Collaborative Research: Strontium Isotope Stratigraphy and Paleohydrology of Miocene Evaporite Sequences

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Marine evaporite sequences constitute a somewhat discontinuous record of seawater chemical compositions through time. The sequence and relative amounts of precipitation of different evaporite minerals in each deposit constrain hypothesized excursions of seawater composition. Mass inventories of ancient evaporites suggest that some major episodes of evaporite deposition decreased mean oceanic salinity and perhaps changed major ion ratios of seawater. In addition, marine evaporite sulfate minerals contain a record of the sulfur and oxygen (in sulfate ion) isotopic composition of seawater through time. The major disadvantage in evaporite geochemical records is the lack of stratigraphic age control within sequences. Sequence ages are usually interpolated from those of underlying and overlying marine strata, but biota in these and in interbedded units are commonly restricted or endemic and have little stratigraphic utility. This problem may be alleviated by measuring strontium isotopic ratios of the gypsum/anhydrite mineral fractions of the evaporites. The dominant source of Sr in most marine evaporitic brines is seawater and, as for marine carbonates, no significant Sr isotopic fractionation takes place during incorporation of Sr into the precipitating sulfate minerals. Sr isotope redistribution due to diagenesis, excluding dewatering of gypsum upon deeper burial, is probably not as much of a problem as it is in carbonate sequences because sulfate mineral alteration is easily recognized and avoided. Sr-isotopic stratigraphy for middle to upper Miocene evaporite sequences in the Gulf of Suez-northern Red Sea, a deposit that provides a relatively long potential geochemical record will be constructed. Determination of Sr-isotopic ratios for the evaporite sequences and comparison to established Sr- isotopic secular curves for late Cenozoic seawater, based on pelagic carbonate sequences, gives reasonable age estimates for presumed Langhian (ca. 15.5 Ma) and Messinian (ca. 6 Ma) anhydrites from industry wells in the Gulf of Suez. The improved stratigraphy will have important applications for higher resolution dating of secular isotopic curves, for chemical mass balance considerations, for a better understanding of timing and nature of tectonic processes in a developing rift basin, and for hydrocarbon exploration. The technique, if successful, may also be applied to other evaporite sequences. The possibility of an Sr-isotopic signal derived from continental runoff will also be considered and, if present, will provide important information on the paleohydrology of "marine" evaporite basins. Such considerations are crucial if we are to regard ancient evaporites as marine paleochemical monitors.