Il contributo delle ittiofaune alla definizione paleoambientale e stratigrafica della fase post-evaporitica del Messiniano. The contribution of the ichthyofauna in the paleoenvironment and stratigraphic definition of the Messinian post-evaporitic phase.

Thanks to the hypothesized spectacular catastrophic scenario, the Messinian salinity crisis has represented one of the most discussed topic in Earth Sciences in the past 30 years. The possibility that during the Messinian the Mediterranean was at times completely desiccated has received considerable attention in the scientific community and stimulated an unusual quantity of multidisciplinary researches, which generated cogent and often conflicting debates. The amazing development of stratigraphic techniques in the last decade has greatly increased our knowledge of the events related to the Messinian salinity crisis. The refinement of the Neogene bio- and magnetostratigraphy in the Mediterranean region and the consequent improvement of a detailed astronomically-tuned time scale have provided a useful tool for high resolution correlations and paleogeographic reconstructions (Hilgen et al., 1999). Moreover, the accurate correlations available have favoured the interpretation of a reliable sequence of environmental changes and their relationships with global and regional climatic and tectonic events (Krijgsman et al., 1999; Hodell et al., 2001; Rouchy and Caruso, 2006). Based on these studies the base of the Messinian (Tortonian-Messinian boundary) has been fixed at 7.25 Ma and its end (Messinian-Zanclean boundary) dated at 5.33 Ma (Hilgen et al., 2000). The top of this stage corresponds to the so-called ‘Terminal Messinian Flooding', a catastrophic return to fully marine condition synchronous in the whole Mediterranean. The well developed and widely distributed cyclic arrangement of the Messinian deposits suggested the classical three phase subdivision (see Krijgsman et al., 1999): 1) Pre-evaporitic phase, characterized by cyclic diatomite-sapropel sedimentation that affected the basin in response to the precessional forcing of ocean stratification (7.25-5.96 Ma); 2) Evaporitic phase (=Lower Evaporites; 5.96-5.59 Ma); 3) Post-evaporitic phase (5.52-5.33 Ma). The second and the third phases correspond to the Salinity Crisis event. The stratigraphic resolution within the three phases is very different, mainly because of the lack of biostratigraphic control in the evaporitic and post-evaporitic phases, classically included in the so-called ‘Non Distinctive Zone'. The lack of biostratigraphic control limits the possibility to obtain a detailed stratigraphic framework for this interval. Moreover, the scarcity of fossils and the heterogeneity of sedimentary products that characterizes the Salinity Crisis-related deposits makes difficult the understanding of the physico-chemical and palaeoenvironmental conditions of the waterbody during the span of this event and the resulting palaeogeographical setting of the Mediterranean. According to the original hypotheses of Messinian palaeoenvironmental evolution the isolated Mediterranean was affected, during the evaporitic phase (Lower Evaporites), by a cyclic evaporative draw down of the sea level that shallowed up to the complete desiccation. As a consequence of the cyclic progressive desiccation, an increased concentration of the brines culminating in the deposition of sulphate-rich evaporites widely occurred in the basin. The evaporitic phase was followed by a short time interval characterized by non-deposition and erosion (Messinian gap), and by the subsequent deposition of the non-marine sediments of the ‘Lago-mare' event (Krijgsman et al., 1999). The progressive sea-level drop and the successive esiccation of the basin were responsible for the drastic change of the drainage pattern in the peri-Mediterranean region, resulting in the partial refill with fresh- and brackish waters of Paratethyan origin (e.g. Cita et al., 1978; McCulloch and DeDekker, 1989). The post-evaportic ''Lago-Mare'' phase in the Mediterranean realm was used initially to describe a brackish to freshwater environment characterized by a fauna of molluscs (''Congeria beds'', or ''Melanopsis beds'') in sediments deposited ending the Messinian Salinity Crisis immediately prior to Pliocene marine re-flooding. Thus, the desiccation model implies that during the Salinity Crisis the Mediterranean waterbody was characterized by dramatic and rapid environmental changes, from hypersaline to completely dried out, to hyposaline. This study is focused on the post-evaporitic ‘Lago-mare' phase and provides a new palaeoenvironment interpretation of the Mediterranean waterbody during such a interval of the Messinian history using fossil fish remains. Up to date, the interpation of the paleoecological setting of the Mediterranean Basin during the evaporitic and post-evaporitic ‘Lago-mare' phase, has been based analysis of the microfaunas (foraminifera and ostracods) and macroinvertebrates (molluscs). The integrated palaeoecological analysis of the entire fossil assemblage (benthic foraminiferans, ostracods, molluscs, fishes) highlights the key role of fish remains as palaeoenvironmental indicators. Because of their mobility and frequent migratory behaviour, marine fishes can co-occur with fresh- or brackish-water organisms, and, at the same time, provide useful information for the interpretation of the palaeoenvironmental conditions of the areas surrounding those testified by the resident biota. The new paleoichthyological approach presented herein has been performed in four different geodynamic contexts, representative of four different depositional environments, in order to provide a reliable and comprehensive large-scale interpretation. The main objective of this paper is to investigate the fossil fish remains, mostly otoliths, collected from the ‘Lago-mare' deposits of five localities in the Italian peninsula. In this Thesis I have analysed eleven stratigraphic section referred to the postevaporitc ‘Lago-mare' phase. Five of these sections resulted productive for such a study. As documented above, all these sections are representative of a different geodynamic context: Capanne di Bronzo section (Marche basin) is typical of the foreland basin system; Cava Serredi and Podere Torricella (Tuscany basin) are typical to the back-arc basins active since Tortonian times; Le Vicenne (Abruzzi basin) (Cipollari et al., 1999a) and Ciaba'²t Cagna (Piedmont basin) (Gelati and Gnaccolini, 2003) are refered to the small basins located in the inner sector of the Apenninic chain. Otoliths are calcareous structures associated with the ear of fish involved in vestibular and acoustic functions (Popper and Fay, 1993). Although fishes posses three pairs of otoliths, only the largest, the Sagitta, is commonly used in paleontological studies. These calcareous structures are often characterized by a wide interspecific morphological variability, and for this reason are sometime used in species-levels taxonomic study. Fossil otoliths are very common in Cenozoic and Quaternary marine and brackish deposits, representing the most common remains of teleost fishes (Nolf, 1985). Because of their abundance and diversity, fossil otoliths usually provide a large amount of paleoecological and paleogeographical data which are necessary for paleoenvironmental reconstruction (Nolf, 1985). The otoliths usally are been presents in the fossiliferous horizons where the carbonate structure, such as molluscs, bryozoan, corals, etc., are to be well-preserved. In the fossiferous horizons here studied the specimes are extremely well-preserved and fine structures useful for taxonomic determinations are usually clearly recognizable. Because of the excellent preservation of the otoliths, we conclude that the fossil assemblages documented herein reflects autochthonous embedding, thereby excluding the possibilities of reworking from older rocks and contamination from younger deposits. This is also suggested by the preservation of delicate mollusc shells and by the mutual occurrence of ostracod valves at different growth stages. The systematic and palaeoecological analysis have revealed a well diversified fish assemblage, composed by 5517 specimens, among which 44 taxa have been recognized. From an ecological point of view, the total assemblage consists of four different ecological guilds: anadromous, estuarine residents, marine migrants and marine stragglers. The palaeoecological analysis reveals a coastal shallow marine environment strongly influenced by unsteady continental outflow in the deposizional environment recorded in the section here analysed. Based on the the palaecological and palaeoenvironmental data, all depositional areas here considered are closely associated to a basin characterized by normal marine waters. These findings clearly demonstrate that fishes, because of their mobility and migratory behaviour, represent a useful tool for the large-scale interpretation of the environmental conditions of the Messinian Mediterranean waterbody. The repetitive occurrence, in the stratigraphic sections analysed herein, of well diversified assemblages of marine stenohaline and euryhaline fishes necessarily implies that a basin characterized by marine water was present close to the depositional environments. The highly diverse taxonomical composition and the ecological disparity that characterize the fish assemblages of these localities suggest that the Mediterranean Basin has already had a complex and fully developed ecological structure during the late Messinian. Moreover, the statistic analysis carried out on the fish assemblages testify the presence of a marine environment linked to the depositional setting recorded in the stratigraphic section here analysed. This palaeonvironment model is in contrast with the classical model (Hsa'¼ et al., 1973, 1977) that suggested the widespread development of brackish/oligohaline conditions in the Mediterramean Basin during the post-evaporitic phase. In order to better define the paleontological significance of these upper Messinian fish assemblages, the oxygen, carbon and strontium isotopic composition of fish otoliths and other fossils (ostracods and molluscs) has been measured. This measurements is in agree with the palaeoenvironmental analysis that indicate the presence of marginal brakish environment linked to the normal marine condition in the Mediterranean Basin. The benthic macro- and microfauna characteristic of the fossiliferous horizon is clearly indicative of a brackish environment during the post-evaporitic phase. Actually, the presence of the marine taxa in the fish assemblage that characterized the brackish environment testified the permanently connection with a basin characterized by normal marine water body. The common occurrence of estuarine resident and the presence of marine fishes is evidently consistent with the hypothesis of a transitional environment characterized by thalassogenic waters, thereby confirming that the ichthyofaunal assemblages can be considered as a useful complementary tool in defining the levels of marine influence on brackish biotopes (see Mariani, 2001). The interpretation of the composition of the waterbody during the deposition of the post-evaporitic phase has been largely debated and several studies of isotope geochemistry, clay mineralogy, and palaeontology (foraminiferans, ostracods, molluscs) have been produced, resulting in a rather confused scenario of fresh-, brackish-, oligohaline or mesohaline waters. The paleontological and geochemical analysis of fossil fish remains clearly indicates that a new palaeoenvironmental interpretation of the Messinian ‘Lago-mare' event is necessary, and provides an unquestionable demonstration that the re-establishment of normal marine conditions in the Mediterranean preceded the Mio-Pliocene boundary. The possibility of an intra-Messinian marine reflooding has been proposed by several authors (e.g. Butler et al., 1995; Riding et al., 1999; Pedley and Maniscalco, 1999; Steffahn and Michalzik, 2000; Griffin, 2002; Aguirre and Sa'¡nchez-Almazo, 2004), and evidences of at least periodical marine influxes are rather common in the ‘Lago-mare' sedimentary record (e.g. Iaccarino and Bossio, 1999; Iaccarino et al., 1999; Rouchy et al., 2001; Crescenti et al., 2002; Clauzon et al., 2005). However, the hypothesis of pre-Pliocene reflooding has never been commonly accepted, as the proposed evidence has been considered weak or inadequate (e.g. Fortuin and Krijgsman, 2003). Fossil fish remains indicate that the rapid and catastrophic Pliocene inundation from the Atlantic was preceded by a progressive transgression that re-established the marine biotic communities. These findings are in agreement with the scenario postulated by Keogh and Butler (1999) who suggested that during the ‘Lago-mare' phase the base-level of the Mediterranean was within the range of the world's oceans even though the waterbody probably had a different but internally homogeneous isotopic composition.