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of iodine, sulfur and chlorine derived by evaporation from
the Pacific. Ericksen (1981) added that biological activ-
ity, bedrock weathering and volcanic emissions from the
Andes may have played a role, with volcanic sources also
highlighted by other authors (Searl and Rankin, 1993;
Chong, 1994; Searl, 1994; Oyarzun and Oyarzun, 2007).
Ericksen (1981) suggested that atmospherically derived
deposits were leached, enriched in their most soluble com-
ponents and redistributed by rainwater to accumulate on
lower hillsides, at breaks in slopes and in salars. Evidence
suggests that the Atacama may have been arid since the
Early Eocene and hyper-arid since the Middle Miocene or
Late Pliocene (Clarke, 2006). Such extended arid condi-
tions, associated with an almost complete absence of bio-
logical activity and leaching, would have permitted large
amounts of nitrate to build up even from modest rates of
inputs. Stable N, O and S isotope studies in the Atacama,
Antarctica and Mojave (Bohlke, Ericksen and Revesz,
1997; Michalski
et al.
, 2002) support the hypothesis that
nitrate-rich deposits represent long-term accumulations of
atmospheric deposition, with a minor contribution by mi-
crobial fixing and oxidation of reduced N compounds in
areas with slightly higher rainfall (Bohlke and Michalski,
2002).
Coastal fogs (locally known as
camanchaca
)havealso
been proposed as a nitrate source in Chile (Ericksen,
1981). Ridge sites close to the Pacific experience fog on
up to 189 days per year (Cereceda and Schemenauer,
1991), with fog moisture fluxes of about 8.5 L/m/day
at the coast, declining to 1.1 L/m/day 12 km inland
(Cereceda
et al.
, 2002). Analyses of coastal fog mois-
ture (e.g. Eckardt and Schemenauer, 1998) estimate NO
3
concentrations at around 1.6 mg/L. The relative purity
of fog water and the limited amounts that are deposited
inland suggest that fog can only be a minor contributor
to nitrate accumulation. Indeed, S and Sr isotopic stud-
ies of Atacama aerosols and sediments (Rech, Quade and
Hart, 2003) indicate that the spatial distribution of high-
grade nitrate deposits corresponds to areas that receive the
lowest fluxes of ocean- or
salar
-derived salts, which may
dilute atmospheric nitrate fallout.
the surface. These are often white, though the presence of
microorganisms such as the flagellate
Dunaliella salina
can give a pink colour. Periodic influxes of rainwater or
runoff often result in the dissolution and subsequent repre-
cipitation of the deposits. In a lacustrine or lagoonal set-
ting (Figure 8.4), dissolution may increase brine salinity,
thus preventing further dissolution of underlying evap-
orites and allowing thick salt sequences to accumulate
(Morris and Dickey, 1957; Busson and Perthuisot, 1977).
The largest of the terrestrial evaporite deposits, which
include interbedded gypsum and halite, may cover thou-
sands of square kilometres and reach thicknesses of hun-
dreds of metres (e.g. Salar de Uyuni, Bolivia; see Risacher
and Fritz, 2000). In the geological record, preserved halite
deposits may be difficult to distinguish from marine evap-
orites formed by the evaporation of blocked near-coastal
or epicontinental seas.
In addition to lacustrine halite accumulations, powdery
halite efflorescences and encrustations are common in
many arid and hyper-arid environments (e.g. Eswaran,
Stoops and Abtahi, 1980; Pye, 1980; Basyoni and Mousa,
2009). These deposits are usually dissolved by even small
amounts of rainfall, but reform during subsequent dry con-
ditions. There are few examples of halite crusts that have
accreted beneath the land surface, either close to the water
table or in the soil zone, in the same way as many cal-
cretes and gypsum crusts. Pedogenic halite crusts, which
have a strong structural resemblance to columnar gypsum
crusts, have been described in the Namib Desert (Kaiser
and Neumaier, 1932; Watson, 1983a, 1983b, 1985a) and
the Chilean Tacna Desert (Mortensen, 1927). Subsurface,
phreatic crusts have been reported from coastal sabkhas
along the Arabian Gulf (Shearman, 1963; Patterson and
Kinsman, 1978) and in lagoonal environments in Western
Australia (Arakel, 1980).
Halite crust formation associated with lacustrine and
phreatic evaporative processes is usually restricted to ar-
eas where mean annual rainfall is less than about 200
mm (Stankevich, Imameev and Garanin, 1983; Goudie
and Cooke, 1984). In contrast, pedogenic halite crusts in
the Namib and Atacama Deserts occur only where annual
rainfall is less than about 25 mm. Since the sources of salts
forming pedogenic halite crusts are often atmospheric, the
underlying materials can be highly variable. Though pe-
dogenic halite crusts are found in association with gyp-
sum crusts, usually there are no distinct stratigraphic as-
sociations. Lagoonal, lacustrine and sabkha halite crusts
may be interbedded with gypsum (Phleger, 1969) or anhy-
drite. Other characteristic facies, as well as structural and
textural features, are well documented (e.g. Shearman,
1966; Arakel, 1980; Warren and Kendall, 1985; Schreiber,
8.3
Halite crusts
8.3.1
General characteristics and distribution
Halite (NaCl) deposits most commonly develop in
sabkhas (Chapter 15) or in the basins of ephemeral lakes
that are subject to periodic evaporation to dryness (Eugster
and Kelts, 1983; Lowenstein and Hardie, 1985). Here,
