Transition from marine to hypersaline conditions in the Messinian Tripoli Formation from the marginal areas of the central Sicilian Basin

Sedimentary Geology 140 (2001) 87±105

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Transition from marine to hypersaline conditions in the Messinian Tripoli Formation from the marginal areas of the central Sicilian Basin A. Bellanca a,*, A. Caruso b, G. Ferruzza a, R. Neri a, J.M. Rouchy c, M. Sprovieri a, M.M. Blanc-Valleron c a

Dipartimento di Chimica e Fisica della Terra ed Applicazioni alle Georisorse e ai Rischi Naturali, UniversitaÁ di Palermo, Via Archira® 36, 90123 Palermo, Italy b Dipartimento di Geologia e Geodesia, UniversitaÁ di Palermo, Corso Tukory 131, 90134 Palermo, Italy c CNRS-ESA 7073, Laboratoire de GeÂologie, MuseÂum National d'Histoire Naturelle, 43, rue Buffon, 75005 Paris, France Received 12 April 1999

Abstract Three sections of the early Messinian Tripoli Formation from the northern and southern margins of the central Sicilian Basin (Serra Pirciata, Torrente Vaccarizzo, and Marianopoli) have been studied with the aim to reconstruct the sedimentary and environmental changes which occurred during the transition between marine conditions and the evaporitic events of the Salinity Crisis recorded in the overlying Calcare di Base Formation. A detailed biostratigraphic and cyclostratigraphic study provided the opportunity of cycle-by-cycle correlations between the marginal sections and the reference section of Falconara. The main paleoenvironmental changes are recorded by: (1) the evolution of calcareous microfossils towards low diversity and their complete disappearance; (2) the composition of the carbonate fraction which commonly changes from calcite, mostly related to calcareous microfossils, to authigenic carbonate phases consisting of either calcite, dolomite, and/or aragonite; (3) the appearance of shallow water deposits and evaporite pseudomorphs; (4) the variation of the stable isotope composition of the carbonate fraction, indicative of large ¯uctuations of the freshwater dilution/evaporation balance, with a general trend towards hypersaline conditions. Both mineralogical and isotope data indicate that the dolomite precipitated generally from concentrated pore waters while other carbonates formed in the water column submitted to large ¯uctuations of salinity. Except locally, the increase in salinity did not reach concentrations high enough to precipitate signi®cant volumes of evaporites as during the deposition of the overlying Calcare di Base. The transition from normal marine to hypersaline conditions is recorded diachroneously in the three sections, respectively, in cycle 34 (6.32 Ma) at Serra Pirciata, cycle 42 (6.15 Ma) at Torrente Vaccarrizzo, and cycle 44 (6.12 Ma) at Marianopoli. These changes represent the hydrological and sedimentary response to the fragmentation of a large marine basin into different subbasins, which ended in the more or less complete closure recorded by the deposition of the overlying evaporite-rich Calcare di Base Fm (true onset of the evaporitic event). Two hypotheses not exclusive may explain this change, either the regional consequence of the compressive tectonics or the incipient drop of the global sea level which led to the Salinity Crisis. Our results con®rm that the Salinity Crisis was preceded by evaporative episodes in the evolution towards hypersaline conditions which occurred diachroneously as a response to the diversity of paleogeographical settings even if the paroxysmal phase seems to have affected the basin at the same time. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Messinian; Mediterranean; Central Sicilian Basin; Tripoli Fm; Salinity crisis; Biostratigraphy; Stable isotopes

* Corresponding author. Fax: 139-91-616-8376. E-mail address: [email protected] (A. Bellanca). 0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0037-073 8(00)00173-1

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1. Introduction During the Messinian the Mediterranean basin was affected by paleoceanographic changes marked in the sedimentary record by both cyclic variations forced by astronomical factors and periodic or abrupt changes related to water mass exchanges with the Atlantic Ocean driven by tectonic/eustatic controls. The changes are recorded, specially in the Sicilian Basin, by a complex sequence of marine marls, diatomite-bearing deposits (Tripoli Formation), evaporites and brackish sediments (Lago-mare) (Cita et al., 1978; Rouchy, 1982; Cita and McKenzie, 1986; Rouchy et al., 1998). Because the marginal basins were extremely sensitive to the environmental variations, their study provides useful information for a deeper comprehension of the Messinian events in the Mediterranean area. The Messinian basin of Caltanissetta, in central Sicily, displays the most complete sedimentary successions present in uplifted basins, which makes it one of the most important models to reconstruct the Messinian events. This basin corresponds to a broad belt, trending NE±SW across the island, which continued to be affected by compressive deformations during the Messinian. Consequently, active thrust may have formed growing anticlines separating isolated synclines along the margins of the basin (Butler et al., 1995, 1999). The deposition of the major part of the early Messinian Tripoli Fm took place in near normal marine conditions submitted to cyclically controlled variations of productivity. This resulted in the repetition of sedimentary triplets composed of homogeneous marls, laminated marls and diatomites which are usually interpreted as being constrained by the astronomical precession (Hilgen et al., 1995; Krijgsman et al., 1995; Sprovieri et al., 1996a,b; Sierro et al., 1999; Hilgen and Krijgsman, 1999; Krijgsman et al., 1999). The Tripoli Fm grades upward into the Calcare di Base Fm which displays the ®rst evidence of evaporite precipitation (gypsum and halite) and, thus, is commonly considered as the true onset of the Salinity Crisis, preceding the deposition of the evaporitic formations (Lower Gypsum, Salt, and Upper Gypsum) (McKenzie, 1985; Bellanca et al., 1986; Decima et al., 1988; Caruso et al., 1997). A number of earlier works have been centred on the sedimentology and isotope geochemistry of the

Calcare di Base Fm, few of these including also the upper part of the Tripoli Fm (McKenzie et al., 1979± 1980; Bellanca and Neri, 1986; Bellanca et al., 1986; Decima et al., 1988; Pedley and Grasso, 1993; Pierre et al., 1997). The transitional steps in the evolution from marine conditions towards evaporitic settings can be recognized by considering that the hydrological system in the marginal basins was more sensitive to the environmental constraints. For this reason, we have studied three marginal sections (Serra Pirciata, Torrente Vaccarizzo and Marianopoli) comparing them with the reference section of Falconara which is located in deeper areas (Fig. 1). In this paper, we propose to reconstruct the nature and chronology of the environmental changes that occurred before the onset of the Salinity Crisis using an integrated approach combining biostratigraphy, micropaleontology, mineralogy, and the stable isotope geochemistry of the carbonates. 2. Methods The study has been focused on three main sections representative of different marginal subbasins (Serra Pirciata, Torrente Vaccarizzo, and Marianopoli) where we expected to obtain an ampli®ed response to the precursor events of the Salinity Crisis. The bioevents considered in this study are mainly based on changes in the planktonic foraminifera assemblage observed in the fraction .125 mm. The micropaleontology, bulk mineralogy and carbonate contents have been analysed on 85 samples from Serra Pirciata, 35 samples from Torrente Vaccarizzo and 53 samples from Marianopoli, with an average resolution of 3 samples for every cycle. The carbonate content of the samples was measured on 100 mg of powdered sediment using a manocalcimeter (MCM). The bulk mineralogy was determined by X-ray diffraction using a Siemens D-500 instrument (Ni-®ltered CuKa radiation). Normal light microscopy and scanning electron microscopy were used to characterize the carbonate component, biosiliceous deposits and diagenetic features of the studied sediments. The isotopic composition of the carbonates was determined on 56 samples from Serra Pirciata, 36 samples from Torrente Vaccarizzo and 44 samples

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Fig. 1. Location map of the studied sections.

from Marianopoli. The carbonate fraction exhibits a great variability in mineralogy (calcite, aragonite, dolomite) and origins of the various components. In the lower part of the sections, the calcite comes predominantly from marine calcareous nanno- and microfossils (planktonic and/or benthic foraminifera), carbonate debris and minor proportions of micrite, while in the upper part of the sections it is mainly related to authigenic phases and carbonate debris. The aragonite is related to pteropods, oolites, and authigenic phases. The oxygen and carbon isotopic compositions of CaCO3 minerals (calcite and aragonite) and dolomite were obtained after removing organic matter by roasting the samples for 40 min at 3808C in high-vacuum glass tubes. Then, the samples were transferred into glass reaction tubes and dissolved in 100% phosphoric acid at 258C under high vacuum for variable periods (12 h for aragonite and/or calcite, 72 h for dolomite) according to the mineral present. When both Ca-carbonate and dolomite were present, the techniques of Epstein et al. (1964) and Becker and Clayton (1972) were used. Taking advantage of the differing reaction rates of the carbonates, CO2 gas was collected at time intervals of 20 min for more reactive Ca-carbonates and 72 h for less reactive dolomite. The obtained CO2 was cryogenically separated from other gases and measured with a Finnigan Delta S mass spectrometer. The isotopic results are expressed in d ½ units and reported against the PDB-1 standard.

The reproducibility for the isotopic determinations was ^0.1½ (1s ) for d 18O and ^0.07½ (1s ) for d 13C. 3. Location and stratigraphy of the sections The depositional changes are constrained chronologically by a biostratigraphic study which provided direct correlations with the accurate frame time control available in the reference section of Falconara (Sprovieri et al., 1996a,b; Caruso 1999). The use of the same bioevents recognized both in the reference and in the marginal sections allowed us to obtain cycle-by-cycle correlations between the different sections, providing an accurate timing for the environmental changes identi®ed in the marginal sections (Fig. 2). The numbering of the lithologic cycles for the studied sections refers to that proposed by Hilgen and Krijgsman (1999) and Krijgsman et al. (1999) for the Falconara section. 3.1. Falconara section The Falconara section is located on the southern slope of Cantigaglione Mount, about 3.5 km NW of the Falconara castle, near the Licata town (Fig. 1). In this section, the Tripoli Formation, 25 m in thickness, includes 45 cycles (Fig. 2) with thickness ranging from few decimetres up to 2 m (Sprovieri et al., 1996a,b; Pierre et al., 1997). Due to tectonic events,

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several cycles are missing in the lowermost part of this section. By correlating the sections of Falconara and Gibliscemi Hilgen and Krijgsman (1999) and Krijgsman et al. (1999) have recognized 49 cycles in the Tripoli Fm which are overlain by several beds interpreted as belonging to the Calcare di Base Fm. The cycles 13 to 49 are generally composed of triplets of homogenous grey marls, laminated red marls, and laminated white diatomites. From cycle 50 upwards, the composition of the cycles changes signi®cantly in relation to the transition towards the overlying Calcare di Base Formation. Cycle 50 consists of grey dolomitic marls, laminated carbonates containing ooids and diatomites whereas cycles 51 is predominantly composed of dolomitic limestones and dolostones alternating with marls, without diatomites. An uppermost, 30 cm-thick, layer of marls underlying a thick interval of brecciated limestones, rich in calcite pseudomorphs after gypsum and in celestite, has been considered as cycle 52 that locally represents the base of the Calcare di Base Fm (Sprovieri et al., 1996a). 3.2. Serra Pirciata (Tallarita) section The Serra Pirciata section (Figs. 1, 2 and 3A) outcrops near the old Tallarita mine, about 5 km from the Riesi town, along the road from Riesi to Sommatino. Previous descriptions of this section were published by Pedley and Grasso (1993), Butler et al. (1995, 1999) and McClelland et al. (1996). A new stratigraphic interpretation, accounting for the presence of tectonic disturbance in the lower part of the section, is reported by Sprovieri et al. (1996b). Here, the lithologic interval displaying the typical sedimentary features of the Tripoli Fm is only 19 m thick and consists of 25 sedimentary cycles composed of homogenous grey marls, laminated red marls and white diatomites, with a covered part, about 2 m thick (Fig. 2). The basal part of this interval, which overlies the Globigerina Marls (middle Tortonian-lower Messinian), is tectonically disturbed. For this reason, the section lacks the lowermost 15 cycles of the

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Tripoli Fm. According to Sprovieri et al. (1996b), the presence of Globorotalia conomiozea, Globorotalia miotumida, together with a calcareous nannofossil assemblage characterized by Reticulofenestra rotaria, indicates that the lower part of the section belongs to the G. conomiozea biozone. Furthermore, a coiling change (by sinistral to dextral) of the Neogloboquadrina acostaensis, occurring in cycle 32 at Falconara, is present here two cycles above the covered interval. In the interval between 13 and 19 m, the succession becomes enriched in layers of laminated dolomitic carbonates interbedded with clays and diatomitic marls forming a gradual passage towards the Calcare di Base Fm. The cyclic sedimentary pattern continues to be clearly marked in this interval although the white diatomites become thinner, thus permitting the cycle correlation with the Falconara section. A ®rst typical layer of brecciated limestone occurs at the base of cycle 46. This layer has been previously interpreted by Pedley and Grasso (1993) as a marker of incipient evaporitic conditions and referred as First Carbonate Bed (FCB) (Figs. 2 and 3A). On the top of the section (from 19 to 31 m), the true Calcare di Base, rich in celestite and halite moulds, and containing crystalline and nodular gypsum, consists of 9 carbonate cycles. 3.3. Torrente Vaccarizzo section The Torrente Vaccarizzo section (Figs. 1 and 2), 49 m thick, outcrops about 5 km NE of the Santa Caterina Villarmosa town, along the road connecting this and the Villarosa town. Here, the cyclicity is not as well apparent as in the other sections due to the scarcity or absence of true white diatomitic beds. According to lithology, three main parts can be de®ned. The lower part, about 5 m thick, is characterized by alternating grey marls and silty marls locally associated to laminated marly limestones. The middle part, 7 m thick, is characterized in the lower 4 m by the presence of diatomitic silty marls followed by marls, while the upper 3 m commonly consist of marls intercalated with laminated carbonates. The

Fig. 2. Cyclostratigraphic correlation of the studied sections with the reference section of Falconara (modi®ed from Sprovieri et al. 1996b). Numbering of cycles corresponds to lithologic cycles of Falconara Tripoli Formation proposed by Hilgen and Krijgsman (1999) and Krijgsman et al. (1999). Seven main bioevents are recognized: 1 ˆ last occurrence of Globorotolia nicolae; 2 ˆ ®rst common occurrence of N. atlantica; 3 ˆ last occurrence of Globorotolia conomiozea group; 4 ˆ last common occurrence of N. atlantica; 5 ˆ ®rst common occurrence of T. multiloba; 6 ˆ sinistral/dextral coiling change of N. acostaensis; 7 ˆ second in¯ux of T. multiloba; FCB ˆ ®rst carbonate bed.

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Fig. 3. Views of Serra Pirciata section (A), lower (B) and upper part (C) of Marianopoli section; numbers correspond to lithological cycles.

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upper part contrasts with the underlying deposits as it consists of a 1.4 m-thick layer of laminar gypsum whose regular lamination is outlined by dolomitic laminae. The overlying interval (from 18.5 to 49 m) corresponds to the Calcare di Base Fm and consists of a rhythmic succession of 6 beds of brecciated, halite-mould-rich carbonate and layers of aragonitic marls. The succession is truncated on the top by recent deposits. An important bioevent identi®ed in this section is the second in¯ux of Turborotalita multiloba in the second lithologic cycle that is then correlated with cycle 35 of Falconara (Fig. 2). Moreover, the occurrence of right coiling N. acostaensis at the base of the succession supports the lack of the ®rst 33 cycles. The contact between this section and the underlying Terravecchia Fm is tectonic. 3.4. Marianopoli section This section (Figs. 1, 2, 3B and C), 35 m thick, outcrops near the town of Marianopoli, along the road from this to the San Cataldo town. The typical deposits of the Tripoli Fm (consisting of the cyclic repetition of homogeneous grey marls, red laminated marls and white diatomites) are only represented in the lower part of the section, about 10 m in thickness. The contact with the underlying marls is not exposed. The age of this part of the section is given by two main bioevents: the second in¯ux of T. multiloba in the third cycle from the base (cycle 35) and the occurrence of right coiling N. acostaensis at the base. The overlying sedimentary succession shows a great lithologic variability (Fig. 3C). The most striking feature is the presence in cycles 44±46 of ooliterich carbonate layers interbedded with marls and diatomites, while the upper part of the section is characterized by the intercalation of cross-laminated sands and silts, laminated limestones, shales, and laminated dolostones. The section ends with the Calcare di Base that consists of thick beds of brecciated limestone, rich in halite moulds and calcite pseudomorphs after halite and gypsum, alternanting with marls.

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4. Micropaleontology, mineralogy and isotope geochemistry 4.1. Serra Pirciata (Tallarita) section The major event identi®ed in this section occurs in cycle 34 where a drastic change affects simultaneously the microfossil assemblage, the mineralogy of the carbonates and their stable isotope composition (Fig. 4). 4.1.1. Lower part of the section (cycles 16±33) The calcareous nannoplankton and planktonic foraminifers are relatively abundant and diversi®ed, showing the same lithology-controlled variations described for the Falconara section (Sprovieri et al., 1996b). The basal homogeneous marls contain an oligotypic assemblage of calcareous nannoplancton and planktonic foraminifers dominated by forms indicative of colder waters such as Calcidiscus leptoporus and small placoliths, Neogloboquadrina atlantica, Turborotalita quinqueloba, N. acostaensis left coiling, and Globigerina bulloides. The laminated red marls and the lowermost part of the diatomites contain foraminifer species indicative of relatively warm waters such as abundant Globigerinoides spp., discrete percentages of Orbulina universa and relatively high percentages of discoasterids. In the rest of the diatomite layers, the assemblages of foraminifers are dominated by N. acostaensis and G. bulloides, which are considered to be indicative of nutrient-rich waters (Lourens et al., 1992), and characterized by the absence of benthic foraminifer species. The diatom assemblage is generally dominated by Thalassionema nitzschioides and Asterolampra acutiloba is common. The carbonate fraction constitutes up to 40% of the bulk sediment and is predominantly composed of calcite related to the biogenic fraction, various types of carbonate debris with size up to 100 mm and small amounts of micrite fraction. The dolomite content is generally less than 10% (Fig. 4). The stable isotope composition of the calcite (Fig. 4) is characterized by d 8O values, comprised between 21.3 and 2.1½, which re¯ect marine conditions submitted to slight ¯uctuations of salinity and/or temperature. Except for two values as low as 27.7 and 212.0½, the negative d 13C values ¯uctuate around 24½. Whole-rock isotopic data should be

94 A. Bellanca et al. / Sedimentary Geology 140 (2001) 87±105 Fig. 4. Calcareous planktic percentages, mineralogy and stable isotope compositions of calcite and dolomite along the Serra Pirciata stratigraphic sequence (Ce ˆ celestite; J ˆ jarosite).

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Fig. 5. Scanning electron microscope (SEM) images. (A) Oligotypic assemblage of diatoms composed of Thalassiotrix longissima (Serra Pirciata section, cycle 50). (B) Euhedral to subeuhedral crystals of early diagenetic dolomite in marls (Serra Pirciata section).

used with caution in decriving a primary environmental signal because signi®cant geochemical changes may occur in response to post-depositional diagenetic alterations (see discussion in Sass et al., 1991; Marshall, 1992; Spicer and Cor®eld, 1992; Shackleton et al., 1993). For the Tripoli sections, petrographic data (thin section and SEM observations) supported by a lack of correlation between d 18O and d 13C rule out alterations by meteoric waters and intense recrystallization during burial and suggest that cementation in these deposits occurred during early diagenesis, mainly by locally derived carbonate. In this context, some quite negative d 13C values, between 24 and 212½, in the lower part of the Serra Pirciata section could indicate a more severe in¯uence of early diagenetic reactions involving organic carbon and probably related to stagnant bottom conditions, which is consistent with the lack of benthic foraminifers. 4.1.2. Upper part of the section (from cycle 34 upwards) A fall in both the abundance and diversity of the calcareous planktonic assemblages occurs in the cycle 34 followed by the de®nitive disappearance of the foraminifers in the cycle 37. The overlying deposits are barren except for some diatomitic layers which contain marine diatoms with an assemblage generally oligotypic and dominated by Thalassiothrix longissima and Actynocyclus curvatulus (Fig. 5A). This biological event suggests a rapid deterioration of the environmental conditions marked also by an abrupt change in mineralogy of the carbonate fraction

which is characterized by the disappearance of calcite, whereas the dolomite becomes the exclusive component, reaching up to 90% of the bulk sediment in the marl layers (Fig. 4). Under the SEM the dolomite appears as euhedral to subhedral crystals with average size of 5 mm (Fig. 5B). The base of cycle 46 contains signi®cant proportions of celestite while jarosite is common in the interval comprised between cycles 48 and 52. The jarosite formation implies: (i) a source of H2S linked with sulphate bacterial reduction; and (ii) a redox boundary promoting oxidation of the sulphide. The stable isotope composition of the dolomite exhibits high d 18O values that are close to 6½, or heavier, with the exception of a few lower values (Fig. 4). Most data indicate that the dolomite precipitated from highly evaporated solutions, whereas the sudden decreases of d 18O are interpreted as evidence for short dilution events. Between cycles 34 and 47, the d 13C values of the dolomite ¯uctuate mostly around 22.5½ with some more negative values down to 27.3½, and then decrease progressively down to 212.4½ through the transitional interval with the overlying Calcare di Base. This pattern indicates substantially increased availability of 13Cdepleted biogenic CO2 derived from processes involving microbial oxidation of organic matter or, at least for extremely negative d 13C values, microbial sulphate reduction. In the two last cycles at the transition with the Calcare di Base (51±52), the d 18O values cover a wider range of variation (from 7.1 to 24.7½) suggesting that salinity ¯uctuated rapidly between

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highly evaporated and diluted conditions. Such a scenario of a basin subjected to poorly oxygenated to anoxic conditions associated to intense evaporation and periodically refreshed by considerable in¯uxes of continental waters accounts for the occurrence of jarosite-bearing blackish argillites in the upper part of the section and just in proximity of beds marked by a sudden decrease of d 18O concomitant with increase of d 13C. 4.2. Torrente Vaccarizzo section The lower part of the Tripoli Fm, up to cycle 33, is lacking in this section and the lithology is more variable than in the other sections (Fig. 6). The lower part of the section is still characterized by marine conditions and the major change is identi®ed by signi®cant variations of the micropaleontological, mineralogical and stable isotope markers in coincidence with cycle 42, that is to say 8 cycles later than in the Serra Pirciata section (Fig. 6). 4.2.1. Lower part of the section (cycles 34±41) The calcareous nannoplancton and planktonic foraminifers display a high abundance and relatively large diversity. The foraminifers are dominated by N. acostaensis dextral coiling, G. bulloides, T. quinqueloba, T. multiloba (from cycle 35), Globigerinoides obliquus, Globigerinoides quadrilobatus and O. universa. T. quinqueloba is commonly dominant in the marls while the group of Globigerinoides spp. is more abundant in the laminated deposits, suggesting that the water temperature was warmer when the laminites were deposited. The benthic foraminifer association is represented by a few species with forms indicative of shallow waters (Elphidium sp, Florilus sp) and poorly oxygenated bottom water (Bulimina echinata). Calcite is the dominant carbonate mineral with contents generally comprised between 20 and 40% and few higher values reaching up to 60% of the bulk sediment (Fig. 6). Dolomite is present throughout the interval even if in low proportions, mostly lower than 10%, exceptionally reaching up to 20%. Clinoptilolite, a mineral commonly associated to the diagenetic transformation of biogenic opal of the diatom frustules (Rouchy et al., 1998), is present in several layers. The SEM examination reveals that it is

commonly crystallized in the foraminifer chambers where pyrite framboids are also present. The stable isotope composition of calcite (Fig. 6) is characterized by d 18O values comprised between 2.3 and 20.8½ with a mean value of 1.5½ indicative of normal marine conditions. In dolomite, the d 18O values are quite constant around 4.5½. Both calcites and dolomites exhibit moderately negative d 13C values which are relatively constant (around 22.5½) for dolomite and more variable (mostly around 22½) for calcite. Owing to the differential oxygen isotope fractionation during the crystallization of carbonate minerals, the d 18O values recorded in dolomite must be lowered by about 3±4½ for a direct comparison with the calcite (Land, 1980). Therefore, in this part of the section, it can be assumed that the dolomite precipitated from interstitial solutions compositionally similar or slightly more evaporated with respect to the surface waters. 4.2.2. Upper part of the section (from cycle 42 upwards) Both the abundance and diversity of foraminifers fall in the upper part of cycle 41, and cycles 42±45 contain only an oligotypic assemblage dominated by G. bulloides and N. acostaensis. The nannoplancton association is represented by dominant C. leptoporus, small placoliths, and Reticulofenestrids. As in the Serra Pirciata section, marine diatoms are still present after the disappearance of the foraminifers with associations dominated by T. longissima and A. curvatulus. Cycle 42 is rich in pteropods belonging to the species Creseis acicula (95%) and Creseis virgula (5%) which are also observed, although in lesser abundance, in cycles 43 and 45. The presence of these two species of pteropods has been previously recognized in Pleistocene deposits of the Red Sea where they characterize the highly saline conditions of isotopic stage 2 (Almogi-Labin et al., 1986, 1998). The marls of cycles 42±45, in which the pteropods are present, contain aragonite in proportions up to 10%, suggesting that this mineral is related to the tests of pteropods (Fig. 6). On the other hand, some layers show the presence of aragonitic laminae consisting of isolated or aggregated needles, in average about 2 mm in length, which suggest a direct precipitation of the mineral. The calcite content of the sediments decreases signi®cantly in the upper

A. Bellanca et al. / Sedimentary Geology 140 (2001) 87±105 Fig. 6. Calcareous planktic percentages, mineralogy and stable isotope compositions of calcite and dolomite along the Torrente Vaccarizzo stratigraphic sequence (C ˆ clinoptilolite, Py ˆ pyrite).

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part up to the base of the laminar gypsum, while dolomite is increasing in the last cycle underlying the gypsum layer and within this layer. Clinoptilolite is still present in cycles 42 and 44. Calcite exhibits an average isotope composition close to that of the calcite from the underlying interval but with wider ¯uctuations for both oxygen and carbon 23.2 , d 13C½ , 20.9) (21.2 , d 18O½ , 3.3; (Fig. 6). In particular, a negative O-isotope excursion at the base of the cycle 43 could correspond to an episode of increased inputs of continental waters. In contrast, the oxygen isotopic composition of dolomite rises progressively from values of about 5.1 up to 8.1½ in cycle 46. Such high values suggest that in this interval dolomite precipitated from highly evaporated solutions. The d 13C values of dolomite widely vary between 27.6 and 21.4½ indicating large ¯uctuations of the availability of 13C-depleted carbon derived by bacterial remineralization of organic matter. We have no isotopic data for samples from the uppermost cycles of the Torrente Vaccarizzo, but Bellanca and Neri (1986) and Bellanca et al. (1987) analysed laminar gypsum samples from these cycles for their trace element contents and the isotopic compositions of the sulphate crystallization water. Their results suggest that the parent brine of the gypsum was signi®cantly diluted by meteoric waters, thus indicating periodically repeated inputs of continental water also during the deposition of this part of the section. 4.3. Marianopoli section Compared to the sections described above, the Marianopoli section (Fig. 7) shows a greater variability of the different markers (i.e. calcareous planktonic assemblage, carbonate mineralogy and stable isotope composition of carbonates. The major event seems to correspond to the beginning of cycle 45 where a sharp decrease in the calcareous planktic assemblages occurs like that observed in the Torrente Vaccarizzo section. 4.3.1. Lower part of the section (cycles 33±43) As in the other sections, this lower part of the Tripoli Fm is characterized by abundant and relatively diversi®ed assemblages of calcareous nannoplankton

and foraminifers. The dominant species are T. quinqueloba and T. multiloba (from cycle 36) usually considered as indicative of cold surface waters. In addition to the above-mentioned planktonic foraminifers, the marls are characterized by the presence of an oligotypic assemblage of benthic foraminifers adapted to bottom oxygen de®ciency (i.e. B. echinata, B. aculeata, and Bolivina dilatata). The laminated red marls and the basal part of each diatomite layer are dominated by Globigerinoides spp., a form living in warmer surface waters, which can constitute 70±80% of the association. They also contain abundant discoasterids and sphenoliths. The upper part of each diatomite layer is characterized by high percentages of N. acostaensis and G. bulloides. Few layers only contain abundant Orbulina spp. A ®rst reduction of diversity affects the planktonic foraminifers from cycle 42. The carbonate fraction is predominantly composed by calcite which forms 18±63% of the bulk sediment, while small contents of dolomite (generally less than 5%) are observed. The d 18O values of calcite, which slightly vary between 20.9 and 1.9½, are typical of marine conditions (Fig. 7). The d 13C values are also fairly constant around a mean value near 22.2½, which suggests a moderate contribution of carbon originated from oxidation of organic matter. 4.3.2. Upper part of the section (from cycle 44 upwards) Cycle 44 marks the abrupt disappearance of foraminifers, although cycle 45 still contains N. acostaensis, T. multiloba, T. quinqueloba, associated with Ammonia sp. and Elphidium sp., typical of shallow water environments. An association of pteropods dominated by the genus Creseis spp. is present in cycles 45, 46, and 48. Diatoms persist throughout this interval and the assemblages are still dominated by species considered as typically marine (e.g. A. acutiloba, T. longissima, and A. curvatulus). The carbonate fraction shows a great variability along this interval (Fig. 7). The calcite content decreases from cycles 44 to 49 and ¯uctuates above, while dolomite content rises from the upper part of cycle 47 reaching up to 40% of the bulk sediment. The dolomite drastically decreases in the interval above cycle 48 that is characterized by the intercalation of

A. Bellanca et al. / Sedimentary Geology 140 (2001) 87±105 Fig. 7. Calcareous planktic percentages, mineralogy and stable isotope compositions of CaCO3 minerals and dolomite along the Marianopoli stratigraphic sequence (L ˆ plant remains). 99

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Fig. 8. Fibrous-radial aragonite oolites in a carbonate layer (Marianopoli section, cycle 45).

sands and sandy marls. Some layers of cycles 44, 45, 46, and 48 exhibit aragonite values between 25 and 40% which are correlated with the presence of either pteropods and/or oolites. The presence of at least three layers rich in aragonitic oolites displaying a wellpreserved ®brous-radial structure is the most striking sedimentary feature of this upper part of the section (Fig. 8). The stable isotope composition of carbonates greatly varies along this part of the section (Fig. 7). The d 18O values of CaCO3 minerals ¯uctuate between 24.3 and 5.2½ suggesting large ¯uctuations of salinity and/or temperature. The d 13C values are also variable with a very low value (210.6½) at cycle 46. A ®rst minimum (24.1½) in the d 18O curve is recorded in a layer containing plant remains and leaves, consistent with a greater in¯uence of continental water inputs. In contrast, positive peaks of d 18O (up to 5.2½) indicate episodes of hypersaline conditions. Some higher values are relative to aragonite± calcite mixtures. For aragonite±calcite fractionation, the aragonite should be enriched in 18O by 0.6±0.8½ (Epstein et al., 1953; Tarutani et al., 1969; Grossman and Ku, 1986) with respect to cogenetic calcite. However, the shift in d 18O is large enough to rule out that it is a result of Ca-carbonate mineralogy variations. Maxima in the d 18O curve alternate with values of d 18O near 0½ measured at the top of cycle 49 in coincidence with the presence of diatoms and calcareous nannoplankton (Fig. 9), thus indicating episodes of alternating hypersaline and normal marine conditions. At the top of the section, a new trend

Fig. 9. Calcareous nannoplankton and diatom assemblages indicative of episodic inputs of marine conditions in the upper part of the Marianopoli section (cycles 47±48).

towards more negative d 18O values (down to 24.3½) occurs in phase with increased proportions of terrigenous material indicated by the presence of sandy interbeds. The isotopic composition of dolomite (5.1 , d 18O½ , 7.3; 28.6 , d 13C½ , 24.7½) is quite different from that of calcite supporting the hypothesis that the two carbonate minerals are not cogenetic. The high d 18O values of dolomite are consistent with a precipitation from concentrated solutions. The low d 13C values indicate an important contribution of carbon derived from oxidation of organic matter.

5. Discussion 5.1. General evolution towards the evaporites During the Messinian, the Mediterranean basin experienced important environmental changes due to the complex interaction of tectonic, eustatic and climatic constraints that are particularly well recorded in the marginal areas. The main change corresponds to the Salinity Crisis which was responsible for the deposition of widespread saline deposits over the whole paleo-Mediterranean area (HsuÈ et al., 1973, 1978; Cita et al., 1978; Rouchy, 1982). An age comprised between 6 and 5.7 Ma is now commonly admitted for the onset of the evaporite deposition in several basins (Vai et al., 1993; Gauthier et al., 1994; Hilgen et al., 1995; Sprovieri et al., 1996a,b; Caruso et

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al., 1997). Nevertheless, an early deposition of gypsum occurred in some marginal basins as indicated by the presence of gypsum layers interbedded in the preevaporitic Tripoli Fm in southern Spain (Rouchy, 1982; Rouchy et al., 1998) or by reworked gypsum pebbles at the top of this Formation in Cyprus (Orszag-Sperber et al., 1979; Rouchy, 1982). In many basins, the marine deposits of the Tripoli Fm are characterized by periods of enhanced biosiliceous productivity whose duration is forced by the astronomical precession (Hilgen et al., 1995; Sprovieri et al., 1996a,b). These episodes of high biosiliceous productivity have been interpreted as caused by either increased activity of upwelling systems (McKenzie et al., 1979±1980; Pierre et al., 1997) or by nutrient inputs connected with increased runoff (Van der Zwaan, 1979; Van der Zwaan and Gudjonsson, 1986). The transition between the preevaporitic Tripoli Fm and the massive evaporites greatly differs according to the different basins (Rouchy, 1982), but in Sicily it is characterized by carbonate deposits up to 50 m thick, the so-called Calcare di Base, which is rich in moulds and pseudomorphs after gypsum and halite, layers of primary gypsum, as well as in native sulphur ores. For these characteristics, it is now admitted that this carbonate unit represents the true onset of the Salinity Crisis. The great variability of the carbonate mineralogy, the large variations of the isotopic composition of carbonates, and the abundance of evaporites point to sedimentary and diagenetic processes in closed to semi-closed systems submitted to rapid and very large ¯uctuations of the water budget, but with dominant hypersaline conditions (Longinelli, 1979±1980; McKenzie et al., 1979±1980; Bellanca et al., 1986; Decima et al., 1988; Pierre at al., 1997). The formation of large volumes of authigenic carbonates (i.e. calcite, dolomite, and aragonite) in the ®eld of halite precipitation implies that large inputs of CaCO3-rich continental or marine waters were introduced into the hypersaline settings (Decima et al., 1988). The importance of the native sulphur deposits associated with carbonate phases replacing Ca-sulphates and very low carbonisotope composition of these diagenetic carbonates indicate that processes of bacterial sulphate reduction were very active (Dessau et al., 1962; Pierre, 1982; Rouchy, 1982; Bellanca et al., 1986; Decima et al., 1988). Several studies showed that the salinity started

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to increase earlier in the upper part of the Tripoli Fm (McKenzie et al., 1979±1980; Bellanca and Neri, 1986; Pierre 1982). On the whole, these data support the interpretation of a deposition in closed hydrological systems submitted to unstable conditions predominantly under the control of climatic and/or local parameters. The main aim of our study was to better constrain the mechanisms and timing of the transition from the more or less open marine conditions to these closed systems characterized by the coeval deposition of carbonate and evaporites. 5.2. Enhancement of restricted conditions In the three sections investigated in this work, several environmental changes can be recognized by the variations of faunal, mineralogical and isotopic markers, but a major change is recorded by the simultaneous variation of these three types of markers. This is a drastic event that recorded the passage from near normal marine conditions to more restricted conditions which became inhospitable for the marine calcareous plankton and benthos. Although it is expressed by a similar biological and sedimentary response, this change did not occur coevally in the different areas, as it is dated 6.32 Ma (cycle 34) at Serra Pirciata, 6.15 Ma (cycle 42) at Torrente Vaccarrizzo, and 6.12 Ma (cycle 44) at Marianopoli. Before this change, similar faunal and sedimentary features as well as similar cyclic patterns characterized both the marginal sections and the deeper Falconara section, indicating that these different areas were widely connected to form a large marine basin with relatively stable hydrological conditions. The most important feature was undoubtedly the above mentioned cyclic variation of the biosiliceous productivity. The cyclicity is also recorded by variations in the relative abundance of the assemblages of planktonic foraminifera. The basal marls are characterized by assemblages typical of cold waters while the intermediary red laminated silty marls and the basal part of the diatomites are dominated by species indicative of warm waters. The abundance of N. acostaensis and G. bulloides in the rest of the diatomites can be linked to nutrient-rich waters. Similar variations have also been reported from other Sicilian sections, i.e. Falconara (Sprovieri et al., 1996a) and Capodarso (Suc et al., 1995). The oxygen isotopic composition measured

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on both the planktonic foraminifera (Van der Zwaan, 1979; Van der Zwaan and Gudjonsson, 1986) and bulk carbonates (McKenzie et al., 1979±1980; Pierre et al., 1997) also indicate that warmer or more diluted conditions occurred during the deposition of the intermediary laminites and basal diatomites. These data are in good agreement with the hypothesis of the relation of high biosiliceous productivity with inputs of nutrients related to increased runoff (Van der Zwaan, 1979; Van der Zwaan and Gudjonsson, 1986), even if the composition of the radiolarian assemblages would rather argue for a relation with upwelling systems (Caulet et al., 1997; Pierre et al., 1997). Although small contents of dolomite are present, the carbonate fraction of the sediment is dominated by calcite which is predominantly related to marine calcareous nanno- and microfossils associated with variable amounts of calcareous debris and small amounts of micrite. The oxygen isotope compositions vary in the range of marine carbonates, indicating moderate ¯uctuations of the dilution/evaporation balance. The slightly negative carbon-isotope compositions generally indicate a moderate contribution of organically derived carbon. The dolomite associated to calcite exhibits isotopic signature suggestive of normal to moderately evaporated marine waters. Other mineralogical and isotopic data obtained for dolomites from other Sicilian sections show that this phase precipitated interstitially from more evaporated solutions, indicating the incipient evolution towards increasing salinity (Bellanca et al., 1986). 5.3. Transition to semi-closed subbasins After the change, the marine nanno- and microplankton disappear abruptly while the carbonate fraction becomes generally characterized by a signi®cant increase of dolomite content which can be associated to calcite and/or aragonite. The small size and subeuhedral habit of the dolomite both argue for a synsedimentary to early diagenetic growth. The oxygen isotope composition of this mineral characterized by high d 18O values (up to 8½) clearly supports a precipitation from hypersaline ¯uids. The aragonite is present in this upper part of the Tripoli Fm and seems to be related to either biogenic or sedimentary particles (pteropods, oolites) or to inorganic precipita-

tion. In several sections, the oxygen isotopic composition of the calcite covers a wide range of values indicating large ¯uctuations of salinity from diluted to more concentrated conditions caused by large variations of the evaporation/dilution balance. Shallow water conditions are indicated by sedimentary (oolite-bearing layers) and faunal (benthic assemblages of Ammonia and Elphidium) markers, as well as by the extreme sensitivity of the waters to variations of external factors. The persistence of marine diatom assemblages along this interval, in the absence of other marine microorganisms, is a common feature previously reported in many other Messinian basins where typical marine diatomites persist up to the Calcare di Base and locally within the massive gypsum (Rouchy, 1982; Caruso et al., 1997). This indicates that inputs of sea water continued to episodically enter these lagoons by advection from the more central parts of the basin. At Falconara, for instance, this interval does not show any drastic change although the faunal assemblages display a reduction of diversity indicating the trend towards a progressive restriction (Sprovieri et al., 1996a; Pierre et al., 1997). On the whole these data argue for the rapid reduction of the connections between the studied areas and the open parts of the Sicilian Basin resulting in fragmentation into subbasins which started to evolve independently each other. 5.4. The onset of evaporitic conditions Near the base of the Calcare di Base Fm, the d 18O values of the dolomite of the Serra Pirciata section exhibit negative excursions (down to 24½) which can be interpreted as evidence of sedimentary to early diagenetic precipitation of the carbonate from diluted waters. Similar wide ¯uctuations of the isotopic composition of the carbonates were also observed in the uppermost cycle of the Tripoli Fm (cycle 52) underlying the Calcare di Base in the Falconara section where the d 18O values of the calcite range between 24 and 3½ (Pierre et al., 1997). Despite the importance of the dilution episodes, the dominant trend observed in the oxygen isotope composition argues for a global evolution towards hypersaline conditions. However, the salinity generally did not reach concentrations high enough to precipitate signi®cant volumes of evaporites, except for the

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local presence of disseminated lenticular crystals of gypsum (Bellanca et al., 1986). In general, the massive precipitation of gypsum occurred later, just before the deposition of the Calcare di Base at Torrente Vaccarizzo, but most commonly during the deposition of the Calcare di Base which frequently includes precursor beds of gypsum in addition to molds and pseudomorphs of gypsum, anhydrite and halite. Both calcite and dolomite exhibit predominantly negative d 13C values, already observed before the main environmental change, which are interpreted as due to the incorporation of organically derived carbon. The biogenic processes, which culminated during the deposition of the Calcare di Base, were very active in many other Mediterranean basins where organic-rich sedimentation and hypersaline conditions occurred simultaneously near the beginning of the Salinity Crisis (Rouchy, 1982; Rouchy et al., 1998). Based on the above, the evolution towards an environment characterized by predominant hypersaline conditions with episodic phases of refreshening (i.e. the typical scenario of the Calcare di Base deposition) initiated in the upper part of the Tripoli Fm. In the meantime, occasional inputs of marine waters still occurred as indicated by the local presence of marine diatomites.

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fossils and in the deposition of authigenic carbonates. Under these conditions, the water bodies underwent wide ¯uctuations of the hydrochemical parameters, ranging from diluted to hypersaline conditions with episodic inputs of marine waters, as recorded by the variations of the carbonate oxygen isotopic composition. However, the results point out a dominant trend towards hypersaline conditions that ended in the extensive precipitation of evaporites during the deposition of the overlying Calcare di Base. The diachronism of the restriction and onset of the evaporitic conditions in the Messinian Sicilian Basin has been interpreted by Pedley and Grasso (1993) and Butler et al. (1995, 1999) as due to the synsedimentary compressive tectonics which controlled the formation and evolution of perched marine basins developed in front of active thrusts. An alternative interpretation could be the progressive isolation of these marginal subbasins as a consequence of the Mediterranean water shallowing which started to affect the marginal areas and continued progressively till the isolation of the central areas where the hypersaline conditions developed later. This contribution leads to better constrain both the timing and mechanisms of the onset of the Salinity Crisis in the Mediterranean area that involved different degrees of severe restriction which occurred at different times according to the initial paleogeography and tectonic evolution of the various subbasins.

6. Conclusion The integrated analytical approach adopted in this study, which combines cyclostratigraphic correlations, variations of faunal composition, carbonate mineralogy, and stable isotope geochemistry, allowed us to characterize the successive steps in the evolution of the environmental conditions in the Sicilian Basin from near normal marine to hypersaline conditions and to re®ne the knowledge of the onset of the Messinian Salinity Crisis. The successive steps in the restriction occurred quickly and diachroneously through the different areas of the basin, affecting the margins before the central areas. With the increasing restriction the various subbasins tended to evolve independently each other developing speci®c depositional and hydrological conditions. The closure of these subbasins resulted in the development of conditions hostile to the life of the marine calcareous micro-

Acknowledgements This research was supported by grants MURST 40% to A. Bellanca, MURST 60% to R. Neri, MURST 60% to R. Sprovieri, and CNRS (ESA 7073) and BQR MuseÂum to J.M. Rouchy and M.M. Blanc-Valleron. Thanks are due to P. CleÂment for assistance in XRD. The work bene®ted from discussions and help by R. Sprovieri, E. Di Stefano, C. Pierre, C. Taberner and J.P. Caulet. The authors would like to thank C. Pierre, C. Taberner, and an anonymous reviewer for their valuable comments and criticism which greatly improved the initial version of the manuscript.

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