudi coast of the Arabian Gulf. sediments) were carried out for both 210Pb and 226Ra to assess for any contamination and to ensure reproducibility of the results. Sediment ac- 2. Materials and methods cumulation rates (expressed in g dry weight (DW) cm−2 yr−1)werede- termined from the 210Pb dates using the Constant Flux: Constant 2.1. Study design Sedimentation model and accretion rates (mm yr−1) were calculated using the dry bulk density (DBD) data of each core. Further details includ- TPH concentrations with depth in the sediments from the Saudi ing a detailed overview of method used for 210Pb dating and calculation of Arabian coastline of the Arabian Gulf were combined with chronologies long and short term soil accumulation rates in the Arabian Gulf (SAR, established using 210Pb dating from sediment cores to reconstruct the his- expressed in g DW cm−2 y−1) are reported in detail elsewhere tory of petroleum inputs supporting the current sequestration rates and (Saderne et al., 2018). Other analysis, including TPH and PAHs were per- stocks in the region (Fig. 1). In addition, we measured 16 USEPA formed at KFUPM laboratory. polyaromatic hydrocarbons (PAHs) namely naphthalene, acenaphthyl- ene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, 2.3. Analysis of total petroleum hydrocarbons pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k) fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenzo(a,h)an- TPHs were extracted from the sediments using the US EPA 3545A thracene, benzo(g,h,i)perylene recognized as priority pollutants by the method (USEPA, 2007a). Briefly, 5 g freeze-dried sediment sample United States Environmental Protection Agency (USEPA) and one methyl- was extracted using a 1:1 (v/v) mixture of HPLC-grade hexane and di- ated PAH congener namely methylnaphthalene. Totally, 16 PAHS and 1 chloromethane in Dionex ASA 200 accelerated solvent extraction unit. methylated PAH were measured from the sediment cores across their The extract was then concentrated to 10 mL with a Kuderna Danish con- depth. A total of twenty-five sediment cores (140 cm length × 7 cm centrator and then to 1 mL by replacing the solvent with hexane and A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212 207

Fig. 1. Locations of blue carbon habitats sampled along the Saudi Arabian coastline, Arabian Gulf. A total of 25 cores were collected, distributed from NW to SW as 3 seagrass cores at Safaniya; 3 saltmarsh, 3 mangrove and 3 seagrass cores in Abu-Ali; 3 saltmarsh, 4 mangrove and 3 seagrass cores at Ras-Tanura and 3 seagrass cores at Uqair. Locations of individual sediment cores samples are given in Table 1. The inset map of Arabian Gulf, with arrows shows the anti-clockwise circulation pattern. (Adapted from Reynolds, 1993).

purging with N2 gas. Concentrations of TPH in the extract were then dichloromethane respectively. The aromatic fraction containing indi- measured following the USEPA method 8015 C (USEPA, 2000)by vidual PAHs was analyzed based on USEPA Method 8270 D (USEPA, injecting 1 μL of the extracted sample into the Agilent 7890 gas chro- 2007b), using the Agilent 6890 Gas Chromatograph connected to matograph equipped with flame ionization detector (Manufactured by Agilent 5973 Mass Spectrometer (single quadrupole mass detector) Agilent, Location – Santa Clara, California, USA. Serial No. (Agilent GC 6890 and MS 5973 manufactured by Agilent, Location – US10381030). Injector temperature was 250 °C. The chromatographic Santa Clara, California, USA. Serial No. US10423013) equipped with column used was HP 5 (30 m length × 0.25 μmID×0.25μm film thick- the chromatographic column Restek-5-MS (30 m length × 0.25 μmID ness) with carrier gas, helium at a flow-rate of 1.2 mL min−1. The GC ×0.25μm film thickness) in Selected Ion Monitoring (GC/MS-SIM) oven temperature was programmed to ramp from 40 °C for 1 min, to mode. 1 μL of the extracted sample was injected in the GC, with the 300 °C at 10 °C min−1, then hold at 300 °C for 10 min. Detector temper- injector temperature maintained at 300 °C with a helium (carrier ature was maintained at 300 °C. Concentrations of TPH in the samples gas) flow rate of 1.2 mL min−1. The GC oven temperature was pro- were read against a calibration graph prepared with different aliphatic gram was set to ramp from an initial temperature of 60 °C to 240 °C hydrocarbons (C10 to C36) concentrations vs response factors. 5α- at 12 °C min−1, then to 300 °C at 5 °C min−1. The following ions Androstane was used as internal standard. were selected: m/z 128, 142, 152, 154, 166, 178, 202, 228, 252, 276 and 278 for monitoring the PAHs analyzed. Standards containing 2.4. Analysis of polycyclic aromatic hydrocarbons known concentrations of polycyclic aromatic hydrocarbons were run on the GC–MS in Selected Ion Mode (SIM), and the chromato- Polycyclic aromatic hydrocarbons (PAHs) were also extracted from grams obtained was used in the calculation of PAH concentration in the sediments using the US EPA 3545A method as described in TPH thesampleextracts.Theareaunderthepeakcorrespondingtoa analysis (USEPA, 2007a). Sediment extract was concentrated to 10 mL known concentration was plotted for individual PAH. Ortho- in a Kuderna Danish concentrator. The sample solvent was extracted terphenyl was used as internal standard. Based on the calibration with hexane and volume was reduced to 1 mL by purging nitrogen plots of known standards, concentration of individual PAH was de- gas. This extract was further fractionated following USEPA Method termined by considering the mass of the sample used for extraction. 3630 C (USEPA, 1996) into aliphatic and aromatic fractions by passing Concentrations of individual PAHs were summed to report ∑16 + 1 through alumina and silica columns, and eluting using hexane and PAHs in μgkg−1. The detection limit was 1.0 μgkg−1 of sediment 208 A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212

Table 1 Geographical information of sampling locations of sediment cores for chronology and average ± SE values obtained for TOC, TPH, sediment accumulation rate, burial rate, TPH stock, PAHs.

Region Habitat Latitude (°N) Longitude (°E) Mean ± standard error values (averaged across individual core depth)

TOC TPH ∑16 + 1 PAHs TPH stock Sedimentation rate Burial rate (mg kg−1) (mg kg−1) (μgkg−1) (g TPH m−2) (g cm−2 yr−1) (g TPH m−2 y−1)

Safaniya Seagrass 27.97364 48.77404 5935.56 ± 453 30.22 ± 11.34 0.17 ± 0.00 0.44 ± 0.15 0.34 ± 0.04 0.03 ± 0.01 Safaniya Seagrass 27.97679 48.77877 11,780.00 ± 368 59.94 ± 24.62 1.84 ± 0.83 0.77 ± 0.32 n/a 0.11 ± 0.05 Safaniya Seagrass 27.98142 48.78462 11,318.00 ± 1428 26.86 ± 7.21 12.34 ± 6.72 0.36 ± 0.10 0.23 ± 0.03 0.01 ± 0.00 Abu-Ali Mangrove 27.28603 49.56685 18,017.50 ± 2935 67.00 ± 36.59 14.10 ± 4.04 0.97 ± 0.52 0.25 ± 0.02 0.02 ± 0.01 Abu-Ali Mangrove 27.28384 49.56403 16,063.33 ± 3016 13.07 ± 11.93 24.20 ± 4.66 0.16 ± 0.15 0.176 ± 0.013 0.01 ± 0.01 Abu-Ali Mangrove 27.28283 49.56523 4172.50 ± 540 5.30 ± 3.51 10.80 ± 1.57 0.08 ± 0.05 0.37 ± 0.02 0.00 ± 0.00 Abu-Ali Saltmarsh 27.28777 49.56953 13,336.25 ± 1386 31.64 ± 17.34 18.25 ± 7.33 0.40 ± 0.21 0.178 ± 0.008 0.02 ± 0.01 Abu-Ali Saltmarsh 27.31332 49.56401 11,872.86 ± 468 22.33 ± 7.28 17.08 ± 3.80 0.28 ± 0.09 0.102 ± 0.007 0.03 ± 0.01 Abu-Ali Saltmarsh 27.29719 49.48471 22,866.67 ± 5841 117.63 ± 42.08 44.60 ± 37.04 1.70 ± 0.63 0.123 ± 0.012 0.13 ± 0.06 Abu-Ali Seagrass 27.26978 49.54546 7956.67 ± 531 112.73 ± 61.14 8.98 ± 2.94 1.88 ± 1.03 0.21 ± 0.06 0.13 ± 0.07 Abu-Ali Seagrass 27.25976 49.57006 11,510.00 ± 708 71.97 ± 12.70 54.64 ± 22.34 1.02 ± 0.18 0.09 ± 0.02 0.03 ± 0.01 Abu-Ali Seagrass 27.27886 49.54524 12,470.00 ± 2097 20.73 ± 5.55 10.80 ± 1.57 0.28 ± 0.07 n/a 0.02 ± 0.00 Ras-Tanura Mangrove 26.71674 50.02111 21,578.89 ± 4285 73.07 ± 25.33 34.25 ± 8.28 0.98 ± 0.37 0.50 ± 0.05 0.07 ± 0.02 Ras-Tanura Mangrove 26.65781 50.00534 19,781.11 ± 4368 48.26 ± 12.28 8.27 ± 2.34 0.61 ± 0.16 0.26 ± 0.10 0.04 ± 0.01 Ras-Tanura Saltmarsh 26.65761 50.01003 25,790.00 ± 2756 50.58 ± 14.23 6.93 ± 1.37 0.67 ± 0.17 0.082 ± 0.010 0.04 ± 0.01 Ras-Tanura Saltmarsh 26.71998 50.02104 19,580.00 ± 2515 33.71 ± 17.83 4.55 ± 0.83 0.44 ± 0.23 0.13 ± 0.03 0.03 ± 0.02 Ras-Tanura Saltmarsh 26.73849 49.99526 13,393.33 ± 2348 44.57 ± 12.05 10.10 ± 4.43 0.55 ± 0.15 0.119 ± 0.014 0.02 ± 0.01 Ras-Tanura Seagrass 26.72047 49.99885 21,898.89 ± 3741 42.71 ± 13.24 5.29 ± 2.47 0.50 ± 0.17 0.10 ± 0.05 0.03 ± 0.01 Ras-Tanura Seagrass 26.70883 50.01225 8298.89 ± 993 10.29 ± 2.22 15.06 ± 3.95 0.14 ± 0.03 0.08 ± 0.02 0.01 ± 0.00 Ras-Tanura Seagrass 26.68064 50.01184 17,167.78 ± 1523 18.67 ± 6.14 19.11 ± 6.97 0.23 ± 0.08 n/a 0.02 ± 0.01 Ras-Tanura Mangrove 26.64246 50.01462 23,431.67 ± 2873 45.58 ± 25.99 13.84 ± 6.95 0.46 ± 0.23 0.28 ± 0.09 0.01 ± 0.01 Ras-Tanura Mangrove 26.63699 50.01067 13,261.11 ± 892 98.86 ± 28.59 16.02 ± 7.76 1.25 ± 0.38 0.44 ± 0.06 0.41 ± 0.09 Uqair Seagrass 25.72452 50.23141 7558.75 ± 957 68.88 ± 35.78 14.41 ± 8.09 1.03 ± 0.54 n/a 0.05 ± 0.03 Uqair Seagrass 25.72891 50.2299 10,197.14 ± 720 29.11 ± 10.41 6.09 ± 3.16 0.35 ± 0.13 0.16 ± 0.06 0.02 ± 0.01 Uqair Seagrass 25.73403 50.22677 5708.57 ± 581 15.72 ± 11.42 11.63 ± 5.83 0.22 ± 0.15 0.16 ± 0.06 0.01 ± 0.01 sample. When individual PAHs were below detection limit, an arbi- Kramer HSD post-hoc test, p = 0.0117). However, sediment TPH contrary value of 0.01 μgkg−1 was assigned for calculation of total concentrations did not differ significantly when surface (0–5 cm) with centration. 20.7% of sediment slices in this analysis had all individual deeper (5–20 cm) layers (ANOVA, p N 0.05) were compared. The aver- PAHs below detection limit. Analyses of reagent blanks, replicates, age polycyclic aromatic hydrocarbon concentrations in the top 0–5cm spiked, spiked duplicates and a reference material (NIST 1941b estu- of Arabian Gulf sediments ranged from 5.80 ± 1.06 μgkg−1 to 24.61 arine sediment) were carried out for both TPH and PAHs to assess for ±4.45μgkg−1 of dry sediment. Trends in total PAH concentrations any contamination and to ensure reproducibility of the results. Re- paralleled that of TPH, with no significant differences among habitats coveries were found in the range of 85–110% for all analyses (TPH (ANOVA, p N 0.05), while differing significantly among the sites and PAH). (ANOVA, p = 0.0155). Abu-Ali supported an average higher concentration of PAHs (∑16 + 1 PAHs) than Safaniya (Tukey-Kramer HSD post- 2.5. Data analysis hoc test, p = 0.0182). Concentrations of TPH and PAHs measured in each core are given in Table 1. Statistical analysis of all data was carried out using JMP® Pro 13.1.0. The stock of TPH in the Arabian Gulf sediments has increased several Individual tests conducted are described along with each major re- folds over the past century: the average stock sequestered in different ported result. Briefly, Analysis of Variance (two-tail) followed by blue carbon habitat sediments (accumulated between 1870 and 2016) Tukey's Post-Hoc tests were conducted to determine significant differ- increased from 0.60 ± 0.12 g TPH m−2 in saltmarsh sediments ences in average TPH concentrations among sites and habitat types. (Fig. 2c) to 1.98 g TPH m−2 in seagrass sediments (Fig. 2b), with man- For determination of historical pollution trends in the Gulf, total petro- grove sediments having intermediate average values of 1.54 ± leum hydrocarbon concentrations from individual Pb210 dated slices 0.85 g TPH m−2 (Fig. 2a). Nearly 75% of this stock has accumulated in were binned into 10 year time frames. The bins were then segregated the last three decades (Fig. 2). Rates of burial of TPH, calculated as the by habitat type to identify the decadal trends across saltmarsh, seagrass accumulated concentrations of TPH recorded in consecutive dated slices and mangrove habitat types. From the data set, two outliers on total pe- by time (in years), yielded an overall mean ± SE burial rate of 0.12 ± troleum hydrocarbon concentrations, one in seagrass (Core 23-Slice 0.04 g TPH m−2 y−1 for the entire region (Table 1). Mean burial rates 1) and one in mangrove core (Cores 17-Slice 3) were identified as of TPH were 0.02 ± 0.007, 0.13 ± 0.06 and 0.17 ± 0.10 anomalous values not correlated with total organic carbon and removed g TPH m−2 y−1 for saltmarsh (n = 47), seagrass (n = 106) and man- from further analysis. Corresponding PAH concentrations were also re- grove (n = 61) ecosystems, respectively (Table 2). Also, with an in- moved from the analysis. crease in concentrations and stocks of TPH over time, the overall burial rates of TPH have increased on average by over ten fold, from 3. Results and discussion oil exploration times (–1930 - 1960) at 0.025 ± 0.009 g TPH m−2 y−1 to the modern day (1985–2015) at 0.321 ± 0.144 g TPH m−2 y−1 for We reconstructed the history of petroleum inputs in the Arabian the entire region, and also is seen within habitats (Fig. 2d, 2e, 2f). Cur- Gulf and calculated burial rates for TPH supporting current sequestra- rent burial rates of 0.321 ± 0.14 g TPH m−2 y−1 (average ± SE), repre- tion and stocks in Gulf blue carbon habitats sediments (Fig. 2). The sent a total burial of 2300 Mg y−1 of TPH, when scaled to the total area TPH concentrations in the top 0–5 cm of Gulf sediments ranged broadly coverage of blue carbon habitats of the Arabian Gulf for seagrass from 25.18 ± 9.60 mg kg−1 to 85.86 ± 51.93 mg kg−1 and were not (7000 km2;(Erftemeijer and Shuail, 2012)) and mangrove (165 km2; significantly different among the studied habitats (ANOVA, p N 0.05) (Almahasheer, 2018)) respectively. but differed significantly among sites (ANOVA, p = 0.02), with Abu Ali Notably, over time, seagrass sediments have accumulated more TPH supporting, on average, higher concentrations than Safaniya (Tukey- relative to mangroves, despite a higher burial rate experienced by A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212 209

Fig. 2. Accumulated stock of total petroleum hydrocarbons (g m−2) and burial rate of TPH (g TPH m−2 y−1) (mean ± SE), from 1870 to 2015 in blue carbon habitat sediments along the Saudi Arabian coastline of the Arabian Gulf. Total petroleum hydrocarbon stock accumulated (g m−2 of sediment) (mean), from left to right (top row) in (a) mangrove, (b) seagrass and (c) saltmarsh sediments. Legend at top-right margin of the figure denotes the four different time-windows in accumulation of TPHs. Burial rate (g TPH m−2 y−1)(mean±SE),fromleftto right (bottom row) in (d) mangrove, (e) seagrass and (f) saltmarsh sediments. mangrove ecosystems (Table 2). Decadal TPH concentrations show a been derived from natural sources such as marine plants (Vane et al., pattern characterized by two distinct peaks over time, with periods of 2009). We compare our data with data from similar studies in the liter- elevated TPH concentrations in the 1930's coinciding with oil discovery ature (Table 3). Some of the highest values observed in this study and exploration, and in the 1990's coinciding with the major oil spill in (582 mg kg−1) are comparable to sites impacted by major historical 1991 resulting from the Gulf War (Fig. 3). Comparison of the average oil spills, such as Gulf of Mexico sediments affected by the Deepwater sediment TPH concentrations in different periods show that all habitats Horizon well blow-out (Duan et al., 2018; Kirman et al., 2016; experience an increase in TPH concentrations relative to pre-industrial Sammarco et al., 2013) or regions with intense anthropogenic activities times (pre-industrial refers to the time before oil exploitation (1930's) over the past century such as Guangzhou, China for this region), and a delay in the occurrence of increased concentration (1179.3–6354.9 mg kg−1) and Bohai Bay, China (6.3–535 mg kg−1) depending on the position of the habitat types along the tidal range (Li et al., 2012; Zhou et al., 2014)(Table 3). Furthermore, the concentra- (Fig. 3). Seagrass habitats displayed a two-fold increase in concentration tions of sedimentary TPH in the Arabian Gulf is higher even than sedi- during the oil exploration period (1930–1960) relative to pre-industrial mentary concentrations reported from oil refinery sites in the times (Fig. 3a). Mangroves showed a gradual increase in concentration Peninsula de Paraguana in Venezuela (18.7 ± 26.4 mg kg−1), which from the preindustrial to oil exploration period, pre-Gulf war houses the world's largest oil deposits (Croquer et al., 2016)(Table 3). (1960–1985) and contemporary (1986–2015) periods, when concen- Also, the average sediment concentrations of TPH of 50.9 ± trations were 7-fold higher than preindustrial levels (Fig. 3b). Salt- 9.8 mg kg−1 for the western Arabian Gulf is about 70 times larger marsh sediments showed relatively low concentrations until the con- than the concentrations reported in mangrove sediments in the eastern temporary (1986–2015) period, when concentrations were 6-fold shore (Mohebbi-Nozar et al., 2015)reflecting the predominant counter- higher than previous levels (Fig. 3c). Hence, the elevated concentrations clockwise circulation characteristic of the Arabian Gulf (Fig. 1). in the 1990's, associated with the Gulf war, were most prominent in In contrast with the tendency for sediment TPH concentration to in- mangrove sediments (Fig. 3b), while seagrass meadows showed a crease exponentially toward surface sediments (e.g. (Latimer et al., marked peak in the 1930's, associated with oil discovery and explora- 2003; Vane et al., 2011; Zhou et al., 2014)), indicative of petroleum pol- tion (Fig. 3a), and salt-marshes only displayed elevated sediment TPH lution being a relatively recent problem, sediments in the Arabian Gulf concentrations in the past decade (Fig. 3c). The decadal concentrations showed elevated TPH concentrations since 1930. This is a clear result also indicate that the higher stock accumulated in the seagrass sedi- of the discovery of large oil fields in the Arabian Gulf in 1938 ments relative to mangrove sediments, arises from the latter having ex- (Beydoun, 1998) followed by their exploitation, leading to an extended perienced massive oil inputs only over the last three decades (Table 2). period of petroleum pollution in the Arabian Gulf until present day The concentrations of TPH we report here (overall mean concentra- (Sheppard et al., 2010). Indeed, from reconstruction of the time-lines of tion in surface sediments (0–5 cm) of 50.9 ± 9.8 mg kg−1) are substan- TPH concentration in blue carbon habitat sediments in the Arabian Gulf tially higher than those reported for non-polluted estuarine and coastal major inputs corresponded to, 1) oil discovery and the onset of exploita- sediments (2–10 mg kg−1 (Vane et al., 2011)), where TPHs may have tion in the 1930's, most pronounced in seagrass sediments, 2) major oil 210 A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212

spills associated with the Gulf War (Sadiq and McCain, 1993)mostprom- inent in mangrove sediments, and 3) a more recent increase, most prominent in salt-marshes. Lack of awareness and the technology to avoid oil spills led to major inputs into the Arabian Gulf environment during the development of the oil industry in the 1930's (Beydoun, 1998) with spills of different magnitudes reoccurring since. In fact, the spill associated with the Gulf War of 1991 with an estimated release of 1.3 million tonnes of petroleum and/or oil equivalent from deliberate opening of oil pipelines, and destruction of several oil-well heads is still the largest oil spill on record (Sadiq and McCain, 1993). Impact of such oil spills have also been discovered in coral reef cores collected from Saudi Arabia, where high levels of petroleum biomarkers in coral annual bands were found to cor- respond with time periods of major pollution events (Poulsen et al., 2006). Spilled oil intersecting the coast at the intertidal zone occupied by mangroves has been reported to cause widespread mortality of mangrove in Kuwait waters (Bejarano and Michel, 2010). Due to the prevail- ing counterclockwise current of the Arabian Gulf (Fig. 1,(Reynolds, 1993)), most of the oil spilled is advected into Saudi waters leading to major accumulation in intertidal sediments, which is clearly reflected in the mangrove sediments sampled here, that occupy the zone along the coastal slope intersecting with water surface through the tidal cycle (Ehrhardt and Burns, 1993; Fowler et al., 1993; Massoud et al., 1998). The consistent pattern of peak concentrations reached earlier in seagrass meadows (1930's), followed by mangrove sediments (1990's) and more recently in salt-marsh sediments (2010–2015) re- flects their position along the intertidal slope and their exposure to petroleum reaching the coast as a surface slick over time. Rates of local sea- level rise in the western Arabian Gulf have been reported to range from 0.23 ± 0.05 mm y−1 to 15.21 ± 7.42 mm y−1 (Alothman et al., 2014; Hosseinibalam et al., 2007) with values of 4 mm y−1 in Abu Ali, a site sampled here, and maximum values of 15.21 ± 7.42 mm in Iman Hassan, located in the northern-eastern part of the Gulf (Hosseinibalam et al., 2007). About, 1/3 of the local sea-level rise is due to subsidence on average, associated with groundwater extraction for urban use and, particularly important in the study region, industrial use as groundwater pumped into subterranean petroleum wells to push the petroleum to the surface of the wells (Alothman et al., 2014)Consid- ering a sea-level rise rate of 4 mm y−1 for Abu-Ali (Alothman et al., 2014), seagrass meadows, sampled at an average depth of 2–3mat low-tide in this study would have been up to 0.34 m shallower than their current depth in the 1930s, and therefore substantially more exposed to surface oil slicks than they are in the current deeper location. Mangroves remain equally exposed, as they grow along the intertidal zone and can move with the sea-level rise, while salt-marsh soils sampled in this study, would have been elevated relative to the high tide mark in the 1930s compared to the present. Hence, the combination of sea level rise and land subsidence renders seagrasses meadows in the Arabian Gulf increasingly sheltered from petroleum inputs, while, on the contrary, salt-marshes were sheltered in their elevated positions in the past, but are becoming progressively vulnerable as they are more likely to be flooded during high tide and thus more exposed to oil slicks, and hence show a recent increase in accumulation of oil. Blue carbon habitats, accumulating sediments at rates of about 2 to 4mmyear−1, have been identified as globally significant sinks for organic carbon (Duarte et al., 2013, 2004). Our results show that they also act as sinks for petroleum hydrocarbons, with burial rates increas- ing over time. We calculate, based on an average organic carbon stock of 14.3 g C kg−1 of sediment and 0.1139 g kg−1 as petroleum hydrocarbons, that petroleum burial in sediments of Arabian Gulf blue carbon habitats support 0.79% of its total organic carbon sink. This is especially relevant considering that many TPHs are considered highly toxic pollut- Fig. 3. Decadal trends in concentration of total petroleum hydrocarbon (mg kg−1)(mean ants and can cause harm to aquatic life. Modern sediments of seagrass ± SE) in of Arabian Gulf sediments from blue carbon habitats of (a) seagrasses and mangrove sediments of the Saudi coast of the Arabian Gulf (corre- (b) mangroves and (c) saltmarshes. sponding to 1990–2015 period) contains an estimated stock of 1250 A. Ashok et al. / Science of the Total Environment 669 (2019) 205–212 211

Table 2 Average values across habitats (without depth segregation) of concentration, stock and burial rate of total petroleum hydrocarbons, concentration of total polycyclic aromatic hydrocarbons (∑16 + 1 PAHs (μgkg−1)) and total organic carbon. All values shown are mean ± SE.

Habitat Number of sites Number of cores Number of slices TPH TPH stock TPH burial rate ∑16 + 1 PAHs TOC (mg kg−1) (g m−2) (g m−2 y−1) (μgkg−1) (mg kg−1)

Mangrove 2 7 61 101.35 ± 54.62 1.54 ± 0.85 0.17 ± 0.11 17.48 ± 2.28 10,698.21 ± 817.13 Saltmarsh 2 6 47 43.33 ± 8.78 0.61 ± 0.13 0.02 ± 0.01 11.11 ± 1.96 9261.09 ± 553.98 Seagrass 4 12 106 151.47 ± 93.41 1.99 ± 1.23 0.13 ± 0.07 13.04 ± 2.53 18,474.91 ± 866.73

Table 3 Comparison of total petroleum hydrocarbon concentrations (mg kg−1) in sediments among oil affected sites globally.

Location Major oil spill event/sources Year of TPH concentration (mg Remarks Reference sampling kg−1)

Clyde Estuary, UK No specific source identified 2011 34–4386 Core samples (Vane et al., 2011) Guangzhou, China Petrochemical complex 2007 1179.3–6354.9 Surface sediment samples (Li et al., 2012) Bohai Bay, China No specific source identified 2001–2011 6.3–535 Surface sediment samples (Zhou et al., 2014) Jimmy Bay, Gulf of Mexico Deep Water Horizon well blow-out, 2010, 2015 0.04–2.31 Surface sediment samples (Duan et al., 2018) oil refineries etc. Barataria Bay, Gulf of Mexico Deep Water Horizon well blow-out, 2010, 2011 94–2984 Core samples (Kirman et al., 2016) oil refineries etc. Peninsula de Paraguana, Venezuela Oil refineries and extraction 2008–2014 18.7 ± 26.4 Surface sediment samples (Croquer et al., 2016) Arabian Gulf (Saudi coastline) Gulf war oil spill 1991, oil refineries etc 2016 50.9 ± 9.8 Core samples This study Arabian Gulf (near shore) Gulf war oil spill 1991, oil refineries etc 2002 87.8 ± 18.9 Surface sediment samples (Joydas et al., 2017) Arabian Gulf (offshore) Gulf war oil spill 1991, oil refineries etc 2002 54.1 ± 7.0 Surface sediment samples (Joydas et al., 2017) and 45.9 Mg of TPH, respectively considering 370 km2 and 10.4 km2 oc- CRediT authorship contribution statement cupied by seagrasses and mangroves, respectively (Almahasheer, 2018; Erftemeijer and Shuail, 2012)). If the burial rate and stock reported here Ananya Ashok: Writing - original draft, Writing - review & editing. Mi- for the Saudi Coast are representative of the Arabian Gulf, a total stock of chael Cusack: Methodology, Formal analysis, Writing - original draft, 59,799 Mg of TPH would be buried in recent (1990–2015) mangroves Writing - review & editing. Vincent Saderne: Methodology, Formal anal- and seagrass sediments in the Arabian Gulf. ysis, Writing - review & editing. Periyadan K. Krishnakumar: Methodol- Concerns that disturbance of blue carbon habitats can lead to emis- ogy, Formal analysis, Writing - review & editing. Lotfi Rabaoui: sions of CO2 are the main motivation to conserve these habitats under Methodology, Formal analysis, Writing - review & editing. Mohammed “blue carbon” strategies (Duarte et al., 2013; Pendleton et al., 2012). A. Qurban: Methodology, Formal analysis, Funding acquisition, Writing However, our results demonstrate that these sediments are also impor- - review & editing. Carlos M. Duarte: Methodology, Funding acquisition, tant sinks for TPH, which would be released into the environment if Supervision, Writing - original draft, Writing - review & editing. Susana these habitats are disturbed. Avoidance of the remobilization of TPHs Agustí: Conceptualization, Methodology, Funding acquisition, Supervi- trapped from past spill events provides an additional motivation to con- sion, Writing - original draft, Writing - review & editing. serve seagrass, mangroves and salt-marshes in the Arabian Gulf and other coastal areas. Acknowledgements

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