Geoscience Reference
In-Depth Information
15
Pans, playas and salt lakes
Paul A. Shaw and Rob G. Bryant
15.1
The nature and occurrence of pans,
playas and salt lakes
Playas and pans vary in size from the frequently very
small depressions of a few tens of square metres in
the Kalahari, western Australia and Texas (Goudie and
Thomas, 1985; Killigrew and Gilkes, 1974; Osterkamp
and Wood, 1987) to massive tectonic basins, which may
exceed 10 000 km
2
in area, such as Lake Eyre, south cen-
tral Australia, and Lake Uyuni, Bolivia (Lowenstein and
Hardie, 1985). Though pans and playas occupy as little as
1 % of the total landscape, they are important and often
numerous features. In parts of southern Africa pans attain
densities of up to 1.14 pans per km
2
(Goudie and Thomas,
1985) and occupy 20 % of the surface area (Goudie and
Wells, 1995), while there are an estimated 30 000 to
37 000 basins on the southern High Plains of north-
west Texas and adjoining New Mexico (Reddell, 1965;
Osterkamp and Wood, 1987).
Playas and pans have been important to human popula-
tions from prehistoric times to the present day as sources
of water and minerals. In modern times they have been
used for urban development (Cooke
et al.
, 1982), for air-
fields and racetracks (e.g. the Blackrock Desert, Nevada,
USA) and in the case of Lop Nor, China, and China Lake,
USA, for testing nuclear weapons. Regrettably these uses
conflict with the inherent value of pans and playas as ex-
treme, unusual and often valuable habitats (e.g. Haukos
and Smith, 1994; McCulloch
et al.
, 2008), while devel-
opment itself is not without difficulties, as the flooding of
Salt Lake City in the 1980s shows (Atwood, 1994). Scien-
tifically they have become increasingly important for the
elucidation of palaeoenvironmental conditions from their
sediments and landforms (e.g. Telfer and Thomas, 2007),
while on the negative side, they are now recognised as ma-
jor sources of atmospheric dust (Gillette, 1981; Prospero
et al.
, 2002), and are monitored accordingly.
The arid zone contains few perennial lakes; examples such
as the Caspian Sea, the Dead Sea and Lake Aral invariably
have inflowing rivers rising in fringing uplands or distant
humid areas. Instead, the majority of arid and semi-arid re-
gions are characterised by endoreic (internal) drainage or,
in extreme cases, may lack integrated surface drainage al-
together. Under these circumstances surface depressions
become important local and regional foci for the accu-
mulation of water in episodic (termed here
ephemeral
)
lakes. Due to the negative balance between evaporation
and rainfall (often exceeding 10 : 1), these water bodies
are often highly saline and, in some cases, supersaturated
with salts. Such salt lakes, which have a minimum salin-
ity of 5000 mg/L, in turn lie at one end of a spectrum
of otherwise ephemeral and often relict closed basins
of varying scales and origins, frequently termed playas
or pans.
Pans and playas have been described in most hot dry-
land environments, particularly Africa, Australia, Ara-
bia and in western USA (see Shaw and Thomas,
1997), but also occur in cold drylands such as Antarc-
tica (Lyons
et al.
, 1998). Although mostly associ-
ated with aridity - the majority of southern Kalahari
and peri-southern Kalahari pans, for example, occur
on the arid side of the 500 mm mean annual iso-
hyet and the 1000 mm free evaporation isoline (Goudie
and Thomas, 1985) - some comparable features are
found beyond the limits of modern aridity, e.g. the
Plains of Zambia (Goudie and Thomas, 1985; Williams,
1987) and the Darwin region of Australia (McFarlane
et al.
, 1995).
