Aspects of the biogeochemical cycling of mercury in Australia and the Southern Hemisphere
thesisposted on 28.03.2022, 15:55 authored by Dean Howard
The increase in levels of mercury in the environment is a truly global concern, recognised in the preparation and signing of the Minamata Convention on Mercury by 128 countries in 2010, and its enacting into force in 2017. The global nature of the problem lies in the persistence of mercury in the environment, leading to distribution of this neurotoxin to regions far-removed from sources, be they natural or anthropogenic. Understanding of the biogeochemical cycling of mercury is critical in order to estimate the legacy of previously emitted mercury, and to assess the effectiveness of control measures designed to decrease releases to the environment. This biogeochemical cycling is a complex system of processes, as the unique physicochemical properties of mercury allow for constant exchange between atmospheric, aquatic, terrestrial and biological reservoirs. Variation to rates, magnitudes and directions of this exchange take place across temporal and spatial scales, necessitating an increase in the number of biogeochemical cycling investigations implemented across the globe. Investigations such as these are limited in the context of the Australian environment. Modelling efforts have taken place to quantify and characterise biogeochemical cycling in Australia, though these are based on field and laboratory parameterisations undertaken elsewhere around the globe, and may not be suitably applicable to Australia's unique environs. Large, sparsely-populated and girt by sea, Australia is relatively unimpacted by local anthropogenic emissions of mercury. Instead, sources of mercury to this island continent are thought to be dominated by long-range transport, particularly through the atmosphere. Despite this, no long-term monitoring datasets of atmospheric mercury exist over Australia. This thesis presents the first multi-year investigations into atmospheric mercury levels in Australia. These investigations take place at two locations spanning the breadth of Australia's climate zones - one in tropical Northern Territory and the other in temperate Tasmania. Comparison between the two suggests that atmospheric mercury concentrations are largely congruous, despite very different source types and source regions. Further comparison with similar datasets taken elsewhere suggests this is broadly the case for the Southern Hemisphere in general, and that the concentration of atmospheric mercury is lower than previously believed. In this context, this thesis also investigates two important aspects regarding the natural biogeochemical cycling of mercury - delivery of atmospheric mercury to vegetation communities, and its release from these communities as a result of biomass burning. In both cases it is found that exchange of mercury is lower than previously believed, and that storage of mercury in Australian vegetation is up to 90 % lower than parameters currently used in biogeochemical modelling efforts. Depletion of ambient elemental mercury as a result of surface deposition under stable nocturnal boundary layers was observed at two locations, and throughout the year at one of these. Re-emission in the following morning at both sites provides further evidence for the prompt recycling process of atmospheric mercury transport. These investigations represent a first step in developing parameterisations unique to the Australian continent, and will serve as part of the wider global effort to monitor and mitigate the impacts of mercury on both humans and the natural environment.