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Large deposits of gypsum accumulated in the marginal basins of the Mediterranean Sea during the Messinian Salinity Crisis. These form the marginal portions of the Mediterranean Salt Giant (MSG) that also occupies the deep, central Mediterranean basins. Although the marine, evaporitic origin of the MSG is undisputed, the analysis of gypsum fluid inclusions and of gypsum-bound water (d18OH2O and dDH2O) suggest that marginal basin gypsum formed from low- to moderate-salinity water masses (5 - 60 ‰), rather than from high-salinity brines (130 - 320 ‰), as expected during the evaporation of seawater. We present a new set of water isotope and fluid inclusion salinity data that extends the low salinity signature of gypsum to include five Mediterranean Sea marginal basins: Caltanissetta Basin (Sicily), Sorbas Basin (Spain), Piedmont Basin and Vena del Gesso Basin (northern Italy) and Catanzaro Trough (Southern Italy). With a simple geochemical model we explore the salinity-d18OH2O-dDH2O evaporation path and the 87/86Sr and d34SSO4 composition of the Mediterranean Sea subject to a variety of evaporation conditions and mixing ratios with continental runoff. This approach suggests that evaporation and mixing with continental runoff - including freshwater transiting via the Paratethys - cannot lead to the observed geochemical signature of MSC gypsum deposits. An alternative process that decouples the saturation state with respect to gypsum from salinity must have been active. We are exploring the possibility that the biogeochemical sulfur cycle leads to spatially and temporally localized gypsum supersaturation conditions via the production of SO42- by the oxidation and disproportionation of reduced sulfur compounds.

The fresh new cores 3AGN2S02 and 3AGN2S04 located in the deformed foredeep of the Gela Thrust System, locally known as Caltanissetta Basin, represent an opportunity for a better comprehension of the Messinian events, as well as for the reconstruction of the Sicilian evaporitic Basin architecture. The entire ‘early Messinian stage’ (7.2-5.96Ma) preceding the Messinian Salinity Crisis (MSC) has been already investigated in the Caltanissetta Basin. Even though the Tripoli Formation and ‘Calcare di Base’ (‘CdB’) have been widely studied for a long period of time, many aspects remain unclear. The ‘CdB’ has been commonly considered to represent the first evaporitic unit of the Messinian succession in Sicily. Different ages obtained in the underlying Tripoli deposits from various Sicilian outcrops display a diachronous onset of the MSC (Rouchy & Caruso, 2006). However, Manzi et al. (2011) propose an alternative interpretation for the ‘CdB’, suggesting that it does not belong exclusively to the onset of the MSC, but it is made of three carbonate facies belonging to different MSC stages. A detailed sedimentological, geochemical and petrographic study of the two cores allowed us to evidence the paleoceanographic changes that affected the central Mediterranean Sea during the transition from marine to restricted conditions, up to the onset of the MSC, and to observe the differences between the marginal and the deep basins of the Caltanissetta Basin, enhanced by the ongoing regional tectonics. Facies characterization made it possible to confirm the nature of the sediments of the cores, reflecting distinct depositional environments. A lithological transition passing from the Tripoli Formation to the complex ‘CdB’ carbonates alternating with shales is observed (3AGN2S04). This CdB appears to be laterally equivalent to gypsum and salts at site 3AGN2S02. In the brecciated facies of the ‘CdB’, evaporite pseudomorphs are also present, implying early stage diagenesis. Furthermore, our analyses gave us insights of strong oscillations in hypersaline conditions with freshwater inputs controlled by Milankovitch’s cycles. Moreover, the 3AGN2S04 core is characterized by the repetition of sedimentary successions due to the later development of a thrust system, which can be an important hint concerning the morphological and structural evolution of the Caltanissetta Basin. These new data are fundamental for stratigraphic reconstructions, comparing them with the already well-calibrated reference section of Falconara-Gibliscemi but also with other outcrops located in the various depocenters of the Caltanissetta Basin. The local transition from the uppermost part of the Tripoli cycles to the ‘CdB’ reflects the worsening of the marine connections, implying that during late Messinian broadly constant stressed environmental conditions existed in the central Mediterranean shelves. We conclude that since the onset of the MSC, marine inputs were not important enough to balance the effects of the climate fluctuations and the evaporation/precipitation budget in the individualized semi-closed settings.

A detailed biostratigraphical and cyclostratigraphical study provided the opportunity of cycle-by-cycle correlations between sections from the marginal and deep areas of the Caltanissetta Basin (Sicily), and the northern Calabrian Rossano Basin. All the sections were compared with the Falconara-Gibliscemi composite section. We present new mineralogical and geochemical data on the transition from Tripoli to Calcare di Base (CdB), based on the study of several field sections. The outcrops display good record of the paleoceanographical changes that affected the Mediterranean Sea during the transition from slightly restricted conditions to the onset of the Mediterranean Salinity Crisis (MSC). This approach permitted to better constrain depositional conditions and highlighted a new palaeogeographical pattern characterized by separated sub-basins. The sedimentological and geochemical parameters of these basins introduced a different and diachronous response to the global constraints of the MSC. Our preliminary results display already evidences of paleoenvironmental changes: (1) a lithological transition passing from the Tripoli’s triplet (grey marls, reddish laminites and diatomites) to the complex carbonates of CdB; (2) the appearance of evaporite pseudomorphs implying early stage diagenesis; (3) the presence of sulphur-rich deposits involving process of bacterial sulphate reduction. The local transition from the uppermost part of the Tripoli cycles to the CdB reflects the worsening of the marine connections, leading to the individualisation of semi-closed settings where the marine inputs were not great enough to balance the effects of the climate fluctuations and especially of the evaporation/precipitation budget.

The Messinian Salinity Crisis (MSC) is a prominent and still misunderstood event that influenced the Mediterranean basin in the late Miocene leaving behind a Salt Giant (SG) widespread all over the Mediterranean basin. More than 90% of the Messinian Evaporitic deposits are located offshore with reduced access via boreholes, and thus has been studied mainly by seismic imaging. Onshore-Offshore should be considered a key for a better understanding and answering some of the controversies on the MSC.The Balearic Promontory (BP) contains a series of small perched basins presently lying at different water depths, stepped from the present-day coastline down to the deep basin. These topographic lows trapped sedimentary series up to 500m thick, interpreted as MSC in age.The reduced tectonic movements in the BP since the late Miocene (Messinian) till recent days, favored the conservation of the MSC records in this area. Moreover, recent studies revealed the presence of a Salt layer in the Central Mallorca Depression (CMD).Considering: 1- the bathymetry of the BP, classified as an intermediate perched basin; 2- the distribution of the MSC records accumulated in a series of sub-basins more or less connected between each other; 3- the geometries of the evaporitic formations, provided by how these records appear on the seismic data; this might recall similarities between the BP records (especially the ones in the CMD) and the MSC reference records outcropping in Sicily (especially in the Caltanissetta Basin).We perform seismic interpretation of a wide seismic reflection dataset in the study area with the aim of refining the mapping of the Messinian evaporites covering the study area. Four seismic units were identified in the BP based on their seismic facies and their seismo-stratigraphic position. We try to match up these units to the consensus Messinian 3-stages chrono-stratigraphic model proposed during the CIESM in 2008.We also attempt to find similarities in geometries, facies and distribution of the MSC between the sub-basins of the BP and those described in the Sicilian sub-basins.

Large deposits of gypsum accumulated in the marginal basins of the Mediterranean Sea during the Messinian Salinity Crisis. These form the marginal portions of the Mediterranean Salt Giant (MSG) that also occupies the deep, central Mediterranean basins. Although the marine, evaporitic origin of the MSG is undisputed, the analysis of gypsum fluid inclusions and of gypsum-bound water (d18OH2O and dDH2O) suggest that marginal basin gypsum formed from low- to moderate-salinity water masses (5 - 60 ‰), rather than from high-salinity brines (130 - 320 ‰), as expected during the evaporation of seawater. The formation of low-salinity gypsum poses a fundamental geochemical problem: how can gypsum saturation conditions be met in marginal basins if evaporation does not concentrate marine water to high salinity? In other words, can gypsum saturation be attained by adding Ca2+ and/or SO42- ions rather than by extracting water? We are exploring two geochemical scenarios to explain this phenomenon: (1) the addition of Ca2+ and SO42- to marginal basins by continental runoff, and (2) the non-steady state addition of SO42- to marginal basins via the biogeochemical oxidation of reduced sulfur. Both scenarios may lead - at least theoretically - to the decoupling of saturation state from salinity that is suggested by gypsum geochemical signatures.

Sulfur is a key element to understand ocean biogeochemical processes. Since decades, numerous studies have explained sulfur isotopic variations registred in the geological record as reflecting changes in continental sulfur inputs, biogeochemical recycling of sulfur and the relative proportion of oxidized (gypsum) vs reduced (e.g. pyrite) sulfur burial fluxes. Geochemical and petrographic studies have showed that these processes were active during the formation of the Mediterranean Salt Giant (MSG), a giant salt deposit formed at the end of the Messinian period (5.9-5.33 Ma) following the restriction of hydrological exchanges between the Mediterranean Sea and the Atlantic Ocean. To date, the biogeochemical sulfur cycle during the formation of the MSG has been investigated by analyzing the sulfur and oxygen isotope composition (δ34S and δ18O, respectively) of the sulfate ion in gypsum accumulated in the deep and marginal Mediterranean basins. In the uppermost gypsum layers (Upper Gypsum unit), significantly higher δ18O isotopic ratios (averaging at 12,7‰) than Messinian marine values suggest implications of microbial sulfate reduction activity followed by complete re-oxidation of sulfide back to sulfate in evaporated marine waters. Nevertheless, these different microbial processes can overprint each other δ34S and δ18O isotopic signatures and could have been provoked by various type of microbial metabolisms, involving different hydrological and environmental conditions. Here we present for the first time a multiple sulfur isotope (δ34S, Δ33S, Δ36S) investigation of samples from the well-known sections of Vena del Gesso (Apennines) and Pollenzo (Piedmont basin) in order to identify and understand how microbial mechanisms were coupled during the MSG formation. We designed a simple steady-state, three-box model representing the analysed S-bearing fractions (SO42-, S0, FeS2) and the different hydrological and biogeochemical S fluxes involved in marginal basin S-cycling. This system of 18 equations allows us to explain the strong isotopic variations we measured (-40.2 to 25.4‰ in δ34S, -0.001 to 0.160‰ in Δ33S and -1.79 to 0.001 in Δ36S‰) as produced by a huge variability in sulfate reduction activity reflecting fluctuations in the availability of organic matter. Moreover, our results, with relatively high λ33net (0.513 to 0.516) suggest than more than 90% of the hydrogen sulfide produced was re-oxidized by disproportionation reactions. Large, cyclic fluctuations of the Mediterranean hydrological cycle, and the presence of easily accessible S-compounds with a variety of oxidation states, makes the MSG a key system to understand the dynamics of the S biogeochemical cycle in the geological past.