Tracing terrestrial salt cycling using chlorine and bromine

Abstract

Understanding and quantifying terrestrial salt cycling is central to scientific fields such as sedimentary geology, mineral exploration, water resources, palaeontology, atmospheric chemistry and limnology. Dissolved chlorine and bromine concentrations have been utilised for decades as individual tracers or as a ratio to trace geochemical processes in saline environments. The stable isotope variations of these two elements have also been found to be useful for understanding and quantifying geochemical processes. However, both hydrogeochemical techniques could benefit from being applied in new environments and the collection of further data on a local and continental scale, as well as developing quantitative methods to provide further value to their use. This thesis presents findings based on theoretical analysis, large-scale monitoring and a targeted field investigation to improve the understanding of how chlorine and bromine can be utilised as tracers of terrestrial salt cycling. Firstly, bench-top salt dissolution experiments were used to verify a previously established quantitative mixing model that utilises chloride/bromide ratios to correct chloride- or bromide-based tracer methods for other chloride sources. The results show that the model can predict the percentage of alternate salt sources accurately after analytical and endmember uncertainties are considered. The results are used to extend the understanding of the uncertainties and sensitivities of the mixing models, providing scientists with a guide to which environments and scenarios the mixing model would be most appropriate. The mixing model correction provides a useful and cheap method for scientists to improve their use of chloride- or bromide-based tracer techniques in catchment studies. Secondly, a continental-scale dataset of wet deposition compositions spanning six and half years was analysed to identify spatial and temporal trends in chloride/bromide ratios. A recently developed imputation algorithm was applied to estimate the high proportion of censored bromide values, as well as the other eight analytes, based on the multivariate relationships of nine analytes. Chloride/bromide ratios of wet deposition decrease with distance inland following a logarithmic regression. The observations provide further confidence in the findings presented in previous studies that have shown that chloride/bromide ratios systematically decrease with increasing distance from the coast. Lastly, chlorine and bromine tracer techniques were applied in a case study of the Lake George Basin, NSW, to trace modern salt cycling proximal to a saline lake, and to investigate how hydrogeochemical signatures can elucidate palaeohydrologic processes. The Lake George Basin was chosen as the field site because of its long, near-continuous sequence of Cenozoic lake sediments, and its complex salt cycling regime. The chlorine- and bromine-based tracer methods, in combination with other geochemical information, have provided a better understanding of the modern salt cycling regime within the catchment, and have also provided useful constraints on the timing of the recession of the mega-lake that existed in the basin during the last glacial maximum. This study also illustrated the utility of chlorine- and bromine-based tracer methods to delineate salt cycling processes in saline lake environments.