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To understand the redox variations and interactions between hydro-, bio- and atmosphere: the power of bromine stable isotopes.

Periodic Reporting for period 1 - BRISOACTIONS (To understand the redox variations and interactions between hydro-, bio- and atmosphere: the power of bromine stable isotopes.)

Reporting period: 2016-09-01 to 2018-08-31

Bromine (Br) is chemically similar to chlorine (Cl) and also has two stable isotopes (79Br and 81Br). Thus its stable isotope variations can be studied just like for Cl. Br isotopes are studied since about 1995 and show that the isotope geochemistry of Br is very different from that of Cl. Br isotope ratios are generally higher, while Cl isotope ratios are lower. The isotope ratio distribution is also different. Cl shows a very narrow range around the seawater composition with a very long negative tail, Br shows a broader range without extreme values.

As Br is heavier than Cl its isotope fractionation is smaller than for Cl. The only processes known to cause large Br isotope variations are redox processes. BRISOACTIONS was set up to study Br isotope systems in order to understand Br isotope fractionation and to compare with Cl. It was expected to lead to a better understanding of the differences between the two systems. Br isotope fractionation was hardly understood at the moment of writing the proposal. As it was expected that redox processes are the most promising processes it was projected to be important processes to study. It was also planned to study isotope behaviour in evaporation and ion-filtration systems. Primary results obtained were surprising and obliged us to change the planning for the project considerably.
It was expected that Br isotopes in salt deposits show little variation due to small fractionation between brine and salt. But, formation waters formed by dissolution of salt have large Br isotope variations. It was suggested that this could only result through oxidation of Br. Br isotope measurements on salt made during the early phases of the project showed large Br isotope variations without indications for Br oxidation. This suggests that large Br isotope variations are primary, indicating large Br isotope fractionation during salt precipitation. Measurements of Cl isotope fractionation during oxidation showed fractionation equal to the theoretical value. This resulted in adjustments to the project. As it was shown that salt precipitation led to large fractionation, with no indications for loss of Br by oxidation, and oxidation shows fractionation as expected for Cl, which logically will be the case for Br too, it was decided to focus the project on the study of isotope fractionation during salt precipitation. The first step in this research was the development of a method to measure Br isotope ratios in salt samples with very low Br/Cl ratios. These samples are not measurable with classic ion-exchange chromatography methods. A method was developed in which Br was extracted from solution using oxidative distillation with nitric acid followed by reduction of Br by ammonia solution that could be measured directly by MC-ICP-MS.

We examined Br isotope variability within a fully developed salt sequence to examine the total variability in salts that precipitate from ocean water. As the Br/Cl ratio of salt increases during the precipitation of subsequent salts this ratio is a measure for the evolution within salt deposits. The Cl isotope ratio decreased from +0.05‰ in the least to -0.5‰ in the most evolved salt. This trend is well known and indicates that natural Cl isotope variations follow the experimentally determined. We anticipated that Br isotopes would behave the same and found that Br isotope ratios decrease during precipitation of NaCl rich salts and increase during precipitation of MgCl2 rich salt. The slope of the Br isotope ratio versus the Br/Cl ratio is significantly steeper than the slope of the Cl isotope ratio indicating larger fractionation for Br. Cl and Br isotopes were also measured from halites of different ages and related to their Br/Cl ratios. They showed comparable trends. The study also indicated that Cl and Br isotope variations of the oceans during the earth's history were no larger than a few tenths of a per mill. In order to understand the Br and Cl isotope data observed in natural salts experiments were done in which we studied chemistry and isotope composition during evaporation of different salt mixtures. The experiments gave different results depending on variability of orignal salt mixtures. It showed that isotope fractionation between salt and brine depends on the salt and also on composition of the brine. The Ca2+ concentration in the brine seems especially important. At the moment of writing we are evaluating the results of the experiments.

Research on ion-filtration is carried out in close collaboration with TOTAL at the University of the Lorraine in Nancy and the first results have just received. In order to prepare for these experiments multiple visits to the institutes in Nancy and Paris were conducted. This work will help us to understand the extreme differences between Cl and Br isotope characteristics in deep-see pore water samples and related formation waters. We also prepared a complete database of all published samples from which the Br and the Cl isotope ratio has been measured. This database will lead to a better understanding of the differences in behaviour of the two isotope systems.
As Dr Eggenkamp is one of the few specialists in the field of stable Cl and Br isotopes in the world the project made it possible to be involved in halogen isotope projects at other institutions. We collaborated with Utrecht University (UU, NL) on a formation water project in the Dutch underground with Prof. Griffioen and Mr Schout and a volcanic lake project with Dr van Bergen and Dr Rodriguez. Dr Eggenkamp was member of the PhD committee on the graduation of Dr Rodriguez. With Sharood University of Technology (IR) we collaborated with Ms Bagheri on a project on local groundwaters in north Iran. These projects led to 6 visits to UU for scientific discussions as well as visits from Mr Schout and Ms Bagheri to IPGP. Discussions with Dr Marks from the University of Tübingen led to a funded project on halogen isotope behaviour in peralkaline rocks that will be staffed by Dr Eggenkamp. Results of BRISOACTIONS have been presented at 5 international conferences (Goldschmidt in Paris, EGU in Vienna, BASIS in Liège and EAGE in Doha and Muscat). Especially successful was the organisation of a session on halogen isotope geochemistry at the Goldschmidt Conference in Paris. The project will lead to 4 primary publications in ISI journals. The first is published in Rapid Communication of Mass Spectrometry (methodology paper, DOI:10.1002/rcm.8080). The second (Zechstein paper) is under consideration with Geochimica et Cosmochimica Acta, the third (Halite paper) is currently circulated among co-authors and the fourth (salt experiments) is being written. Secondary papers are expected following the ion-filtration experiments conducted in Nancy as well as from the collaborative projects with the Universities in Utrecht and Shahrood. During the course of the EU funded project Dr Eggenkamp reviewed nine papers for international journals such as Geochimica et Cosmochimica Acta, Applied Geochemistry, Chemical Geology and Sustainable Water Resources Management.
Figure showing the experimental process of evaporation of modern seawater. The figure indicates clea