Playas of the Thar Desert: Mineralogical and Geochemical Archives of Late Holocene Climate
This study presents a synthesis of the published data and summarizes new data on mineralogy and geochemistry of the late Holocene playa sediments from the Thar Desert, located in the down stream of southwest monsoon. The eastern margin of the Desert is semi-arid (>400 mm a-1), whereas the western region is arid (200-300 mm a-1). The negative water budget (higher evapo-transpiration/precipitation) has led to the presence of a number of playas in the region. The assemblage of clastic minerals is similar and constitutes quartz, plagioclase, K-feldspar, amphibole, mica and chlorite. The non-clastic mineral assemblage is variable and comprises of both evaporite minerals and carbonates. The geochemical proxies enabled differentiation of the shallow depth profiles at five different playas into horizons of varying chemical weathering, aeolian input and evaporation. Between ca.1.3-3.1 ka, the dominance of proto-dolomite, [Ca>1(Fe,Mg)<1(CO3)2] in the eastern playas and presence of gypsum (CaSO4 . 2H2O) in the western playas suggest that the present day climatic gradient might be exiting during the late Holocene.
Playas are topographically enclosed, saline shallow water bodies formed
in a variety of topographical depression in arid and semi-arid regions,
where evaporation exceeds precipitation (Shaw and Thomson, 1989; Briere,
2000) e.g., Northern Great Plains of Canada (Last, 1989), high plains
of southwest USA (Osterkamp and Wood, 1987; Langford, 2003), northern
and central Spain (Schütt, 1998, 2000), Africa (Jones et al.,
1977) and in the Thar Desert of India (Wasson et al., 1984; Enzel
et al., 1999; Roy et al., 2001; Sinha and Raymahashay, 2004;
Roy et al., 2006, 2007; Roy and Smykatz-Kloss, 2007; Roy, 2007).
The geochemical evolution of the playa brine and composition of minerals
precipitated from the brine are controlled by composition of catchment
lithology, sequential precipitation of evaporite minerals with increasing
evaporation, dissolution and re-precipitation of earlier precipitated
evaporites, reverse weathering of precursor clay minerals, seepage loss
and sulphate reduction in an anoxic environment of deposition (Rosen,
1994; Rouchy et al., 2001; Yan et al., 2002; Yechieli and
Wood, 2002). So the sediments deposited in the basins provide information
about paleo-depositional environments, paleo-hydrology and geochemical
processes responsible for the formation of economically important evaporite
Sedimentary facies, pollen assemblages, evaporite mineralogy and geochemical
proxies from the sediments deposited in the Thar Desert playas have been
used as tools to delineate the phases of varying southwest Indian monsoon
and the paleo-hydrological condition during the late Pleistocene-Holocene
(Singh et al., 1972, 1990; Wasson et al., 1984; Sundaram
and Pareek, 1995; Enzel et al., 1999; Deotare et al., 2004;
Sinha and Raymahashay, 2000, 2004; Sinha et al., 2006; Roy et
al., 2006). Comparison and synthesis of the results suggest that the
entire region experienced hyper-arid, hyper saline condition from LGM
to 15 ka BP. The playa hydrology was fluctuating between hypersaline and
fresh water between 15 and 7-8 ka BP. The climatic condition improved
and the playas turned perennial between 7-8 and 5-6 ka BP. This improved
climatic condition was relatively extensive in the eastern part of the
desert compared to west. The western region is experiencing an ephemeral
condition since last 6 ka BP, whereas the eastern playas are ephemeral
since last 3.2 ka BP.
In this study, we summarize the comparative mineralogy and geochemistry
of late Holocene sediments from ten different playas across the Thar Desert.
Phases of varying chemical weathering, aeolian influx and evaporation
are identified in the five different shallow depth profiles using the
elemental ratios. We propose that a paleo-humidity gradient was present
during late Holocene and support it by the relative abundance of evaporite
mineralogy in the shallow core sediments of eastern and western playas.
The Great Indian Sand Desert, better known as the Thar Desert, covers
an area of ca. 4,50,000 km2 and is divided between the state
of Rajasthan (India) and the province of Sindh (Pakistan). To the east,
it is bounded by the Aravalli Range of mountains, which divides Rajasthan
physiographically into two parts: the semi humid eastern part and the
semi arid to arid western part. The region experiences variable annual
southwest rainfall and the annual average rainfall shows a gradual decrease
from 500 mm in the eastern margin to 100 mm in the western margin (Fig.
1). During the summer, the temperature of the region rises up to 50°C
and during the winter mornings the temperature falls to as low as 6°C.
The evapo-transpiration in the region ranges from three to twenty times
higher than the precipitation resulting negative water budget in the hydrologically
closed, saline playas.
The playa catchments are characterized by rocks belonging to the Archaean
(3.3 Ga) and Proterozoic (0.75 Ga) ages (Abu-Hamatteh, 2002). The Archaean
amphibolite, granite and gneisses overlain by Proterozoic quartzite, mica-schists,
gneisses (Biswas et al., 1982; Misra, 1982; Sen and Sen, 1983;
Dassarma, 1988; Sinha and Raymahashay, 2004) are exposed along the Aravalli
mountain system. The Malani basalt and rhyolites, Palaeozoic Bap boulder
bed and Proterozoic sandstone are exposed in the central and western Rajasthan.
The Pleistocene sand dunes are exposed in the surroundings of the eastern
and western playas. Our recent work (Roy and Smykatz-Kloss, 2007) suggested
that playas of the entire region receive clastics from the eastern Aravalli
Mountains. Additionally, the western playas also receive sediments from
the surrounding basalt and sandstone outcrops.
The tectono-geomorphic evolution of these playas has been related to
excessive siltation in the river confluence, dune segmentation of former
streams during the late Pleistocene climatic transition (Agarwal, 1957;
Ghose, 1964; Ghose et al., 1977; Kar, 1990; Singhvi and Kar, 1992)
and tectonic movements along the lineaments that caused formation of horst
and graben structures (Sinha-Roy, 1986; Dassarma, 1988; Roy, 1999). These
playas are replenished by surface runoff
||Location map of the Thar desert, India. The (sampled)
playas (except Phulera) are scattered in the region west of the Aravalli
during the 3-4 months of southwest monsoon and derive sediments and soluble
ions from the chemical weathering of rocks from Aravalli mountains (Sinha
and Raymahashay, 2004; Roy and Smykatz-Kloss, 2007). The hypersalinity
of the brine is attributed to the higher evapo-transpiration and negative
water budget in the region (Ramesh et al., 1993; Yadav, 1997).
MATERIALS AND METHODS
Sediment samples were collected from the surface of a total of ten playas
namely, Phulera, Sambhar, Sargot, Kuchaman, Didwana (all sampled in May
2001), Bap-Malar, Thob, That, Pachapadra and Pokhran (all sampled in October
2002) (Fig. 1). Samples from shallow (1-1.5 m) profiles
were also collected through hand augering and dug pits from the Phulera,
Sambhar, Didwana, Pachapadra and Pokhran playas at an interval of 10-25
cm (Fig. 2). All the playas have an evaporite enriched
crust at the surface but sampled sediments along the shallow depth profile
consist of sandy silt and silty-clay (Fig. 2). The clastic
and non-clastic minerals were identified with the X-ray diffraction (XRD)
analysis from the dried, powdered, bulk sediments using Cu target for
the 2θ range of 3° to 63° in the Siemens Diffractometer.
Taking into consideration the area below the characteristic major peak
as indicative of the quantity of the mineral present, the clastic and
non-clastic minerals were quantified separately after recalculating the
integrated XRD intensities of characteristic major peaks (area below the
peaks) of all the identified minerals to 100%. In a few samples, the compositions
of the clastic grains were determined with a SX50 Cameca electron microprobe
analysis and the types of feldspars were
||Generalised lithostratigraphy of sampled shallow profiles
from Thar Desert playas and OSL chronology
identified. Major elements were measured in SRS 303 AS XRF instrument
and trace elements were measured by Spectrace 5000 XRF. For the major
element analysis, soluble evaporite minerals were removed from the bulk
sediments by washing repeatedly in double distilled water. Trace elements
were measured in the total sediments. So the major element concentrations
represent the composition of clastics and carbonate fractions and trace
elements concentrations mirror the bulk composition of the sediments.
Total amount of sulfur and carbon present in the playa sediments were
measured from powdered sediments by the Leybold 5003 Carbon Sulfur Analyser
(CSA). The amount of organic carbon present in the sediments was estimated
by measuring the amount of inorganic carbon with the Carbon Water Analyser
(CWA) and then subtracting that from the total amount of carbon.
A total of six OSL ages (2 samples from the Phulera playa and 4 samples
from the Pokhran playa) from the fine grain fractions (4-11 μm) were
dated using the Infrared Stimulated Luminescence (IRSL) dating technique
in the Daybreak 1150 automated TL/OSL system at the Physical Research
Mineralogy and Core Lithostratigraphy
Playa sediment mineralogy allows us to reconstruct lake phases (ephemeral,
perennial) as well as depositional conditions (e.g., salinity, brine composition).
These sediments essentially consist of a detrital fraction, derived by
chemical weathering from the playa catchment and the evaporite fraction,
formed by precipitation from aqueous solution. The evaporite fraction
is significantly influenced by lake water salinity and brine composition
and therefore, these minerals provide very important clues for reconstruction
of paleolimnic environments. Based on thermodynamic considerations, evaporites
form in a predictive sequence with increasing evaporation (aridity) (Eugster
and Hardie, 1978; Ingebritsen and Sanford, 1998). The first mineral to
precipitate in most cases is calcite (CaCO3). Subsequent precipitation
of a mineral sequence of sulphates (e.g., gypsum, CaSO4.2H2O),
silicates (e.g., smectite) and chlorides (e.g., halite, NaCl) is controlled
by relative concentration of Ca+2, Mg+2, HCO3-1,
SO4-2 and Cl-1 in the brine. It follows
therefore that calcite would represent the onset of salinity and aridity
and halite would form in late stages of evaporation under high salinity
conditions. Gypsum would represent an intermediate stage. In some instances
when evaporation does not reach up to gypsum saturation, as calcite continues
to be precipitated, Mg/Ca ratio in the water increases and Mg content
of the subsequent carbonate precipitates rises. In other instances, however,
evaporation may continue beyond the halite stage and depending upon the
availability of K+ ion, minerals like carnallite (KMgCl3.
6H2O) and sylvite (KCl) may also precipitate. The presence
of these minerals would therefore indicate extremely hypersaline conditions.
However in some cases, the highly soluble chloride bearing minerals can
be present through out the sediment sequence caused by their dissolution
from the playa surface, percolation of Cl rich hypersaline brine from
playa surface into the shallow profiles and their reprecipitation in the
sediment pores. Table 1 and 2 shows comparative detrital
and evaporite mineralogy of the bulk sediments of several playas of the
The detrital fraction of all the playas shows very similar assemblage
(Table 1, 2). They consist of quartz, feldspars, sheet
silicates (micas and chlorites), amphiboles and garnets. Figure 3 shows
the relative abundance of quartz, feldspars and sheet silicates in the
surface and shallow core samples of the playas considering that the semi-quantitative
determination from the XRD charts possibly include an error of 10-20%.
Quartz is the most abundant mineral and constitutes up to 90% of the clastic
fractions. Feldspars (K-feldspar and plagioclases) are the next clastic
component in abundance. They are relatively higher in the surface sediments
compared to the deeper samples. In bulk samples, the amount of plagioclase
is much more in comparison with K-feldspars. The K-feldspars are of microcline,
orthoclase and adularia types, whereas plagioclases vary between albite
and CaO-rich labradorite. Sheet silicates are present in traces (up to
10%) in the surface, but it is relatively more (up to 20%) in the deeper
sediments (Fig. 3). The chlorites are characterized
by high intensities of the even-order reflections (002, 004) and weak
intensities of the odd-order reflections (001, 003), which indicate it
to be a Fe-rich type (Moore and Reynolds, 1997). Trace amounts of amphiboles
and garnets are also identified in the bulk sediments.
The following sections describe the distinctive features of these playas,
lithostratigraphy and evaporite mineralogy of near-surface sediments in
shallow profiles. The summarized lithostratigraphy of the playas along
with OSL ages are shown in Fig. 2.
The Phulera is a small and dry playa (ca. 6 km2) located
in the east of the Aravalli mountains that form the eastern margin of
the Thar Desert. The playa receives annual rainfall of ca.500 mm which
dries out during the summer months forming desiccated polygonal cracks
on the surface.
A shallow profile of 110 cm at Phulera shows that the near surface sediments
are carbonate-rich and the concentrations of sulphate and chloride are
extremely low (Table 2). The lowermost lithostratigraphic
unit (110-60 cm) consists of laminated calcareous mud. In this unit, calcite
(CaCO3) and non-stoichiometric Fe-bearing proto-dolomite (Ca>1.0(Mg,Fe)<1.0(CO3)2)
are the major carbonate minerals. Huntite [CaMg3(CO3)4]
and anhydrite (CaSO4) are present in very minor amount. Between
70 and 80 cm, there is a distinct layer of dominant proto-dolomite with
traces of halite. This horizon is also characterized by desiccation cracks
filled with fine sand. Though the OSL ages are similar, an extremum age
bracket is derived using (age+error) for the basal sample and (age-error)
for the upper sample. This yields an age range of <3.1 ka to >1.5
ka for this litho unit (Fig. 2).
The topmost litho unit (60 cm-surface) consists of moderately laminated
silty clay and medium sand intercalations capped by a thin horizon (5
cm) of well developed polygonal cracks. Between 60-20 cm, calcite is dominant
compared to proto-dolomite. Trace amounts of halite (NaCl), thenardite
(Na2SO4), anhydrite (CaSO4) and proto-dolomite
are present along with minor amounts of calcite (CaCO3) in
the surface sediments (Table 1).
||Mineralogy of surface sediments of Thar desert playas
|(Q = Quartz, F = Feldspar, M = Mica, Chl = Chlorite,
Am = Amphibole, P = Palygorskite, Cc = Calcite, Pro-dol = Protodolomite,
Anhy = Anhydrite, Th = Thenardite, Gyp = Gypsum, Hal = Halite, K-evap
= K-bearing evaporites) (++++++ = >40%, +++++ = 30-40%, ++++ =
20-30%, +++ = 10-20%, ++ = 5-10%, + = <5%). Trace amounts of Illite-smectite
and palygorskite, K-evaporite from Sambhar, trona from Kuchaman and
Didwana, glauberite from Sargot are identified
||Mineralogy of shallow-surface sediments of Thar desert
|(Q = Quartz, F = Feldspar, M = Mica, Chl = Chlorite,
Am = Amphibole, P = Palygorskite, Cc = Calcite, Pro-dol = Protodolomite,
Anhy = Anhydrite, Th = Thenardite, Mir = Mirabilite, Gyp = Gypsum,
Hal = Halite, K-evap = K bearing evaporites), (++++++ = >40%, +++++
= 30-40%, ++++ = 20-30%, +++ = 10-20%, ++ = 5-10%, + = <5%)
||Ternary diagram showing the distribution of quartz,
feldspars and sheet-silicates in (a) playa surface sediments and (b)
shallow core sediments
The Sambhar is the largest playa (ca. 225 km2) of the Thar
Desert, situated in the wind gaps of the Aravalli hills. The playa receives
an annual rainfall of ca.500 mm and is fed by two ephemeral streams, the
Mendha from north-east and the Rupangarh from south-west, apart from several
rivulets. This playa is extensively used for commercial production of
common salt, halite. The Quaternary sediments in the playa are >15
m thick spanning for more than 30 ka in age (Sinha et al., 2006).
Much of our earlier work has been focused on this playa (Sinha and Raymahashay,
2000, 2004; Roy et al., 2001; Sinha and Smykatz-Kloss, 2003; Sinha
et al., 2006; Roy et al., 2006, 2007; Roy, 2007) and a very
comprehensive account of evaporite mineralogy is available.
Three shallow profiles correspond to three different geochemical zones
in our earlier work (Roy et al., 2006) and show similar lithostratigraphical
distribution (Fig. 2 shows the summarized log). These
zones contain assemblages of carbonates, sulphates and chlorides (Table
2). But the total sulphate content is much lower in comparison to
chlorides and carbonates. The bottommost litho unit (125-50 cm) consists
of laminated calcareous mud. This unit shows a calcite-protodolomite-halite
assemblage with minor amounts of thenardite/mirabilite and traces of anhydrite
and polyhalite. A distinct enrichment of proto-dolomite is observed at
75 cm depth which might have formed due to dolomitization of a precursor
calcite (Sinha and Smykatz-Kloss, 2003). This litho unit is similar to
the bottommost litho unit (110-60 cm) of the Phulera playa. Considering
the proximity of the Sambhar and Phulera playas, rate of sedimentation
in both the playas can be assumed to be similar. This places the unit
(125-50 cm) in to the age bracket of 1.5-3 ka, which is also supported
by our AMS 14C chronology from a deeper bore hole from Sambhar
(Sinha et al., 2006).
The topmost litho unit (50 cm-surface) consists of moderately laminated
silty-clay capped by a thin horizon of evaporite rich crust. In this unit,
the abundance of proto-dolomite is much lower compared to calcite. Halite
(NaCl) is the most abundant evaporite mineral in the surface sediments
and thenardite is the main sulphate mineral, whereas mirabilite, polyhalite
and anhydrite are present in traces (Table 1). A significant
spatial variability in evaporite mineralogy exists across the playa surface
and a zonal distribution as per relative solubility was reported in our
earlier work (Roy et al., 2001). A distinctive feature of the evaporite
mineralogy of the Sambhar playa is the presence of K-bearing evaporites
such as polyhalite, carnallite and sylvite in the surface sediments. It
is important to note that the Sambhar playa has a distinctly different
mineralogy compared to the Phulera even though they are only a few kilometers
apart. The Sambhar playa is enriched both in chlorides and carbonates
but the Phulera is enriched only in carbonates (Table 1).
The Kuchaman and Sargot playas are located north of Sambhar playa.
Both Kuchaman and Sargot receive ca. 400 mm of annual rainfall and are
small playas with an area of 8.5 and 2.0 km2, respectively.
The Kuchaman-Sargot basin is fed mainly by the Palara river and some minor
drainage trending east-west. In Kuchaman surface sediments, calcite is
the most dominant along with traces of proto-dolomite and trona (Table
1). Thenardite occurs in significant quantities in some samples and
anhydrite is recorded in traces. Kuchaman surface sediments are generally
poor in halite but at least one sample recorded significant quantities
of halite indicating that there may be some pockets (topographic lows)
where the brine reaches halite saturation. On the other hand, halite is
the major evaporite mineral present on the Sargot playa surface sediments
(Table 1). Calcite is next in abundance. Anhydrite,
glauberite and proto-dolomite are the other evaporites present in traces
in the Sargot surface sediments. Gypsum is not present in the sediments
of either of the playas. The main difference between Kuchaman and Sargot
is that trona and thenardite are present only in Kuchaman but are absent
in Sargot. Similarly, glauberite is present in Sargot but absent in Kuchaman.
Both these playas are completely desiccated with polygonal cracks on
the surface. A hard lithified carbonate layer at 5-7 m depth in Kuchaman
marks extreme aridity and a break in sedimentation (Rai and Sinha, 1990).
The overlying dark colored clay with dolomite and feldspar represents
the last phase of sedimentation in the playa which was eventually covered
by aeolian sand.
The oval-shaped Didwana playa lies in the semi arid (ca.330 mm of
annual rainfall) part of western Rajasthan near the eastern edge of the
Thar Desert dune fields, ca.50 km NW of the Sambhar playa. It is the second
largest playa in the Thar Desert with an area of ca.13.5 km2
and is commercially exploited for salt production. The playa is flanked
by isolated hills of quartzite and slate to its SW margin and by longitudinal
dunes to its NW margin. The playa is mainly fed by rain water during monsoons
and there is no major drainage feeding the playa. The playa is filled
with water during the rainy season but remains dry for most parts of the
year. The litho units of the shallow depth profile (Fig.
2) correspond to the three geochemical zones of our earlier work (Roy
et al., 2006). The bottommost litho unit (120-60 cm) consists of
laminated calcareous mud and contains calcite as the major mineral (Table
2). Halite, thenardite and proto-dolomite are also present in significant
quantities along with traces of anhydrite and thenardite. The intermediate
litho unit (60-15 cm) consists of moderately laminated silty-clay and
contains calcite and halite. Anhydrite and thenardite are present in traces
and proto-dolomite is almost absent in this litho unit. The presence of
anhydrite all along the profile may in fact be the dehydration product
of gypsum. The uppermost litho unit (0-15 cm) consists of sandy silt and
is enriched in halite. Thenardite is present in minor amounts and the
presence of trona is restricted to surface sediments. The absence of K-bearing
evaporites and presence of thenardite and trona in higher amount in surface
sediments distinguishes the Didwana playa from the Sambhar.
A deeper profile (>6 m) at Didwana was studied by Wasson et al.
(1984) and they clearly distinguished surface evaporites (precipitated
from shallow standing water) and sub-surface evaporites (precipitated
in pore spaces) based on their relationship to bedding and mud content.
In addition, they reported a very complex evaporite, northupite (MgCO3.Na2CO3.NaCl),
from depths >3 m occurring in association with dolomite and conformable
to the bedding (surface evaporite). The conventional radiocarbon chronology
of Singh et al. (1974) and Wasson et al. (1984) put the
sediments from the top 120 cm in the age bracket of last 3-4 ka.
Pachapadra and Thob
The Pachapadra playa lies ca.100 km NW of the city of Jodhpur with
a catchment area of ca.82 km2 (Deotare and Kajale, 1996). The
playa occupies a much smaller area (ca.10 km2) and receives
an annual rainfall of 300 mm. It has been suggested that this playa formed
due to disorganization of the old Luni river (Ghose et al., 1966).
The Thob is also a smaller playa (ca.11 km2) lying ca.25 km
NE of Pachapadra and is surrounded by the Aravalli mountains and aeolian
sand dunes (Deotare and Kajale, 1996). Both Pachapadra and Thob have very
similar surface mineralogy dominated by halite followed by calcite (Table
1). The playas have very low concentrations of gypsum/anhydrite and
quite a few samples from Pachapadra show traces of proto-dolomite.
A shallow profile from the Pachapadra playa (Fig. 2)
consist of alternate horizons of pale yellow to dark brown medium sand
and dark brown sandy silt (Roy et al., 2007). Minor fissures (desiccation
cracks) filled with medium sand has been observed in the sandy silt horizons.
Halite and calcite are present through out the shallow profile in comparable
amounts (Table 2). Gypsum occurs in varying amounts
in near surface sediments but its concentration increases significantly
below 1.3 m with a corresponding decrease in calcite concentration. Deotare
and Kajale (1996) and Kajale and Deotare (1997) also reported gypsum-rich
horizons below 1 m depth at Thob playa associated with deep water facies.
Bap-Malar and That
The Bap-Malar playa is situated 140 km NW of Jodhpur, west of Aravalli
ranges in the western margin of the Thar Desert. The playa occurs in the
arid core of the Thar Desert, where the mean annual rainfall is 200 mm.
The playa is primarily fed by the rainwater and groundwater and some minor
ephemeral streams from the SW side. That is a small playa located SW of
the Bap-Malar and this again has no influx from streams. It is fed mainly
by summer rains (<200 mm). The surface mineralogy of both the Bap-Malar
and That playas is halite-dominated followed by calcite (Table
1). That has a much lower carbonate fraction than the Bap-Malar and
gypsum/anhydrite are present in traces in both the playas.
We do not have any new data from the shallow profiles at Bap-Malar but
an earlier work from a 6 m thick section showed that gypsum is the major
evaporite mineral below 1.2 m depth while calcite and halite occur in
low amounts (Deotare et al., 2004). Such sharp changes were interpreted
to be climatically-induced.
The Pokhran is an elongated playa (ca.12 km2), located
in the western Rajasthan and receives an annual rainfall of 200 mm. The
lacustrine sediments, variable in thickness from 2 to 5 m, overlie the
sandstone basement, which are exposed in the north and northeastern side
of the playa. In the eastern and western parts of the playa, there are
outcrops of basalt and rhyolite (Rai, 1990). Similar to the Bap-Malar,
the Pokhran playa is an arid core playa and halite is the most dominant
evaporite mineral followed by calcite and traces of gypsum/anhydrite and
proto-dolomite (Table 1).
||Concentration of CO3, SO4 and
Corg (maximum in mass percent) in the Thar Desert playa
The shallow depth profile at Pokhran (Fig. 2) is well
constrained with 4 OSL ages. The bottommost litho unit (150-90 cm) consists
of gritty and gravelly sand. This unit is enriched in calcite. Proto-dolomite,
anhydrite and halite are present in traces. The assorted nature of the
sediments suggests it to be high energy depositional event. So using linear
extrapolation, this unit can be placed between 4.0 and ca.6.6 ka. The
intermediate litho unit (90-50 cm) consists of moderately laminated silty-clay
and is well constrained between 4.0 and 2.7 ka. It has a calcite-halite
dominated assemblage with gypsum and anhydrite as traces (Table
2). The topmost unit (50 cm-surface) consists of sandy silt. A gypsum-rich
lamina was recorded between 40 and 50 cm depth. It is devoid of calcite
(Table 2) indicating its formation from evaporation
of sulphate-rich standing water. The OSL dates place this horizon between
2.5 and 1.3 ka. The sediments younger than 1.3 ka are enriched in calcite.
Anhydrite, gypsum and halite are present in traces. The surface mineralogy
is halite dominated with traces of calcite, proto-dolomite, anhydrite
Table 3 presents the summary of organic carbon (Corg),
carbonate and sulphate contents in playa sediments. Among the eastern
playas, Sambhar has the highest concentration of Corg, Phulera
has intermediate and Didwana the lowest concentration. The surface sediments
of Sambhar playa contains up to 1.7 mass percent of Corg and
sediments in shallow profile up to 3.1 mass percent of Corg.
The western playas contain very low contents of Corg. The inorganic
geochemistry is very similar to that of carbonate and evaporite mineralogy.
The sediments from Phulera and Sambhar exhibit 19.4 mass percent and 21.2
mass percent of CO3, respectively. In the sediments of Pachapadra
and Pokhran playas, the maximum CO3 content are 10.8 and 11.9
mass percent, respectively. The trend is completely opposite when it comes
to the sulphate contents. The sediments of western playas have higher
concentration of SO4. The shallow depth core sediments of the
Pachapadra and Pokhran playas contain up to 14 and 26.3 mass percent of
SO4, respectively. In the eastern playas the SO4
content varies between 0.1 and 1.5 mass percent.
We present here a synthesis of both the published (Roy et al.,
2006) and new data on characteristic elemental ratios i.e. Na/Al, Sr/Ba
and Zr/Al along the depth profiles to identify horizons of varying chemical
weathering, aeolian activity and evaporation. Based on the elemental ratios,
the shallow profiles of Phulera, Sambhar, Didwana, Pachapadra and Pokhran
are divided into three different geochemical zones (I-III) (Fig.
4). As Na+ is more soluble compared to Al3+
(hydrolysate), the higher values of Na/Al in the clastics indicate lower
chemical weathering (sediment-water interaction) and vice versa (Mason
and Moore, 1982; Nesbitt and Young, 1982; Sinha et al., 2006).
On the other hand, the ratio of Zr/Al reflects the abundance of zircon
to feldspars (Jones and Bowser, 1978; Tripathi and Rajamani, 1999). As
zircon is mainly present in silt size fractions and easily transported
by wind activity, the higher values of Zr/Al indicate continental input
by aeolian activity in an arid environment. Similarly, Ba is associated
with clastics and Sr with carbonate and sulphate minerals, so the higher
values of Sr/Ba suggest higher evaporation and salinity. The geochemical
||Geochemical proxies (a) Na/Al (b) Sr/Ba and (c) Zr/Alx10-4
along the playa profiles
are comparable in the depth profiles of all the playas. The sub recent
zone (I) and zone III show higher values of Na/Al compared to zone II.
The ratio of Sr/Ba is highest in zone III. Except the Sambhar playa, all
other profiles show high Zr/Al in zone I (Fig. 4).
Mineralogical Assemblage of Surface Sediments: Spatial Variation
The variation in the composition of evaporite mineralogy in playa
sediments are controlled by the inflow composition (catchment lithology)
and the sequential saturation of evaporites with the
||(a) Distribution of carbonate, sulphate and chloride
minerals in the surface sediments of the playas (Type I-Phulera; Type
II-Didwana, Type III-Pachpadra, Type IV-Sambhar, That, Thob, Sargot-Kuchaman)
and (b) Distribution of carbonate, sulphate and chloride minerals
in the shallow core sediments (Type A-Phulera, Type B-Pokhran, Type
C-Sambhar, Didwana, Pokhran, Type D-Pachpadra, Pokhran)
progressive evaporation of the brine following the principle of chemical
divide (Eugster and Hardie, 1978; Ingebritsen and Sanford, 1998). The
other factors which may also influence the evaporite mineralogy are sulphate
reduction (Eugster and Jones, 1979; Decima et al., 1988; Burns
et al., 2000; Rouchy et al., 2001; Yan et al., 2002),
ground water leakage (Wood and Stanford, 1990) and clay mineral regradation
i.e., reverse weathering of precursor clay minerals (MacKenzie and Garrels,
The surface sediments of the Thar Desert playas show varying assemblages
and abundances of evaporite minerals. The relative concentrations of carbonate,
sulphate and chloride minerals in the surface sediments have been renormalised
and plotted as triangular plots in Fig. 5 (considering
an error of 10-20% in the semi-quantitative data from the XRD charts).
There are four distinct clusters of non-clastic mineral assemblages. Type
I is the carbonate-rich assemblage represented by Phulera, Bap-Malar and
Kuchaman playas. Type II is a mixed assemblage, represented by Didwana
and shows the highest concentrations of sulphate minerals amongst all
playas. Type III is a sulphate-poor assemblage and chlorides are relatively
higher than carbonates. This assemblage is mainly represented by Pachapadra
and Pokhran. Finally, the Sambhar, That, Thob and some samples from Sargot
playas represent type IV assemblage characterised by dominance of chloride
minerals (mainly halite) and much lower concentrations of both carbonates
The assemblages of clastic minerals (quartz, feldspars, amphiboles, garnets,
micas and Fe-chlorites) and geological-geochemical studies suggest an
igneous-metamorphic provenance for the playa sediments (Sinha and Raymahashay,
2004; Roy and Smykatz-Kloss, 2007). The exposures of silicate rocks (quartzite,
granites, gneisses, schist and basalt) present in the playa catchments
suggest that chemical weathering of these rocks provide HCO3
rich inflow into the playa basins (Eugster and Hardie, 1978; Yan et
al., 2002). We assume that the CO2 saturated rainwater
is the principal weathering agent and contributes cations i.e., Na, Ca,
K, Mg and Fe, into the playa basins by the chemical weathering of the
primary minerals (plagioclases, K-feldspars, amphiboles, micas and Fe-chlorites).
Considering the similar assemblages and abundances of clastics present
in playa sediments, it is obvious that the playa basins receive inflow
with similar composition irrespective of their geographic locations. This
is supported by the studies on petrography and REE geochemistry of playa
sediments (Roy and Smykatz-Kloss, 2007). These rule out the effects of
different catchment lithologies on the different assemblages of evaporite
minerals in different playas. Apart from that the sulphate or chloride
bearing rock types are absent in the playa surroundings. Both Cl and SO4
could be contributed into the playa basins by the dissolution of
the aerosol sea salts by rain water. So, we consider that the carbonates
and evaporite minerals are authigenic and precipitated as a result of
enhanced aridity in the region.
The relatively higher abundance of halite in the surface sediments (Table
1) can be attributed to the process of evaporative pumping of the
sediment pore brine to the playa surface and its precipitation due to
enhanced present day aridity (Yechieli and Ronen, 1997; Yechieli and Wood,
2002). But the relative concentration of halite with respect to other
evaporites i.e., carbonates and sulphates, is extremely variable in different
playas across the Thar Desert (Fig. 5). Several adjoining
playas e.g., Sambhar, Phulera, Kuchaman and Sargot, show distinctly different
abundances of evaporite mineralogy. Such small-scale variations, where
influxes of cations, rainfall are generally similar, might be controlled
by local hydrological and topographic conditions. Phulera is a much smaller
playa compared to Sambhar and it receives very little runoff most of which
evaporates in early summer much before the brine is concentrated to precipitate
halite in significant quantities. The Kuchaman, although larger than Sargot,
may drain off all its inflow through rainfall before peak evaporative
conditions necessary for halite saturation. The Sargot, on the other hand,
seems to be located in a topographic low where the brine stays at least
in the near surface conditions to precipitate significant quantity of
The presence of thenardite (Na2SO4) in the surface
sediments of Phulera, Sambhar, Didwana and Kuchaman, glauberite (CaSO4.Na2SO4)
in Sargot and gypsum (CaSO4 . 2H2O) in Bap-Malar,
That, Thob, Pachapdra and Pokhran suggest present day aridity and different
mechanisms of sulphate removal from the brines of different playas (Table
1). Unlike gypsum, mirabilite (Na2SO4 . 10 H2O),
the precursor to thenardite, precipitates at a much lower temperature
(4 to 6°C) (Eugster and Hardie, 1978; Cooke, 1981), suggesting that
in the eastern playas a part of the SO4 from the brine freezes
out as mirabilite and later dehydrates to thenardite as a consequence
of higher temperature in the region or seasonal variability in the water
availability and temperature change. The minimum temperature of ca. 6°C
during the winter months at the Thar Desert very well supports the mirabilite
formation in the eastern region. During the monsoonal months a part of
the mirabilite redissolves and reprecipitates as glauberite in the surface
sediments of some of the eastern playas e.g., Sargot. The present day
higher water availability in the eastern region is also reflected by the
presence of H2O-bearing evaporites, e.g., carnallite, polyhalite
and trona (Table 1) in the Sambhar and Didwana playas.
Compared to eastern region, the playas of western region receive less
rain fall (200-300 mm a-1) and do not retain inflow till the
winter months. So the inflow coming into the playas dries out completely
during the hot summer months forming gypsum.
Mineralogical Assemblage in Depth Profiles: Temporal Variation
A triangular plot for the shallow core sediments (Fig.
5) indicates a different clustering and a mixed assemblage of evaporite
minerals. Phulera still represents a carbonate playa (Type A). A majority
of samples from Pokhran playa fall in carbonate-rich type B. Type C represents
a mixture of carbonates and chlorides with sulphate minerals less than
25% of the total. The playas such as Sambhar, Didwana, Pachapadra and
some samples from Pokhran fall in this category. Type D represents mostly
of Pachapadra samples and shows variable amounts of carbonates and chlorides
with sulphates more than 25% of total. This plot clearly shows that the
brine composition of all playas has undergone significant variation with
time and two playas i.e., Pachapadra and Pokhran, show very large variations.
Other playas have also varied in brine composition but the variation has
been limited between carbonate and chloride fractions.
Along the shallow depth profiles (Table 2), halite
is near-uniform and present as sub-surface precipitates. As halite is
highly soluble, the occurrence of halite can be due to the percolation
of Na and Cl rich hypersaline brine from playa surface into the shallow
profiles and precipitation in the sediment pores. Again, non-stoichiometric
Fe-bearing proto-dolomites are abundant and gypsum is absent in the shallow
core sediments of eastern playas (Phulera, Sambhar and Didwana), whereas
gypsum is present in variable abundances in the sediments of western (Pachapadra
and Pokhran) playas. The formation of ferroan modern dolomites and absence
of gypsum in Sabkha el Melah de Zarsis (Tunisia) (Perthuisot et al.,
1990) and Lagoa Vermelha (Brazil) (Vasconcelos and Mckenzie, 1997) have
been related to SO4 reduction in an anoxic environment by microbial
activity which contributes abundant HCO3-for the precipitation
of dolomites. Except Sambhar playa, the preservation of Corg
is low in the sediments of eastern playas. This suggests an oxygenated
environment of deposition in most of the Thar Desert playas. Although
the geochemical process of SO4 reduction can not be ruled out
completely in Sambhar playa, the formation of proto-dolomites in the sediments
of Phulera and Didwana shallow-surface sediments can not be explained
by SO4 reduction by microbial activity.
Wood and Sanford (1990) and Sanford and Wood (1991) demonstrated that
the groundwater leakage (outflow to inflow ratio between 0.01-0.001) does
not disturb the sequence at which dolomite, gypsum and halite are precipitated.
Similarly, the process of clay mineral regradation (formation of illite
and smectite at the expense of kaolinite and removal of K, Mg and Na from
the brine) does not affect the precipitation of proto-dolomite and gypsum
(Yan et al., 2002). So the presence of gypsum in shallow profiles
of Pachapadra and Pokhran and its absence in eastern playas can alone
be related to different extents of evaporation in two different margins
of the desert. The OSL chronology of this work and comparison with radiocarbon
chronology of Singh et al. (1974), Wasson et al. (1984)
and Sinha et al. (2006) put the proto-dolomite enrich horizon of
the eastern playas and gypsum enriched horizon of Pokhran playa in to
the same age bracket (ca.1.3-3.1 ka). Considering that Pachapadra is located
in a paleo-river bed, we assume that the sedimentation rate must have
been higher at Pachapadra basin. So the gypsum horizon between 130 and
150 cm can be comparable to the proto-dolomite horizon of eastern playas.
Regional Paleoclimate and Paleolimnology
For the paleo-climatic and paleo-hydrological development of the region,
mineralogy and geochemical proxies for chemical weathering, aeolian influx
and evaporation are taken into consideration. During the deposition of
zone III (ca.1.3-3.1), the higher values of Na/Al and higher Sr/Ba suggest
low chemical weathering (sediment-water interaction) and higher evaporation
in all the playas. This suggests a regional weak monsoonal rainfall. During
this event, higher abundance of proto-dolomite in the eastern playas and
gypsum in the western playas suggest that the eastern desert margin experienced
evaporation only to the extent of carbonate precipitation, whereas the
western playas experienced further evaporation till the precipitation
of gypsum. But in an arid environment, proto-dolomites were formed by
the dolomitisation-reprecipitation of earlier precipitated calcite and
Mg enriched brine in the eastern playas. The minor SO4 contents
in eastern playa brines were removed by freezing out of mirabilite during
winter months. This is reflected in the presence of minor amounts of mirabilite
and its dehydration product, thenardite in the eastern profiles.
During the deposition of zone II (ca. 1.3 ka-present day), the relatively
improved monsoon and humid condition is reflected by the lower Na/Al,
Sr/Ba and Zr/Al and higher abundance of calcite. In the eastern playas,
the abundance of proto-dolomite is low, whereas in western playas gypsum
is present as traces. The modern conditions of low chemical weathering
and high aeolian input are mirrored by the higher values of Na/Al and
Zr/Al in the sub-recent zone I. This is also supported by the abundance
of halite and presence of relatively unstable (easily weatherable) amphiboles.
The absence of zircon in the modern sediments of Sambhar can be related
to the location of the sampled profile in the playa centre, which might
not have received any aeolian influx.
The playas of the Thar Desert show similar clastics, but varying assemblages
of carbonate and evaporite minerals. After ruling out the effects of catchment
lithology, reverse weathering, seepage loss and sulphate reduction, the
varying non-clastic assemblages in the surface and shallow surface sediments
is explained by the sequential precipitation of evaporite minerals as
a results of topography, moisture availability, mechanisms of SO4
removal and evaporation. The absence of gypsum (CaSO4 . 2H2O)
in the late Holocene shallow core sediments of Phulera, Sambhar, Didwana
playas and its presence in the sediments of Pokhran and Pachapadra suggest
that during the late Holocene the eastern margin playas experienced relatively
humid conditions compared to playas of western region. Between ca.1.3-3.1
ka, the geochemical proxies for chemical weathering and evaporation suggest
regional weakening of monsoonal rainfall. During this arid event, proto-dolomites
were precipitated as a result of dolomitisation of precursor calcite in
the eastern playas and minor SO4 were removed by precipitation
of mirabilite. In the western region, the playa brines evaporated up to
the precipitation of gypsum.
PDR acknowledges the financial support from German Academic Exchange
Service (DAAD), Ministry of Science, Baden Wuettenberg (Germany) and Physical
Research Laboratory (Ahmedabad) during this research. RS acknowledges
the support from the Department of Science and Technology, New Delhi and
the Alexander von Humboldt Foundation. The authors are thankful to Dr.
Utz Kramar for XRF analysis and Dr. Navin Juyal for his help during the
field work. The comments and suggestions of the three anonymous referees
are thankfully acknowledged.
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