Formation waters from a number of sedimentary settings show δ37Cl and δ81Br values are characterized by variations in the order of -1.3 to +1.82 for chlorine (SMOC) and -1.50 to +3.35 for bromine (SMOB). Interestingly, a positive trend was found, where enrichment of δ81Br is coupled to δ37Cl enrichment.
The ability to use these two isotopic tools parallel to traditional chemistry, elemental ratio trends (e.g. Br/Cl) and isotopic parameters such as 18O, 2H and 87Sr has revealed a number of hitherto unknown trends in several well studied basins. In the Paleozoic sequences in southern Ontario and Michigan, the δ37Cl and δ81Br signatures of formation water collected from northwest of the Algonquin Arch are distinct, with depleted isotopic values in comparison with those collected from southeast of the Arch. The δ81Br signatures of the two brines show total separation with no overlaps.
The δ81Br and δ37Cl signatures of Williston Basin brines suggest several different isotopically distinct brines exist in different stratigraphic units, despite chemical similarities. The relatively wide range of δ37Cl and δ81Br of the formation waters suggests that the ocean isotopic signatures were variable over geologic time. A seawater temporal curve for δ81Br and δ37Cl was proposed with a larger variation of δ81Br in comparison with &delta37Cl, which agrees very well with 87Sr/86Sr seawater variation during the same period.
In general, the use of chlorine and bromine stable isotopes can be very useful in assessing the origin and evolutionary processes involved in evolving formation waters and also in distinguishing different brines (end members).