Onset of the Messinian Salinity Crisis: Insights from molecular organic geochemistry

Abstract

The Messinian Salinity Crisis (MSC) generated extensive anoxic sediments under the evaporitic deposits in marginal Mediterranean basins. In an interdisciplinary approach combining field work with multiple laboratory techniques, the oil source rock potential and biomarkers of pre-evaporitic Messinian sediments from the Caltanissetta Basin (Sicily) were evaluated using Rock-Eval pyrolysis, gas chromatography-mass spectrometry, isotopic analysis and optical and electron microscopy. Organic-rich samples from the 45 m Serra Pirciata section consist of cycles of diatomaceous and carbonate layers alternating with organic shales of variable TOC. The organic matter is mostly optically amorphous, increasing in content upwards in the section. Of the 43 samples analyzed within the section, an especially organic- and sulfur-rich interval shows TOC values up to 10 %, and oil generation potential (Rock-Eval S2) up to 75 mg HC/g rock. Samples show Rock-Eval Hydrogen Indices (HI) up to ca. 800 mg HC/g TOC, but most fall in the range of 300–600 mg HC/g TOC, indicating type II kerogen. Within this section, a specific interval is especially remarkable for its unusual biomarker assemblage, having abundant sulfur compounds including isoprenoid, steroid, and hopanoid thiophenes and thiolanes, as well as microscopic sponge fossils. We propose that this organic-rich interval with its distinctive biomarkers represents an initial episode of basin restriction and, based on cyclostratigraphic interpretations, constitutes a non-evaporitic record of the onset of the MSC in the deeper part of the Caltanissetta Basin, where gypsum deposits were initially absent. If the depocenter of the Caltanissetta Basin is an analog for the deep Mediterranean basin, these results suggest that a large volume of organic matter of bacterial origin accumulated in the deep Mediterranean prior to evaporitic deposition.