Hatem Zaghden
email: hatem.zaghden@gmail.com

Marc Tedetti

Sami Sayadi

Mohamed Moncef

Boubaker Elleuch

Alain Saliot

Origin and distribution of hydrocarbons and organic matter in the surficial sediments of the Sfax-Kerkennah channel (Tunisia, Southern Mediterranean Sea)

Keywords: Sediment, hydrocarbons, organic matter, Sfax, Gulf of Gabès, Mediterranean Sea

We investigated the origin and distribution of aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs) and organic matter (OM) in surficial sediments of the Sfax-Kerkennah channel in the Gulf of Gabès (Tunisia, Southern Mediterranean Sea). TOC, AH and PAH concentrations ranged 2.3-11.7%, 8-174 µg g -1 sed. dw and 175-10,769 ng g -1 sed. dw, respectively. The lowest concentrations were recorded in the channel (medium sand sediment) and the highest ones in the Sfax harbor (very fine sand sediment). AHs, PAHs and TOC were not correlated for most of the stations. TOC/N and δ 13 C values revealed a mixed origin of OM with both marine and terrestrial sources. Hydrocarbon molecular composition highlighted the dominance of petrogenic AHs and the presence of both petrogenic and pyrogenic PAHs, associated with petroleum products and combustion processes. This work underscores the complex distribution patterns and the multiple sources of OM and hydrocarbons in this highly anthropogenized coastal environment.

Introduction

The study of the composition of coastal sediments represents one of the main pathways to highlight the level and source of contamination within marine ecosystems. Indeed, sediments are receptacles for a wide variety of organic contaminants emitted from land and which reach the sedimentary layer through their adsorption onto particles in the water column.

In addition, sediments may accumulate biogenic particulate material issued from surface waters and thus may provide fruitful information about autochthonous biological activity of marine ecosystems.

Hydrocarbons, including aliphatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs), are abundant components of the organic material in coastal sediments [START_REF] Volkman | Identification of natural anthropogenic and petroleum hydrocarbons in aquatic sediments[END_REF][START_REF] Gogou | Marine organic geochemistry of the Eastern Mediterranean: 1. Aliphatic and polyaromatic hydrocarbons in Cretan Sea surficial sediments[END_REF], PAHs being among the most ubiquitous organic contaminants in the marine environment [START_REF] Louati | Hydrocarbon contamination of coastal sediments from the Sfax area (Tunisia), Mediterranean Sea[END_REF][START_REF] Roose | Monitoring organic microcontaminants in the marine environment: principles, programmes and progress[END_REF].

AHs, which may be of biogenic or anthropogenic origin, consist of a series of resolved nalkanes (R) and of unresolved complex mixture (UCM). Their biogenic sources include terrestrial plant waxes, marine phyto-and zoo-plankton, bacteria and diagenetic transformations [START_REF] Cripps | Problems in the identification of anthropogenic hydrocarbons against natural background levels in the Antartic[END_REF][START_REF] Rieley | The biogeochemistry of Ellesmere Lake, UK-I: source correlation of leaf wax inputs to the sedimentary lipid record[END_REF][START_REF] Volkman | Identification of natural anthropogenic and petroleum hydrocarbons in aquatic sediments[END_REF][START_REF] Wang | Oil spill identification[END_REF][START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF], whereas their anthropogenic sources comprise essentially unburned petroleum/oils [START_REF] Mazurek | Characterization of biogenic and petroleum-derived organic matter in aerosols over remote rural and urban areas[END_REF][START_REF] Bouloubassi | Investigation of anthropogenic and natural organic inputs in estuarine sediments using hydrocarbon markers (NAH, LAB, PAH)[END_REF][START_REF] Wang | Using systematic and comparative analytical data to identify the source of an unknown oil on contaminated birds[END_REF][START_REF] Readman | Petroleum and PAH contamination of the Black Sea[END_REF][START_REF] Zaghden | Hydrocarbons in surface sediments from the Sfax coastal zone, (Tunisia) Mediterrranean Sea[END_REF].

Parent PAHs and their alkylated homologues (i.e. mono-, di-, tri-or tetra-methyl PAHs) may be also of biogenic or anthropogenic origin. PAHs are synthesized during the formation of oil (petrogenic PAHs), during the incomplete combustion of fossil fuels and biomass (pyrogenic PAHs) [START_REF] Wang | Oil spill identification[END_REF][START_REF] Wurl | A review of pollutants in the sea-surface microlayer (SML): a unique habitat for marine organisms[END_REF], and biologically produced in soils from woody plants, termites or from the microbial transformation of organic matter [START_REF] Chen | Termites fumigate their nests with naphthalene[END_REF][START_REF] Wilcke | Carbon isotope signature of polycyclic aromatic hydrocarbons (PAHs): evidence for different sources in tropical and temperate environments?[END_REF]. Petrogenic PAHs consist of low molecular weight (LMW) 2-3 ring compounds with a high proportion of alkylated homologues, while pyrogenic PAHs comprise high molecular weight (HMW) 4-6 ring compounds with a low proportion of alkylated derivatives [START_REF] Neff | Polycyclic aromatic hydrocarbons in the aquatic environment sources, fates and biological effects[END_REF]. PAHs are known to be harmful to living organisms, with reprotoxic, carcinogenic and mutagenic effects [START_REF] Kennish | Poly-nuclear aromatic hydrocarbons[END_REF]. Therefore, they are recognized for a long time as highly priority contaminants by various international Hence, hydrocarbons (AH and PAHs) in sediments may originate from numerous sources including petroleum inputs, incomplete combustion of fuels (PAHs), forest and grass fires (PAHs), biosynthesis by marine and terrestrial organisms, and early diagenetic transformation processes (UNEP/IOC/IAEA, 1992; [START_REF] Clark | Marine Pollution[END_REF][START_REF] Readman | Petroleum and PAH contamination of the Black Sea[END_REF].

Anthropogenic hydrocarbons are introduced into marine waters mainly via the direct discharge of crude oil and petroleum products during sea-based activities (spills from tankers, platforms and pipelines, ballast water discharge, drilling…) or via industrial and urban wastes (fuel combustion, traffic exhaust emissions, varied spills) routed by rivers, surface runoffs, effluents and the atmosphere [START_REF] Wang | Oil spill identification[END_REF][START_REF] Wurl | A review of pollutants in the sea-surface microlayer (SML): a unique habitat for marine organisms[END_REF][START_REF] Dachs | Organic Pollutants in Coastal Waters, Sediments, and Biota: A Relevant Driver for Ecosystems During the Anthropocene?[END_REF]. Thus, investigating the concentrations and molecular composition of hydrocarbons in surficial sediments allows for a better understanding of the levels and sources of anthropogenic contaminations and of the origin (marine versus terrestrial) of natural organic matter in coastal waters [START_REF] Budzinski | Evaluation of sediment contamination by polycyclic aromatic hydrocarbons in the Gironde estuary[END_REF].

The Gulf of Gabès (Southeast Tunisia, Southern Mediterranean Sea) presents two major characteristics. First, it is one of the most productive coastal environments of the Mediterranean Sea due to nutrient availability and the main fishing area of the Tunisian coasts (Jabeur et al., 2001;Bel Hassen et al., 2009;D'Ortenzio and Ribera d'Alcalà, 2009;[START_REF] Mermex | Marine ecosystems responses to climatic and anthropogenic forcings in the Mediterranean[END_REF]. Second, the Gulf of Gabès is submitted to very high anthropogenic pressures, especially around Sfax city, the second largest city in Tunisia and the one with the most important fishing and harbor activities. In this highly urbanized and industrialized area, organic contaminants are issued from a multitude of sources [START_REF] Illou | Impact des rejets telluriques d'origines domestiques et industrielles sur les environnements côtiers : cas du littoral nord de la ville de Sfax (Tunisie)[END_REF][START_REF] Serbaji | Utilisation d'un S.I.G. multi-sources pour la compréhension et la gestion intégrée de l'écosystème côtier de la région de Sfax (Tunisie)[END_REF][START_REF] Louati | Etude de la contamination par hydrocarbures des sédiments de la région de Sfax (Tunisie)[END_REF]. However, the studies dealing with the assessment of the hydrocarbon levels in the surficial sediments of the Gulf of Gabès/Sfax coastal area remain few compared to those conducted in the Northwestern Mediterranean Sea. They have really begun in the 2000's with the first reports of concentrations in AHs [START_REF] Louati | Hydrocarbon contamination of coastal sediments from the Sfax area (Tunisia), Mediterranean Sea[END_REF][START_REF] Zaghden | Hydrocarbons in surface sediments from the Sfax coastal zone, (Tunisia) Mediterrranean Sea[END_REF][START_REF] Elloumi | Detection of Water and Sediments Pollution of An Arid Saltern (Sfax, Tunisia) by Coupling the Distribution of Microorganisms With Hydrocarbons[END_REF][START_REF] Aloulou | Even-numbered nalkanes/n-alkenes predominance in surface sediments of Gabes Gulf in Tunisia[END_REF][START_REF] Amorri | Organic matter compounds as source indicators and tracers for marine pollution in a western Mediterranean coastal zone[END_REF], in PAHs [START_REF] Kessabi | Possible chemical causes of skeletal deformities in natural populations of Aphanius fasciatus collected from the Tunisian coast[END_REF], and in both AHs and PAHs [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF]. Interestingly, the AH and PAH concentrations found in the surficial sediments of the Sfax coastal area are rather located in the upper range of those recorded for the whole Mediterranean Sea.

The main objectives of this study were 1) to determine the contents in several biogeochemical parameters (C/H/N/S, TOC, CaCO 3 and δ 13 C) and in hydrocarbons (AHs and PAHs) in surficial sediments of the Sfax-Kerkennah channel and to compare these contents to those recorded in other regions of the Mediterranean Sea. 2) To assess the spatial distribution of these parameters in relation to the Sfax-Kerkennah channel geomorphology/bathymetry and to examine the degree of correlation between these parameters, especially between TOC, AHs and PAHs. 3) To evaluate the origin (marine versus terrestrial versus anthropogenic) of organic matter and hydrocarbons regarding molecular/isotopic ratios and indices.

Material and methods

Study area

This study was conducted in the coastal area between the Sfax city and the Kerkennah Islands, in the Northern part of the Gulf of Gabès (Tunisia, Southern Mediterranean Sea) (Fig. 1). The Gulf of Gabès, located in the Southeast of Tunisia, extends from the city of Chebba (35°14'N, 11°09'E) to the Tunisian-Libyan border (33°10'N, 11°34'E) and shelters the Kerkennah Islands in the Northeast and Djerba Island in the Southeast. The climate in the Gulf of Gabès is arid to semiarid, i.e. dry (average annual precipitation: 210 mm) and sunny with strong easterly winds (INM, 2008), resulting in a severe aeolian erosion and the transport of Saharan dusts into the sea [START_REF] Jedoui | Etude hydrologique et sédimentologique d'une lagune en domaine méditerranéen: Le bahiret el Boughrara (Tunisie)[END_REF][START_REF] Bouaziz | Néotectonique affectant les dépôts marins tyrrhéniens du littoral sud-est tunisien: implications pour les variations du niveau marin[END_REF]. The gulf is characterized by a wide continental shelf (~ 250 km) with a very low slope and an important network of channels and wadis. Hence, its basin is very shallow, with in some areas, a water depth not exceeding 1 m over several kilometres. Also, in the Gulf of Gabès, the tide is semidiurnal and one of the highest in the Mediterranean Sea, with a maximum range of approximately 2.3 m at spring low tides [START_REF] Sammari | Sea level variability and tidal resonance in the Gulf of Gabes, Tunisia[END_REF]. The sediment of the gulf is rich in organic matter and principally composed of sand with a high density of plants [START_REF] Ben Othman | Le sud tunisien (golfe de Gabès) : Hydrologie, sédimentologie, flore et faune[END_REF]. The Gulf of Gabès is an important nursery for several species of fish and accounts for 65% of the Tunisian fish production [START_REF] Dgpa | Direction Générale de Pêche et de l'Aquaculture[END_REF]. These favourable geomorphologic and climatic conditions have led to the development of one of the most extensive marine habitats of seagrass

Posidonia oceanica.

Sfax (34°43'N, 10°46'E; Fig. 1), with a population of about 730,000 inhabitants distributed over about 20,000 ha, represents the second largest city and the second economic pole in Tunisia. Sfax is heavily industrialized with important fishing and harbor activities. Its main industrial activity domains are phosphates, chemical products, textiles, olive oil, food, soap and paint. Therefore, the sources of pollution in the Sfax coastal zone are numerous: atmospheric depositions, ship traffic, fishery activities, rivers, wadis, wild landfills, municipal sewage effluents and industrial wastewaters, especially those coming from the storage of crude oil, phosphogypsum and olive oil wastes at the coast [START_REF] Ben Mustapha | Bionomie des étages infra et circalittoral du golfe de Gabès[END_REF][START_REF] Louati | Hydrocarbon contamination of coastal sediments from the Sfax area (Tunisia), Mediterranean Sea[END_REF]. The Kerkennah Islands (34°42'N, 11°11'E; Fig. 1), which include the two principal islands Gharbi and Chergui, are situated ~ 20 km offshore Sfax. They have an area of 160 km 2 and are low-lying being no more than 13 m above the sea level. These islands are characterized by a -10 m isobath few kilometers away from the shoreline and by a lithology dominated by smooth rocks [START_REF] Katlane | Recent dynamics of submerged shoals and channels aroud the Kerkennah archipelago (Tunisia) from LANDSAT TM and MODIS[END_REF]. Since there is neither industrial activity nor urban concentration in the Kerkennah Islands, the latter are much less submitted to anthropogenic pressures than the Sfax coastal area.

The Sfax-Kerkennah channel is an underwater channel, Northeast-Southwest oriented, which cut the shelf at depths higher than 20 m. It allows for conducting the ferry boat crossings Sfax-Kerkennah (Fig. 1). Bottom sediments are largely composed of sand, muddy sand and shell-sand with a high density of plants. This structure extends from the beaches up to about 20 m depth. Muddy sands are the most widespread sediment of the Sfax shelf. This sediment is the substrate of mixed Cymodocea and Posidonia meadows. In some places (southern Kerkennah), areas with gravel or concretions of calcareous algae are distinguished.

At the Kerkennah islands, the shelf is characterized by the presence of sandy shoals.

Sampling, storage, sieving and granulometric analysis of samples

Twenty stations were sampled in January 2005 at low tide on board a vessel from the Fishing Professional Training Centre of Sfax. Stations were located along two transects: one between the North of Sfax -Sidi Mansour area (station R101) and the Kerkennah harbor (station R111), and the other one between the Kerkennah harbor and the Sfax harbor (station R201) (Fig. 1; Table 1). Stations R104-R107 and R203-R210, displaying depths from > 5 to 20 m, were comprised within the Sfax-Kerkennah channel, whereas stations R101-R103, R108-R111, R201 and R202, with depths < 5 m, were positioned outside the channel (Table 1). At each station, surficial sediments (0-1 cm) were collected by professional divers.

Surficial sediments were transferred into pre-combusted (450 °C, 6 h) glass bottles and stored on board in the dark in the cold (~ 6 °C).

Back in the laboratory, samples were immediately frozen at -20 °C. They were maintained frozen for few days and then freeze dried. Afterwards, each sediment sample (200 g) was dry sieved sequentially during 30 min, using an electric shaker, through twenty AFNOR standard stainless steel screens of mesh sizes ranging from 4 mm to 50 µm [START_REF] Aloulou | Benthic foraminiferal assemblages as pollution proxies in the northern coast of Gabes Gulf, Tunisia[END_REF]. Before sieving, stainless steel forceps were used to remove vegetal fragments from sediment samples. The percentages of sediment mass corresponding to each particle size were determined. Grain Size Analyzer (GSA) software was used to establish granulometric curves and to derive the parameters describing the grain size distribution, i.e. the index of mean trend, expressed by the mean of the size (M z ), and the index of classification, expressed by the standard deviation of the size (σ). M z and σ are given in unit ϕ, where ϕ = -log 2 D/D 0 (D 0 being a reference diameter and D the diameter of the particle in mm) [START_REF] Folk | Brazos River bar: a study in the significance of grain size parameters[END_REF][START_REF] Blott | GRADISTAT: a grain size distribution and statistics package for the analysis of unconsolidated sediments[END_REF][START_REF] Ghannem | Spatial Distribution of Heavy Metals in the Coastal Zone of ''Sfax-Kerkennah'' Plateau, Tunisia[END_REF]. All chemical analyses (hydrocarbons, elemental composition, organic carbon, calcium carbonate, δ 13 C, see below) were performed on the fraction < 63 µm, which represents the fraction of silts and clays.

Extraction, purification and GC-FID/MS analysis of hydrocarbons

Sediment samples (around 5 g dry weight of the fraction < 63 µm) were transferred into a pre-combusted glass tube, in which were also added 30 ml of a dichloromethane (CH 2 Cl 2 )/methanol (CH 3 OH) (3:1 v/v) solvent mixture, deuterated (internal) standard mixtures (n-C 24 -d 50 for AHs and p-terphenyl-d 14 for PAHs) and activated copper to remove sulphurs. Extraction was carried out in an ultrasonic bath for 15 min. The sediment was then isolated from the lipid extract by centrifugation (3500 rpm for 10 min). The entire extraction procedure (ultrasounds and centrifugation) was repeated 3 times. The extracts (supernatants) were combined and concentrated with a rotary evaporator at T < 30 °C, dried with magnesium sulphate and filtered on a pre-combusted glass fiber filter of porosity 4 µm. The remaining solvent was changed to n-hexane [START_REF] Ke | Determination of polycyclic aromatic hydrocarbons in mangrove sediments: Comparison of two internal standard surrogate methods and quality-control procedures[END_REF][START_REF] Bouloubassi | PAH transport by sinking particles in the open Mediterranean Sea: a 1 year sediment trap study[END_REF][START_REF] Parinos | Occurrence, sources and transport pathways of natural and anthropogenic hydrocarbons in deep-sea sediments of the eastern Mediterranean Sea[END_REF].

Hexane-solubilised extracts were purified to separate AHs and PAHs (hydrocarbon fraction) from more polar compounds. The extracts were fractionated on a column (6 mm i.d.) filled with 400 mg of silica gel (extra pure Merck 60), beforehand Soxhlet extracted with CH 2 Cl 2 , activated 1 h at 150 °C and partially deactivated with 4% water by weight. The AH fraction was first eluted with 3 ml n-hexane, followed by the elution of the PAH fraction with 9 ml hexane/toluene (9:1 v/v). Finally, purified extracts were concentrated to 150-200 µl with a rotary evaporator and a gentle stream of nitrogen.

Analyses of hydrocarbons (AHs and PAHs) were performed with a Delsi DI 200 gas chromatograph coupled with a flame ionisation detector (GC-FID) (Perichrom, France) [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF]. The GC-FID was equipped with a DB-5 MS fused-silica capillary column (30 m × 0.25 mm × 0.25 µm, J&W Scientific, Agilent Technologies, USA) and used helium as a carrier gas at a flow rate of 1.5 ml min -1 . The injector (used in splitless mode) and detector temperatures were 250 and 320 °C, respectively. The initial column temperature was held for 2 min at 60 °C, next ramped at 25 °C min -1 (ramp 1) to 100 °C and then at 2 °C min -1 (ramp 2) to a final temperature of 310 °C, which was held for 10 min. With each set of samples to be analysed, AH and PAH calibration standards were run for peak identification and quantification. Compounds were identified mainly by their retention times.

To confirm the structure of several hydrocarbons, some samples were also analysed using a HP 6890 gas chromatograph coupled with a HP 5973 MSD mass spectrometer (GC-MS) (Agilent Technologies, Wilmington DE, USA), equipped with a DB-5 MS fused-silica capillary column coated with 5% phenyl methyl siloxane [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF].

Helium was used as a carrier gas. The injector temperature and column temperature program were the same than those used for the GC-FID. GC-MS analyses were run in the electron impact mode at 70 eV with a 0.6 scan s -1 time over a 50-550 atomic mass unit (amu) range resolution.

Quality assurance and quality control of hydrocarbon analyses

During the procedures described above, nitrile gloves were worn and care was taken to avoid contaminations. All the glassware was cleaned with ultrapure water (Milli-Q water from Millipore system, final resistivity: 18.2 MΩ cm -1 at 25 °C, pH ~ 5), combusted at 450 °C during 6 h and finally cleaned with CH 3 OH and CH 2 Cl 2 before use. The precision glassware, which could not be baked, was cleaned in a bath of sulfochromic acid for at least 4 h, and then rinsed with ultrapure water, CH 3 OH and CH 2 Cl 2 . All the (Teflon-lined) caps were wrapped with Teflon tape for the storage of samples. All organic solvents were of trace-analysis quality (Merck, Darmstadt, Germany) and were further distilled before use. Silica and magnesium sulphate were purified by soxhlet extraction with CH 2 Cl 2 for 24 h and then dried in the oven. Deuterated (internal) standards mixtures (04071, Fluka, and 47543-U, Supelco) were introduced before ultrasonic treatment to assess the recoveries of the analytical procedure (including extraction, evaporation and purification processes). The latter were on average > 75% for the different AHs and PAHs investigated. Calibration (external) standards (04071, Fluka, and 47543-U, Supelco) as well as procedural and solvent blanks were run with each set of samples to check for contamination and for quantification. Calibration curves were constructed for all target hydrocarbons analysed except for the alkylated PAHs, which were quantified with their parent-compound calibration curves. For the different hydrocarbons, the detection limits ranged from 0.01 to 0.5 ng g -1 . Instrumental reproducibility, evaluated from samples R110 and R207, was on average ± 10%. All the concentration values, given in ng g -1 (PAHs) or µg g -1 (AHs) sediment dry weight (sed. dw), were blank-and recovery-corrected.

Determination of individual hydrocarbons and molecular diagnostic ratios

For AHs, we determined the concentrations of resolved n-alkanes (R), including linear n-alkanes from n-C 15 to n-C 34 and two isoprenoids, pristane (Pr, C 19 ) and phytane (Phy, C 20 ), and the concentrations of UCM. We computed different ratios and indices allowing to distinguish biogenic AHs (issuing from biological activity) and petrogenic AHs (coming from uncombusted petroleum): 1) the UCM/R ratio as indicator of the presence of degraded petroleum products (when > 3-4) [START_REF] Simoneit | Organic matter of the troposphere-II.* Natural background of biogenic lipid matter in aerosols over the rural western United States[END_REF][START_REF] Mazurek | Characterization of biogenic and petroleum-derived organic matter in aerosols over remote rural and urban areas[END_REF]. 2) The Pr/Phy ratio as indicator of biogenic AHs (when >> 1), even though values < 1 do not necessarily reflect the presence of petrogenic AHs [START_REF] Cripps | Problems in the identification of anthropogenic hydrocarbons against natural background levels in the Antartic[END_REF][START_REF] Commendatore | Natural and anthropogenic hydrocarbons in sediments from the Chubut River (Patagonia, Argentina)[END_REF][START_REF] Cincinelli | Natural and anthropogenic hydrocarbons in the water column of the Ross Sea (Antarctica)[END_REF]. 3) The n-C 17 /Pr and n-C 18 /Phy ratios as indicators of degraded (when < 1) or less degraded or relatively fresh AHs (when > 1) [START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF][START_REF] Asia | Occurrence and distribution of hydrocarbons in surface sediments from Marseille Bay (France)[END_REF]. 4) The carbon preference index, the ratio of odd to even carbon-numbered n-alkanes, in the ranges n-C 15 ˗n-C 24 and n-C 25 ˗n-C 34 (CPI 15-24 and CPI 25-34 ), which are indicators of crude oil/petrogenic AHs (when ~ 1) or biogenic AHs (when >> or << 1) [START_REF] Eglinton | Leaf epicuticular waxes[END_REF][START_REF] Rieley | The biogeochemistry of Ellesmere Lake, UK-I: source correlation of leaf wax inputs to the sedimentary lipid record[END_REF][START_REF] Wang | Oil spill identification[END_REF][START_REF] Harji | Sources of hydrocarbons in sediments of the Mandovi estuary and the Marmugoa harbour, west coast of India[END_REF]. 5)

The terrigenous/aquatic ratio (TAR), the ratio between the concentrations of long-chain n-

alkanes (n-C 27 + n-C 29 + n-C 31 ) to short-chain n-alkanes (n-C 15 + n-C 17 + n-C 19 )
, as index of the relative importance of terrestrial (higher plants) and aquatic (algae, phyto-and zooplankton) materials [START_REF] Bourbonniere | Sedimentary geolipid records of historical changes in the watersheds and productivities of Lakes Ontario and Erie[END_REF][START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF] (Table 2).

Concerning PAHs, we investigated the concentrations of 17 PAHs: 12 parent PAHs, namely naphthalene (Nap), phenanthrene (Phe), anthracene (Ant), thiophene (Thi), fluoranthene (Flt), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), perylene (Per) and benzo[g,h,i]perylene (BgP), and 5 alkylated homologues, i.e. the sums of mono-, di-and tri-methyl compounds (∑Me) of the five target compounds Nap, Phe, Ant, Pyr and Chr. Thi is a 1-ring sulfur heterocyclic compound. Naph and its alkylated homologues are 2-ring compounds. Phe, Ant and their alkylated homologues are 3-ring compounds. These 2-3 ring compounds represent low molecular weight (LMW) PAHs. Flt, Pyr, BaA, Chr and alkylated homologues of Pyr and Chr are 4-ring compounds. BkF, BaP and Per are 5-ring compounds, whereas BgP is a 6-ring compound. These 4-6 ring compounds represent high molecular weight (HMW) PAHs. We determined several ratios allowing to differentiate petrogenic PAHs and pyrogenic PAHs (issuing from the incomplete combustion of fossil fuels), PAHs from fuel combustion and those from the combustion of grass, coal or wood, and PAHs from traffic emissions and PAHs from non-traffic emissions: ∑LMW/∑HMW, ∑MePhe/Phe, ∑MePyr/Pyr, Flt/Flt+Pyr and BaP/BgP [START_REF] Yunker | PAHs in the Fraser River basin: a critical appraisal of PAH ratios as indicators of PAH source and composition[END_REF][START_REF] Brändli | Critical evaluation of PAH source apportionment tools using data from the Swiss soil monitoring network[END_REF][START_REF] Tobiszewski | PAH diagnostic ratios for the identification of pollution emission sources[END_REF][START_REF] Katsoyiannis | Model-based evaluation of the use of polycyclic aromatic hydrocarbons molecular diagnostic ratios as a source identification tool[END_REF] (Table 2).

Analysis of C/H/N/S, TOC, CaCO 3 and δ 13 C

Several parameters were measured on sub-samples of freeze-dried and homogenized sediments (fraction < 63 µm). Content in total carbon, hydrogen, nitrogen and sulphur (C/H/N/S) was determined using a SC-144 LECO Elemental Analyzer at combustion temperatures of 1050 (C, H, N) or 1350 °C (S) in oxygen (C, H, S) or helium (N) atmosphere.

For total organic carbon (TOC) determination, sediment sub-samples were acidified with 1 N HCl and oven dried at 60 °C. This procedure (acidification and drying) was repeated twice in order to remove inorganic carbon. Then, sub-samples were run in the Elemental Analyzer the same way as for total carbon. Two replicates of each sample were run for these elemental analyses. Content in calcium carbonate (CaCO 3 ) was estimated as the difference between total carbon and TOC contents. Concentrations in C, H, N, S, TOC and CaCO 3 are expressed in percentages (%) for 1 g sed. dw.

δ 13 C, given in ‰, is the ratio of stable isotopes 13 C/ 12 C. It was determined on OC (acidified) sub-samples using the following formula: [( 13 C/ 12 C) sample / ( 13 C/ 12 C) standard -1] x 1000, where ( 13 C/ 12 C) standard corresponds to the isotopic ratio of the international standard Pee Dee Belemnite (PDB), and ( 13 C/ 12 C) sample to the isotopic ratio measurements of sediment subsamples conducted by using an elemental analyzer coupled to an isotopic ratio mass spectrometer (EA-IRMS).

Statistical analyses

Ocean Data View (ODV) software version 4.6.5 (Schlitzer, R., http://odv.awi. de, 2014.) was employed for the spatial representation of TOC; δ 13 C, R and PAH concentrations as well as R/TOC and PAH/TOC ratios. The spatial interpolation/gridding of data was performed using Data-Interpolating Variational Analysis (DIVA) [START_REF] Barth | A web interface for griding arbitrarily distributed in situ data based on Data-Interpolating Variational Analysis (DIVA)[END_REF][START_REF] Troupin | Generation of analysis and consistent error fields using the Data Interpolating Variational Analysis (Diva)[END_REF]. Pearson correlation matrices were performed using XLSTAT 2011.2.05 (Microsoft Excel add-in program). The significance threshold was set at p < 0.05.

Results and discussion

Granulometry and content in C/H/N/S, TOC, CaCO 3 and δ 13 C

Mean grain size (M z ) ranged from 4 to 3 ϕ (i.e. 62.5-125 µm; very fine sand) in stations R104, R111, R201, from 3 to 2 ϕ (i.e. 125-250 µm; fine sand) in stations R101-R103, R106, R108-R110, R202, R210, and from 2 to 1 ϕ (i.e. 250-500 µm; medium sand) in stations R105, R107, R203-R209 (Table 3). The index of classification (σ) was comprised between 2 and 1 ϕ for all stations, which reflected poorly sorted sands (Table 3). Therefore, the granulometric distribution was closely related to the bathymetry and geomorphology of the Sfax-Kerkennah channel. The fact that sediments were essentially constituted of sand is in accordance with the hydrodynamic properties of this intertidal area where the combined action of waves and tide prevent the deposition of the finest fraction along with promoting a sandy facies [START_REF] Aloulou | Benthic foraminiferal assemblages as pollution proxies in the northern coast of Gabes Gulf, Tunisia[END_REF]. Apart from stations R104, R106 and R210, which were situated at the border of or within the channel, fine and very fine sands were found outside the channel, i.e. in the most coastal and the shallowest stations, including harbors (Fig. 1; Tables 1,3). In contrast, medium sand was observed in the deepest stations, in the area where the channel is the widest.

In this zone, fines particles, coming from the North-East South-West-oriented current, cannot reach the bottom and are transported further southward (Fig. 1; Tables 1,3).

The content in C, H, N and S ranged from 7.9 (R202) to 14.6% (R201), from 0.57 (R202) to 1.91% (R201), from 0.19 (R203, R202) to 1.23% (R201) and from 0.10 (R107) to 1.33% (R201), respectively (Table 3). The highest percentages were thus recorded in the Sfax harbor (R201) and the lowest ones close to the Sfax harbor (R203, R202) or in the Sfax-Kerkennah channel (R107). The C content was comparable to that determined in surficial sediments of Moroccan coastal waters (Southern Mediterranean Sea), i.e. 1.3-15.1% [START_REF] Pavoni | Environmental pollutants and organic carbon content in sediments from an area of the Moroccan Mediterranean Coast[END_REF]. In contrast, the N and S contents were much higher than those reported for these Moroccan waters or in the Todos os Santos Bay, Brazil (0.01-0.3 and < 0.01-1.3%, respectively) [START_REF] Pavoni | Environmental pollutants and organic carbon content in sediments from an area of the Moroccan Mediterranean Coast[END_REF][START_REF] Venturini | Characterization of the Benthic Environment of a Coastal Area Adjacent to an Oil Refinery, Todos os Santos Bay (NE-Brazil)[END_REF].

The TOC content varied between ~ 2.3 (R203, R202) and ~ 11.7% in the Sfax harbor (R201) and, quite surprisingly, in the Sfax-Kerkennah channel (R107) (Table 3; Fig. 2a).

High values of ~ 7.2% were observed in the Kerkennah harbor (R111) and the Sfax-Kerkennah channel (R104) (Table 3; Fig. 2a). The TOC content determined here was higher than that found in the surficial sediments of the Northeastern (0.02-2.4%) and Northwestern (0.35-6.2%) Mediterranean Sea (Lipiatou and Saliot, 1991a;[START_REF] Benlahcen | Distribution and sources of polycyclic aromatic hydrocarbons in some Mediterranean coastal sediments[END_REF][START_REF] Kucuksezgin | Distribution and sources of polycyclic aromatic hydrocarbons in Cilician Basin shelf sediments (NE Mediterranean)[END_REF] and the Bizerte Lagoon (0.4-3.9%) [START_REF] Barhoumi | Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Bizerte Lagoon, Tunisia: levels, sources, and toxicological significance[END_REF].

Nonetheless, it was lower than that recorded in the Abu Qir Bay (Egyptian coasts), where it reached up to 20% [START_REF] El Deeb | Distribution and Sources of Aliphatic and Polycyclic Aromatic Hydrocarbons in Surface Sediments, Fish and Bivalves of Abu Qir Bay (Egyptian Mediterranean Sea)[END_REF]. Our quite high TOC concentrations were very likely related to the enhanced marine productivity (eutrophication) of this coastal area due to diverse nutrient inputs (Bel Hassen et al., 2009;D'Ortenzio and Ribera d'Alcalà, 2009;[START_REF] Mermex | Marine ecosystems responses to climatic and anthropogenic forcings in the Mediterranean[END_REF].

The TOC/N ratio ranged from 9.4 (R201) to 56.6 (R107) (Table 3), which was higher than that measured in the surficial sediments from the Northwestern Mediterranean Sea (~ 7) [START_REF] Charles | Ecodynamics of PAHs at a peri-urban site of the French Mediterranean Sea[END_REF], the Northern Adriatic (3-37) [START_REF] Guerra | Polycyclic Aromatic Hydrocarbons, Polychlorinated Biphenyls and Trace Metals in Sediments from a Coastal Lagoon (Northern Adriatic, Italy)[END_REF][START_REF] Acquavita | The PAH level, distribution and composition in surface sediments from a Mediterranean Lagoon: The Marano and Grado Lagoon (Northern Adriatic Sea, Italy)[END_REF] and the Todos os Santos Bay, Brazil (8.8-27.6) [START_REF] Venturini | Characterization of the Benthic Environment of a Coastal Area Adjacent to an Oil Refinery, Todos os Santos Bay (NE-Brazil)[END_REF]. The TOC/N ratio may provide information about the origin of organic matter. A high TOC/N ratio (> 20) reveals rather a terrestrial origin of organic matter due to the low N percentage in the higher vegetation [START_REF] Muller | C/N ratios in Pacific deep-sea sediments: effects of inorganic ammonium and organic nitrogen compounds sorbed by clays[END_REF][START_REF] Emerson | Processes controlling the organic carbon content of open ocean sediments[END_REF][START_REF] Meyers | Preservation of elemental and isotopic source identification of sedimentary organic matter[END_REF]. On the other hand, a low TOC/N ratio (5-7) implies a marine origin (plankton or seaweeds) [START_REF] Muller | Productivity, sedimentation rate and sedimentary carbon content in the oceans, 1-Organic carbon preservation[END_REF][START_REF] Monoley | Modelling carbon and nitrogen flows in a microbial plankton community[END_REF][START_REF] Meyers | Preservation of elemental and isotopic source identification of sedimentary organic matter[END_REF]. In our case, TOC/N ratio was comprised between 9.4 (R201) and 18 (R110) for almost all the stations, which suggested a mixed origin with both autochthonous marine and terrestrial sources. For station R111 (22.3) and above all R107 (56.6), the high ratio was in favour of a dominance of the terrestrial origin of organic matter. The TOC/S ratio varied between 2.6 (R202) and 118.9 (R107) (Table 3) and was more important than that observed in Todos os Santos Bay (2.8-13.6) [START_REF] Venturini | Characterization of the Benthic Environment of a Coastal Area Adjacent to an Oil Refinery, Todos os Santos Bay (NE-Brazil)[END_REF].

Except in station R202, the TOC/S ratio was > 2.8, which put forward the occurrence of oxic conditions in the surficial sediments of the Sfax-Kerkennah channel [START_REF] Leventhal | An interpretation of carbon and sulfur relationships in Black Sea sediments as indicators of environments of deposition[END_REF][START_REF] Berner | Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over Phanerozoic time[END_REF]. It is worth noting that the high TOC/N and TOC/S ratios observed in R107 were due to an elevated TOC content relative to low N and S concentrations (Table 3).

The content in CaCO 3 was minimal (~ 40%) in the Sfax-Kerkennah channel (R104) and the Sfax harbor (R201), and maximal (~ 95.6%) on the Sidi Mansour-Kerkennah transect (R105, R108) (Table 3). This content was on average higher than that recorded in surficial sediments of Todos os Santos Bay, Brazil (0-93.2%) [START_REF] Venturini | Characterization of the Benthic Environment of a Coastal Area Adjacent to an Oil Refinery, Todos os Santos Bay (NE-Brazil)[END_REF] and Athens coastal area, Greece (24-86.3%) [START_REF] Kapsimalis | Organic contamination of surface sediments in the metropolitan coastal zone of Athens, Greece: Sources, degree, and ecological risk[END_REF]. The presence of CaCO 3 in surficial sediments is related to marine organisms. In our case, the two mains contributors of CaCO 3 would be benthic foraminifera and green seaweeds [START_REF] Aloulou | Benthic foraminiferal assemblages as pollution proxies in the northern coast of Gabes Gulf, Tunisia[END_REF]. The highest δ 13 C signature was found in R101 (-20.9‰). It decreased from R101 to R107 (-25.1‰) and increased from R107 to R111 (-22.2‰). Then, the δ 13 C signature tended to decrease to reach -25.5‰ in R203. It finally slightly increased towards R201 (-24.0‰) (Table 3; Fig. 2b).

Carbon isotopic ratio (δ 13 C) is useful to distinguish between marine and terrestrial sources of sedimentary organic matter. Marine organic matter typically has δ 13 C values between -22 and -20‰, whilst terrestrial organic matter (C 3 land plants) has an average δ 13 C value of -27‰ (Meyer, 1994). It thus appears that our sediments displayed overall a mixed origin with a stronger marine fingerprint in stations R101-R103, R109 and R111 (Table 3; Fig. 2b).

Concentrations in hydrocarbons, comparison with other regions of the

Mediterranean and Sediment Quality Guidelines

Total R concentration (sum of n-C 15 to n-C 34 + Pr and Phy) ranged from 8.1-14.2 µg g -1 sed. dw in the Sfax-Kerkennah channel (R102, R105-R107, R208, R209) to 173.9 µg g -1 sed. dw in the Sfax harbor (R201). High values were also recorded in the Sfax-Kerkennah channel (45.7 µg g -1 sed. dw, R104) and the Kerkennah harbor (72.1 µg g -1 sed. dw, R111) (Table 3; Fig. 2c). These R concentrations (8.1-173.9 µg g -1 sed. dw) were quite high compared to those reported in the surficial/surface sediments from other regions of the Mediterranean Sea (Table 4). Indeed, for the sediments from Abu Qir Bay (Egypt), Gulf of Lions, Gulf of Fossur-mer, Berre lagoon (France), Gulf of Trieste (Italy), Catalan coast (Spain), Cretan Sea (Greece) and Coastal Aegean Sea (Tukey), concentrations did not exceed 10 µg g -1 sed. dw (Lipiataou and Saliot, 1991b;[START_REF] Tolosa | Aliphatic and polycyclic aromatic hydrocarbons and sulfur/oxygen derivatives in Northwestern Mediterranean sediments: Spatial and temporalvariability, fluxes, and budgets[END_REF][START_REF] Gogou | Marine organic geochemistry of the Eastern Mediterranean: 1. Aliphatic and polyaromatic hydrocarbons in Cretan Sea surficial sediments[END_REF][START_REF] El Deeb | Distribution and Sources of Aliphatic and Polycyclic Aromatic Hydrocarbons in Surface Sediments, Fish and Bivalves of Abu Qir Bay (Egyptian Mediterranean Sea)[END_REF][START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF][START_REF] Gonul | Aliphatic and polycyclic aromatic hydrocarbons in the surface sediments from the Eastern Aegean: assessment and source recognition of petroleum hydrocarbons[END_REF][START_REF] Kanzari | Aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, organochlorine, and organophosphorous pesticides in surface sediments from the Arc river and the Berre lagoon, France[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF][START_REF] Mandalakis | Distribution of aliphatic hydrocarbons, polycyclic aromatic hydrocarbons and organochlorinated pollutants in deep-sea sediments of the southern Cretan margin, eastern Mediterranean Sea: A baseline assessment[END_REF] (Table 4). In the Gulf of Tunis, Khniss coast (Tunisia), Tangier coastal zone (Morocco), Rhône delta (France), Patroklos and Sitia areas (Greece) and Aliağa Bay (Turkey), maximal concentrations were comprised between 10 and 57 µg g -1 sed. dw (Lipiatou and Saliot, 1991b;[START_REF] Tsapakis | PAHs and n-alkanes in Mediterranean coastal marine sediments: aquaculture as a significant point source[END_REF][START_REF] Mzoughi | Distribution and partitioning of aliphatic hydrocarbons and polycyclic aromatic hydrocarbons between water, suspended particulate matter, and sediment in harbours of the West coastal of the Gulf of Tunis (Tunisia)[END_REF][START_REF] Bouzid | Distribution and origin of aliphatic hydrocarbons in surface sediments of strategical areas of the western moroccan mediterranean sea[END_REF][START_REF] Neşer | Polycyclic aromatic and aliphatic hydrocarbons pollution at the coast of Aliağa (Turkey) ship recycling zone[END_REF][START_REF] Zrafi | Aliphatic and Aromatic Biomarkers for Petroleum Hydrocarbon Investigation in Marine Sediment[END_REF] (Table 4). In Sfax ponds (Tunisia) and Eastern harbour of Alexandria (Egypt), maximal concentrations were 128 and 143 µg g -1 sed. dw, respectively (Aboul-Kassim and [START_REF] Aboul-Kassim | Petroleum hydrocarbon fingerprinting and sediment transport assessed by molecular biomarker and multivariate statistical analyses in the Eastern Harbour of Alexandria, Egypt[END_REF][START_REF] Elloumi | Detection of Water and Sediments Pollution of An Arid Saltern (Sfax, Tunisia) by Coupling the Distribution of Microorganisms With Hydrocarbons[END_REF]. In the Sfax coastline (Tunisia), close to our study area, [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF] found concentrations reaching up to 430 µg g -1 sed. dw (sediments collected in 2003). Finally, Amori et al. ( 2011) observed concentrations that were largely higher, i.e. 3,886 µg g -1 sed. dw., in Gabès, Kettana and Al-Zar coastline (Tunisia). Hence, this comparison underscores that n-alkane concentrations in the surficial/surface sediments of the Tunisian coasts, particularly of the Gulf of Gabès, were among the highest measured in the Mediterranean Basin (Table 4).

Total PAH concentration (sum of 17 PAHs) varied between 175-245 ng g -1 sed. dw in the Sfax-Kerkennah channel (R109, R209, R208) and 10,769 ng g -1 sed. dw in the Sfax harbor (R201), with concentrations > 1,000 ng g -1 sed. dw recorded in the Sfax-Kerkennah channel (R104) and in the vicinity of Sfax (R204, R203, R202) and Kerkennah (R110, R111) harbors (Table 5; Fig. 2d). These total PAH concentrations (175-10,769 ng g -1 sed. dw) mirror a moderate to very high pollution level according to the pollution level classification proposed by [START_REF] Baumard | Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records[END_REF] (Table 5) and were situated in the mid-range of those reported in the surficial/surface sediments from other regions of the Mediterranean Sea (Table 6). Actually, for the sediments from Bizerte lagoon, Khniss coast (Tunisia), Al Hoceïma coastal area (Morocco), Bay of Banuyls-sur-mer (France), Chioggia and Ancona coastal zones (Italy), Cretan Sea (Greece), Candarli Gulf (Turkey) and Cilician Basin (Cyprus), concentrations did not exceed 1,000 ng g -1 sed. dw [START_REF] Magi | Distribution of polycyclic aromatic hydrocarbons in the sediments of the Adriatic Sea[END_REF][START_REF] Pavoni | Environmental pollutants and organic carbon content in sediments from an area of the Moroccan Mediterranean Coast[END_REF][START_REF] Trabelsi | Polycyclic aromatic hydrocarbons in superficial coastal sediments from Bizerte Lagoon, Tunisia[END_REF][START_REF] Charles | Ecodynamics of PAHs at a peri-urban site of the French Mediterranean Sea[END_REF][START_REF] Kucuksezgin | Marine organic pollutants of the Eastern Aegean: Aliphatic and polycyclic aromatic hydrocarbons in Candarli Gulf surficial sediments[END_REF][START_REF] Kucuksezgin | Distribution and sources of polycyclic aromatic hydrocarbons in Cilician Basin shelf sediments (NE Mediterranean)[END_REF][START_REF] Zrafi | Aliphatic and Aromatic Biomarkers for Petroleum Hydrocarbon Investigation in Marine Sediment[END_REF][START_REF] Barhoumi | Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Bizerte Lagoon, Tunisia: levels, sources, and toxicological significance[END_REF][START_REF] Mandalakis | Distribution of aliphatic hydrocarbons, polycyclic aromatic hydrocarbons and organochlorinated pollutants in deep-sea sediments of the southern Cretan margin, eastern Mediterranean Sea: A baseline assessment[END_REF] (Table 6). Also, in the Sfax, Luza, Sousse, Jarzouna-Bizerte coastal areas, Gulf of Tunis (Tunisia), Abu Qir Bay (Egypt), Rhône Delta, Port Vendres harbor (France), Gulf of Trieste (Italy) and coastal Aegean Sea (Turkey), maximal concentrations were comprised between 1,000 and < 10,000 ng g -1 sed. dw [START_REF] Bouloubassi | Investigation of anthropogenic and natural organic inputs in estuarine sediments using hydrocarbon markers (NAH, LAB, PAH)[END_REF][START_REF] Baumard | Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records[END_REF][START_REF] Khairy | Risk assessment of polycyclic aromatic hydrocarbons in a Mediterranean semi-enclosed basin affected by human activities (Abu Qir Bay, Egypt)[END_REF][START_REF] Zrafi-Nouira | Distribution and Sources of Polycyclic Aromatic Hydrocarbons around a Petroleum Refinery Rejection Area in Jarzouna-Bizerte (Coastal Tunisia). Soil and Sediment Contamination[END_REF][START_REF] Mzoughi | Distribution and partitioning of aliphatic hydrocarbons and polycyclic aromatic hydrocarbons between water, suspended particulate matter, and sediment in harbours of the West coastal of the Gulf of Tunis (Tunisia)[END_REF][START_REF] Gonul | Aliphatic and polycyclic aromatic hydrocarbons in the surface sediments from the Eastern Aegean: assessment and source recognition of petroleum hydrocarbons[END_REF][START_REF] Kessabi | Possible chemical causes of skeletal deformities in natural populations of Aphanius fasciatus collected from the Tunisian coast[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF][START_REF] Zaghden | Evaluation of hydrocarbon pollution in marine sediments of Sfax coastal areas from the Gabes Gulf of Tunisia, Mediterranean Sea[END_REF] (Table 6). Nevertheless, in Egypt coastal areas, Lazaret Bay, Gulf of Fos-sur-mer (France), Taranto Gulf, Coastal Ligurian Sea, Venice Lagoon, Naples harbor (Italy), Santander Bay, Catalonia coast (Spain), Gulf of Corinth, North Evoikos and Saronikos Gulfs, Drapetsona-Keratsini coastal zone (Greece), Izmit and Aliağa Bays (Turkey), Rovinj coastal area and Rijeka Bay (Croatia), maximal concentrations were much higher than 10,000 ng g -1 sed. dw (La [START_REF] Rocca | PAHs content and mutagenicity of marine sediments from the Venice lagoon[END_REF][START_REF] Benlahcen | Distribution and sources of polycyclic aromatic hydrocarbons in some Mediterranean coastal sediments[END_REF][START_REF] Eljarrat | Toxic potency assessment of non-and mono-ortho PCBs, PCDDs, PCDFs, and PAHs in northwest Mediterranean sediments (Catalonia Spain)[END_REF][START_REF] Viguri | Environmental assessment of polycyclic aromatic hydrocarbons (PAHs) in surface sediments of the Santander Bay. Northern Spain[END_REF][START_REF] Bertolotto | Polycyclic aromatic hydrocarbons in surficial coastal sediments of the Ligurian Sea[END_REF][START_REF] Bihari | PAH content, toxicity and genotoxicity of coastal marine sediments from the Rovinj area, Northern Adriatic, Croatia[END_REF][START_REF] Tolun | Polycyclic aromatic hydrocarbon contamination in coastal sediments of the Izmit Bay (Marmara Sea): case studies before and after the Izmit Earthquake[END_REF][START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF][START_REF] Sprovieri | Heavy metals, polycyclic aromatic hydrocarbons and polychlorinated biphenyls in surface sediments of the Naples harbor (southern Italy)[END_REF][START_REF] Annicchiarico | PCBs, PAHs and metal contamination and quality index in marine sediments of the Taranto Gulf[END_REF][START_REF] Alebic-Juretic | Polycyclic aromatic hydrocarbons in marine sediments from the Rijeka Bay area, Northern Adriatic, Croatia, 1998-2006[END_REF][START_REF] Barakat | Distribution and characteristics of PAHs in sediments from the Mediterranean coastal environment of Egypt[END_REF][START_REF] Botsou | Polycyclic aromatic hydrocarbons (PAHs) in marine sediments of the Hellenic coastal zone, eastern Mediterranean: levels, sources and toxicological significance[END_REF][START_REF] Neşer | Polycyclic aromatic and aliphatic hydrocarbons pollution at the coast of Aliağa (Turkey) ship recycling zone[END_REF][START_REF] Kapsimalis | Organic contamination of surface sediments in the metropolitan coastal zone of Athens, Greece: Sources, degree, and ecological risk[END_REF] (Table 6). It should be noticed that our total PAH concentrations were similar to those recorded by [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF] for the same area (113-10,720 ng g -1 sed. dw) (sediments collected in 2003).

Sediment quality guidelines (SQGs) are used to assess the contamination level of marine and estuarine sediments [START_REF] Long | Incidence of adverse biological effects with ranges of chemical concentrations in marine and estuarine sediments[END_REF][START_REF] Barhoumi | Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Bizerte Lagoon, Tunisia: levels, sources, and toxicological significance[END_REF]. [START_REF] Long | Incidence of adverse biological effects with ranges of chemical concentrations in marine and estuarine sediments[END_REF] proposed two guideline values, an effects range low (ERL) and an effects range median (ERM) to determine the sediment quality. Our PAH concentrations were thus compared to these ERL and ERM values (Table 7). Total PAH concentrations in all stations were below ERL, except in R202 (close to Sfax harbor) and R201 (Sfax harbor) where they were comprised between ERL and ERM. At the level of individual compounds, concentrations in Nap, BaA and BaP were below ERL in all stations, while concentrations in Phe, Ant, Flt and Chr were below ERL or between ERL and ERM depending on the stations. Concentration in Pyr was > ERM in R201 (Table 7). Consequently, PAH concentrations in surficial sediments in this Sfax-Kerkennah channel area may be harmful for marine biota mainly in the Sfax harbor.

Sediment geochemistry and relationships between hydrocarbons and biogeochemical parameters

Stations displaying high C, H, N, S, TOC, R and PAH contents were the Sfax (R201) and Kerkennah (R111) harbors as well as station R104 (in the channel), and, in a lesser extent, station R110 (close to Kerkennah harbor; outside the channel). Except R110 (fine sand), these stations were characterized by very fine sand (Tables 3,5; Fig. 2a,c,d), which may be linked to the relatively high PAH levels encountered. Indeed, PAHs are known to be mainly adsorbed onto very fine particles because of their higher specific surface area [START_REF] Xia | Effect of sediment particle size on polycyclic aromatic hydrocarbon biodegradation: importance of the sediment-water interface[END_REF]. TOC/N and δ 13 C values emphasized a mixed (marine and terrestrial) origin of organic matter (Tables 3; Fig. 2b), while the hydrocarbon levels suggested significant anthropogenic inputs in this sites. In the Sfax and Kerkennah harbors, anthropogenic inputs were rather evident with ship traffic and petroleum wastes. Nevertheless, specific anthropogenic sources were less obvious in station R104. The latter was located in a cuvette that might receive particular industrial wastewaters. In general, sediments with high organic carbon content contained high PAH concentrations [START_REF] Barhoumi | Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Bizerte Lagoon, Tunisia: levels, sources, and toxicological significance[END_REF]. However, station R107 (in the channel) presented a high TOC concentration and, in the meantime, low R and PAH levels compared to those of stations R201, R111 and R104. R107 was also typified by medium sand and by a terrestrial fingerprint of the organic matter with regard to its TOC/N and δ 13 C values. An inverse pattern was observed in stations R202-R204 with high R and PAH concentrations and a very low TOC content. TOC/N and δ 13 C showed a marine and terrestrial origin of organic matter (Tables 3,5; Fig. 2a-d). These stations, located outside (R202) or at the channel border (R203-204), were very likely under the influence of anthropogenic (PAH) inputs from the Sfax harbor and city. All other stations of the channel (R105, R106, R205-R210) as well as stations R108 and R109 (outside the channel) were characterized by fine or medium sand and presented the lowest R and PAH concentrations, a low TOC content, a high CaCO 3 content and TOC/N and δ 13 C values revealing both marine and terrestrial organic matter (Tables 3,5; Fig. 2a-d). These sites were thus much less impacted by anthropogenic inputs. This was due to their relative greater distance from the coast and their higher bathymetry as well as to the North-East South-West-oriented current crossing the channel, which transports the finest particles further southward.

As mentioned above, strong decoupling occurred between hydrocarbon and TOC contents depending on the stations, especially for R107. As seen from Fig. 2e, f, the R/TOC and PAH/TOC ratios were not constant: low ratios were observed inside the channel (< 8 mg g -1 and < 60 µg g -1 sed. dw, respectively), while high ratios were found in coastal stations outside the channel (> 10 mg g -1 and > 500 µg g -1 sed. dw, respectively). When taken into account all stations (n = 20), significant positive linear correlations appeared between total R concentration and C, H, N, S and TOC contents (r = 0.49-0.93, p < 0.05, n = 20), and between total PAH concentration and H, N and S contents (r = 0.73-0.81, p < 0.05, n = 20). A significant correlation was also found between total R and PAH concentrations (r = 0.85, p < 0.05, n = 20) (Table 8). When removing station R107 from the dataset, for which the decoupling between hydrocarbons and TOC was very pronounced (this station appeared as outlier when plotting the data), correlations between R, PAHs and TOC clearly increased (r = 0.66-0.88, p < 0.05, n = 19) (Table 8). However, when station R201 (i.e. the station showing "extreme" hydrocarbon concentrations) was also excluded in addition to R107, the degree of correlation between parameters dropped: PAHs were not correlated anymore to TOC and R (r = -0.08-0.21, p > 0.05, n = 18), whereas the correlation between the two latter substantially decreased (r = 0.68, p < 0.05, n = 18) (Table 8). Finally, when eliminating R107 and all stations with total PAH concentration > 1000 ng g -1 sed. dw (i.e. R201, R104, R110, R111, R202-R204), the correlation between R and TOC disappeared as well (r = -0.02, p > 0.05, n = 12) (Table 8). This correlation study shows that the significant correlations occurring between R, PAHs and TOC were almost merely due to the "extreme" station R201. Therefore, the distribution and concentrations of R, PAHs and TOC were decoupled in the surficial sediments of the Sfax-Kerkennah channel. R and PAHs were very likely more influenced by specific inputs rather than by organic matter content. The absence of significant linear relationship between PAH and TOC concentrations or between R and PAH concentrations in surficial/surface sediments were also reported for the Bizerte Lagoon (Tunisia), Marano and Grado Lagoons (Italy) and Egyptian coastal areas [START_REF] El Nemr | Aliphatic and polycyclic aromatic hydrocarbons in the surface sediments of the Mediterranean: assessment and source recognition of petroleum hydrocarbons[END_REF]Aquavita et al., 2014;[START_REF] Barhoumi | Polycyclic aromatic hydrocarbons (PAHs) in surface sediments from the Bizerte Lagoon, Tunisia: levels, sources, and toxicological significance[END_REF]. Moreover, [START_REF] Simpson | Composition and distribution of polycyclic aromatic hydrocarbon contamination in surficial marine sediments from Kitimat Harbor, Canada[END_REF] proposed that PAH and TOC concentrations in sediments are significantly correlated solely in greatly contaminated sites where total PAH concentrations > 2,000 ng g -1 , which is in accordance with our results.

Composition and sources of hydrocarbons

The UCM/R ratio was > 3 for stations R106, R111 (Kerkennah harbor), R201 (Sfax harbor), R202, R208-R210 (Sfax-Kerkennah channel) (Table 3), which reflects the presence of degraded petroleum products in these stations [START_REF] Simoneit | Organic matter of the troposphere-II.* Natural background of biogenic lipid matter in aerosols over the rural western United States[END_REF][START_REF] Guigue | Occurrence and distribution of hydrocarbons in the surface microlayer and subsurface water from the urban coastal marine area off Marseilles, Northwestern Mediterranean Sea[END_REF][START_REF] Parinos | Occurrence, sources and transport pathways of natural and anthropogenic hydrocarbons in deep-sea sediments of the eastern Mediterranean Sea[END_REF]. CPI 15-24 was < 1 for all the stations, except for the Sfax harbor (R201) where it was close to 1 (Table 3). This shows the predominance of even carbon number in short chain n-alkanes. Indeed, n-C 16 , n-C 18 and, in a lesser extent, n-C 20 were the dominant compounds over the whole range n-C 15 -n-C 34 apart from the coastal stations R101-R103, the Kerkennah harbor area (R110, R111) and the Sfax harbor (R201), in which n-C 31 and n-C 33 were the major compounds. The predominance of even light n-alkanes n-C 16 , n-C 18 and n-C 20 in sediments is not so common albeit it has been observed for instance in sediments from the Arabian Gulf [START_REF] Grimalt | n-Alkane distributions in surface sediments from the Arabian Gulf[END_REF], the Gulf of Fos-sur-mer [START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF] and the Taihu Lake, China [START_REF] Yu | Distribution and sources of n-alkanes in surface sediments of Taihu Lake, China[END_REF]. It has been suggested that these even light n-alkanes were issued from bacteria, but also fungi, and yeast species, and from petroleum-derived inputs [START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF][START_REF] Harji | Sources of hydrocarbons in sediments of the Mandovi estuary and the Marmugoa harbour, west coast of India[END_REF][START_REF] Yu | Distribution and sources of n-alkanes in surface sediments of Taihu Lake, China[END_REF]. CPI 25-34 displayed values > 3 for stations R101-R110 and R208 (Table 3). This underscored the presence of AHs from mixed petroleum and biogenic (terrestrial higher plant debris) sources, the latter having a higher contribution [START_REF] Rieley | The biogeochemistry of Ellesmere Lake, UK-I: source correlation of leaf wax inputs to the sedimentary lipid record[END_REF][START_REF] Harji | Sources of hydrocarbons in sediments of the Mandovi estuary and the Marmugoa harbour, west coast of India[END_REF]. In contrast, for almost all the stations from the Kerkennah-Sfax city transect, CPI 25-34 was < 3 (Table 3), which emphasised an increase in the contribution of petroleum inputs [START_REF] Mille | Hydrocarbons in coastal sediments from the Mediterranean sea (Gulf of Fos area, France)[END_REF][START_REF] Guigue | Occurrence and distribution of hydrocarbons in the surface microlayer and subsurface water from the urban coastal marine area off Marseilles, Northwestern Mediterranean Sea[END_REF]. TAR was > 1 in most of stations, being comprised between 1.2 and 2.8, with high values of 6.6 and 10.4 detected in R102 and R103. TAR was ~ 1.0 in stations R106 and R205 and < 1 in R104, R107, R202 and R207 (Table 3). Hence, this ratio mirrored the higher contribution of terrestrial higher plants compared to aquatic material (algae, phyto-and zoo-plankton).

Ratios involving Pr and Phy (Pr/Phy, n-C 17 /Pr and n-C 18 /Phy) have to be taken with extreme caution because these branched alkanes have multiple origins and are sensitive to diagenetic conditions and thermal maturity [START_REF] Peters | The Biomarker Guide, 2nd edn[END_REF]. They are both derived from the phytol side chain of chlorophyll a, either under reducing conditions (Phy) or oxidizing conditions (Pr). They are thus abundant in weathered crude oils. However, Pr can also originate from zooplankton, and Phy from Archaebacteria, such as methanogens [START_REF] Volkman | Identification of natural anthropogenic and petroleum hydrocarbons in aquatic sediments[END_REF]. It has been proposed that Pr/Phy < 1 could be taken as an indicator of petroleum origin and/or highly reducing (anoxic, hypersaline) depositional environments. Pr/Phy > 3 may reflect the presence of biogenic AHs, while Pr/Phy between 1 and 3 may be the sign of oxidizing depositional environments [START_REF] Volkman | Identification of natural anthropogenic and petroleum hydrocarbons in aquatic sediments[END_REF][START_REF] Ten Haven | Applications and limitations of Mango''s light hydrocarbon parameters in petroleum correlation studies[END_REF][START_REF] Peters | The Biomarker Guide, 2nd edn[END_REF]. Here, we found Pr/Phy < 1 for all the stations, except R110, R201, R208 and R209, which presented values comprised between 1.19 and 2.85 (Table 3). These Pr/Phy values (< 3) underlined the absence of biogenic AHs. Although no significant correlation was found between Pr/Phy ratio or Phy concentration and % S (data not shown), we may assume that the relatively high sulphur content in these sediments (underscoring the occurrence of anoxic conditions) contributed to the relatively low Pr/Phy values observed, in addition to the petroleum signature. These results are in agreement with those from [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF] in the same area. The n-C 18 /Phy ratio was > 1 for all the stations. Values comprised between 2 and 5 were observed for a majority of stations, whereas higher values (6-9) were found in stations R107, R205, R209, R210 (Table 3). Interestingly, the highest value was detected in R107 suggesting a very recent hydrocarbon input that could be related to the relatively high TOC concentration found in this station. In the Sfax harbor area, n-C 18 /Phy decreased (~ 1.7 in R201 and R202) (Table 3), which reflected the presence of more degraded AHs. The n-C 17 /Pr ratio, which showed values around 1 for several stations from coastal areas and the Sfax-Kerkennah channel, did not follow the same trend than the n-C 18 /Phy ratio (Table 3). Its interpretation seems thus more complicated. This analysis of AH ratios and indices suggests the presence of both petrogenic and biogenic materials, the petrogenic fingerprint being more accentuated in the harbor areas and in the Kerkennah-Sfax city transect compared to North of Sfax-Kerkennah transect.

Concerning PAHs, the most abundant compounds were ∑MePyr (in R101, R104, R110, R111, R207, R203, R201), ∑MePhe (in R102, R106, R107, R204) or Chr and/or ∑MeChr (in R103, R105, R210, R205), which accounted on average for 20% of total PAHs (Table 5). Per (35%), BgP (34%), BaP (30%), Flt (17%) and BkF (19%) dominated in R108, R109, R209, R208 and R202, respectively. Nap, ∑MeNap, Ant and BaA were not detected, except in R103, R209, R204, R203 and/or R202 (Table 5). High proportions of alkylated Pyr and Phe in the Sfax sediments have already been pointed out by [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF]. Hence, PAH molecular profiles illustrated the predominance of 4-ring compounds apart from stations R204 (dominance of 3 rings), R108, R209 (dominance of 5 rings) and R109 (dominance of 6 rings) (Fig. 3). Fig. 4 presents the cross plot of ∑LMW/∑HMW versus Flt/Flt+Pyr ratios for the different samples, except stations R106, R209 and R210, for which Flt/Flt+Pyr could not be determined. Most of the stations (R101-R105, R107, R109-R111, R205, R201) displayed ∑LMW/∑HMW ratio < 1 and Flt/Flt+Pyr ratio < 0.4, which highlighted both petrogenic and pyrogenic sources of PAHs. In station R204, the inverse pattern (∑LMW/∑HMW > 1 and Flt/Flt+Pyr < 0.4), suggested this mixed source as well. On the contrary, stations R108, R202, R203, R207 and R208 presented ∑LMW/∑HMW ratio < 1 and Flt/Flt+Pyr ratio > 0.4, putting forward the dominance of the pyrogenic source [START_REF] Soclo | Origin of polycyclic aromatic hydrocarbons (PAHs) in coastal marine sediments: case studies in Cotonou (Benin) and Aquitaine (France) areas[END_REF][START_REF] Yunker | PAHs in the Fraser River basin: a critical appraisal of PAH ratios as indicators of PAH source and composition[END_REF][START_REF] Li | Distribution and sources of polycyclic aromatic hydrocarbons in the middle and lower reaches of the Yellow River, China[END_REF][START_REF] Zhang | Source diagnostics of polycyclic aromatic hydrocarbons in urban road runoff, dust, rain and canopy throughfall[END_REF]. R108, R202, R203 and R207 had Flt/Flt+Pyr ratio < 0.5, which might be attributed to fuel combustion. R208, with Flt/Flt+Pyr ratio of 0.74, was distinguished by a contribution of grass, coal, and/or wood combustion [START_REF] De | Soil-borne polycyclic aromatic hydrocarbons in El Paso, Texas: analysis of a potential problem in the United States/Mexico border region[END_REF]Fig. 4; Table 2). On the other side, ∑MePhe/Phe ratio was > 2 with the exception of stations R109 and R210, which implied the dominance of the petrogenic source [START_REF] Prahl | Polycyclic aromatic hydrocarbon (PAH)-phase associations in Washington coastal sediments[END_REF][START_REF] Garrigues | Pyrolytic and petrogenic inputs in recent sediments: a definitive signature through phenanthrene and chysene compound distribution[END_REF]. In the same way, ∑MePyr/Pyr was > 1 apart from R107, R205 and R203, underscoring the petrogenic fingerprint [START_REF] Zaghden | Sources and distribution of aliphatic and polyaromatic hydrocarbons in sediments of Sfax, Tunisia, Mediterranean Sea[END_REF][START_REF] Bajt | Aliphatic and Polycyclic Aromatic Hydrocarbons in Gulf of Trieste Sediments (Northern Adriatic): Potential Impacts of Maritime Traffic[END_REF]. At last, BaP/BgP was > 0.6, except for R103, R109 and R201. This could reflect traffic emissions [START_REF] Katsoyiannis | On the use of PAH molecular diagnostic ratios in sewage sludge for the understanding of the PAH sources. Is this use appropriate?[END_REF]Table 2). Consequently, from these different indices and ratios, it appears that the Sfax-Kerkennah channel area was characterized by various petrogenic and pyrogenic sources of PAHs, with no clear trend highlighted between coastal stations (outside the channel) and stations inside the channel.

Conclusion

This study investigated the origin and distribution of hydrocarbons (AHs and PAHs) and organic matter in the surficial sediments of the Sfax-Kerkennah channel. Sediments, mainly composed of sand, displayed a grain size distribution in relation with the geomorphology, bathymetry and hydrodynamic properties of the Sfax-Kerkennah channel. Fine and very fine sands were generally found outside the channel (coastal stations and harbors), whereas medium sand was observed within the channel (deepest stations). Compared to other regions of the Mediterranean Sea, we recorded high TOC concentrations (> 11%), quite high R (up to 174 µg g -1 sed. dw) and PAH concentrations (> 10,000 ng g -1 sed. dw). According to [START_REF] Baumard | Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records[END_REF], PAH pollution was moderate to very high in the Sfax-Kerkennah channel. Moreover, with regard to sediment quality guidelines [START_REF] Long | Incidence of adverse biological effects with ranges of chemical concentrations in marine and estuarine sediments[END_REF], the pyrene concentration in the Sfax harbor sediment may be detrimental for marine ecosystems. In the Sfax and Kerkennah harbors as well as in stations R104 and R110, we found high contents in C, H, N, S, TOC, R and PAHs. Except for station R110, these high contents were associated with very fine sand. In contrast, most of the stations located within the channel were characterized by fine or medium sand and by low TOC, R and PAH concentrations.

Nevertheless, by examining in details the degree of correlation between parameters, we put forward that R, PAHs and TOC were actually decoupled for most of the stations. This suggested that hydrocarbons were very likely more influenced by specific inputs rather than by organic matter content in the surficial sediments of the Sfax-Kerkennah channel. TOC/N and δ 13 C values revealed a mixed origin of organic matter with both autochthonous marine and terrestrial sources. Index and ratio diagnostic emphasized the dominance of petrogenic origin of n-alkanes (relatively to biogenic origin) and the presence of both petrogenic and pyrogenic PAHs. AH ratios revealed the presence of both biogenic and petrogenic materials, the petrogenic fingerprint being more important and more degraded in the harbor areas and in the Kerkennah-Sfax city transect. However, PAH diagnostic did not reveal any clear relationship between the geographical repartition of stations and the molecular composition/origin of hydrocarbons. This work underscores the complex distribution patterns and the multiple sources (marine, terrestrial, anthropogenic) of organic matter and hydrocarbons in the Gulf of Gabès. Also, further investigations should consider the molecular composition of hydrocarbons in the water column (particulate and dissolved phases) for a global view of organic pollutant dynamics in coastal waters of the Gulf of Gabès. Table 3. Granulometric parameters (Mz and σ in unit ϕ), percentages (for 1 g sediment dry weight) of elemental constituents (C, H, N, S), calcium 26 carbonate (CaCO 3 ) and total organic carbon (TOC), isotopic signature of organic carbon (δ 13 C in ‰), and concentrations in total n-alkanes (R in µg g -1 27 sed. dw) and associated molecular diagnostic ratios (UCM/R, CPI, Pr/Phy, n-C 17 /Pr, n-C 18 /Phy, TAR) in the fraction < 63 of surficial sediments (0-28 1 cm) collected along the Sfax-Kerkennah channel (Southeast Tunisia, Southern Mediterranean Sea). 29 30

R101 R102 R103 R104 R105 R106 R107 R108 R109 R110 R111 R210 R209 R208 R207 R205 R204 R203 R202 R201 Mz (ϕ)
3 to 2 3 to 2 3 to 2 4 to 3 2 to 1 3 to 2 2 to 1 3 to 2 3 to 2 3 to 2 4 to 3 3 to 2 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 3 to 2 4 to 3 σ (ϕ)

2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 2 to 1 C (%)

9 -20.9 -21. 2 -22.4 -23.7 -24.6 -25.1 -25.1 -24.4 -22.7 -23.6 -22.2 -24.7 -25.2 -25.4 -25.0 -25.0 -25.4 -25.5 -24.8 -24

organizations: 8

 8 PAHs are included in the list of the 45 priority regulated substances by the European Union (Official Journal of the EU 24/08/2013, Directive 2013/39/EU) and 16 PAHs are included in the list of 126 priority regulated substances by the US Environmental Protection Agency (US EPA, 40 CFR Part 423, Appendix A to Part 423).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure2. Spatial distribution of a) the concentration in TOC (in % for 1 g sed. dw), b) the isotopic signature of organic carbon (δ 13 C in ‰), c) the concentration in total n-alkanes (R in µg g -1 sed. dw), d) the concentration in total (∑17) PAHs (in ng g -1 sed. dw), e) the R/TOC ratio (in mg g -1 ) and f) the PAH/TOC ratio (in µg g -1 ) in surficial sediments of the Sfax-

Figure 3 .

 3 Figure 3. Distribution pattern of PAHs in the surficial sediments of the Sfax-Kerkennah

Figure 4 .

 4 Figure 4. Cross plot of ∑LMW/∑HMW versus Flt/Flt+Pyr ratios for the different samples,

  Figure 2

Table 2 .

 2 Hydrocarbon molecular diagnostic ratios investigated in this study with typical values from the literature. Adapted fromTobiszewski 23 and Namieśnik (2012) and[START_REF] Katsoyiannis | Model-based evaluation of the use of polycyclic aromatic hydrocarbons molecular diagnostic ratios as a source identification tool[END_REF].

	Station Area	Position	Depth of the	Type of bottom
				water column (m)
	R101	North of Sfax -Sidi Mansour 34°48'52"N, 10°53'05"E 1	Dense meadows of Posidonia
	R102		34°47'56"N, 10°54'00"E 1.1	Meadows of Posidonia
	R103		34°47'19"N, 10°54'30"E 1.7	Meadows of Posidonia
	R104	Channel	34°46'00"N, 10°54'57"E 7.6	Muddy sand
	R105	Channel	34°45'05"N, 10°55'25"E 9.5	Dense meadows of Posidonia
	R106	Channel	34°44'08"N, 10°55'50"E 8.5	Sandy with Posidonia
	R107	Channel	34°43'08"N, 10°56'20"E 5.5	Dense meadows of Posidonia
	R108		34°42'10"N, 10°56'48"E 2.8	Dense meadows of Posidonia
	R109		34°41'10"N, 10°57'18"E 1	Dense meadows of Posidonia
	R110		34°40'19"N, 10°57'43"E 0.8	Muddy sand
	R111	Kerkennah harbor	34°39'26"N, 10°58'00"E 2.7	Sandy with Posidonia
	R210	Channel	34°40'19"N, 10°56'05"E 7	Sandy
	R209	Channel	34°40'52"N, 10°55'05"E 9	Meadows of Posidonia
	R208	Channel	34°41'25"N, 10°54'00"E 15.7	Meadows of Posidonia
	R207	Channel	34°41'48"N, 10°52'55"E 20	Meadows of Posidonia
	R205	Channel	34°42'15"N, 10°50'28"E 7.3	Muddy sand
	R204	Channel	34°42'23"N, 10°49'15"E 8.5	Muddy sand
	R203	Channel	34°42'31"N, 10°48'00"E 7.9	Muddy sand
	R202		34°42'39"N, 10°46'50"E 4	Muddy sand
	R201	Sfax harbor	34°42'46"N, 10°46'05"E 4.5	Muddy sand

Table 4 .

 4 Comparison of total n-alkane concentrations (R in µg g -1 sed. dw) in surficial/surface sediments from different regions of the 36

	.0

Table 5 .

 5 Concentrations in polycyclic aromatic hydrocarbons (PAHs in ng g -1 sed. dw) and associated molecular diagnostic ratios in the fraction < 63 µm of 39 surficial sediments (0-1 cm) collected along the Sfax-Kerkennah channel (Southeast Tunisia, Southern Mediterranean Sea).

	40
	41

Table 6 .

 6 Comparison of total PAH concentrations (in ng g -1 sed. dw) in surficial/surface sediments from different regions of the Mediterranean Sea.

	Country	Site

Acknowledgements.

We acknowledge the service central d'analyses du CNRS (Vernaison, France). We are grateful to Pr. J.-L. Reyss (LSCE, UMR CEA-CNRS 1572, France) for providing access to CHN and isotope ratio equipment. We warmly thank A. Lorre for her help and assistance for OC and δ 13 C analyses. One anonymous Reviewer is acknowledged for his relevant comments and corrections. This work was conducted in part in the framework of the IRD French-Tunisian International Joint Laboratory "LMI COSYS-Med".

a The pollution levels are those defined by [START_REF] Baumard | Origin and bioavailability of PAHs in the Mediterranean Sea from mussel and sediment records[END_REF]: low, 0-100 ng g -1 ; moderate, 100-1,000 ng g -1 ; high, 1,000-5,000 ng g -1 ; very high, > 5,000 ng g -1 ; bld: below detection limit.