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GSA TODAY | JUNE 2015 BACKGROUND Figure 3. Ternary diagram in equivalents for Ca-SO4-HCO3 showing the
dominant carbonate and sulfate chemical divides that commonly occur as
Hundreds of ephemeral saline lakes with changing colors in water evaporates. These types of diagrams are typically used to describe brine
southern Western Australia were noted by Gregory (1914). The evolution in alkaline, neutral, or Ca-Cl type waters. The lack of HCO3− and
acidity of these saline lakes was recognized by Mann (1983). calcite in the Western Australia (WA) acid lakes demonstrates that these
McArthur et al. (1989, 1991) analyzed pH, salinity, and major and systems are endmembers. WA lake waters are shown as orange squares and WA
minor ions in acid saline lakes and groundwaters near Salmon groundwaters are red circles (Bowen and Benison, 2009). Typical WA rain
Gums, in particular Lakes Gilmore and Swann. Alpers et al. water values after Hingston and Gailitis (1976). Other examples of global
(1992) reported stable isotopes from two of the same samples. extreme endmember saline systems are illustrated in comparison to the WA
More recent work illustrated the spatial and temporal complexity system. 1–3: Spring waters from China; 4–11: Surface brines from China lakes;
of these dynamic systems (Benison et al., 2007; Bowen and average seawater and average river water; all after Lowenstein et al. (1989);
Benison, 2009). 12: Salton Sea; 13: Great Salt Lake; 14: Deep Springs Lake; 15: Walker Lake;
16: Mono Lake; 17: Pyramid Lake; all after Spencer (2000).
In southern Western Australia, the acidity has been attributed
to ferrolysis, weathering, and oxidation of Fe2+ (Gray, 2001; reactions. Some lakes have low dissolved Fe and little Fe-minerals
McArthur et al., 1989, 1991; Mann, 1983). The abundance of (Bowen and Benison, 2009), suggesting that ferrolysis is not as
secondary iron oxides in the region points to the importance of important as sulfide oxidation at these lakes.
iron redox cycling in this geochemical system (Anand and Paine,
2002). McArthur et al. (1991) suggested that ferrolysis is the main Another secondary acidification process that likely occurs in
source of the acidity, mainly due to the high iron content in the southern Western Australia is due to acidophilic microorganisms.
lowest pH waters analyzed at that time. Alpers et al. (1992) and Macrofauna are noticeably absent from the acid saline lakes, and
Long et al. (1992) called for sea spray aerosols as contributors to nearby vegetation is of low diversity, especially compared to
the chemistry of acid saline waters. Long and Lyons (1990) theo- nearby neutral saline lakes (Benison, 2008). However, evidence of
rized that acid saline waters might be the natural late-stage microorganisms has been detected in the field (Benison, 2008).
product of continental evolution. If so, the recognition of other Molecular methods have documented diverse communities of
acid saline lake systems in the rock record may not only help us to prokaryotes in the acid saline lakes (Mormile et al., 2009). The
interpret local and regional climate histories of the past, it can majority of these prokaryotes are novel, but some of their closest
also help us to make interpretations about large-scale processes, matches are S- and Fe-oxidizing bacteria. Other microbiological
such as tectonic evolution and continental weathering. studies of these acid saline lakes suggest the additional presence of
eukaryotes, such as acidophilic and halophilic alga and fungi
SOURCES OF ACIDITY (Benison, 2012; S.S. Johnson, 2014, pers. comm.). In addition,
microorganisms have been detected as solid inclusions and within
The most likely initial, and perhaps most important, acidifica- fluid inclusions in halite and gypsum precipitated from these acid
tion process is the oxidation of sulfides. This interpretation is saline lakes (Benison et al., 2008; Conner and Benison, 2013). It is
supported by the presence of sulfides in host rocks, high sulfur in known that many acidophilic microorganisms can promote
waters, abundant sulfate minerals, and oxidizing environment biochemical processes such as Fe- and S- oxidation, resulting in
(Benison and Bowen, 2013). We have observed sulfur veins in even lower pH (e.g., Langworthy, 1978; Oren, 2010). Although
Archean greenstone rocks from outcrops and mine tailings near more work is needed to better understand the specific microbial–
Norseman. We also have observed disseminated pyrite in felsic water geochemistry relationships in the Western Australia acid
igneous and metamorphic rock cores from the subsurface near saline lakes, it is likely that the microorganisms are influencing
Kalgoorlie. Disseminated sulfides, including Fe- and Cu- sulfide the pH of the lakes and groundwaters.
minerals, are found in the Archean rocks (R. Whittem, 2006,
pers. comm.). There is high sulfate content (up to 35,000 ppm)
and excess sulfur in waters (more S than can be accounted for
with sulfate), indicating the presence of other S species (Bowen
and Benison, 2009). Acid lakes and adjacent environments are
characterized by abundant sulfate minerals, including gypsum,
jarosite, and alunite (Benison and Bowen, 2013; Benison et al.,
2007). In addition, the byproducts of ferric- and copper-sulfide
oxidation are found here, including high Fe and Cu in waters and
iron oxide minerals (Bowen and Benison, 2009).

Secondary acidification processes that likely occur in southern
Western Australia involve combined oxidation and hydrolysis.
Ferrolysis is a combined process of oxidation and hydrolysis that
occurs in waters enriched with dissolved iron and yields addi-
tional H+, causing water pH to decrease. Similar chemical reac-
tions that influence pH occur with waters enriched with dissolved
aluminum. Repeated precipitation and dissolution of hematite,
jarosite, alunite, and gibbsite, as well as other Fe- and/or
Al-bearing minerals, may provide varying concentrations of
dissolved Fe and Al that can contribute to these pH-lowering

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