Daniel W Müller

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

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 (2σ).

Abu Dhabi sabkha, Persian Gulf, revisited: Application of strontium isotopes to test an early dolomitization model

A study of brines and diagenetically altered minerals from coastal sabkha of Abu Dhabi, Persian Gulf, was undertaken to test the flood recharge-evaporative pumping model for early dolomitization using strontium isotopes as a tracer. The {sup 87}Sr/{sup 86}Sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model. The {sup 87}Sr/{sup 86}Sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters. The {sup 87}Sr/{sup 86}Sr ratio of gypsum from the dolomitization area (1.4 to 4.4 km from high-tide mark) and seaward reflects precipitation from marine brines, but more landward (6 to 9 km), gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence (dolomite {yields} gypsum {yields} anhydrite). The dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes.