Ciclicità sub-milankoviana nelle Evaporiti Messiniane: analisi sedimentologiche, petrografiche, geochimiche e geochimico-isotopiche

An integrated multi-proxy study (stratigraphy, facies analysis, petrography, sedimentology, isotope geochemistry, microbiology) of the different geological unit deposited during the Messinian salinity crisis (MSC), is here proposed. The main purpose of this research is the reconstruction of the paleoceanographic and paleoenvironmental conditions that controlled this event and the identification and investigation of different depositional cyclicities at milankovian and sub-milankovian scale. The main results of this multidisciplinary investigation aimed at a number of issues of the Messinian salinity crisis are here summarized. Pre-MSC – Three different geochemical proxies (87Sr/86Sr, δ18O, δ13C) were investigated in order to reconstruct the conditions of the Mediterranean at the onset of the MSC. Fifty samples, consisting of planktonic foraminifers, evaporites and mollusks belonging to different sections of the Sorbas basin were analyzed. The isotopic data show a long-term trend with the major part of values plotting within the Global Ocean range and distinct oscillations at precessional scale. These variations reflect an important influence of continental fresh water during the humid phase of the precessional cycle. These results suggest that the Western Mediterranean was efficiently connected with the Atlantic ocean before and during the first stage of the MSC. Different climate-driven forcings played a fundamental role in the evolution of water geochemistry, in sediments deposition and in the ecological response of planktonic communities of the Sorbas basin. 1st Stage MSC – 87Sr/86Sr high resolution profiles were realized for five cycles of the Primary Lower Gypsum unit (PLG) of the Vena del Gesso basin (Northern Apennines). The new values show a geochemical trend shifting from the Global Ocean Sr signature, suggesting a progressive paleohydrologic isolation of the basin. The trend of values within a single cycle permitted to identify an astronomical control on the precipitation of gypsum and show geochemical oscillations at sub-milankoviana scale. Different type of lithological cyclicity were identified in different primary selenite crystals. Spectral analyses on the thickness variations of these phases of growth suggest a strict relationship with pluriannual climate forcings. Micro-sedimentological studies, the relation between the number of growth phases and the total thickness of a single gypsum bank and the clear presence of a pluriannual climate forcings suggest that these growth bands represent episodes of annual deposition and that the gypsum deposition within a single precessional cycle represent less than 3000 years. Several other PLG outcrop were geochemically (87Sr/86Sr) characterized. Furthermore, the biological content of PLG crystals was studied, in order to confirm the high potential of these rocks as a biological fossil archive. 2nd Stage MSC – The investigation of the lower unit of the Realmonte (AG) salt deposit permitted to identify more than 280 depositional cycles, consisting of the alternation of clay minerals, polyhalite, halite and kainite, characterized by a seasonal cyclicity. Spectral analysis on thickness variations shows the influence of pluriannual climate forcings on salt deposition. The deep Messinian deposits of the Levantine basin were geochemically (87Sr/86Sr) characterized. These analyses allowed to verify that the salt deposit belong to the second stage of MSC and that the basal portion of the investigated sequence belong to the pre-MSC stage. These values provide an important support to the interpretation of well logs and seismic data in order to reconstruct this Messinian succession. 3rd Stage MSC – The geochemical (87Sr/86Sr) study of different foraminifer assemblages found within hypoaline successions of the Lago Mare period permitted to demonstrate that the Mediterranean sea could be sporadically connected with the Atlantic ocean during the last stage of the MSC.