Abstract
AbstractDuring Messinian time, the Mediterranean underwent hydrological modifications culminating 5.97 Ma ago with the Messinian salinity crisis (MSC). Evaporite deposition and alleged annihilation of most marine eukaryotes were taken as evidence of the establishment of basin-wide hypersalinity followed by desiccation. However, the palaeoenvironmental conditions during the MSC are still a matter of debate, chiefly because most of its sedimentary record is buried below the abyssal plains of the present-day Mediterranean Sea. To shed light on environmental change at the advent and during the early phase of the MSC, we investigated the Govone section from the Piedmont Basin (NW Italy) using a multidisciplinary approach (organic geochemical, petrographic, and carbon and oxygen stable isotope analyses). The Govone section archives the onset of the crisis in a succession of organic-rich shales and dolomite-rich marls. The MSC part of the succession represents the deep-water equivalent of sulphate evaporites deposited at the basin margins during the first phase of the crisis. Our study reveals that the onset of the MSC was marked by the intensification of water-column stratification, rather than the establishment of widespread hypersaline conditions. A chemocline divided the water column into an oxygen-depleted, denser and more saline bottom layer and an oxygenated, upper seawater layer influenced by freshwater inflow. Vertical oscillations of the chemocline controlled the stratigraphic architecture of the sediments pertaining to the first stage of the MSC. Accordingly, temporal and spatial changes of water masses with different redox chemistries must be considered when interpreting the MSC event.
Highlights
During Messinian time (7.25–5.33 Ma) the Mediterranean area underwent progressive environmental and hydrological changes, which culminated c. 5.97 Ma ago with the Messinian salinity crisis (MSC; Hsü et al 1973; Roveri et al 2014; Camerlenghi & Aloisi, 2020)
The powders were split in two aliquots: one fraction was heated to 1350°C and total carbon (TC) contents were measured using a LECO SC-144DR Carbon Analyser equipped with an infrared detector
Archaeol and Glycerol dibiphytanyl glycerol tetraethers (GDGTs) distribution and caldarchaeol/ crenarchaeol ratio GDGTs and C20-20 archaeol were detected in the Govone samples. The former are mainly represented by GDGT0 and GDGT-5, with relative contents ranging from 18.0% to 31.2% and 18.3% to 32.9% of the total GDGTs plus archaeol assemblage, respectively (Fig. 6, Table 1; online Supplementary Material, available at http://journals. cambridge.org/geo)
Summary
During Messinian time (7.25–5.33 Ma) the Mediterranean area underwent progressive environmental and hydrological changes, which culminated c. 5.97 Ma ago with the Messinian salinity crisis (MSC; Hsü et al 1973; Roveri et al 2014; Camerlenghi & Aloisi, 2020). The analysis of molecular fossils (lipid biomarkers) preserved in Messinian evaporites and their intermediate- to deep-water lateral equivalents provides fundamental information for the reconstruction of the Mediterranean hydrological cycle and palaeoenvironments (Vasiliev et al 2017; Natalicchio et al 2019; Sabino et al 2020) This approach is a valuable tool to unveil the conditions in the water column during the MSC. The onset of the crisis is archived in a sequence of organicrich shales and marls, representing the deep-water equivalents of primary sulphate evaporites deposited at the basin margins (Gennari et al 2020; Sabino et al 2020) These sediments have recently been investigated to reconstruct the palaeoclimate and palaeohydrologic variability in the northern Mediterranean across the onset of the MSC (Sabino et al 2020). This approach allows us to reconstruct the environmental changes across the onset of the MSC, and reveals how these changes influenced the stratigraphic architecture of the sediments deposited during times of change in the Mediterranean realm
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