An improved sample preparation method for strontium isotope stratigraphy: case study of early Miocene oysters and foraminifera from the Gulf of Suez, Egypt

Abstract

Strontium isotope stratigraphy (SIS) is a widely employed method for constructing age models in marine sedimentary sequences. This technique relies on the assumption that the strontium isotope (⁸⁷Sr/⁸⁶Sr) composition of seawater is globally homogeneous but evolves predictably over geologic time. A critical requirement for SIS is that the Sr isotope ratios measured in marine calcareous fossils reflect those of the original seawater at the time of biomineralization. However, post-depositional diagenesis can alter these ratios, rendering SIS-derived ages unreliable. To address this issue, careful sample selection and tailored pretreatment protocols are essential. Through parallel leaching experiments, we introduce an improved sample preparation method for SIS. Our approach enhances conventional protocols by using controlled acetic acid concentrations (0.01–0.5 M) for sequential leaching, monitoring Fe/Ca and Mn/Ca ratios of leachates to identify the leachate most likely to preserve the original seawater signatures, and incorporating an ammonium carbonate rinse between leaches to remove loosely bound cations and acid-mobilized contaminants. Contrary to traditional practices, optical observations and geochemical evidence suggest that the final third of unpowdered material dissolved by dilute acetic acid often retains the least-altered primary calcite. We validate our method using oysters and foraminifera from early Miocene deposits in the central Gulf of Suez, Egypt, where independent age constraints confirm the accuracy of our SIS ages. Our results underscore the importance of tailored leaching strategies to mitigate diagenetic biases, thereby enhancing the reliability of SIS in chronostratigraphic applications.