The marine sedimentary record of natural and anthropogenic contribution from the Sulcis-Iglesiente mining district (Sardinia, Italy)

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The marine sedimentary record of natural and anthropogenic contribution from the Sulcis-Iglesiente mining district (Sardinia, Italy) Elena Romanoa,⁎, Giovanni De Giudicib, Luisa Bergamina, Stefano Andreuccib, Chiara Maggia, Giancarlo Pierfranceschia, Maria Celia Magnoa, Antonella Ausilia a b

ISPRA, National Institute for Environmental Protection and Research, Via Vitaliano Brancati 60, 00144 Rome, Italy Università di Cagliari, Department of Chemical and Geological Sciences, Via Trentino 51, 09127 Cagliari, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Local background Enrichment factor Mineralogy Mine exploitation Metals and trace elements Sulcis-Iglesiente

Intensive exploitation of base metal deposits in the Sulcis-Iglesiente district (Sardinia, Italy), lasted from the 1850s to the 1990s, determined a high environmental impact on the coastal area, but the effects on marine environment have never been investigated. A marine sediment core, dated with 14C, was characterized for grain size, chemical and mineralogical composition, in order to reconstruct the sedimentary history of the area and to assess the environmental impact of mining. The comparison of chemical and mineralogical characteristics of recent sediments with those of pre-industrial age allowed discriminating the real anthropogenic impact from the natural metal enrichment. The correspondence, in the upper core, of anthropogenic trace metal enrichment with the presence of mine waste minerals is attributed to the exploiting over industrial scale; the still high metal enrichment in sediment surface levels suggests a still existing impact due to mine dumps and tailings weathering.

1. Introduction The operations carried out in mining districts, including exploiting, treatments and storage, have strong consequences on the surrounding territory, not only during the period of the activities, but also after their end. Consequently, studies about soil, stream sediment/water, and groundwater are commonly carried out to evaluate the environmental degradation on the mainland (Cidu and Fanfani, 2002; Garcia-Ordiales et al., 2017). Differently, the effects of mining activities on marine environment are scarcely investigated and often considered only in case of submarine tailings disposal (Elberling et al., 2003; Blackwood and Edinger, 2007), or after disastrous events (Hudson-Edwards, 2016). Since marine sediments are the final destination of metals and trace elements, both derived from the natural erosion of the upstream basins and from anthropogenic supply, they can be considered as an archive where temporal changes of these contributions are recorded (Apitz et al., 2009). For this reason, the study of sediment cores is a suitable method to investigate the impact of human activities on marine environment and to compare impacted conditions with the reference ones recorded in the ancient levels, ascribable to pre-industrial times. In marine areas impacted by mining activities, the study of vertical distribution of trace metal concentrations along sediment cores, as done by Shumilin et al. (2013), can help to discriminate contaminated intervals from pristine layers with concentrations lower than natural

⁎

background. For a reliable assessment of metal enrichment due to anthropogenic contribution, the local background concentrations should be considered, especially in areas affected by natural geochemical anomalies (Ligero et al., 2002; Wang et al., 2008; Liu et al., 2010; Romano et al., 2015, 2017). In this study, marine sediment cores were collected in a coastal zone of Sulcis-Iglesiente (West Sardinia, Italy), impacted by pre-industrial and industrial-time past mining activities (Cidu, 2011; Medas et al., 2012; De Giudici et al., 2014, 2017). The analytical work was focused to characterize the sediment core by means of chemical and physical composition and to identify the primary and secondary mineral phases. The study of pre-industrial uncontaminated levels offered the possibility to determine the degree of enrichment for some elements by means of the determination of local background values. 2. Geological setting and study area Sardinia is the second biggest island of the West Mediterranean Sea and represents the eastern margin of the Balearic Basin. It is mainly characterized by Paleozoic (Lower Cambrian to Lower Permian) rocks deformed during the Hercynian orogenesis (Carmignani et al., 2001). The style of deformation and metamorphic grade within the orogen change systematically from South to North, producing distinctive tectonic zones: (i) parautochthonous tectonic unit of the foreland zone in

Corresponding author. E-mail address: [email protected] (E. Romano).

http://dx.doi.org/10.1016/j.marpolbul.2017.06.070 Received 10 May 2017; Received in revised form 22 June 2017; Accepted 23 June 2017 0025-326X/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Romano, E., Marine Pollution Bulletin (2017), http://dx.doi.org/10.1016/j.marpolbul.2017.06.070

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Fig. 1. Geological map of the study area. This is indicated with a star (after Boni et al., 2000 modified).

season (from October to April), accounting for 80% of the yearly precipitation and frequent heavy rainstorm events (Delitala et al., 2000). The dominant wind is Mistral which is responsible for NW/SE long shore drift (Manca et al., 2013). However, secondary winds from SW may determine episodic, but extreme, storm events with waves up to ca 7 m high (APAT, 2010). The sea is microtidal, never exceeding 1 m of fluctuation (Donda et al., 2008).

the southwest, overthrust by (ii) geometrically complex external and internal napped zones in the central part and (iii) highly deformed inner zone in the northern portion of the Island. Mesozoic rocks, that recorded repetitive marine transgressive and regressive episodes during their sedimentation, locally overlie the Hercynian orogen (Fig. 1). The Sardinia crustal block detached from the southern European plate during the lower Miocene and drifted to its present position because of a NW-dipping subduction, associated with the collision of European and African plates (Doglioni et al., 1999). This subduction produced an extensive Oligo-Early Miocene calcalkaline volcanic complex developed along the island. During this migration, Sardinia was dissected by variously oriented transcurrent and normal faults that bound a series of half graben, filled with marine and continental deposits. During the Early Pliocene, widespread volcanism and basin uplift occurred and the island suffered severe erosive phases (Carmignani et al., 2001). In the study area, Cala Domestica bay, the meta-sedimentary rocks, belonging to the Sulcis-Iglesiente tectonic unit, dominate the bedrock along the coast (Fig. 2). In particular, meta-sandstones and claystones with minor limestone lenses (Nebida Group) are capped by a thick carbonate succession (Gonnesa Group) which host the major mineralization extensively carved since the Roman time. Along the Riu Guttu valley, Late Carboniferous orthogneiss locally crop out. Further south, along the coast, andesitic pyroclastic flows, belonging to the OligoMiocene volcanic calcalkaline complex, extensively occur. Cala Domestica bay is characterized by a small cliff-bounded embayment (100 m wide, 450 m long), with a well-developed sandy pocket beach (Fig. 2). The seafloor is covered by a sandy shore, up to the average depth of 15 m, and further down extensive sea-grass (Posidonia oceanica) meadows. The coastal area is dominated by an incipient dune field system, occasionally interested by an ephemeral stream (Riu Guttu). The bulk of sediment feeding this system is partly constituted by bioclastic sand, produced in the sea-grass meadows and deposited on the beaches during major storms, and partly by silico-clastic sand, transported to the shore by NW/SE long shore current or by the ephemeral stream. The area is characterized by a warm-temperate climate with wet

3. Mine activity The study area is included in the district of Sulcis-Iglesiente (SW Sardinia) where the past mine activity occurred in pre-roman times, and an intensive exploitation started in the 1850s closing in the 1990s. Over 40 mines exploited base metal (Zn, Pb, Ag, Ba) and calamine (Zn, Pb) deposits (Aversa et al., 2002 and references therein) leaving on site a large volume of mine residues. Since centuries, such areas have been affected by erosion, causing significant dispersion of the mine residues that, as transported by the river, have been re-deposited into the Mediterranean Sea (Medas et al., 2012; De Giudici et al., 2014; Frau et al., 2015). Most of the exploited base metal orebodies are pre-variscan stratiform and/or strata-bound deposits, hosted in the Lower Cambrian carbonate rocks (Gonnesa Group). They can be distinguished into syngenetic and early diagenetic massive sulphides and void-filling, breccia cement, and late-diagenetic replacement bodies (Boni et al., 1996); Montevecchio area hosts the largest vein system, where the most widespread minerals are galena, sphalerite, pyrite, and barite, while the common ones are arsenopyrite, chalcopyrite, sulphosalts and oxides. The strata-bound ores were strongly tilted with their host carbonate rocks, thus rendering easier the recharge and circulation of meteoric waters, responsible for dissolution and secondary oxidation. In the calamine deposits, smithsonite, hydrozincite, and hemimorphite are the principal zinc-bearing minerals (Aversa et al., 2002). Cerussite and anglesite also occur, often associated with nodules and lenses of residual or supergene galena. A complex association of iron and manganese oxy-hydroxides, with a characteristic redbrown staining (goethite, lepidocrocite, hematite), and residual clay minerals, host this nonsulphide ore. In the Cala Domestica bay, Canal Grande mine was active 2

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Fig. 2. Location map of the study area: A) satellite view of SW Sardinia coast and location of the studied area (Cala Domestica bay). The dashed circle refers to the main mine district of Sardinia; B) geological map of the study area; C) detailed satellite view of Cala Domestica bay with sampling station.

extracting Zn, Pb and Ag until the first decade of 20th century (Salvadori and Zuffardi, 1956). Canal Grande ore comprises sphalerite and galena, associated to supergene alteration minerals such as cerussite, smithsonite, iron oxides, dolomite and anglesite.

University, Department of Chemical and Geological Sciences (DCGS), in order to perform a standard facies analysis and to carry out X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive Spectrography (EDS) analysis through 40 samples. In addition, in order to confirm the correspondence of two cores, the grain size analyses and qualitative mineralogical study along the core were repeated by ISPRA laboratory on the second core. To establish a chronological framework for the sedimentary sequence, two marine bivalve shells (S1 and S5) were collected at −36 and − 246 cm, respectively, for radiocarbon dating purpose.

4. Material and methods 4.1. Sampling Two replicates of sediment core (SI69ISPRA and SI69DCGS) were collected in July 2014, at around 13 m water-depth, by means of vibrocorer (mod. SHSBD) equipped with an internal HDPE liner (Fig. 2). The SI69ISPRA (260 cm long) was sub-sampled at ISPRA laboratories, to determine grain size and chemical concentrations, also defining the qualitative mineralogical characteristics of sediments along the core depth. For this purpose, a total of 29 levels of 2 cm thickness were collected: 11 levels were continuously taken in the first 22 cm of the core to guarantee the highest resolution of more recent times, 1 level every 10 cm from 22 to 100 cm depth along the core, and 1 level every 20 cm below 100 cm to detect the main environmental changes in the previous times. The SI69DCGS (273 cm long) was longitudinally opened at Cagliari

4.2. Sediment description Sediment colour and texture, type and concentration of accessory materials, including aligned rounded pebbles, marine shells, sea-grass plant fragments and sea-grass “balls”, when present, were used as basic tools for identification of the main sedimentary facies. Primary sedimentary structures are poorly preserved. Characterization of depositional environment was also based upon vertical lithofacies relationships together with main erosive surfaces.

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Fig. 3. Core SI69DCGS. Note the overall homogeneity of textural characteristic of the sediments and the change in colour along the studied core.

4.3. Grainsize and qualitative mineralogical study Each sample was treated twice with a solution of hydrogen peroxide (30%), and then washed twice with natural water. Then, samples were wet-separated in the > 63 μm and < 63 μm fractions, and the first one was dry-sieved by means of ASTM series sieves (Romano et al., 2009). The fine fraction was not analyzed because it was absent or lower than 5%. The sand was examined under a stereomicroscope (M165C, Leica) to qualitatively determine the composition of biotic and abiotic component. In particular, the major mineral components and the possible anthropogenic material were identified. 4.4. Metals and trace elements Some metals and trace elements (As, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn) were determined in this study. Metal dissolution was conducted by means of microwave-assisted digestion (Milestone MLS Ethos TC high performance microwave digestion unit) using oven-dried

Fig. 4. Stratigraphic reconstruction of core SI69. The main storm-related bodies and radiocarbon ages are shown.

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Fig. 5. Sand size distribution of replicate cores showing a mode around 2.5 ϕ (fine sand).

conventional dates were calibrated following the methodology of Reimer et al. (2013; INTCAL09 and MARINE13 calibration curve), and converted into calibrated ages ( ± 2σ; Table 3). Finally, Δ13C values were reported in parts per thousand (per mil) relative to PDB-1 measured on a Thermo Delta Plus IRMS. Typical Δ13C error is ± 0.3‰.

homogenized sample aliquots digested in Teflon bombs with 3 ml of HNO3 and 9 ml of HCl super-pure. Cd and As were determined in digested solution by a graphite furnace atomic absorption with Zeeman background correction technique (SpectrAA - 220Z, Varian). Ba, Cr, Cu, Fe, Mn, Ni, Pb and Zn were measured with coupled emission plasma ICP-OES (5100 Agilent) (Romano et al., 2009). Hg was analyzed by means of a Direct Mercury Analyzer (DMA-80, FKV) through direct atomic absorption with gold amalgamation, without chemical pretreatment (Maggi et al., 2009). To guarantee quality assurance/quality control (QA/QC), quality parameters such as accuracy, quantification limit, and repeatability were estimated. The accuracy for total content of metals was evaluated using certified reference materials (PACS-2 and MESS). Quantification limits were: 0.01 μg g− 1 for Hg and Cd, 1.5 μg g− 1 for all the other elements.

5. Results 5.1. Core description The core SI69 DCGS underwent a macroscopic sedimentological analysis in order to define the main facies and depositional environment. The core was uniformly composed of well sorted medium-fine grained sand, consisting of marine bioclastic materials and of various silico-clastic and rock fragment grains (Fig. 3). Below the first 30 cm the sediment showed a colour change from yellowish to dark gray/black. Similar change was also observed in the SI69ISPRA core. No clear sedimentary structures were identified along the core but occasional aligned pebbles and/or shells and/or sea grass “balls” suggested the presence of erosive surfaces. Six surfaces were recognized along the core, bounding six main sedimentary bodies ranging from 30 up to 50 cm in thickness (Fig. 4).

4.5. Mineralogy After drying at room temperature, 200 mg of powder of each sample were lightly ground in an agate mortar and subjected to XRD analysis, using conventional θ-2θ equipment (PANalyticalXpert Pro) with Cu Kα wave-length radiation (λ = 1.54060 Å), operating at 40 kV and 40 mA, using the X'Celerator detector. SEM imaging and EDS analyses were carried out using an Environmental Scanning Electron Microscope (ESEM QUANTA 200, FEI) to investigate dried uncoated samples. 40 samples were investigated by XRD but, as many samples provided the same information, only part of the data are shown hereafter. The detection limit of XRD for crystalline mineral phases can be assumed to be about 1%, thus detection of a mineral phase below this abundance is not allowed for trace minerals.

5.2. Grain-size and qualitative mineralogical study The results of grain size analysis highlighted the homogeneity of textural characteristic of the sediments along the two replicate cores, with sand content nearly always above 99%. A light decrease of sandy fraction below this percentage was recorded in core SI69ISPRA, between 9 and 13 cm, due to the presence of about 6.5% of gravel, at 139 cm and at the bottom of the core, due to a very slight increase of pelite (1.3% at both levels). Also in core SI69DSCG the pelitic fraction was slightly higher below 139 cm, with a maximum of 1.5% in the bottom sample. The sediments of both cores were constituted by well-sorted medium to fine grained sand with a distribution curve showing a mode included in a narrow range, between 2 and 2.5 ϕ (Fig. 5). Observation under stereomicroscope of the sandy fraction highlighted the prevalence, among abiotic constituents, of quartz, calcite and dolomite, and lithic fragments of carbonate rocks; less abundant were feldspars, micas (biotite and muscovite), metamorphic rock fragments, and heavy minerals such as magnetite, ilmenite, rutile, zircon, amphiboles and pyroxenes. The organic fraction was generally scarce, whereas the biotic constituents are marine shells and fragments of bivalves, gastropods, echinoids, bryozoans, ostracoda and foraminifera. In the upper core, above 8 cm, anthropogenic fragments (carbon, hydrocarbons) were present.

4.6. Radiocarbon dating 14 C dating on shells of marine bivalves was carried out by Beta analytic radiocarbon laboratories of Miami. Accelerator Mass Spectrometry (AMS) measurements were made on one of 4 in-house National Electrostatics Corporation (NEC) Single Stage AMS accelerator mass spectrometers. The reported age is the “Conventional Radiocarbon Age”, corrected for isotopic fraction using the Δ13C. Age is reported as RCYBP (RadioCarbon Years Before Present, abbreviated as BP, “present” = 1950 CE). By international convention, the modern reference standard was 95% the 14C signature of NBS SRM-4990C (oxalic acid) and calculated using the Libby 14C half life (5568 years). Quoted error on the BP date is 1 sigma (1 relative standard deviation with 68% probability) of counting error (only) on the combined measurements of sample, background and modern reference standards. Total error at Beta (counting + laboratory) is known to be well within ± 2σ. The

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137 14,064 333 3,272 4,448 67 16 533 128 203 201 17

Basic statistics of metal and trace element concentrations is reported in Table 1 together with the mean Earth's upper crust concentrations by Li and Schoonmaker (2003) for comparison, while full results are given in supplementary material (Table 1S). Elements exceeding these reference values may be considered as affected by enrichment. The pattern along core-depth offered indications about the origin of this enrichment (Fig. 6). From basic statistics it may be deduced that As, Cd, Pb and Zn exceed these values in all, or nearly all the samples. Ba showed mean and median below reference value, while Hg mean concentration exceeded mean crustal value; both these elements displayed considerable standard deviation. Differently, Cr, Fe and Ni were always far below the mean crustal values. Observing the down-core variation of concentrations, most analyzed elements (Ba, Cd, Cu, Hg, Mn, Pb and Zn) showed typical profiles, characterized by low and rather steady concentrations in the lower part of the core and higher and more variable values in the first 20 cm (Fig. 6), attributable to anthropogenic enrichment (Fukue et al., 2006). Nevertheless, the very low Cu concentrations indicate no environmental significance for this element. A significant peak was recorded at 50 cm for Hg and Pb. Arsenic did not show any significant pattern displaying the highest concentrations in the lower part of the core, while Fe, Cr and Ni showed very low variability along the core depth, without any significant pattern in the vertical profile.

121.76 354.98 144.59 204.33 91.24 770 0.02 1.29 0.06 0.40 0.50 0.08

0.75 3.96 0.75 1.35 0.92 55

Pb (mg kg− 1) Mn (mg kg− 1) Hg (mg kg− 1)

Ni (mg kg− 1)

5.3. Heavy metals and trace elements

66 2510 147 515 703 570 11.49 60.21 23.77 25.42 9.28 1.6

0.54 79.41 2.56 22.73 30.22 0.1

3.14 7.93 5.48 5.59 1.34 69

0.75 8.81 0.75 2.87 2.83 39

0.49 0.80 0.59 0.59 0.08 4.17

XRD analysis recognized both primary and secondary minerals in the sediments (Table 2; Fig. 7). Within the primary rock forming minerals, silica, feldspars, carbonates and phyllosilicates and their XRD detectability through the sediment core were found. Particularly, three different phases of K-feldspar, sanidine, orthoclase and microcline were identified. According to the geological setting, orthoclase and microcline are abundant in the lithology (granites) outcropping in East and North land areas respect to the location of SI69 core (Fig. 1). Sanidine, the disordered phase of K-feldspar, however is abundant in rocks (volcanites, comendites) outcropping in the southern areas. Based on the detection of XRD peaks, the abundances of Mg-calcite and aragonite bioclasts showed some variability along the core. Zn minerals were detected by XRD analysis in the first part (about 10 cm) of the core: smithsonite (ZnCO3) and hemimorphite [Zn4(Si2O7) (OH)2·H2O]. The recognized Pb bearing minerals were lead-oxide-carbonates and lead-oxide-sulphates. Cd bearing minerals were not found, likely due to their low abundance. The fine fraction (< 63 μm) was collected along the core, at first, for discrete intervals (27–100 cm, 100–200 cm, and 200–270 cm). Then, shorter levels (50–70 cm, 150–170 cm, and 250–270 cm) were sampled to sieve and collect fine fraction. It contained quartz, clinochlore-illite, barite, pyrite (Fig. 7) and accessory minerals such as zircon. Clinochlore and illite are clay minerals deriving from rock belonging to schist lithology. The origin of pyrite, i.e. residual or biogenic, is crucial to understand the stability of metal sulphides and the redox properties of the porewater-sediment system. Several minerals not recognized by XRD were identified by SEMEDS. Framboidal pyrite was recognized in the fine fraction by both instruments (Fig. 7). Overall, the formation of framboidal pyrite indicated that sulphur reducing bacteria were effective in the reduction of sulphate ions, and, after contact with ferrous-ions, precipitation of pyrite occurs. Zircon was found several times during SEM-EDS backscattered electrons imaging associated to chemical EDS analysis. Barite was identified in the first centimeters of the core and in the fine fractions of the core samples. Precipitates, made of iron oxides with Zn, Pb and Cl, were found by SEM-EDS. These zinc and lead ferrites generally form as secondary minerals during weathering of metal sulphides, and

Min Max Median Mean SD Mean Earth's upper crust

Ba (mg kg− 1)

Cd (mg kg− 1)

Cr (mg kg− 1)

Cu (mg kg− 1)

Fe (%)

5.4. Mineralogy

As (mg kg− 1)

Table 1 Basic statistics of metal and trace element concentrations. Mean Earth's upper crust concentrations are reported as reference values from Li and Schoonmaker (2003). Concentrations exceeding reference values are in bold.

Zn (mg kg− 1)

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Fig. 6. Down-core distribution of metal and trace element concentrations.

for these elements and can be considered as anthropogenically influenced. The outliers below 20 cm, determined by means of box-plots, were excluded from the dataset (Fig. 9). Among these, the significant peaks of Hg and Pb at 50 cm, could be attributable to mining activity during pre-industrial times, in accordance also to radiocarbon dating. So, the background concentrations were obtained from the dataset including only natural concentrations, deprived from outliers (Table 4). These values were used to determine the Enrichment Factor (EF) of metals and trace elements. Because no significant changes of sediment texture were detectable in the core, nearly constituted by sand (mean 99%), normalization with conservative element to avoid grain size effect was not necessary (Middleton and Grant, 1990; Covelli and Fontolan, 1997). Consequently, the EF was determined as EF = Cx / Cb where Cx is the concentration in sample X and Cb is the BGV. Elements with EFs > 1.5 (Ba, Cd, Hg, Mn, Pb and Zn), were considered as anthropogenically enriched (Zhang et al., 2007) and plots of EF values along core-depth are shown in Fig. 10. The systematic enrichment started at 22 cm for Cd and Zn; at 20 cm for Ba and at 18 cm for Hg and Mn. The highest EFs were shown by Zn (up to 39 in the top level) and Cd (up to 28 at 13 cm) and, secondly, by Ba (up to 12 in the top level). Among these elements, only Cd significantly decreased in the uppermost core levels with respect to the ones below, while Ba and Zn increased significantly.

can represent the chemical elements remnant from the alteration of primary metal sulphides under oxidizing and alkaline conditions in present and ancient seawater. Manganite was also found. 5.5. Radiocarbon ages Two well-preserved shells of marine bivalves (S1 and S5) were collected for dating purpose along the SI69DCGS core at the depth of 36 and 246 cm respectively. The resultant 14C data were Cal 960 to 865 years BP and Cal 4435 to 4290 BP years respectively, indicating Middle to Late Holocene ages (Table 3). 6. Local background values and enrichment factors The comparison of As, Cd, Pb and Zn concentrations with mean Earth's upper crust values (Li and Schoonmaker, 2003) indicated a clear enrichment also down to the core bottom. Thus, it may be supposed that these elements were affected by a geochemical anomaly due to the geological features of the catchment basin. This implies that, for a correct evaluation of anthropogenic contribution, local BackGround Values (BGV) should be determined. Romano et al. (2015) defined a method for determining the BGV, from a dataset of natural concentrations of sediment cores (Cobelo-García and Prego, 2003; Guo and Yang, 2016). It took into account the study of chemical profiles, supported by sediment dating, to identify the core levels affected by anthropogenic contribution and by Principal Component Analysis (PCA) (Wang et al., 2008). After the elimination of levels corresponding to the anthropogenic enrichment during industrial times, the dataset was also deprived of outliers, and finally the BGV was determined as “mean + 2σ”, where σ is the standard deviation. The outliers elimination was necessary, not only because anthropogenic contribution may have occurred in the pre-industrial times, but also because the definition of background concentration implies the absence of anomalies. This formulation took into account 97% of natural variability so that the threshold value was able to discriminate between natural and anthropogenic contribution (Matschullat et al., 2000). Applying this method in the present study, a PCA was carried out (Fig. 8). It identified a group of highly correlated elements (Ba, Cd, Cu, Hg, Mn, Pb and Zn) clearly plotting on the first component, which are the same elements showing a vertical concentration profile with a clear anthropogenic enrichment (Fig. 6). The core levels with positive PC1 scores were those from the top of the core to 16–18 cm; these levels had the highest concentrations

7. Discussion The two replicate sediment cores clearly showed similar unimodal grain size distribution and textural characteristics. Given the above presented data, the studied sedimentary sequence likely developed in the lower shoreface zone of a beach system, below the fair-weather wave base (> 3.5 m below the sea level). The recognized six erosive surfaces, bounding six sedimentary bodies, can be interpreted as the results of multiple storm events. The bioclastic-rich sediments were transported from the close by sea-grass meadows into the study area by the long shore current and frequent storms due to Mistral wind. Conversely, the presence of metamorphic and carbonate fragments, microcline and minor orthoclase witnesses the “abiotic fraction” coming from the close inland rocks. Moreover, the presence of aligned pebbles at the base of some erosive surfaces clearly indicated that huge floods of the Riu Guttu stream episodically occurred. Finally, the presence of sanidine crystals in some layers is somehow difficult to explain. This mineral is very common in Oligo-Miocene calcalkaline andesitic 7

Iron oxides with Zn, Pb, Cl

8

x

x

x

x

x

x

x

x

x

x

x

x

x

10_12

12_14

14_16

16_18

18–20

20_22

28_30

x

x

x

8_10

x

x

x

x

x

6_8

x

x

Calcite dolomite

x

x

x

Mg Calcite

x

x

Quartz

4_6

2_4

0_2

Sample (cm)

Table 2 Primary rock forming and biogenic minerals.

x

Anorthite ord

x

x

x

Muscovite 1

x

x

x

x

x

x

x

Albite

x

x

x

x

x

x

Microcline int

x

x

x

Orthoclase

x

Sanidine

x

x

x

x

Illite

x

Ilmenite

x

x

Fayalite

x

Enstatite

x

x

x

x

x

Smithsonite

x

Emimorphite

x

Lead Oxide/ carbonate shannonite

x

Lead Oxide Sulphate

x

x

x

Barite

x

Rutile

x

Magnesium sulphate hydrate

Pyrite

Manganite

(continued on next page)

x

Posphate Ca Na

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Iron oxides with Zn, Pb, Cl

9

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

68_70

78_80

88_90

98_100

118_120

138_140

158_160

x

x

x

x

Calcite dolomite

x

x

48_50

Mg Calcite

58_60

x

Quartz

38_40

Sample (cm)

Table 2 (continued)

Anorthite ord

x

x

Muscovite 1

x

x

x

x

Albite

x

x

Microcline int

x

x

x

Orthoclase

x

Sanidine

x

x

x

x

x

x

x

Illite

Ilmenite

Fayalite

Enstatite

Smithsonite

Emimorphite

x

Lead Oxide/ carbonate shannonite

Lead Oxide Sulphate

Barite

Rutile

Magnesium sulphate hydrate

Pyrite

Manganite

(continued on next page)

Posphate Ca Na

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10

x

x

x

200,270

100–199

x

x

x

x

x

x

x

258_260

x

x

x

238_240 x

x

x

x

Albite

218_220

Muscovite 1

x

x

Anorthite ord

x

x

Calcite dolomite

198_200

Mg Calcite

x

Quartz

178_180

< 63 mm 30–99

Iron oxides with Zn, Pb, Cl

Sample (cm)

Table 2 (continued)

x

Microcline int

x

Orthoclase

Sanidine

x

x

x

x

Illite

x

Ilmenite

Fayalite

Enstatite

Smithsonite

x

Emimorphite

Lead Oxide/ carbonate shannonite

Lead Oxide Sulphate

x

x

Barite

x

Rutile

Magnesium sulphate hydrate

Posphate Ca Na

x

x

x

Pyrite

x

Manganite

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be suggested. Finally, 14C ages indicated that the study site was a shoreface zone (below the fair-weather wave base) already at the end of the Middle Holocene (ca. 4300 yr BP). Of particular interest is the change in sediments colour from yellowish to gray/dark observed in both cores most likely due to the increasing of sub-oxic/anoxic condition toward the base of sedimentary sequences. Moreover, the presence of secondary pyrite seems to indicate pervasive early diagenetic processes and to confirm reducing (sub-oxic/anoxic) condition for the porewater-sediment system. Overall, the occurrence of heavy metals bearing minerals can be interpreted as follows: i) smithsonite and hemimorphite have low solubility (Medas et al., 2014; Li et al., 2014) in marine environment, and a long time persistency, thus they likely come from dismantling of rocks and mine activity in the area; ii) zinc and lead ferrites generally form as secondary minerals during weathering of metal sulphides in marine seabed, representing the chemical elements remnant from the alteration of primary metal sulphides and witness the likely fate of metal sulphides under the seabed oxidizing conditions; iii) framboidal pyrite indicates the actual minerogenetic conditions active below the modern sedimentary body. The very scarce or null presence of fine sediment fraction along the entire core, implies that changes of concentration levels may be exclusively attributed to changes in metal contribution. The increase of concentrations in the first 20 cm of the core for Ba, Cd, Hg, Mn, Pb and Zn was attributed to an anthropogenic input, based on the study of vertical profiles and PCA. Among these elements, the presence of very high concentrations of Zn, Cd, Pb and Hg with respect to the lower part of core, was recorded. Based on radiocarbon dating (Table 3) it may be deduced that the high concentration values of metals in the lower section of the core, have to be attributed to a natural contribution and, consequently, a positive geochemical anomaly is highlighted in the sediments of the study area. For this reason it was necessary to determine local background concentrations in order to carry out a correct assessment of anthropogenic input, through the determination of the EF. Most of the

Fig. 7. Fine fraction in SI69 core sediment contains quartz, phyllosilicate minerals and framboidal pyrite. Table 3 Summary of the radiometric (14C) ages. Calibrated dates are based on Reimer et al. (2013). Sample

Depth (cm)

Conventional radiocarbon agea

Calibrated 2ϭ radiocarbon agea

13

SI 1 SI 5

− 36 − 246

1420 ± 30 BP 4320 ± 30 BP

960–865 Cal BP 4435–4290 Cal BP

+ 0.1 0/00 + 1.2 0/00

a

C/12C ratio

BP = 1950 CE.

pyroclastic flows, cropping out southward of the Cala Domestica bay (Carmignani et al., 2001). Thus, episodic storms coming from SW determined the mixing of sanidine grains into the local-derived sands can

Fig. 8. Principal Component Analysis of metal and trace element concentrations. PC1 accounts for 68.7%, while PC2 accounts for 13.6% of variance.

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Fig. 9. Box plot of concentrations in levels below 20 cm. Outliers are highlighted.

positive geochemical anomaly, due to the extensive outcropping of sulphide and oxide ores (De Vivo et al., 1997). Also As, although to a lesser extent, was naturally enriched because it may be associated to pyrite and/or occurs as arsenopyrite in rocks and ores.

determined BGVs (Ba, Cr, Cu, Fe, Mn and Ni) were well below the mean Earth's upper crust concentrations, while BGV of Hg was similar to the reference value (Table 4). Cd, Pb and Zn exceeded considerably these values, indicating a strong natural enrichment, and a consequent 12

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215 73 361 67 49 41 131 17 1.30 0.96 3.21 55 140 10 160 770 0.56 0.04 0.65 4.17 0.75 0.00 0.75 39

0.05 0.07 0.20 0.08

Fe (%)

Fig. 10. Plot of Enrichment Factors along core depth. Only elements anthropogenically enriched (EF > 1.5) are reported.

23.26 2.55 28.35 1.6

119 45 209 570

1.56 0.65 2.87 0.1

6.24 1.12 8.48 69

The EFs > 1.5 recorded for Ba, Cd, Hg, Mn, Pb and Zn, correspond to anthropogenic enrichment. Some of these may be attributed to extensive mining activities in the Sulcis-Iglesiente area. Ores close to the surface, mainly containing Zn (smithsonite, hydrozincite) and Pb carbonates (cerussite), were exploited up to 1960s, while deeper levels, enriched in Zn (sphalerite), Pb (galena) and Ba (barite) sulphides were exploited later. Greenockite (Cd sulphide), mostly related to Zn minerals, is particularly enriched in correspondence of treatment plants and tailing areas (Boni et al., 1999). Although affected by minor mining activities, Ba and Pb-Ag ore bodies are present in rocks of the lower Paleozoic carbonate succession of the region (Boni et al., 2000), and Aggalena outcrops in Canal Grande ore at Cala Domestica (Salvadori and Zuffardi, 1956). Data indicated gradually increasing EFs from 20 cm to the top of the core, witnessing the first metal enrichment due to the industrial mine activity, which started in 1850s. Several elements show maximum EFs in the uppermost levels although exploitation ceased in the 1990s; it may be deduced that the most recent contamination is due to the presence of ore treatment and deposit areas rather than to the exploitation (Cidu et al., 2012). During ore exploitation, large volumes of minewaste-materials were deposited along or close to the streambeds which were eroded and transported to the sea-floor. The anomalous and huge amount of anthropogenic sediments banked along streambeds during the recent industrial activity has been only partially re-eroded and fed to the seabed (Medas et al., 2012). As pointed out by Medas et al. (2012) and De Giudici et al. (2017), after several tens of years from the mine closure, these mine wastes have been often deeply eroded and/or

Mean natural SD BGV Mean Earth's upper crust

Cd (mg kg− 1) Ba (mg kg− 1) As (mg kg− 1)

Table 4 Mean natural concentrations. Standard deviation and background values (in bold).

Cr (mg kg− 1)

Cu (mg kg− 1)

Hg (mg kg− 1)

Mn (mg kg− 1)

Ni (mg kg− 1)

Pb (mg kg− 1)

Zn (mg kg− 1)

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Acknowledgments

underwent any kind of natural stabilization. Significant amount of mine waste materials remain widespread in the mine district, representing potential or active sources of mine waste dispersion. Therefore, remediation on the dismissed mining areas is still needed in order to attain environmental restoration and reduce the risk related to the presence of active or potential source of mine waste material dispersion. Anthropogenic contribution, corresponding to EF > 1.5, which was recognized for some elements (Hg, Pb and Zn) between 50 and 60 cm, thus deposited prior than 960–865 years Cal BP, may be attributed to mining in pre-industrial times, because metal ores were exploited since Phoenician times and after by Romans, Pisans and in the Middle Ages (Boni et al., 1999). The identification of pre-industrial anthropogenic contribution confirmed the necessity of removing outliers from dataset also below the industrial interval, characterized by systematic enrichment, as it was done in the present study for the determination of BGVs, in accordance with Romano et al. (2015).Very serious contamination was recognized by Boni et al. (1999) in stream sediments of the southern Iglesiente area for Ba, Cd, Pb and Zn, which are abundant in ore deposits of the carbonate succession. According to Da Pelo (1998), 80% of Pb and 60% of Zn present in mining tailings were found easily soluble when subjected to sequential extractions. High concentrations of Zn, Cd and Pb were also recorded in stream waters flooding from Pb-Zn deposits, hosted in both carbonate and silicate rocks of the Sulcis-Iglesiente mining district, that contribute to spread the contamination to 10 km downstream of the mines (Cidu, 2011; Cidu et al., 2012; Medas et al., 2012). A similar mechanism may be supposed also for Cala Domestica, where waters may be drained by Riu Guttu from contaminated mining, treatment and deposit areas, and finally transferred to marine sediments (Fig. 2). After fed to marine seabed, as some of the minerals recognized by sediment analysis in this study, episodic storms coming either from S or N can admix sediments from Riu Guttu with other drift-transported sediments. This study suggests that the length of marine sediment path transported along the coast could be longer than previously expected.

The authors are grateful to Regione Sardegna (RAS), Assessorato Difesa Ambiente, which allowed this research in the framework of the environmental characterization project of the Sulcis-Iglesiente coastal area. In particular, GDG and SA are grateful to RAS and Fondazione Banco di Sardegna for research funding (F72F16003080002). References APAT, 2010. Rete Ondametrica Nazionale. http://www.idromare.com (accessed December 2012). Apitz, S.E., Degetto, S., Cantaluppi, C., 2009. The use of statistical methods to separate natural background and anthropogenic concentrations of trace elements in radiochronologically selected surface sediments of the Venice Lagoon. Mar. Pollut. Bull. 58, 402–414. Aversa, G., Balassone, G., Boni, M., Amalfitano, C., 2002. The mineralogy of the “Calamine” Ores in SW Sardinia (Italy): preliminary results. Period. Mineral. 71, 201–218. Blackwood, G.M., Edinger, E.N., 2007. 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8. Conclusions Two replicate cores collected on shallow marine (lower shoreface) deposits, far off Cala Domestica bay, were analyzed using a multidisciplinary approach. Thanks to the results of this study, the environmental impact of mining exploitation activity and of ore treatment was demonstrated for the first time in marine sediments of the SulcisIglesiente mining district. In accordance with the homogeneous textural characteristics of the core, the increase of the element concentrations along core-depth cannot be associated to the variation of fine sediment fraction, but it have to be ascribed to changes in the contribution from mainland. Natural enrichment due to the outcropping of metal based ores in the area was recognized for As, Ba, Cd, Pb and Zn, and so local background concentrations were determined for a correct assessment of the anthropogenic contribution. The simultaneous increase of anthropogenic elements was associated to the start of exploitation over industrial scale during the 1850s. This study recognized the anthropogenic enrichment for Ba, Cd, Hg, Mn Pb and Zn, mainly attributed to mining activity and, in particular, to the contribution of mine waste erosion supplying the stream of the area. It is evident along the cores that, below the modern sedimentary body, early diagenesis processes were recognized while, in the modern sedimentary body, evidences for weathering of mine waste minerals were found. Finally, this work provided quantitative and significant insight about the impact of past mine activity in SW Sardinia. Further studies would be needed to better assess the fate of mine waste minerals in the modern sedimentary body and the impact of climate changes and current presence of mine dumps and tailings on the inland and coastal area. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.marpolbul.2017.06.070. 14

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