Fluid Inclusions in Marine Halite as a Window into the MG Isotopic Composition of Past Oceans

Abstract

Evidence from marine carbonates, evaporites, and fossil echinoderms has conclusively shown that seawater composition varied considerably during the Phanerozoic. Seawater has oscillated on 100- to 200-million-year cycles between “calcite seas,” periods with predominant carbonate formation of low-Mg calcite and Mg/Ca ratios as low as 1, and “aragonite seas,” periods with predominantly aragonite formation and Mg/Ca ratios of up to 5. Proposed explanations for the oscillations between calcite and aragonite seas include changes in the rate of Mg uptake at mid-ocean ridges and variations in shallow-water dolomitization rates. This study is the first to analyze the validity of these proposed explanations using direct measurements of seawater magnesium isotope ratios within evaporites. δ26Mg values of Phanerozoic seawater were obtained by developing a new method for analysis of brine inclusions trapped within ancient marine halite. Dissolution of fluid-inclusion-rich halite followed by magnesium separation using ion chromatography and isotope ratio analysis using an MC-ICP MS led to highly reproducible δ26Mg values for modern, Messinian, and Silurian-age seawater samples. Silurian and modern samples showed similar δ26Mg values, suggesting that similar processes may drive seawater composition during the two time periods. Further adjustments to the methodology and additional sample analysis can achieve a more complete dataset for seawater δ26Mg values and a better understanding of the processes that have controlled the chemistry of seawater throughout the Phanerozoic.