Chronology with a pinch of salt: Integrated stratigraphy of Messinian evaporites in the deep Eastern Mediterranean reveals long-lasting halite deposition during Atlantic connectivity

Abstract

The Messinian Salinity Crisis (MSC; 5.97–5.33 Ma) is considered an extreme environmental event driven by changes in climate and tectonics, which affected global ocean salinity and shaped the biogeochemical composition of the Mediterranean Sea. Yet, after more than 50 years of research, MSC stratigraphy remains controversial. Recent studies agree that the transition from the underlying pre-evaporite sediments to thick halite deposits is conformal in the deep Eastern Mediterranean Basin. However, the age of the base and the duration of halite deposition are still unclear. Also disputed is the nature of the intermediate and upper MSC units, which are characterized as periods of increased clastic deposition into the Eastern Mediterranean based on marginal outcrops and seismic data. We provide a multidisciplinary study of sedimentary, geochemical, and geophysical data from industrial offshore wells in the Levant Basin, which recovered a sedimentary record of deep-basin Mediterranean evaporites deposited during the MSC. In combination with previous observations of the MSC throughout the Mediterranean Basin, our results promote the need for a new chronological model. Remarkably, the one-kilometer-thick lower part of the evaporitic unit is composed of essentially pure halite, except for 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. We find that distinct, meters-thick non-evaporitic intervals interbedded with the halite, previously thought to be clastic layers, are diatomites. While XRD analysis confirms an increase in clastic components in these sediments, they are composed primarily of well-preserved marine and freshwater planktonic diatoms. The occurrence of marine planktonic diatoms in these intervals indicates the input of Atlantic waters into the Mediterranean Basin during the deposition of the massive halite unit. Seismic stratigraphy and well-log cyclostratigraphy further support deep basin halite deposition, which started about 300 kyr earlier than widely assumed (~5.97 Ma). We propose that halite deposition in the deep Mediterranean took place during stage 1 of the MSC, rather than being limited to the short 50 kyr MSC acme when sea level was presumably 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 yet open Mediterranean-Atlantic connection that allowed inflow of oceanic water. We observe an increase in heavy minerals and reworked fauna within the clastic-evaporitic, Interbedded Evaporites of the basinal MSC section, and argue that these settings correspond in the deep basins with a significant sea-level drawdown during stage 2 of the MSC, as observed in the marginal sections. This correlation is corroborated by astrochronology and chemostratigraphic markers, such as the distribution of n-alkanes and biomarker-based thermal maturity indices. The Levant deposits indicate that high sea level and partial connectivity with global oceans promoted the deposition of deep-basin deep-water halite, while sea-level drawdown promoted deposition of reworked and transported material from the margins into deep Mediterranean basins. This study modifies the current understanding of the mechanisms governing salt deposition throughout the MSC with implications for other evaporitic events in the geologic record.