Isotopic Study of Bromine: Determining Bromine Isotope Fractionation During Evaporation and Diagenesis

Abstract

Stable isotopes of bromine (δ⁸¹Br) and chlorine (δ³⁷Cl) provide new opportunities to trace geochemical processes in various natural systems. While chlorine isotopes have long been applied in diverse hydrologic and geochemical fields, bromine isotopes have only recently drawn attention. Recent analytical advances have revealed unexpectedly large δ⁸¹Br variations in formation waters and evaporite deposits, yet the controls on the δ⁸¹Br variability remain incompletely understood. Resolving the fractionation mechanisms and the relationship between bromine and chlorine isotopes is essential. Understanding δ⁸¹Br-δ³⁷Cl systematics can support interpretations of evaporite basin evolution, and groundwater sources and mixing, and identification of diagenetic overprints in the subsurface. This thesis integrates three complementary approaches, including a global synthesis of published δ⁸¹Br-δ³⁷Cl datasets, controlled seawater evaporation experiments, and analyses of paleo-evaporite sequences. The integrated work provides insights into when and how stable bromine isotopes fractionate, and when and why its isotopic behavior decouples from that of stable chlorine isotopes. The synthesis of published datasets harmonizes unconnected studies to enable a comparison of the isotopic behaviors of bromine and chlorine across hydrologic and lithologic settings. The laboratory experiments isolate mineral-specific isotopic behavior and incorporation of bromine during sequential mineral precipitation under well-constrained conditions relevant to natural evaporative concentration of seawater. The analyses of paleo-evaporite sequences reveal how depositional signatures can be subsequently modified by post-depositional processes, and how such processes alter the isotopic and geochemical signature of bromine from chlorine. The global comparison of δ⁸¹Br and δ³⁷Cl values in groundwater and surface waters reveals a consistent pattern: δ⁸¹Br often diverges from δ³⁷Cl, with broader scatter and distinct behaviors across settings. The data show that δ⁸¹Br values often cannot be predicted from δ³⁷Cl values alone, highlighting that the two halogens respond to overlapping but non-identical processes differently. Multiple regression approaches and the comparisons among them indicate additional fractionation pathways, beyond simple mass-dependent behavior, differentially affect bromine and chlorine. Relatively high δ⁸¹Br values often cluster under specific geologic or hydrologic contexts, including organic complexation/decomposition, atmospheric interaction, elevated temperature-pressure regimes, and cryogenic processes. The comprehensive review provides a structured map of where bromine and chlorine behave similarly, where they diverge, and what that implies for interpreting natural datasets. Controlled seawater evaporation experiments demonstrate that bromine isotopes fractionate during sequential mineral precipitation, and crucially, that detectable shifts are observed before halite saturation. To better understand the underlying mechanisms, a laboratory evaporation series using synthetic seawater was conducted with continuous monitoring and stepwise sampling of brines, precipitates, and gas traps. Precipitates were characterized mineralogically, and both precipitates and coexisting brines were analyzed for δ⁸¹Br and δ³⁷Cl values to evaluate isotope fractionation during evaporation and mineral formation. Carbonates, gypsum, and halite have distinct δ⁸¹Br signatures consistent with differing pathways of Br incorporation, while δ³⁷Cl values are consistently higher in minerals than in coexisting brines. Capture of bromine in the gas phase sampled during the experiment confirms volatilization as an additional fractionation pathway for bromine isotopes. These experimental results explain why δ⁸¹Br values can vary widely, and differently from δ³⁷Cl, during progressive evaporation, mineral formation, and volatilization. Analyses of paleo-evaporite sequences show that primary depositional signals can be systematically reshaped by post-depositional modification. To evaluate depositional and post-depositional signals, basin-wide stratigraphic sampling for Br, Cl, δ⁸¹Br and δ³⁷Cl analysis across evaporite horizons and locations in the Salina Formation in the Michigan Basin was combined with lithological characterization. Br/Cl ratios were examined with petrographic indicators of diagenetic overprints, and basin-evolution context from previous studies was integrated. Paired measurements of δ⁸¹Br and δ³⁷Cl were then linked to these signatures. Consistent with the compiled groundwater and surface water datasets, evaporite δ⁸¹Br and δ³⁷Cl values often show weak correlation, reflecting multiple fractionation pathways rather than a single control acting on both isotopes simultaneously. Diagenetic processes often produce path-dependent shifts: either δ³⁷Cl or δ⁸¹Br values can increase, decrease, or remain unchanged depending on the processes involved (e.g., dissolution-reprecipitation, fluid-salt interaction, thermal regimes, organic interaction). The δ⁸¹Br–δ³⁷Cl dataset provides clear basin-evolution interpretations in complex evaporite records. The combined results establish that bromine isotopes are particularly sensitive to specific geochemical processes and that their variability cannot be explained by chlorine isotope systematics alone. This research contributes three main advances: (1) it provides a systematic review and defines the variability of bromine isotopes associated with diverse hydrogeologic settings; (2) it demonstrates experimentally that bromine isotopes fractionate significantly during evaporation and mineral precipitation, including prior to halite saturation; and (3) it shows that natural evaporite systems preserve complex isotopic signatures that integrate depositional and post-depositional processes, and that these signals are better understandable when both halogens are considered. Overall, this thesis develops a conceptual framework for bromine isotope geochemistry and demonstrates the value of Br-Cl dual isotope systematics to disentangle overlapping depositional and diagenetic processes, with implications for groundwater studies and evaporite basin evolution.