The bromine and chlorine isotope composition of primary halite deposits and their significance for the secular isotope composition of seawater

Abstract

We determined the chlorine and bromine isotope compositions of 83 halite samples from nine different geological periods between the Orosirian and the present in order to study the secular Cl and Br isotope variations in the ocean during the last 2 billion years. Relatively large Cl (−0.24 to +0.51‰ vs. SMOC) and Br (−0.24 to +1.08‰ vs. SMOB) isotope variations are found in these halite samples. Two different methods, one in which the isotope fractionation between the brine and the salt is used, and a second in which the relationship between the isotope compositions and the Br/Cl ratios in the halite samples is used were applied to establish the original Cl and Br isotope compositions of the ocean. Both approaches showed that the Cl and Br isotope compositions of the ocean have always been close to the modern value (which is by definition 0‰ for both isotope systems) and that at most very small variations in Br and Cl isotope composition of seawater have occurred during the last 2 billion years. This indicates that, unlike in other isotope systems that often show significant isotope variations over geologic time, Cl and Br isotope compositions can be used directly to determine processes that occurred in the deposits of interest without need for correction for secular variations.