Origin and age of the Mediterranean Messinian evaporites: implications from Sr isotopes

Abstract

Strontium isotope ratios from the Messinian (late Miocene; 6.4 to 4.85 Ma) evaporites of the Mediterranean basin [1,2] support a subdivision into stratigraphically lower, predominantly marine deposits (lower evaporites = LEV: lower gypsum and main salt) characterized by Sr isotopic ratios generally equivalent to those of Messinian seawater, and stratigraphically higher deposits (upper evaporites = UEV), with 87Sr/ 86Sr ratios lower than Messinian seawater. The range of 87Sr/ 86Sr ratios of the LEV unit from the eastern Mediterranean overlap with values obtained from essentially coeval Red Sea evaporites [3]. Strontium isotopic ratios measured from the upper evaporites, however, are lower, and a high proportion of this Sr was probably derived from low 87Sr/ 86Sr continental runoff that characterizes much of this basin [4]. These results, therefore, strongly support depositional models utilizing a deep desiccated basin [1] with a constant marine inflow during the early stages, including salt formation, followed by a series of long-lasting and sometimes isolated desiccation events that produced a strong, but variable, continental influence on the Sr isotopic compositions of the UEV. Strontium isotopic measurements can also be used to asses the actual timing of the deposition of the LEV. Ratios obtained from oyster fragments and foraminifers from pre-evaporitic units at ODP Site 654 (Tyrrhenian Sea) suggest that the major rifting phase in this region occurred between 8 and 6.5 Ma. Ratios obtained from the early LEV from the Mediterranean Sea suggest that initiation of evaporite deposition began about 5.65 Ma ago. Results from units immediately overlying the UEV in Sicily suggest termination of evaporite deposition about 4.9 Ma ago. Errors associated with these age estimates are about 0.5 Ma (2 σ).