Geochemical and Isotopic Characterisation of Actual Lacustrine Sediments from the Hydrothermal Lake Specchio di Venere , Pantelleria Island ( Italy )

Hydrothermal lakes are a very common feature in volcanic environments, and among these lake Specchio di Venere (Pantelleria island, Italy) has attracted the interest of several researchers due to its peculiar characteristics. With the aim of improving the knowledge of its mineralogy, our work pointed out the characterisation of the bottom lake sediments. We collected and analysed 5 sediments cores around the shoreline, determining the mineralogical phases, concentration of major, minor, and trace elements, and the isotopic composition of carbon and oxygen in the carbonate phases. Our findings remarked a general compositional homogeneity in both the vertical and horizontal distribution of mineral phases, with the exception of peculiar geological niches connoted by biological and hydrothermal activities.


Introduction
Hydrothermal lakes are a very common feature in volcanic environments.Their chemistry is controlled by the interaction between fluids of different origin: hydrothermal vents, rainfall, groundwater and runoff inputs, and seawater, if located in proximity of the sea.Lake Specchio di Venere, located in the northern sector of the volcanic Pantelleria island (Italy), has attracted during the last decades the interest of several researchers due to its peculiar characteristics.Among the others, the chemistry of its water, the deposition of mineral phases, and the interaction with biological processes were investigated by Azzaro et al. [1], Parello et al. [2], Aiuppa et al. [3], Cangemi et al. [4,5], and Censi et al. [6].
Most of these studies were focused on the geochemistry of lake water and its implication for volcanic hazard and geobiological processes; less attention was paid to the characterisation of its sediments.Azzaro et al. [1] described 2 sediment cores mainly composed of carbonates.Mineralogical studies were resumed by Aiuppa et al. [3], which presented the semiquantitative analyses of 3 cores of lake sediments.
With the aim of filling the gap of development between geochemical and mineralogical studies, our study pointed out the mineralogical and geochemical characterisation of sediments deposited in lake Specchio di Venere.We collected 5 cores around the shoreline, successively analysed for the determination of the mineralogical and chemical composition, including minor and trace elements, and of the isotopic composition of carbon and oxygen in the carbonate phases.

Study Area
Pantelleria island represents the emerging part of a volcanic structure, located in the Strait of Sicily, situated along the tectonic trench which takes the same name as the island itself.High-density volcanic products are mainly found in the submerged part of the island, whereas alkaline and peralkaline products with high silica content (more than 67% as reported in [7]) constitute its subaerial portion.
The oldest subaerial products date at 320 ka [8,9].The successive cycles of explosive activity were followed by caldera collapses at 114 ka (caldera "La Vecchia") and 45 ka (Green Tuff eruption).The last eruption formed the "Cinque Denti" caldera.Intracalderic activity in the last 45 ka [8,9] and geophysical data [10]indicate the presence of an active magma chamber at crustal depth.
Presently, Pantelleria shows diffuse hydrothermal activity characterised by low temperature fumarole vents and hydrothermal springs.Lake Specchio di Venere, located inside the calderic depression of Caldera Cinque Denti (Figure 1) in the northeastern part of the island, is one of the most peculiar places where interaction between hydrothermal and surface processes takes place.
Lake Specchio di Venere shows a subcircular shape, being ca 450 m long and ca 350 m wide, and a maximum depth of 12.5 m [11, and references therein].The lake is endorheic and located few hundreds of meters apart the sea shore; since the elevation of its surface is 1 m a.s.l., most of the water body permanently lies below the sea level.The inflow of fluids (and related dissolved and transported materials) to the lake is driven by three main mechanisms [3]: (i) direct recharge due to rainfall; (ii) surface runoff, essentially from the streams running on the western portion of its catchment area; (iii) fluid vents, mainly located close to the shoreline in the southwestern portion of the lake, discharging hydrothermal waters associated with CO 2 -dominated gaseous phases [2,3,12].
Since the lake has no emissaries and its annual hydrological deficit is positive, its quite stationary level is regulated by an underground outflow toward the sea [3].
The lake water has a pH of about 9 and its chemical composition is dominated by Na and Cl.
Chemistry is driven by the endorheic regime under a semiarid climate, the CO 2 -weathering of the Na-rich rocks outcropping in the catchment basin, and the chemical exchanges with seawater [5, and references therein].The lake chemistry shows a considerable variability due to rainfall dilution, evaporation, and precipitation of mineral phases, especially carbonates.

Sampling Strategy and Analytical Methods
The studied sediments were recovered using acrylic tubes, in summer 2008, in two different areas of the lake (Figure 1): the first one in the southern side (cores 2, 4, and 6, with a length of 11, 19, and 19 cm, resp.) and the second one in the north one (cores 11 and 12, 9 and 17 cm of length).Sediment cores were immediately subsampled, slicing them every 1 cm for the upper 5 cm and every 2 cm for the deeper portion.The subsamples were sealed off in polyethylene flasks and stored at −20 ∘ C until analysis.In the laboratory the samples were oven dried at 40 ∘ C prior to geochemical and mineralogical analyses.A total of 50 samples were investigated.
Bulk sample mineralogy was determined by powder Xray diffraction (XRD) using a Philips PW14 1373 with Cu-K radiation filtered by a monochromator crystal at a scanning speed of 2 ∘ 2/min.The relative proportions of minerals were established according to methods and data by Schultz [13] and Barahona et al. [14].
Pseudo-total trace elements concentrations of sediments were obtained by digesting 0.5 g of dried sample with 10 mL of freshly prepared aqua regia solution (HNO 3 /HCl, 1 : 3 v/v) in a Teflon vessel using a microwave oven (CEM-MSD 2000).This method has been widely employed in environmental geochemical studies [15][16][17][18][19], to remove non-lattice-bound metals.Blanks, duplicate samples, and standard reference material (Mess 3) were prepared with the same amount of acids for quality control purposes.Working calibration standards were prepared with serial dilution of stock standard solutions of each metal containing 10 mg kg −1 , using the same acid matrix utilized for digestion of the sediment samples.Caution was used in preparing and analysing samples to reduce contamination from air, glassware, and reagents.Only reagents of Suprapur quality and MilliQ water were used during the laboratory procedures.All glassware and the reaction vessels were previously soaked overnight with 10% nitric acid solution and then rinsed with ultrapure water.
The obtained solutions were analysed by inductively coupled plasma optical emission and mass spectrometry (ICP-OES and ICP-MS) using a Horiba Jobin Yvon equipment, model ULTIMA 2, and an Agilent 7500ce.All determinations were performed with the external standard calibration method, using In and Re as internal standards.The accuracy and precision of analytical procedures have been checked by analysing replicated measures of an international reference material (Mess 3), reagent blanks, and duplicated samples.The quality control gave good precision (S.D. < 10%) for all analytes.
Oxygen and carbon isotope compositions were determined from bulk sediment using an Analytical Precision AP2003 continuous flow mass spectrometer connected online with an automated acid dosing system.Samples were reacted with anhydrous phosphoric acid (100% H 3 PO 4 ).Calibration has been performed by the use of an in-house standard (calcite).The isotopic data obtained are expressed in delta notation () in permil (‰), relative to the conventional international standard Vienne Pee Dee Belemnite (V-PDB).Reproducibility of replicated standards was ±0.1‰ for both  18 O and  13 C.

Data Presentation and Discussion
The mineralogical composition of sediment cores is illustrated in Figure 2

Detrital Flux-Indicative Elements.
To evaluate the detrital inputs of the lake the geochemical relationship, has been determined [20].D * enhance the antagonism between Al, characterised by low geochemical mobility and typically of continental origin [21], and Fe and Mn of mainly deep-sea origin [21][22][23].Moreover, Fe and Mn concentrations can be influenced by variations in redox conditions or by volcanic rock weathering.The mean D * values are 0.42 in core 2, 0.37 in core 4, 0.26 in core 6, 0.18 in core 11, and 0.28 in core 12 (Figure 3); the higher value is found in core 2, which is the closest to the submerged continuation of the main fluvial channel of the studied area.However, all of the cores are below 0.63, reported by Wedepohl [24] for typical terrigenous    shales in marine environments.The detrital influence can be also deduced by the relationship by which a decrease of this ratio means an increasing of terrigenous supply [21], because Ti is linked to detrital material, whereas Fe is considered characteristic of deepsea sediments [21][22][23].The expected anticorrelation between D * and C * is not found in core 6, collected in a marsh in the southeastern part of the lake in close proximity of an hydrothermal vent [2], hence reflecting the geochemical interaction with a different fluid.The ratios Ti/Al, K/Al, Na/Al, and Rb/Al (Figure 3), commonly used as reliable tracers of terrestrial supplies [21], were also calculated.Element concentrations were normalized to Al because this element is generally assumed to represent a reliable measure of the terrigenous load in sediments.Another reason for using Al is that this element is typically not diagenetically labile.Ti/Al vertical distribution is quite constant along each core, with a mean value of 0.07, similar to those reported for sediment cores from the Strait of Sicily by Böttcher et al. [25], Cangemi et al. [19], and Tranchida et al. [26].The origin of titanium is related both to aeolian inputs [27,28] and to the weathering of the surrounding volcanic rocks [21].Very similar patterns are shown by K/Al, Na/Al, and Rb/Al ratios, with mean values of 0.63, 4.63, and 0.01, respectively.
The homogenous vertical distribution of the above mentioned parameters, with the exception of core 2, indicates constant depositional conditions in each site.The same homogeneity is also found between different cores, characterised by the same vertical patterns, indicating that depositional conditions do not significantly change thorough the lake.The higher fluctuations observed in core 2 can be explained with the proximity to the main drainage channel of the area, transporting a more heterogeneous detrital load during different runoff episodes.

Redox Sensitive Elements.
Changes in redox conditions in the sediment column, from oxidant to reducing, can occur moving from the water-sediment interface to major depths.These changes control the solubility of some elements as Mn and Fe, which can be used as geochemical tracers of redox conditions for their relatively rapid oxidation in the sediments [21,29].Following Machhour et al. [20] we used the relationship in order to detail these conditions.Figure 4 shows the vertical distribution of Mn * in all the cores, which are characterised by positive values, indicating an oxidizing environment [20,30].Despite the relatively homogeneous vertical pattern shown by Mn * , concentrations of V, Ni, and Co highlight more fluctuation, generally anticorrelated to Mn * (Figure 4).These elements generally show higher concentration in reducing depositional environment [31], suggesting possible formation of enriched metallo-organic complexes [32,33] due to the presence of biogenic matter [4][5][6].

Elements of Hydrothermal Origin.
As, Mo, Cd, and Se vertical profiles (Figure 5), as far as V and Ni, are generally constant in the first centimetres of all the cores, showing major fluctuations downward.As concentration ranges from 9.65 (in core 6) to 35.20 mg kg −1 (in core 2), with a mean value of 16.31 mg kg −1 , within the typical range reported in literature [34, and reference therein] for unconsolidated lake sediments.High As concentrations are usually associated with pyrite or Fe oxides or sulphide minerals.
Mo shows a mean concentration of 9.59 mg kg −1 with very high values up to 54.41 mg kg −1 in core 4, sensibly elevated if compared to hydrothermal and lake waters.Mo usually forms oxyanions and has a strong affinity with hydrogen sulphide under reducing conditions and it is coupled with organic carbon accumulation.
Analogue considerations can be done for Se and Cd vertical profiles.vertical distributions (Figure 6) show similar behaviour in all the sediment cores.The most abundant concentrations reflect the presence of carbonate mineralogical phases.Ca concentrations range from 5.39 in core 2 to 26.93% in core 6, with a mean value of 13.97%, whereas Mg content spans from 3.08 in core 6 to 11.21% in core 2, showing a mean value of 6.75%.
High Sr concentrations are observed in all the cores, showing a mean value of 2547.1 mg kg −1 and a particular enrichment in cores 6 and 11 with the highest value up to 4957.5 mg kg −1 in core 6.This enrichment reflects the abundance of aragonite as main carbonate mineralogical phase, being the preferential phase where Sr is incorporated.
Ba concentrations range from 55.26 in core 2 to 329.74 mg kg −1 in core 6, with a mean value of 152.74 mg kg −1 .
18 O vertical distributions in the sediment cores show values ranging from −0.23 to 5.92‰.Cores 2 and 4, located close to the hydrothermal vents, exhibit a quite homogeneous profile without significant fluctuation with a mean value of 0.7‰.On the opposite, cores 6, 11, and 12 are characterised by more pronounced variations along the sediment depth.
13 C shows a mean value of 6.0‰ and its vertical variations are generally contained in a range of ±2‰, with a maximum value of 7.38‰ in core 12.
The observed positive  13 C values were already found in carbonate microbial mats from this lake by Cangemi et al. [5, and references therein].The authors suggested that not only simple fractionation processes take place, invoking the selective sequestration of 12 C in the biomass or other disequilibrium effects induced by microbial activity, either metabolic or kinetic [35,36].Moreover, the presence of pyrite in cores 11 and 12 indicates reducing conditions that could affect the C exchange and isotopic fractionation between biogenic and abiogenic phases in the sediments.
Oxygen and carbon isotopic composition seems to be influenced by the environmental conditions under which the sediments are deposited, with particular reference to the buffering effect due to the continuous CO 2 bubbling from the hydrothermal vents, as remarked by the stability shown by core sediments 2 and 4. Cores 6, 11, and 12 (Figure 6), less affected by the direct interaction with hydrothermal fluids, exhibit a higher variability of the isotopic signal, especially  18 O.

Concluding Remarks
Previous investigations on the mineralogical composition of lake Specchio di Venere sediments presented very general semiquantitative data, highlighting the presence of carbonate phases, mainly aragonite, and clay minerals as the most abundant.Our analyses, carried out on five new cores located in three different sectors of the lake, confirmed the previous findings (Figure 2) and remarked a general compositional homogeneity in both the vertical and horizontal distribution of mineral phases.With the exception of peculiar biogeological niches, where biological activity leads to the formation of different phases (mainly silica, [4][5][6]), chemical sedimentation can be ascribed to a common process thorough the lake.This process is mainly dominated by the precipitation of the phases that exceed the saturation indexes in lake water [4,5], with a minor contribution of detrital origin transported by the stream network.
The novel data about major, minor, and trace elements and isotopic composition (carbonate phases) of these sediments give new insights about the different factors contributing to the chemical sedimentation into the lake.
The presence of significant concentration of Ti, K, Na, and Rb (Figure 3), typical of the rocks outcropping around the lake, indicates a clear detrital load transported into the lake by the stream network.
The environment is generally oxidizing, as remarked by the positive values of Mn * (Figure 4), which decrease with profile depth in some sites, as expected.As for the mineralogical composition, the chemistry of sediments is, in general, vertically and spatially quite regular, indicating that no significant variations affect the sedimentation processes.Correlated fluctuations are observed for V and Ni, especially in cores 4 and 12 where hydrothermal activity (core 4) and alternating dark laminae associated with pyrite (core 12, Figure 2) suggest more reducing local conditions.Vertical fluctuations are observed in the same cores for other hydrothermal sensitive elements, like As and Mo (Figure 5).
Vertical profiles and Ca, Sr, and Ba concentrations (Figure 6) show that the site with the higher aragonite content (core 6, Figure 2) is the most enriched in Sr and Ba, vicariants of Ca in the aragonite lattice.Their concentrations decrease downward as the content in aragonite.
Oxygen isotopic composition generally indicates evaporative conditions (Figure 6), compatible with a normal chemical sedimentation driven by evaporation.Conversely, positive values of  13 C, already found in carbonate microbial mats from this lake by Cangemi et al. [5], suggest the effect of biological activity on the carbonate deposition.
As a final remark, the mineralogical analyses confirm that the sedimentation in lake Specchio di Venere is mainly driven by normal evaporation processes, with minor contribution of a detrital input.However, hydrothermal and biogeochemical activity are responsible for local variations, as already observed in previous studies [4][5][6].

Figure 1 :
Figure 1: From the upper left counter-clockwise: location of Pantelleria island, location of lake Specchio di Venere, particularly of the lake with bathymetry (in meters below lake level), and location of sampling sites.

Figure 2 :
Figure 2: Relative abundance of the mineralogical phases of sediment cores.

Figure 4 :
Figure 4: Depth profiles of Mn * and concentrations of V, Ni, and Co (all expressed in mg kg −1 ) for lake Specchio di Venere sediment cores.

Figure 5 :
Figure 5: Depth profiles of concentrations of As, Mo, Cd, and Se (all expressed in mg kg −1 ) for lake Specchio di Venere sediment cores.

4. 2 . 4 .
Geochemistry of Ca, Mg, Sr, and Ba Vertical Distribution and Oxygen and Carbon Isotopic Composition.Ca, Sr, and Ba

Table 1 :
Mean values (%) of semiquantitative mineralogic analysis of core sediments from lake Specchio di Venere.
and Table 1.Concentration of major, minor, and trace elements and carbon and oxygen isotopic composition of sediments are reported in Table 2.
4.1.Bulk S4,5,and 6.neralogy.The analysed sediments reveal a quite homogeneous mineralogical composition, both vertical and horizontal (Figure2).All the cores contain large amounts of carbonates, with aragonite as the main mineralogical phase (from 49% in core 12 up to 69% in core 6); dolomite is present as an accessory mineral in cores 6 and 12.The other mineralogical components are represented by variable amounts of clay minerals (mean values ranging from 15%, in core 11, to 39% in core 2), accompanied by moderate amount of feldspars (from 6% in core 2 to 24% in core 12), quartz (no more than 2%), and halite, ubiquitously present in low percentages (no more than 2%).Pyrite is also present in cores 11 and 12, collected in the northern side of the lake.4.2.SedimentGeochemistry.Vertical profiles of selected elements, indicative of detrital delivery, depositional redox conditions, hydrothermal supply, and carbonate precipitation are illustrated inFigures 3,4,5,and 6.

Table 2 :
Mean, minimum, and maximum values of geochemical proxies for lacustrine sediment samples from lake Specchio di Venere, Pantelleria island; n is the number of samples for each sediment core.