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Figure 5. Flow chart showing proposed evolution of
geology and geochemistry of acid brine lake waters
and groundwaters in southern Western Australia.
GSA TODAY | JUNE 2015 and field observations show that evapo-concentration decreases and opal. Even some marginal marine environments have moder-
pH if the starting solution was moderately acid (Foster and ately acid (pH ~5) waters (i.e., Sammut et al., 1996). Southern
Benison, 2006). Western Australia, and in particular the Yilgarn Craton, may simply
be more advanced in acid brine evolution than the remainder of the
MAINTENANCE OF ACID BRINE ENVIRONMENTS continent. A combination of Australia’s old cratonic rocks, relatively
low tectonic activity, relatively few carbonate rocks, and long wet-to-
Geochemical cycling and physical reworking operate dynami- dry climate trend, as proposed by Long and Lyons (1990, 1992) over
cally on both varying temporal and spatial scales on the Yilgarn the past tens of millions of years, has likely resulted in the acid brine
Craton. An understanding of physical and chemical sedimento- development for much of the continent.
logical processes is necessary for interpreting acid brine evolution.
Flooding-evaporation-desiccation cycles controlled by local FUTURE OF ACID BRINES IN RESPONSE TO GLOBAL
weather at individual acid saline lakes drive much of this recycling CLIMATE CHANGE AND HUMAN LAND USE
(Benison et al., 2007). For example, lake water may precipitate
gypsum, halite, iron oxides, and kaolinite. When the lake dries In the twentieth century, both agriculture and mining had local
up, winds can entrain and transport those chemical sediments, influence on acid brine groundwater. A government-sponsored
depositing them in the same lake or tens to hundreds of kilome- effort to turn the semi-arid eucalypt forests of inland southern
ters away. Flooding of a lake due to a rainstorm will carry sedi- Western Australia to crop and ranchland promoted the deforesta-
ments in sheet floods to lakes from dunes, soils, and sandflats. In tion of the “wheat belt” region. With fewer trees to soak up the
addition, the meteoric water may dissolve some soluble minerals, acid saline groundwater, the water table rose. Ranchers realized
such as halite and gypsum, releasing their ions back into the lake that cattle and sheep did not thrive with acid brines. Farmers
water. These dynamic surface processes cause the lake waters to found the only successful crops were grown high above the water
fluctuate in pH and salinity, thus crossing geochemical divides table and irrigated with desalinized seawater piped a distance of
that determine precipitation versus dissolution of specific hundreds of kilometers. Mining efforts have also used desalinized
minerals. Geochemical cycling of sulfur, as well as other elements, seawater pipelines. Both mining and agriculture import fresher
particularly Cl, Fe, and Al, is intense (Benison and Bowen, 2013). water to the groundwater system and may be responsible for
Changes in dissolved Fe and S, in particular, play a role in keeping changing the volume of groundwater slightly, as well as potentially
the pH low. The acid brine lake water–groundwater systems seem causing dissolution of some subsurface halite and other chemical
to be maintaining themselves by these processes. sediments, and, perhaps in turn, increasing groundwater salinity.
The limited volume of groundwater, in combination with its acidity,
WIDESPREAD ACID BRINES THROUGHOUT AUSTRALIA salinity, and high concentrations of some metals, make southern
Western Australia a difficult place for human habitation.
Acid sulfate minerals, soils, and weathering profiles have been
reported across Australia. Lake Tyrrell in northern Victoria is Predictions for future climate suggest continued aridification in
famous for its localized acid brines (e.g.., Dickson and Giblin, Australia due to global warming. Frederiksen et al. (2009) noted
2009; Long et al., 1992, 2009). Thiry et al. (2006) interpreted decreasing peak jet stream winds at ~30°S over southern Australia
acidic groundwater alteration in south-central Australia to have and predicted that this would cause increased temperatures,
produced weathering profiles rich in kaolinite, gypsum, alunite, decreased autumn and winter rainfall, and prolonged droughts.
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