Divergent behaviors in global geochemical cycling of bromine and chlorine

Global geochemical cycles of bromine (Br) and chlorine (Cl) largely parallel one another. Oceanic emissions dominate the atmospheric fluxes of both halogens, augmented by terrestrial biogenic sources and volcanic emissions. Wet and dry deposition and biological uptake transfer the halogens to soil and vegetation. Ultimately, the halogens return to their marine origins via surface runoff or are carried into the subsurface with groundwater. Cl/Br ratios are widely used as natural tracers of groundwater history due to generally conservative behavior in this environment. In other parts of their geochemical cycles, however, multiple physical and chemical processes lead to differing rates of transformation and transport and subsequent fractionation of Cl/Br ratios. These include bursting bubbles at the ocean surface, partitioning between gas and particulate phases in the atmosphere, partitioning between organic and inorganic atmospheric compounds, photodissociation, aerosol acidification, precipitation scavenging, sorption onto organic and inorganic surfaces, plant uptake, and precipitation of salts from brines. A conceptual box model lays the groundwork for assessing past, present and future variations in Cl/Br ratios in global environmental compartments.