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Integrated Stratigraphy of Messinian Evaporites in the Deep Eastern Mediterranean Reveal Long Lasting Halite Deposition During Atlantic Connectivity

Abstract

The Messinian Salinity Crisis (MSC) is considered to be an extreme environmental event driven by climate and tectonism, which affected global ocean salinity and shaped the biogeochemical composition of the Mediterranean Sea. Yet, after more than 50 years of research, the MSC stratigraphy is still controversial. Recent studies agree that the transition from the underlying pre-evaporites to thick halite deposits is continuous in the Levant Basin. However, the age of the base and the duration of halite deposition are still unclear. The nature of the upper MSC unit, characterized from seismic data as a period of increased clastic deposition into the eastern Mediterranean, is still disputed. We report a multidisciplinary study of sedimentary and geophysical data from industrial offshore wells in the Levant Basin, which recovered a continuous sedimentary record of deep-basin Mediterranean evaporites deposited during the MSC. Remarkably, the evaporite part of the 1,600 m thick unit is composed solely of halite, other than a thin transitional anhydrite layer at its base. The halite is undisturbed and homogeneous, lacking diverse features apparent in more proximal sections, indicating a deep sea depositional environment. Distinct several meters thick non-evaporitic intervals interbedded with the halite, previously thought to be clastic layers, are identified here as diatomites. While XRD analysis confirms an increase in clastic components, these sediments are composed primarily of very well preserved marine and freshwater planktonic diatom species. The occurrence of marine planktonic diatoms, reported only from Pre-Evaporite and Lower Gypsum marginal deposits, indicates Atlantic input to the Mediterranean basin during early stages of the MSC. Seismic and well log cyclostratigraphy further support deep basin halite deposition, which started about 300 kyr earlier than widely believed. Consequently, our results on lithological, faunal, geophysical, and geochemical proxies indicate that halite deposition in the deep basin took place during stage 1 and 2 of the MSC, rather than being limited to the short 50 kyr MSC acme when sea level was at its lowest. Thus, brine formation, salt precipitation, and faunal extinction occurred at least in part in a deep, non-desiccated basin, with a restricted Mediterranean-Atlantic connection that allowed inflow of oceanic water. We observe an increase in heavy minerals and reworked fauna within the clastic-evaporitic upper part of the basinal MSC section, correlating well to the Upper Gypsum and Lago-Mare interval in marginal sections. This correlation is also substantiated through chemostratigraphic markers, such as n-alkane distribution and maturity indices as indicated by sterane and hopane biomarkers. Our results modify the current understanding of the mechanisms governing salt deposition throughout the MSC in particular and the geological record in general.