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In-Depth Information
and von der Borch, 1995), supports the contention that
many playas have cycles of erosion and sedimentation
with a distinct local signature. In turn this can be inter-
preted in terms of decreased precipitation, or changes in
wind strength or circulation, and can be dated using an
OSL assay of sedimentary cores (e.g. Telfer and Thomas,
2007; Telfer et al. , 2008).
termine the sources of the geochemical evolution of ter-
restrial evaporites deposited in a range of playas from
Australian, USA and Africa.
In order to elucidate geomorphic processes and dat-
ing of recent surficial deposits on playas, Reynolds et al.
(2007) have utilised radionuclide analyses ( 137 Cs) to de-
termine recent erosion and sediment accumulation rates
for playas in the southwestern USA. At the same time,
the geochemistry and regional impact of dust emanating
from playas has also received much attention (e.g. Gill
et al. , 2002; Reheis, 2006). Although organic materials
do not display the same degree of preservation that oc-
curs in temperate environments, studies involving pollen,
diatoms (Burrough et al. , 2007), stromatolites (Casanova
and Hillaire-Marcel, 1992), ostracods (Lister et al. , 1991)
and even ostrich shells (Miller et al. , 1991) have added to
the debate.
15.5.3 Stable isotope studies and pan
hydrochemical evolution
Recent advances in the multidisciplinary study of playas
as three-dimensional features, with distinct hydrological,
sedimentary, chemical and organic budgets, has allowed
the identification of distinct facies associated with the
saline pan cycle, and thus allowed the interpretation of
their sedimentary record, even where it is discontinuous
(Chivas, 2007; Yechieli and Wood, 2002). Geochemical
studies have contributed greatly to this, in particular the
recognition that common minerals such as gypsum may
take on different crystal forms in lacustrine, groundwa-
ter, aeolian and pedogenic environments (Magee, 1991;
Magee et al. , 1995). Building on the work of Eugster
and Jones (1979), a number of workers (e.g. Risacher
and Fritz, 1991; Bryant et al. , 1994a; Yan, Hinderer
and Einsele, 2002; Eckardt et al. , 2008) have undertaken
systematic analyses of surface waters in order to under-
stand and model sources of solutes and geochemical brine
evolution processes associated with playa systems (Fig-
ure 15.8). In order to look more closely at the geochem-
ical pathways associated with brine evolution over long
time periods, trace element and stable isotope approaches
have been used to trace the often complex geochemical
provenance of evaporite deposits (e.g. groundwater, sur-
face water or atmospheric sources; see Figure 15.5). Typ-
ical trace element analyses include determining the Br/Cl
content of halite (e.g. Hardie, 1984) and the strontium
content of gypsum (e.g. Rosell et al. , 1998). More gen-
eral bromine geochemistry of playa halite has also been
undertaken (e.g. Bryant et al. , 1994a; Risacher and Fritz,
2000) to attempt to understand their source, preserva-
tion and diagenetic history. However, given that modern
playa evaporites can display a marine-like geochemical
signature, these approaches have fundamental limitations
(Bryant et al. , 1994a). As a consequence, Chivas et al.
(1991), Vengosh et al. (1992), Ramesh, Jani and Bhushan
(1993) and Eckardt et al. (2008) successfully use a com-
bination of strontium, oxygen, boron and sulfur isotope
compositions of waters and mineral phases (e.g. calcium
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