Detecting long-term temporal trends in sediment-bound metals in the western Adriatic (Mediterranean Sea)

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Detecting long-term temporal trends in sediment-bound metals in the western Adriatic (Mediterranean Sea) Marilia Lopes-Rochaa,b,⁎, Leonardo Langonec,⁎⁎, Stefano Miserocchic, Patrizia Giordanoc, Roberta Guerrab,d a Departamento de Química-Física, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, UNESCO/UNITWIN WiCoP, Campus de Excelencia International del Mar (CEIMAR), Polígono Río San Pedro s/n, Puerto Real 11510, Cádiz, Spain b Centro Interdipartimentale di Ricerca per le Scienze Ambientali (C.I.R.S.A.), University of Bologna, Ravenna Campus, 48123 Ravenna, Italy c National Research Council-Institute of Marine Sciences (CNR-ISMAR), 40129 Bologna, Italy d Department of Physics and Astronomy, University of Bologna, 40127 Bologna, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Trace metals Background levels Sediment records Historical trends of contamination Inventories of trace metals excess Adriatic Sea

Major and trace metal concentrations were determined in western Adriatic sediment cores. Based on sediment chronology, the earliest anthropogenic influence appeared as a Zn and Pb increase in the Po River prodelta starting from ~ 1914. The increasing contamination signal of these trace metals propagated southward as far as 450 km with a growing delay, taking ~10 years to reach the south Adriatic Sea. Although greater inventories of excess trace metals in the northern sector pointed to the influence of the intense human activities in the Po River drainage basin and Venice lagoon system, we observed a reduction of excess trace metals from mid-1980s, related to the implementation of stricter environmental regulations on chemical wastewaters. In contrast, an increase in trace metal accumulation in surficial sediment from the 2000s in front of the cities of Ancona and Bari suggested a recent local input of trace metals, probably due to harbor activities.

1. Introduction Human activities have accelerated the cycling of trace metals and increased metal deliveries to coastal zones in the last decades (Han et al., 2002). Because metals can be toxic to aquatic life, all changes in their loads or concentrations may have ecosystem-wide implications (Boyd, 2010). Therefore, it is important to understand how anthropogenic activities can change the concentrations of potentially toxic metals, which processes can affect such changes and which activities have the greatest effects (Richir and Gobert, 2016). In coastal and marine systems, most of the trace metals associated with the surfaces of particles are preferentially transported, deposited and eventually buried with fine grained sediments. Dated cores through sediment deposits can provide chronologies of metal concentration or input in areas of net sediment deposition (Hornberger et al., 1999; Miller et al., 2014). On the other hand, interpretation of human influences requires the determination of trace metal natural baselines (preindustrial levels) in the sedimentary archives. The Adriatic Sea is a land-locked marginal sea where the intensity of the human pressure on its coastal areas enhances the natural

concentrations of trace metals in sediments. In this basin, the main source of contaminants is the Po River, the largest Italian river along with the inputs of the Adige and Brenta rivers and the Venice Lagoon (Frignani et al., 1997; Cochran et al., 1998; Ianni et al., 2000; Bellucci et al., 2002). The Po River drainage basin is one of the major drainage basin in Europe and of great economic importance due to the presence of numerous large industries and small and medium-sized enterprises, as well as intensive agricultural and zootechnical activities. For these reasons, the effects of contamination and pollution are more pronounced in the northern Adriatic sector than in the central and southern ones (Guerzoni et al., 1984; Frascari et al., 1988; Faganeli et al., 1991; Covelli et al., 2016; Lopes-Rocha et al., 2017). Although the Adriatic Sea is an important and interesting area for contamination studies encompassing heavily industrialized, urbanized and agriculturally productive areas (Cibic et al., 2008; Guerra et al., 2014; Migani et al., 2015; Mali et al., 2017), only few authors have assessed historical trace metal reconstructions based on sediment cores, which were mostly focused on restricted areas (Price et al., 1992; Romano et al., 2013; Ilijanić et al., 2014; Spagnoli et al., 2014). Despite the scientific effort made so far, important questions remain

⁎ Correspondence to: M. Lopes-Rocha, Departamento de Química-Física, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, UNESCO/UNITWIN WiCoP, Campus de Excelencia International del Mar (CEIMAR), Polígono Río San Pedro s/n, Puerto Real 11510, Cádiz, Spain. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (M. Lopes-Rocha), [email protected] (L. Langone).

http://dx.doi.org/10.1016/j.marpolbul.2017.07.026 Received 4 June 2017; Received in revised form 10 July 2017; Accepted 11 July 2017 0025-326X/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Lopes-Rocha, M., Marine Pollution Bulletin (2017), http://dx.doi.org/10.1016/j.marpolbul.2017.07.026

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Fig. 1. Location map and bathymetry of the Adriatic Sea. Triangles represent sampling stations located in: the Adige River prodelta (core 10), the Po River prodelta (core 9), offshore Ancona (core AN2), offshore Pescara (core 7), Pomo Pit (core 57), offshore Gargano (core GG2), offshore Bari (core BA5) and Otranto Strait (core 13).

2. Material and methods

unresolved: a) what are the historical trends of trace metals that were released into the Adriatic Sea in the past and how much does it remain in the Adriatic sediments today? b) Does the application of Italian environmental regulations on chemical treatments have helped to reduce the discharge of anthropogenic trace metals into the Adriatic Sea? c) Being the Po River the main source of anthropogenic trace metals to the Adriatic Sea, how far is the anthropogenic signal detecting along the Po River dispersion system and how long does this signal take to move southward from the Po River prodelta?. To answer these questions, we analyzed trace metal concentrations on 210Pb-dated sediment cores collected at several locations along the western Adriatic Sea. We established trace metal background levels indicating preindustrial times, when the influence of human activities was still limited. Then, we reconstructed the historical trends of Zn and Pb concentrations and we dated the onset of their increase, and the beginning of their decreasing shift. This approach has been taken in an attempt to evaluate temporal patterns of the inventories of excess trace metals and to elucidate if the increasing and decreasing variations of trace metal concentrations are synchronous, progressive or scattered at basin scale. This work was part of PERSEUS (Policy-oriented marine Environmental research in the Southern European Seas) project which aims to identify the most relevant pressures exerted on the ecosystems of the Southern European Seas (SES), linking them to the Marine Strategy Framework Directive (2008/56/EC, European Commission, 2008) (MSFD) descriptors, criteria and indicators. Based on the descriptor 8, this work aims to ensure that the levels of trace metal contaminants in the marine environment do not to give rise to pollution effects at regional and subregional scale.

2.1. Study area The Adriatic Sea is a shallow semi-enclosed basin connected to the Mediterranean Sea through the Otranto Strait (Manca et al., 2002). It is commonly divided into three sub-basins (North, Central and South), which are characterized by different sedimentary settings, organic matter inputs and metal concentrations. The northern Adriatic has a wide and shallow continental shelf (down to 100 m deep); the Central Adriatic extends from Ancona to the Gargano Promontory, including the Pomo Pit (270 m depth); whereas the southern Adriatic basin extends from the Palagruza Sill (Split-Gargano transect) to the Otranto Strait, including the deepest areas of the whole Adriatic basin (down to 1200 m) (Fig. 1). The Po River, located in the northern Italy, is one of the dominant drainage basins in Europe and the primary fluvial dispersal system entering the Adriatic Sea, along with additional inputs from several smaller Alpine rivers in the North, and Apennine rivers in the Central and South sectors (Dinelli and Lucchini, 1999; Frignani et al., 2005a). The geology of the Po River watershed presents mineral deposits and ultramafic rocks that are naturally enriched in metals (e.g. chromium and nickel) relative to the main composition of the continental crust (Amorosi, 2012). The Adriatic basin is characterized by a microtidal regime and the hydrodynamics is dominated by a cyclonic circulation driven by thermohaline density differences, which in turn, are modulated seasonally (Nittrouer et al., 2004). This cyclonic circulation, known as the Western Adriatic Current (WAC), restricts the freshwater discharges by the Po River and Apennine rivers to a narrow band along the western side of 2

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41.1 16.54 74 0.30 ± 0.02 93–99 97 ± 1.6 0.6–0.9 0.8 ± 0.08 0.08–0.11 0.10 ± 0.01 8.6–11 9.5 ± 0.4 −23.7/−21.6 −22.9 ± 0.4

39.35 18.47 599 0.067 ± 0.004 89–97 93 ± 2 0.3–0.7 0.4 ± 0.08 0.05–0.09 0.06 ± 0.01 9.5–7.0 8.7 ± 0.6 − 23.7/−22.56 − 23.2 ± 0.3

the basin down to the Otranto Strait (Poulain, 2001). Due to the counter-clockwise gyre of the Adriatic waters, fine material supplied by rivers is accumulating in a modern mud-wedge, which extends south of the Po River delta. This mud-wedge is parallel to the western coast and extends to water depths between 10 and 30 m in the North Adriatic, and progressively deeper (down to 80 m) going southward (Correggiari et al., 2001; Cattaneo et al., 2003). The sediment mass balance of the western Adriatic Sea has established that only ~10% of total riverine sediment supply may escape the accumulation in the Adriatic continental shelf. This material is transported to the south Adriatic Sea and eventually exported to the Ionian Sea through the Otranto Strait (Frignani et al., 2005a). In addition, the cascading of North Adriatic Dense Water (NAdDW) through the Bari canyon also enhances particle transfer towards the deepest part of the south Adriatic basin (Turchetto et al., 2007; Langone et al., 2016). 2.2. Field and laboratory work The sampling plan was drawn up based on detailed morphobathymetric maps (Trincardi et al., 2014). Sediments were collected along the western Adriatic Sea during two cruises: in November 2013 onboard the R/V Dallaporta (ADRI-13) and in October 2014 onboard the R/V OGS Explora (ADX-14). Undisturbed sediment cores were retrieved within the mud-wedge in the following stations: the Adige River prodelta (core 10) and Po River prodelta (core 9), offshore Ancona (core AN2), Pescara (core 7), Gargano (core GG2) and Bari (core BA5), and in the deep-sea areas of the Pomo Pit (core 57) and Otranto Strait (core 13) (Fig. 1 and Table 1). Sediments were collected using a cylindrical box-corer or a gravity corer SW104, specially designed to preserve the sediment-water interface. The sampler was washed in clean water after every sampling to avoid any contamination between sample sites. Sediment cores (length ≤ 50 cm; diameter: 10 cm) were sectioned onboard at 1 cm intervals and samples were stored immediately after collection in polyethylene bags and frozen at −20 °C. An aliquot of sediment was dried at 60 °C to measure the water content and to calculate porosity (ϕ) according to Berner (1971) assuming a sediment density of 2.6 g cm− 3 and a water density of 1.034 g cm− 3. Grain size was determined after the removal of organic matter using H2O2 and wet sieving at 63 μm to separate sand from silt and clay fractions. Another aliquot of sediment was freeze-dried and homogenized. Total nitrogen (TN) and organic carbon (OC) contents were determined by elemental analysis (EA) of combusted aliquots using a CHNS Elemental Analyzer (Thermo Fisher Flash 2000 IRMS). Subsamples for organic carbon (OC) were first decarbonated with 1.5 M HCl. Stable isotopic analyses of OC (δ13C) were carried out on samples by using a FINNIGAN Delta Plus mass spectrometer directly coupled to the EA. Stable isotope data were expressed in ‰ relative to the variation (δ) from the international PDB standard (Tesi et al., 2007):

(13C 12C )sample 3 − 1⎤ δ13C = ⎡ ⎢ (13C 12C ) ⎥ × 10 PDB ⎣ ⎦ For the analysis of major and trace metals, a total digestion technique was carried out according to Loring and Rantala (1992) using HCl, HNO3, and HF (3:1:1 v/v) suprapure acids. Copper (Cu), lead (Pb), nickel (Ni) and chromium (Cr) were analyzed by graphite furnace atomic absorption spectrophotometry (GF-AAS, Perkin Elmer A Analyst 100); zinc (Zn), iron (Fe) and manganese (Mn), were determined by flame atomic absorption (FAAS), according to the EPA methods 7010 and 7000B (USEPA, 2007a, 2007b). Aluminum (Al) and titanium (Ti) were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES, Thermos ICAP series 6000). For analytical quality control, reagent blanks, a random replicated sample and a certified reference material Sco-1 (Silty Marine Shale from the United

δ13C ‰

C:N ratio

TN%

OC %

Lat. °N Long. °E Depth m SAR cm year− 1 Mud %

Min − max Mean Min − max mean Min − max mean Min − max Mean Min − max Mean

44.41 12.31 27 0.52 ± 0.08 90–100 99 ± 1.9 0.5–1.0 0.8 ± 0.13 0.08–0.13 0.1 ± 0.01 7.5–10 9.6 ± 0.5 −24.0/−23.2 −23.7 ± 0.2 45.8 12.25 23 0.48 ± 0.04 41–90 75 ± 11 0.4–1.5 0.9 ± 0.2 0.03–0.11 0.08 ± 0.02 11–21 14 ± 2 − 24.1/−21.7 − 23.2 ± 0.5

43.39 13.34 42 0.35 ± 0.04 98–99 99 ± 0.3 0.8–0.6 0.7 ± 0.07 0.07–0.10 0.08 ± 0.008 9.1–10.5 10 ± 3 −24.20/−23.26 −23.6 ± 0.2

42.3 14.22 65 0.26 ± 0.01 98–100 99 ± 0.5 0.9–0.4 0.6 ± 0.1 0.06–0.12 0.08 ± 0.01 6.7–10 9 ± 0.6 − 24.69/−23.36 − 23.9 ± 0.3

42.5 14.44 268 0.07 ± 0.02 97–99 99 ± 0.6 0.5–0.6 0.5 ± 0.03 0.08–0.09 0.08 ± 0.003 5.0–8.0 7.3 ± 0.9 − 21.67/−22.0 − 21.8 ± 0.1

41.59 16.9 34 0.46 ± 0.06 93–99 97 ± 1 0.4–0.7 0.6 ± 0.05 0.06–0.08 0.07 ± 0.005 7.0–11 9.7 ± 0.7 − 24.7/−23.3 − 23.9 ± 0.3

Core 13 Core BA5 Core AN2 Core 9 Core 10

Core 7

Core 57

Core GG2

Otranto Strait Offshore Bari Offshore Gargano Pomo Pit Offshore Pescara Offshore Ancona Po River Prodelta Adige River Prodelta

Table 1 Latitude, longitude, water depth, estimated SARs and minimum, maximum and mean and standard deviation ( ± SD) in sediment cores from the western Adriatic Sea: Organic carbon (OC), Total nitrogen (TN), C:N atomic ratio, stable carbon isotopic signature (δ13C).

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In addition, we calculated the modified pollution index (MPI), which is an improvement of the Nemerow pollution index (PI) (Nemerow, 1991). This multiple element index uses enrichment factors instead of concentration factors and it is effective for sediment qualification because of its sensitivity in detecting successive graduation of sediment class (Brady et al., 2015). MPI takes into consideration different sediment behavior that is likely to occur in complex environments due to the use of a normalization element (Duodu et al., 2016). The MPI was calculated according to the formula:

States Geological Survey, USGS) were analyzed in parallel with each batch of samples. The overall analytical precision and accuracy were typically between 5 and 10% (Lopes-Rocha et al., 2017). 2.3. Dating Sediment accumulation rates (SARs, cm year− 1) over a 100-yr time scale have been extensively assessed along the Adriatic Sea (n = 246) from the Gulf of Trieste down to the Gargano Promontory (Frignani et al., 2005a; Palinkas and Nittrouer, 2006, 2007; Tesi et al., 2013) based on short-lived radioisotope geochronology (mainly 210Pb and 137 Cs). Reference sediment accumulation rates for each core location were taken from these datasets, except for sediment cores sampled at offshore Pescara (core 7) and in the Otranto Strait (core 13), for which 210 Pb activities were specifically measured in this study. Alpha counting of daughter isotope 210Po, considered in secular equilibrium with its grandparent 226Ra, was used for 210Pb analyses, following the procedure depicted in Frignani and Langone (1991). In order to estimate the date for each section of the sediment cores, the sediment accumulation rate reported for each key-station was used, as follows:

MPI =

(EF average)2 + (EF max)2 2

Six degrees of contamination are defined: MPI < 1: unpolluted; MPI > 1 and < 2: slightly polluted; MPI > 2 and < 3: moderately polluted; MPI > 3 and < 5: moderately - heavily polluted; MPI > 5 and < 10 severely polluted and MPI > 10 heavily polluted (Brady et al., 2015).

2.5. Excess inventories of trace metals

b Estimated date [anno Domini (A. D.)] = α − ⎛ ⎞ ⎝c⎠

We calculated the excess of trace metal concentration (Mex) in the sediment cores as follows:

where α is the year in which the core was collected, b is the depth of the section in the core and c is the SAR of each core.

Mex = Mez − [Al z (M Al) bkg] where Mez is the total trace metal concentration at depth z, Al is aluminum concentration at the same depth and (M/Al)bkg is the metal/ Aluminum ratio relative to preindustrial sediments. This calculation subtracts the amount of an element (Al) that is mineralogically associated with the terrigenous component from the total concentration of that element (Schroeder et al., 1993). The excess inventory gives the total amount of trace metals accumulated in a given area in excess of the natural levels (background levels), and that has been retained in the sediment column as a result of anthropogenic contaminant inputs (Bricker, 1993). In this study, the excess inventories of trace metals were estimated to evaluate the potential of sediments as a new source of contamination to the Adriatic Sea marine ecosystem, using the following equation:

2.4. Enrichment factor and modified pollution index In order to provide an assessment of the sediment quality, we first established the background levels on the basis of preindustrial concentrations of trace metals (Fig. 2). For this purpose, we considered the average trace metal concentrations detected in the deepest layers of each sediment core deposited before the great Italian industrial growth of the twentieth century (i.e., older than ~1910). We also assumed that these concentrations were fairly similar to the natural trace metal levels and thus significant and identifiable with the actual background levels of the sampled locations. The only exception was the Adige River prodelta station; here the grain size distribution showed a great vertical variability with fine sediment at the top that becomes relict sands towards the bottom of the core. Given that, in this station we did not use the deepest core values from sands, but the trace metal concentrations by the transition zone (Fig. 2 and Fig. S1a; Supplementary material). This pattern suggests the occurrence of important physical and biological mixing processes. As a consequence, we reconstructed the contamination history of this area only for the uppermost 21.5 cm (~ 1970 A.D), that is the last 35 years. Then, we calculated the Enrichment factor (EF) using a standard formula as the ratio of the element of interest to a conservative element (assumed to have no anthropogenic input or not affected by weathering) (e.g. Fe, Al, Ti) in a given sample, to the same ratio in a local/ regional background. For this study, we used Al as the normalization element due to both its abundance and representation of aluminosilicates, which commonly adsorb trace elements (Morelli et al., 2012). The EF was calculated according to the formula:

EF =

Inventory = Σ(Mex Di ρi) where Mex is the trace metal excess in the sediment layer (mg kg− 1 dry weight), Di is the thickness of the sediment layer (cm) and ρi is the dry bulk density of the sediment layer (g cm− 3).

2.6. Statistical analysis A multivariate statistical analysis based on similarity matrices was carried out (non-metric MultiDimensional Scaling, nMDS) in order to check for possible similarities among the sampled stations and their trace metal concentrations (Zn, Pb, Cu, Ni and Cr). This analysis is based on the measurement of the similarity between objects in a database and the grouping of objects according to these similarities. Euclidean distances were calculated between each pair of objects. The reliability of nMDS was assessed using the stress value, with values near 0 indicating better fit (Clarke and Warwick, 2001). Permutational multivariate analysis of variance (PERMANOVA; Anderson, 2001) based on the Euclidian resemblance matrix was used to test for differences in the trace metal concentrations (Zn, Pb, Cu, Ni and Cr) among the sampled core stations (8 levels). The p-values were obtained by 9999 permutations of residuals under a reduced model. Pair-wise tests were performed to explore significant interactions among levels (p < 0.01). These analysis were performed using the software PRIMER v6 (Clarke and Gorley, 2006).

(Ci Ciref )sample (Ci Ciref )background

where Ci is the concentration of the element of interest and Ciref is the concentration of the normalization element. Five degrees of contamination are commonly associated with enrichment factors: EF < 2, deficiency to minimum enrichment; 2 ≤ EF < 5, moderate enrichment; 5 ≤ EF < 20, significant enrichment; 20 ≤ EF < 40, very high enrichment; and EF > 40, extremely high enrichment (Sutherland, 2000). 4

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Fig. 2. Dated profiles of trace metal content (mg kg− 1) in the studied cores. See Fig. 1 for location of sampling stations.

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Fig. 2. (continued)

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Table 2 Minimum, maximum and mean and standard deviation ( ± SD) of major and trace metals in sediment cores from the western Adriatic Sea. Metals

Al % Ti % Fe % Mn mg kg− 1 Zn mg kg− 1 Pb mg kg− 1 Cu mg kg− 1 Ni mg kg− 1 Cr mg kg− 1

Min − max Mean Min − max Mean Min − max Mean Min − max Mean Min − max Mean Min – max Mean Min − max Mean Min – max Mean Min – max Mean

Adige River Prodelta

Po River Prodelta

Offshore Ancona

Offshore Pescara

Pomo Pit

Offshore Gargano

Offshore Bari

Otranto Strait

Core 10

Core 9

Core AN2

Core 7

Core 57

Core GG2

Core BA5

Core 13

2.8–4.0 3.5 ± 0.2 0.10–0.20 0.16 ± 0.02 1.6–2.7 2.2 ± 0.3 342–445 394 ± 27 29–151 100 ± 32 13–35 25 ± 6 6–17 12 ± 2 12–35 25 ± 6 28–88 65 ± 16

5.2–7.3 6.1 ± 0.5 0.18–0.26 0.22 ± 0.02 2.9–4.1 3.5 ± 0.2 534–917 617 ± 56 84–203 127 ± 22 13–39 28 ± 6 26–39 32 ± 3 50–86 63 ± 7 139–165 153 ± 5

5.4–6.4 5.0 ± 0.4 0.24–0.31 0.27 ± 0.02 3.0–3.5 3.2 ± 0.1 723–969 787 ± 41 64–134 95 ± 16 17–23 20 ± 2 16–23 19 ± 2 34–59 50 ± 7 103–166 120 ± 14

2.8–5.6 4.9 ± 0.4 0.11–0.22 0.19 ± 0.02 2.7–3.3 3.0 ± 0.1 636–840 721 ± 45 44–90 65 ± 13 11–26 16 ± 3.6 16–23 18 ± 1.5 35–48 42 ± 3 63–110 83 ± 9

4.6–5.3 5.0 ± 0.2 0.15–0.18 0.17 ± 0.010 2.7–3.1 2.9 ± 0.1 1396–2015 1590 ± 210 41–65 55 ± 7 10–13 12 ± 1 22–25 24 ± 1 76–108 85 ± 9 85–110 94 ± 7

4.7–6.2 5.2 ± 0.3 0.19–0.27 0.23 ± 0.02 2.7–3.2 3.0 ± 0.1 571–715 656 ± 25 64–174 89 ± 21 12–19 16 ± 2 15–24 19 ± 3 24–68 38 ± 8 97–120 108 ± 6

6.2–7.6 6.9 ± 0.3 0.26–0.35 0.31 ± 0.03 2.4–3.6 3.3 ± 0.3 692–1015 749 ± 70 76–118 95 ± 11 19–36 24 ± 4 20–26 22 ± 2 18–32 26 ± 4 129–156 142 ± 6.0

5.2–6.9 6.3 ± 0.4 0.21–0.29 0.26 ± 0.02 2.9–3.4 3.1 ± 0.1 509–1910 656 ± 359 57–93 71 ± 12 9–18 11 ± 2 14–37 20 ± 4 67–143 94 ± 23 85–118 97 ± 10

3. Results

important deposition sites are offshore Ancona (core AN2; 0.35 ± 0.04 cm year− 1) and in front of the Gargano promontory (core GG2; 0.46 ± 0.06 cm year− 1). The lowest SARs were observed in the Otranto Strait (core 13; 0.067 ± 0.004 cm year− 1) and in the Pomo Pit (core 57; 0.07 ± 0.02 cm year− 1).

3.1. Sediment characteristics A summary of grain size distribution, organic carbon (OC), C:N atomic ratio, total nitrogen (TN) and carbon isotopic signature (δ13C) is given in Table 1 and Fig. S1a, b (Supplementary materials). Fine fractions (< 63 μm) were predominant in the sampled sediments accounting for ~40 to ~99%. These results are consistent with other studies accomplished in the western Adriatic Sea (Frignani et al., 2005a; Romano et al., 2013; Spagnoli et al., 2014; Combi et al., 2016), where the sediment released by the Po and Apennine rivers consists primarily of silt and clay particles (> 90%) (Nittrouer et al., 2004). Grain size distribution presented a distinct trend in the Adige River prodelta station (core 10), with decreasing mud content towards the bottom of the sediment core (Fig. S1a; Supplementary material). This vertical profile suggests the existence of two different sediment layers. The upper one (< 21.5 cm depth) is the result of the modern progradation of the Adige River prodelta, while the deeper layers (> 27 cm depth) represent the relict sands deposited during the last sea level rise, with fine material (silt + clay) contents lower than 70%. OC and TN distribution contents varied between 0.3 and 1.5%, and 0.03 and 0.013%, respectively, displaying low variations with depth among the stations and increasing trends upward. Similarly, the vertical distribution of C:N ratios were relatively constant (9 ± 1), except for the Adige prodelta station (core 10) where C:N ratios presented an increasing trend downward. Overall, OC and TN contents, C:N molar ratios were higher in the northern sector. Accordingly, stable carbon isotopic signatures (δ13C) are comparatively uniform with slightly depleted values in the Po River prodelta station (core 9; −23.7 ± 0.2‰) when compared to the stations located southwards, such as those at the Pomo Pit (core 57; −21.8 ± 0.1‰), offshore Bari (core BA5, − 22.9 ± 0.4‰) and in the Otranto Strait (core 13; −23.2 ± 0.3‰) (Fig. S1a,b;Supplementary materials). These results suggest that the patterns observed in the northern Adriatic Sea are related to the high discharge of the Po River and that the proportion of marine-derived OC increases as the material moves southwards (Tesi et al., 2013) and to deeper areas. Calculated SAR for each station is reported in Table 1. The highest SARs were observed in the Po River prodelta (core 9; 0.52 ± 0.08 cm year− 1), where sediments accumulate preferentially in two depocenters (Palinkas and Nittrouer, 2006), followed by the Adige River prodelta (core 10; 0.48 ± 0.04 cm year− 1). Other

3.2. Sedimentary profiles of major and trace metals Sedimentary profiles of measured major and trace metals are shown in Figs. S2a, b (Supplementary materials) and Fig. 2, respectively. A summary of these data is presented in Table 2. Most of the stations show relatively homogeneous vertical distributions of Al, Fe, Ti and Mn levels with a similar range of values and mean concentrations (Fig. 2; Table 2). However, their concentrations are lower in the Adige River prodelta station (core 10) and a sharp increase in Mn concentrations in the first ~ 3 cm depth is observed at Otranto station (core 13; 1910 mg kg− 1, respectively). At the Pomo Pit station (core 57), Mn concentrations (1590 ± 210 mg kg− 1) were about two times higher than the levels observed in the other stations (Table 2; Fig. S2b; Supplementary material). Trace metal distribution profiles showed a similar trend at the Adige River prodelta, Po River prodelta, offshore Pescara and offshore Gargano stations presenting upward increasing concentrations from the beginning of the 20th century to the mid-1980s with a subsurface decrease (Fig. 2). Conversely, at stations offshore Ancona (at 4.5 cm depth; 100–133; 17–23; 16–20; 134–166 mg kg− 1 for Zn, Pb, Cu and Cr, respectively) and Bari (at 3.5 cm depth; 83–105, 20–36, 22–26 mg kg− 1 for Zn, Pb and Cu, respectively), sediment cores displayed an upward increasing trend from 2000 towards the present (Fig. 2). On the other hand, the Pomo Pit station exhibited fairly constant trace metal profiles and similarly to the Otranto station, exhibited concentrations close to background levels (55 ± 7 and 71 ± 12; 12 ± 1 and 11 ± 1, 24 ± 1 and 20 ± 4, 85 ± 9 and 94 ± 22, 94 ± 7 and 97 ± 10 mg kg− 1 for Zn, Pb, Cu, Ni and Cr, respectively; Table 2). 3.3. Background and excess trace metals The spatial distribution of the background levels are shown in Fig. 3 and given in Table 3. Estimated background levels of trace metals are different core by core. The geological and pedological characteristics of the Italian peninsula are very different from region to region, this results in a great compositional variability of locally discharged materials 7

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Fig. 3. Spatial patterns of trace metal background levels in the western Adriatic Sea (mg kg− 1).

exceeded the threshold of 1.0 in the central and southern sectors of the western Adriatic Sea, thus evidencing a slightly contaminated status of the sediments in the entire basin. Comparison between the average EF and the MPI (Table 4), suggests that the MPI is more useful as single index for defining the cumulative contamination status of the trace metals studied in each sector because of its sensitivity in detecting graduation of sediment class (Brady et al., 2015; Duodu et al., 2016). On the other hand, EFs of each trace metal are more informative regarding the actual origin of trace metal contamination. Inventories related to the excess of trace metals are shown in Fig. 4. The Po River prodelta station (core 9) displayed the highest inventories for all trace metals (1987, 608, 149, 245 and 352 μg cm− 2 for Zn, Pb, Cu, Ni and Cr, respectively), followed by the Adige prodelta (644, 102, 24, 120 and 316 μg cm− 2), offshore Pescara (525, 156, 25, 147 and 252 μg cm− 2) and offshore Gargano stations (443, 92, 23, 210 and 151 μg cm− 2). Inventories were one order of magnitude lower at stations located further south, such as those found at the offshore Bari and Otranto Strait stations (71 and 79, 28 and 12, 11 and 3, 28 and 85, and 31 and 46 μg cm− 2 for Zn, Pb, Cu, Ni and Cr in core BA5 and core 13, respectively). The Pomo Pit station (core 57) exhibited the lowest trace metal inventories along the western Adriatic Sea (15, 3, 2, 17 and 9 μg cm− 2 for Zn, Pb, Cu, Ni and Cr, respectively).

Table 3 Background levels of trace metals estimated for each sediment core. Stations

Zn mg kg− 1

Pb mg kg− 1

Cu mg kg− 1

Ni mg kg− 1

Cr mg kg− 1

Adige River Prodelta Po River Prodelta Offshore Ancona Offshore Pescara Pomo Pit Offshore Gargano Offshore Bari Otranto Strait

86 84 79 54 55 77 95 61

23 14 21 14 12 13 23 10

12 30 20 18 24 21 22 21

23 61 49 39 85 33 26 103

57 152 113 79 96 109 143 100

and sediments accumulating along its coastline. In fact, as shown in Fig. 3, trace metal background levels computed in this study did not show a clear pattern of distributions along the western Adriatic coastline. The highest variations were found for Ni (52 ± 29 mg kg− 1) and Pb (16 ± 5 mg kg− 1) followed by Cr (106 ± 31 mg kg− 1), Cu (21 ± 5 mg kg− 1) and Zn (75 ± 15 mg kg− 1). The stations located in deeper waters (depth > 200 m), such as Pomo Pit and Otranto Strait, exhibited high Ni and low Zn and Pb background levels. The highest background levels of Zn and Pb were found in the Adige River prodelta and Bari stations. In contrast, the Po River prodelta station exhibited the highest Cu and Cr concentrations. The background levels found in this study are in good agreement with those previously reported for the Po River prodelta (Correggiari et al., 2016) and Bari coastal zone (Mali et al., 2015a) and, due to the basin-scale approach of this study, could be useful as a reference for any future research in the western Adriatic sea (Table 3). The EF and MPI results calculated for all stations are shown in Table 4. In general, trace metals displayed minor average EF in all sediment cores (EF < 2.0). However, Pb, and to a lesser extent Zn, are the most important trace metal contaminants in the western Adriatic Sea, since they displayed the highest EF absolute values in the Po River prodelta station (core 9: EFPb = 3.1 and EFZn = 2.7). These results are consistent with the MPI of 2.4 found at this station, which is an indicative of moderately contaminated sediments. The MPI levels always

4. Discussion 4.1. Trace metal spatial patterns Although Ni and Cr background concentrations (Table 3) found in this study, especially in the Po River prodelta station (61 and 152 mg kg− 1, respectively), are higher than the upper continental crustal composition (47 and 92 mg kg− 1; Rudnick and Gao, 2003), the EF results are essentially indicative of an unpolluted status (EF < 2) (Table 4). Therefore, the high Cr and Ni concentrations are probably related to the widespread presence of the ultramafic complexes cropping out in the western Italian Alps (Amorosi, 2012), where soil erosion by the Po River and its tributaries carry anomalously high amount of these metals to the Adriatic Sea (Picone et al., 2000). Both trace metal 8

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Table 4 Minimum, maximum and mean enrichment factors (EF) of the trace metals and modified pollution index (MPI) results from the sediment stations in the western Adriatic Sea.

Zn (EF) Pb (EF) Cu (EF) Ni (EF) Cr (EF) MPI

Min − max Mean Min − max Mean Min − max Mean Min − max Mean Min − max Mean

Adige River Prodelta

Po River Prodelta

Offshore Ancona

Offshore Pescara

Pomo Pit

Offshore Gargano

Offshore Bari

Otranto Strait

Core 10

Core 9

Core AN2

Core 7

Core 57

Core GG2

Core BA5

Core 13

0.9–1.8 1.3 0.9–1.5 1.2 0.8–1.3 1.0 0.9–1.5 1.2 0.9–1.6 1.2 1.5

1.0–2.7 1.6 1.0–3.1 2.1 0.9–1.4 1.1 0.9–1.6 1.0 0.9–1.2 1.0 2.4

0.8–1.6 1.2 0.8–1.1 1.0 0.8–1.2 1.0 0.7–1.2 1.0 0.9–1.4 1.1 1.4

0.8–1.8 1.2 0.8–2.1 1.2 0.9–1.3 1.0 0.9–1.3 1.1 0.7–1.3 1.1 1.7

0.8–1.2 1.0 0.9–1.1 1.0 1.0–1.04 1.0 0.9–1.2 1.0 0.9–1.1 1.0 1.1

0.9–1.8 1.2 0.9–1.4 1.2 0.8–1.2 1.0 0.8–1.5 1.2 0.9–1.1 1.0 1.5

0.9–1.2 1.0 0.9–1.8 1.0 0.9–1.3 1.0 0.7–1.3 1.0 0.9–1.1 1.0 1.4

0.9–1.7 1.2 0.9–1.8 1.2 0.6–1.2 0.9 0.7–1.4 0.9 0.8–1.2 1.0 1.5

(Tables 2 and 4, Fig. 4) further highlight that Pb and Zn are the most significant anthropogenic trace metals in the western Adriatic sea. Anthropogenic contaminants mostly accumulate in sediments close to the source areas and their concentrations and inventories gradually decline with increasing distance from the inputs (Frignani and Belluci, 2004). The highest inventories were found in the Po River prodelta, followed by the Adige prodelta station, and decreasing towards the southern stations (Fig. 4). Inventories calculated for the Po River prodelta station (core 9) are ~80% higher than the other stations, whereas the Pomo Pit and Otranto Strait stations accounted for only ~7% of the total trace metal amount stored in the Po prodelta sediments. These findings confirm the importance of the Po River drainage system and the highly industrialized zones of North Italy, as Porto Marghera in the Venice Lagoon (Frignani et al., 2005b; Zonta et al., 2007), as the major sources of contaminants to sediments of the western Adriatic Sea. In many sedimentary systems, such as the Adriatic Sea, the magnitude of along-shelf particulate transport is much greater than the corresponding across-shelf component (Palanques et al., 2008). In these systems, the riverborne contaminated material follows the dispersion pattern controlled by the main currents and accumulates when and where energy decreases (Cattaneo et al., 2007). The relatively high

loads delivered by the Po River account for ~50% of their total inputs into the western Adriatic Sea (Lopes-Rocha et al., 2017). These trace metals are commonly associated to the residual phase of sediments being strongly bounded to mineral crystals (Morillo et al., 2004; Yuan et al., 2007; Reza et al., 2016), thus suggesting a typical geological signature rather than an anthropogenic source to the sediments. The EF results of Cu are frequently close to 1.0 in all stations (Table 4), evidencing unpolluted levels in the sediments of the western Adriatic Sea. Cu was previously found to be associate with surface active organic substances in the Adriatic Sea, being remineralized during partial oxidation of organic matter and kept in solution longer than other metals (Tankere et al., 2000b). This might suggest that while other trace metals conserve their enriched levels because of their stronger affinity with fine particles, Cu is marked by the loss of its anthropogenic component that is progressively transferred to the dissolved phase, most likely mediated by organic ligands (Roussiez et al., 2011). High Zn and Pb concentrations have been previously found in the northern Adriatic sector (Romano et al., 2013; Ilijanić et al., 2014; Lopes-Rocha et al., 2017) and related to anthropogenic influences. In this study, the results of total concentrations, EFs and excess inventories

Fig. 4. Spatial patterns of excess trace metal inventory (μg cm− 2) in the western Adriatic Sea.

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Fig. 5. nMDS ordination plot based on the trace metals analyzed (Zn, Pb, Cu, Ni and Cr), showing the similarity among the sediment cores. The stress value is indicated (0.08).

(p = 0.036; Table 1S; Supplementary material). These stations have similar trace metal concentrations (close to the background levels), low SARs (~ 0.07 cm year− 1), OC and TN contents, and Zn and Pb concentrations, with high concentrations of Mn with respect to the other sampled stations, thus evidencing little anthropogenic influence and suggesting that both stations are subject to similar trace metal depositional processes. Consistently with our findings, previous studies in the Pomo Pit area have reported high Mn concentrations in the sediments related to the presence of manganesiferous particles (Dolenec, 2003; Spagnoli et al., 2014). This station is located in a shelf basin that represents a transition zone from the shallow north to the deep Southern Adriatic Sea. Such complex morphology provides areas where a branch of the North Adriatic Dense Water (NAdDW) water is captured and temporarily stored (Marini et al., 2016). The NAdDW is the densest water of the whole Mediterranean, formed over the northern Adriatic shelf during cold months and spreading southward along the western Italian coast (Vilibić and Supić, 2005). As it approaches the western side of the Pomo Pit, a part of NAdDW sinks and fills the bottom layer of the basin (Marini et al., 2016). The new-formed dense shelf water is oxygen-rich, that along with the low sedimentation rates, OC and TN contents and hence, low degradation rates, would allow deeper oxygen penetration into the sediment (Hedges and Keil, 1995). Apparently, the major zone of Mn reduction is not sufficiently close to the sediment water interface and Mn produced is not likely to escape. This is consistent with the low dissolved Mn levels previously found in bottom waters in this area (Price et al., 1999; Tankere et al., 2000b). Due to the ability of certain trace metals to complex with this metal, the increase of Cu and Ni concentrations found in this core (Sensarma et al., 2015), might have reflected the Mn cycle in deep sediments (McManus et al., 2012). In the Otranto Strait, the Mn enrichment detected in the uppermost 3 cm (Fig. S2b; Supplementary material) suggests Mn oxide dissolution in sub-oxic subsurface sediment layers and the migration of their dissolved cations upward, where they are re-precipitated under oxic conditions (Thomson et al., 1993). The similarity in the upper few centimeters between all trace metals and, to a lesser extent, to Zn profiles probably reflects analogies in their environmental fate related to the existence of diagenetic processes in this sediment core (Fig. 2b).

excess of trace metal inventories found at the Gargano station (Fig. 4), indicates an efficient transfer of anthropogenic particulate trace metals from the Po prodelta towards south. On the other hand, the Po River sediment is accumulated only in a minor fraction in the middle of the Central Adriatic Sea (Pomo Pit), but keeps moving further southwards parallel to the coastline and it is finally deposited in the subaqueous Gargano delta (Cattaneo et al., 2003). The inner shelf of the south Adriatic Sea (offshore Bari) was found to be one of the final repositories of these contaminants (Lopes-Rocha et al., 2017). Another possible sink could be the deep-sea region of the southern Adriatic basin, triggered by cascading events of dense shelf waters (Turchetto et al., 2007; Langone et al., 2016), although up to now no systematic measurement is available, while a net export of Zn and Pb from the Otranto Strait has been previously excluded (Tankere et al., 2000a). 4.2. Trace metal dynamics in deep areas of the Adriatic basin Trace metal geochemistry within the western Adriatic sediments seems to be complex and certainly subject to the influence of natural and anthropogenic factors such as: proximity to the coast and populated areas, the WAC influence and the location of each sampled core within the Adriatic mud-wedge (Price et al., 1993). The pairwise PERMANOVA results based on trace metal concentrations found at each sampled station revealed that the geographical location of the sediment cores is significant (p < 0.01; Table 1S; Supplementary material). This is further confirmed by the nMDS ordination plot in which all stations are relatively separated from each other (Fig. 5). The coastal stations of Po River prodelta and the Adige River prodelta (~ 50 km distance) have particularly low similarity. Although both stations showed relatively similar Zn and Pb concentrations, the average concentrations of Cr, Ni and Cu levels are two times lower at the Adige River prodelta station (Table 2). North of the Po River mouth, sediment composition is influenced by the predominance of dolomitic facies in the drainage basin of the eastern Alps and the dispersion of Po River sediment is prevented northward due to the strong WAC circulation. Thus, the trace metal dynamics in the two areas is different in spite of their geographical proximity. Stations located south of the Po River prodelta in the inner shelf, such as offshore Ancona, Pescara and Gargano are quite grouped together. Although trace metal concentrations are significantly different in these stations (p < 0.01; Table 1S; Supplementary material), pointing to the predominance of local inputs of trace metals, there is a certain similarity in the trace metals concentrations, which in turn, might be related to the WAC regime of water circulation promoting the southward sediment transport of trace metals (Price, 1993). Interestingly, stations located far apart (~ 460 km distance) at depths > 200 m (as the Pomo Pit and Otranto Strait stations) are grouped together

4.3. Pb and Zn historical trends The trace metal vertical profiles can be influenced by sediment texture and composition, input from polluting sources, and diagenesis. Therefore, it is important to establish the key processes that contribute to metal concentration profile. The history of contamination can be accurately reconstructed from the sedimentary record only if trace metal distributions are mainly controlled by the input variability 10

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sediment grain size, as the mud content of most sediment cores was similar, but by a decrease on the anthropogenic pressure. This overall temporal trend is expected according to the environmental Kuznet Curve with an inverse U-shaped pattern (Romano et al., 2013). The pronounced increasing shift in the bottom of the sediment cores corresponds to the beginning of the anthropogenic impact, which started around 1910s and peaked in the late 1960s-early 1970s (Felice and Carreras, 2012; Gürlük, 2009). Given that, based on sediment accumulation rates, we calculated the ages of the onset of the trace metal increase as well as those of the more recent decreasing concentration shift in the above mentioned sediment cores. This approach was carried out in an attempt to elucidate if the

(Frignani et al., 1997). Excess trace metal profiles are displayed in Fig. 6. Although the excess of Zn and Pb profiles from the Adige River prodelta, Po River prodelta, offshore Pescara and Gargano stations did not exhibit the same vertical distribution, their general trends are similar. Collectively, sediment cores are characterized by increasing concentrations from the bottom layers upwards, an intermediate section with relatively high values and a decreasing trend towards the top layers (Fig. 6), where excess Zn and Pb have approximately halved with respect to the subsurface peak values. The highest reduction was displayed in the Adige River prodelta station with results down to 5% in the top of the sediment core. This decrease cannot be explained by changes on the surface

Fig. 6. Excess of trace metals (mg kg− 1) calculated for each sampled station.

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Fig. 6. (continued)

first contamination signal by Pb and Zn in the Adriatic Sea sediments. On the other hand, trace metals started to decrease around 1984–1985 in the Adige prodelta station (Fig. 7). The same signal was then observed in the Po River prodelta between 1986 and 1989, later (1990–1992) offshore Pescara and finally detected in 1996–1997 offshore the Gargano promontory. In response to increasing pollution levels affecting marine ecosystems, many countries introduced environmental regulations over the last few decades. Some studies have shown a reduction on trace metal levels (Hornberger et al., 1999) or in the rate of increase of pollution in the uppermost sediments of some coastal zones (Bay et al., 2003; Palanques et al., 2017). This decrease has been linked to causes such as

timing of both variations were synchronous, progressive or scattered at basin scale. Individual Zn and Pb profiles for the selected cores are shown in Fig. 7. The onset of trace metals was not detected in the Adige River prodelta. This was expected since in this part of the progradational unit, the modern accumulation of fine sediments was inferred to have begun not before of 40–50 years ago. In the Po River prodelta, the onset of excess trace metals was estimated to be in 1914, whereas in the offshore Pescara station it was observed at ~1920 and in the Gargano station at ~1922. Therefore, the time interval between 1914 and 1922 is representative of the beginning of the influence of industrial activities in all these areas. Based on these age estimates, the Italian industry's war effort during World War I probably constitutes the source of the 12

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Fig. 7. Zn and Pb profiles (mg kg− 1) of Adige River prodelta, Po River prodelta, offshore Pescara and Gargano. Right-oriented arrows near the bottom indicate the onset of trace metal concentration increase; left-oriented arrows close to the core top indicate the start of reduction of trace metal concentration in recent years.

4.4. Insights on particle transfer mechanisms and rates along the Adriatic mud-wedge

coastal zone management programs (Díaz-Asencio et al., 2011), economic contractions and environmental regulations enforced by national governments (Dai et al., 2007). However, in places where the growing industrial development has surpassed the government regulation, especially after 2000s (e.g. Pearl River estuary, Wang et al., 2015), trace metal profiles are still increasing together with urban and industrial development. In the Adriatic Sea, the decline of Zn and Pb concentrations and inventories in the last decades reflects the enforcement of the first national regulation on wastewater discharges (Merli Law n. 319/76). This regulation concerns wastewater emission standards and limits for chemical substances discharged from both urban and industrial wastewater treatment facilities. Although this legislative act was adopted in 1976, its full implementation required several years (4–8 years). It is noteworthy that the decrease of trace metal contamination is recorded at least 2 years earlier in the Adige River prodelta station with respect to the Po River prodelta station. Camusso et al. (2002) and Pettine et al. (1994) have claimed that the patterns of trace metal supply from the Po River had not substantially changed within the 10 years following the maximum trace metal fluvial supply. We suggest that the small drainage basin of the Adige River responded more rapidly to the reduction of trace metal inputs, while the larger drainage basin of the Po River (75,000 km2) retain for a longer time the memory of contamination with slow releases over time. The progressive glacier melting further increases the river supply of those elements for which anthropogenic inputs are mainly by atmospheric fallout (e.g., Pb). In addition, at the offshore Ancona station, Zn and Cr have slightly increased since 2000. Zn, in Ancona coastal zone has been detected also in the bioavailable fraction (Dell'Anno et al., 2003), which could be accounted by the harbor activities developed in this area, while high Cr concentrations was found to be mainly in the residual fraction (> 90%). At offshore Bari station, the accumulation of excess Zn and Pb approximately doubled since 2000. In Bari coastal zone, contamination is mainly caused by water washout from the intensively cultivated areas along the coastline and transferred to the sea through the local hydrographic network. This area shelters the Port of Bari, which is one of the most important commercial ports in the Mediterranean Sea (Mali et al., 2015a, 2016), where relatively high metal concentrations and other contaminants had been previously detected (Accornero et al., 2004; Mali et al., 2015b; Combi et al., 2016). Recent works of enlargement of the port of Bari, as well as that of the nearby Molfetta, could have promoted metal remobilization from the inner areas and final accumulation in the offshore sediments.

As in the marine environment contaminants are mostly adsorbed onto the particulate matter, the study of their fate can contribute to elucidate the depositional processes of riverborne material from the riverine source to the final sink into the sedimentary record. Thus, the propagation of the increasing or decreasing contamination signals from the Po River dispersion system provided important insights also on sediment dynamics, such as the estimate of the particle transfer rates along the Adriatic mud-wedge. Many studies have made significant contribution to the understanding of sediment transport mechanisms involved in the early deposition, resuspension, along-coast transport and final burial of sediment, and in the architecture construction of subacqueous prodeltas and late-Holocene mud-wedge (Gilbert, 1886; Rich, 1951; Bates, 1953; Asquith, 1970; Pirmez et al., 1998; Adams and Schlager, 2000; Cattaneo et al., 2003; Patruno et al., 2015). For the Adriatic continental shelf, recent studies have elucidated some of the sediment transport processes actively shaping the shelf clinoform. The late-Holocene mud-wedge in the Adriatic encompasses three connected depositional elements (Cattaneo et al., 2003; Palinkas and Nittrouer, 2006): (a) the Po delta system, including the major subaerial deltaic system and its related delta plain and prodelta; (b) the central Apennine mud-wedge fed by numerous coalescing rivers along the eastern coast of Italy; and (c) the Gargano subaqueous delta located east and southeast of the Gargano promontory, away from any direct river source. Satellite imagery records recurrent flood events, particularly off the Po delta, that generate plumes moving typically to the southeast along the coast. The dominant cyclonic circulation traps fluviatile waters along the western side of the basin. Southeastward flow induced by the WAC tends to redistribute sediment originating from individual fluvial sources (Po and Apennines rivers) along the entire shelf. Acrossshelf sediment transport occurs by bottom Ekman transport. This transport is relatively weak when compared to the along-shelf transport, but is significant during Bora storms, which strengthen the WAC, increase wave shear stresses, and resuspend sediments (Fain et al., 2007; Puig et al., 2007). The result is that during a river flood, part of the fluvial sediment quickly flocculates and accumulates on the prodelta area. Sediment that escapes early deposition, is dispersed along the shelf as a hypopycnal plume, which progressively loses suspended material. Later, during Bora storms, newly-deposited sediment can be resuspended and the WAC transports it southeastward until the energy decreases and the sediment accumulates again. How many steps are necessary for sediment to travel the entire Po dispersion system (from 13

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at least 2 years earlier in the Adige prodelta with respect to the Po River prodelta, suggesting that the small drainage basin of the Adige River responded more rapidly to the reduction of trace metal inputs, while the larger drainage basin of the Po River retain for a longer time the memory of contamination with slow releases over time. e) A first estimate of ~ 10 years is provided for the mean transfer time of sediment particles travelling from the Po River mouth to the Gargano subaqueous delta, which is based on the delay of propagation of the signal of the onset and decreasing shift of excess Zn and Pb recorded in selected sediment cores collected along the Po river dispersion system. f) Finally, the background and excess concentrations of Zn, Pb, Cu, Ni and Cr, provided in this study for the different regions of the western Adriatic Sea, can be a useful reference for future monitoring efforts, as those requested by the MSFD application.

the Po River delta to the Gargano subaqueous delta) and how long it takes is unknown. From a geological point of view this is a rapid process, but there is no estimate in terms of duration (years, decades, centuries or millennia?). Based on Pb and Zn vertical profiles of the 4 selected cores, sediment recording the onset of the excess metals would have taken ~ 6 years to be transported from the Po prodelta up to the offshore Pescara and after further ~2 years to be deposited in the offshore Gargano station (Fig. 7). Thus, the contamination signal from the Po River propagates southwards along the Adriatic basin with an increasing delay. In addition, also the lag of the shift of the declining metal concentrations was in the order of 8–10 years between the Po River prodelta and the offshore Gargano station, substantially confirming the results of the contamination onset. Based on the delay of the contamination signal transferred along the Adriatic continental shelf, we can thus provide for the first time a transport duration of ~10 years for particles entering the Adriatic Sea through the Po River to finally accumulate in the mud-wedge in front of the Gargano Promontory after repeated steps of deposition and resuspension by marine storms. Due to the nature of data on which the transit time was calculated, this estimate can be considered reliable during the last century, while we cannot extrapolate it for older ages as no information about the temporal variability of the environmental forcing, such as the WAC strength or the wind regime, is available.

Acknowledgments Special thanks to the Captains and the crews of the R/Vs Dallaporta and OGS Explora for the support during sediment sampling and to the Lab of CNR-ISMAR for the measurement of N and C contents and stable isotopes. MR wishes to thank the ‘Programa Ciências sem Fronteiras’ for the PhD scholarship (Capes 1133136/2013). This work has been partially funded by the EC FP7 PERSEUS Project (Grant Agr. 287600). This is contribution number 1936 of CNR-ISMAR of Bologna.

5. Conclusions Appendix A. Supplementary data In the western Adriatic Sea, trace metal dynamics seems to be strictly influenced by inputs from the Po River and highly industrialized coastal cities, by the SE-ward flow induced by the WAC and by the fine material accumulating in the modern mud-wedge zone. Despite of some uncertainties caused by the complex history of sediment transport in coastal areas, the temporal evolution of the sediment-bound metals accumulating in the western Adriatic mud-wedge (Mediterranean Sea) were reconstructed for the last century. The main inferences can be summarized as follows: a) The historical trends of trace metals in the Adriatic sediments coincided with the industrial production activities and their past use. Our results showed that Zn and Pb concentrations started to increase from the World War I. Trace metal concentrations and excess inventories continued to increase from 1950s to 1980 followed by a slightly decreasing trend towards the present days. At present, there is no indication of recent massive contamination of the considered trace metals at basin scale, although the human influence remains apparent. b) The Adriatic Sea is being covered by surficial sediments that are less contaminated than those accumulated before ~1985, that is, shortly after the introduction of the national environmental regulations on wastewaters. Definitely, the decrease in concentrations of trace metals in the surficial coastal sediments showed that the application of a stricter Italian environmental legislation has helped to reduce the anthropogenic trace metal discharge into the Adriatic Sea. c) EF together with MPI and excess inventories found in the Po River prodelta station suggest that the trace metal inputs (especially Zn and Pb) from the Po River are still significant. Surficial trace metal peaks recorded starting from ~2000 at Ancona and Bari stations, both sheltering local ports, pointed to a recent influence of human activities. On the contrary, the trace metal profiles of Pomo Pit and Otranto Strait stations showed levels close to the background values. This suggests that in the deep and distal part of the Adriatic depositional system diagenetic reactions and current dynamics (NAdDW cascading events) play the key role on the trace metal biogeochemistry. In summary, with the exception of the local sources of contamination and the restricted areas of accumulation in sediments found in different Adriatic coastal areas, the Po River remains the main source of contaminants at basin scale. d) After 1985, the decrease of trace metal contamination is recorded

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