Why are variations in bromine isotope compositions in the Earth's history larger than chlorine isotope compositions?

Abstract

In this paper we discuss the marked discrepancy in global chlorine and bromine isotope variations. While, based on experimentally and theoretically determined fractionation factors, it is expected that bromine isotope variations should be, depending on the process, 2 to 7 times less than chlorine isotope variations it is observed that in formation brines the isotope variations of bromine are at least of the same size as chlorine isotope variations, and regularly even larger. In this paper we argue that this is caused by the fact that oxidation-reduction processes play a more important role in bromine isotope geochemistry than in chlorine isotope geochemistry. Due to the fact that the bromide ion is more easily to oxidise than the chloride ion Rayleigh effects can cause the observed larger variations in bromine isotope geochemistry. In this paper we propose that biochemical reactions (oxidation of bromide ions to methyl bromide) may be the major cause for this effect. Although we do not yet understand the full processes that take place we show that oxidation-reduction processes must be the main effect to explain the differences between the two isotope systems and propose that more research is developed to understand how the processes cause the observed variations.