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Multiple episodes of aggradation and calcrete formation in Late Quaternary aeolian sands, Central Thar Desert, Rajasthan, India
Journal of Asian Earth Sciences 37 (2010) 10–16
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Multiple episodes of aggradation and calcrete formation in Late Quaternary aeolian sands, Central Thar Desert, Rajasthan, India R.P. Dhir a, A.K. Singhvi b,*, J.E. Andrews c, A. Kar a, B.K. Sareen a,e, S.K. Tandon d, A. Kailath b, J.V. Thomas b a
Central Arid Zone Research Institute, Jodhpur 342003, India Physical Research Laboratory, Ahmedabad 380009, India c School of Environmental Sciences, University East Anglia, Norwich NR4 7TJ, UK d Department of Geology, University of Delhi, Delhi 110007, India e Geological Survey of India (WR), 15–16 Jhalana Doongri, Jaipur 302017, India b
a r t i c l e
i n f o
Article history: Received 7 August 2008 Received in revised form 10 June 2009 Accepted 2 July 2009
Keywords: Thar Desert Calcretes Aeolian sediments Palaeoclimate Luminescence dating
a b s t r a c t A 12 m thick section in a dune-sandy plain terrain of the Central Thar in Rajasthan, has provided a near continuous record of environmental change for the past 160 ka. The site presently receives 280 mm rainfall, almost entirely from the summer monsoon. The base of this section comprises a gravel bed of an ephemeral stream and the overlying six litho-units, each with discrete boundaries, comprise a succession of aeolian sands. Luminescence dating provided an estimate of the timing of the sand aggradation periods to 160, 90, 60, 27 and 17–14 ka and helped constrain the timing of calcrete formation periods. In each aggradation unit, discrete nodular calcretes formed by the leaching of carbonate from the overlying solum. This is analogous to present-day conditions in sandy plains during periods of increased rainfall and landscape stability. Several of these calcretes are, however, devoid of their donor solum, suggesting solum removal during a subsequent period of decreased rainfall and resultant surface instability. This is supported by the presence of reworked nodules on the surfaces of some calcretes. A prominent phase of calcrete development followed the aeolian sand aggradation at 60 ka, suggesting climate amelioration that also caused the formation of groundwater-related calcrete and mottling. The study suggests that stage II calcrete nodules form in a time frame of 10–20 ka, and confirms limited data on the duration and stage of calcrete development in the literature. The d13C values of calcrete carbonate lie in a narrow range (+0.5 to 1.1‰) suggesting formation under soils with C4 vegetation. This implies that even during phases of climatic amelioration, the high temperatures and increased seasonality of rainfall did not permit significant development of C3 plants in the Central Thar. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Sedimentary records from deserts and semi-deserts have been used to reconstruct climatic changes during the Quaternary (Glennie, 1987; Dhir et al., 2004; Singhvi and Kar, 2004; Lancaster, 2007; Singhvi and Porat, 2008). In these regions, climate amelioration leads to stabilization of aeolian sediments, soil profile development, activation of local fluvial systems, the formation of playas, and changes in the hydrology and geochemistry of lakes. Conversely, more arid episodes lead to desiccation, decreased vegetation cover and mobilization of sands. Soil carbonates (calcretes) in semi-deserts form during phases of climatic amelioration when sediment stabilization allows calcium carbonate to mobilize within * Corresponding author. Address: Physical Research Laboratory, Planatory and Geosciences, Navrangpura, Ahmedabad, Gujarat 380009, India. Tel.: +91 79 26314366; fax: +91 79 26301502. E-mail address: [email protected] (A.K. Singhvi). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.07.002
the solum (Wright, 2008). Calcretes are thus valuable palaeoclimatic indicators. So far, numerous ‘stacked’ calcrete profiles have been described from pre-Quaternary sediments (e.g. Watts, 1980; Tandon et al., 1995); however, these are often without chronometric controls. Consequently, the temporal significance of stacked calcrete profiles is largely undetermined. In this study, we have dated a stacked Quaternary calcrete profile from the Thar Desert (India) to provide chronometric constraints on the development of various calcrete types (Wright, 2008) and to understand their palaeoclimatic significance in both local, regional and global contexts. 2. Study area Modern mean annual rainfall in the Thar Desert ranges from 500 mm (eastern margin) to <100 mm (western margin) (Fig. 1). The potential evapotranspiration is 1650 mm (east) and 2000 mm (west), implying a high moisture deficit. Of the total
R.P. Dhir et al. / Journal of Asian Earth Sciences 37 (2010) 10–16
Fig. 1. Sketch map of Thar Desert, India and location of Chamu Site (marked +). Also shown are rainfall isohyets (in mm).
rainfall, over 85% is received in a few discrete events, during the monsoon season (July–September). Although rainfall is sparse, high atmospheric humidity and moderate temperatures during the monsoon provide conditions favorable for seasonal plant growth and biological activity. Considerable diurnal and seasonal temperature variations occur during the year: mean maximum and minimum temperatures during summer (May–June) are 41.0 °C and 26.3 °C respectively, and during winter (January) are 24.8 °C and 10.2 °C respectively. The study site is in the Central Thar Desert at Chamu village (60 km northwest of Jodhpur), with a mean annual rainfall of 280 mm (Fig. 1). The terrain comprises dunes and sandy plains overlying Late Proterozoic sandstone. Groundwater in this sandstone aquifer is 50 m below surface. A few phreatophytic trees, shrubs and a variety of grasses constitute the modern flora and the area is seasonally cultivated today. The sand grains in the aeolian sediments are mainly 100– 150 lm size, dominated by quartz along with some orthoclase and traces of plagioclase. Silt and clay fractions constitute 2–4% and 5% respectively of the dune sands, and 8–10% of each of these fractions in the soils of the sandy plains. The sands are weakly calcareous with 2–4 wt% calcium carbonate. The carbonate can occur as sand-sized grains, but is commonly silt-sized material or as coatings on coarse grains (Courty and Fedoroff, 1986; Dhir et al., 2004). The Quaternary sedimentary record of the Thar shows that climatic conditions fluctuated through time (Anonymous, 2000; Singhvi and Kar, 2004). Large areas of alluvial sediments in the region suggest less arid climatic conditions in the past (Ghose, 1965; Kar, 1995; Jain and Tandon, 2003) and archaeological evidence supports this inference (Allchin et al., 1978; Misra et al., 1982). Studies on Didwana and other lakes (Singh et al., 1974; Wasson et al., 1984) have demonstrated accentuated aridity during the Last Glacial Maximum (LGM) that persisted to about 13 ka, the time when the SW monsoon re-established itself. These studies, and also those of Enzel et al. (1999) and Roy et al. (2008), further suggest a climate optimum phase around 7.5–5 ka. Singhvi and Kar (2004) provided a synthesis of chronological work on the aeolian sands. A peak in aeolian sand dune aggradation occurred during a narrow time-window 13 ka which coincided with re-establish-
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ment of the SW monsoon (Chawla et al., 1992; Sirockko et al., 1993; Thomas et al., 1999; Singhvi and Kar, 2004). The discovery of Middle and Lower Palaeolithic tools in a deep trench in an obstacle dune at Didwana indicates that dune formation episodes predated 100 ka (Misra et al., 1982). This is corroborated by similar luminescence ages from the eastern margin and the Lower Luni Basin, of the Thar (Chougaonkar et al., 1999; Kar et al., 2001). The new luminescence dates presented in this paper add to this limited chronometric data on the long-term aeolian dynamics and palaeoclimatic changes in the Thar. Andrews et al. (1998) combined stable isotope data from calcrete-bearing aeolian sediments with optical dating to suggest that periods of weakened monsoon occurred during the LGM and at around 60 ka, with a more intense monsoonal period between 50 and 30 ka. Similar inferences were also drawn from optically dated fluvial records on the southern margins of the Thar (Juyal et al., 2006). More recent work by Achyuthan et al. (2007) presents a calcrete stable isotope record apparently extending beyond 163 ka, which in the absence of methodological details on dating can only be regarded as preliminary. These authors suggest that the longer term d13C variation was controlled by expansion of C4 vegetation during warmer and wetter monsoon-dominated interglacials, rather than by variability in atmospheric pCO2 (as proposed by Andrews et al., 1998).
3. Materials and methods The landscape at Chamu comprises coalesced parabolic sand dunes and sandy plains. A 3.5 km long section, up to 12 m deep was exposed during excavation for a lift canal. The exposure was mapped for lithofacies distribution and a representative section was logged and sampled in detail. Sand samples were dated using the infrared stimulated luminescence (IRSL) of feldspar grains which offered the only possibility for dating due to their higher saturation doses, compared to quartz. Although the stability of the luminescence signals in feldspars has been debated (Huntley and Lamothe, 2001), our earlier work from a nearby Thar site indicated that fading of feldspars luminescence during burial was not a serious problem (Andrews et al., 1998; Kar et al., 2001). Feldspar grains were extracted by standard pre-treatment with 1 N HCl and 30% H2O2 followed by density separation. Etching of the grains was not carried out so as to avoid uneven breakdown of grains. A monolayer of grains was stuck on stainless steel discs using a silicone spray. The measurements were made in a Riso TL–OSL reader with stimulation at 880 nm and observation in the blue window defined by a Corning 7–59 + BG-39 filters coupled to an EMI 9235 QA photomultiplier tube. A pre-heat of 220 °C for 1 min was used. Multiple aliquot additive dose analysis with short shine normalization was carried out for the paleodose estimation. This was considered appropriate, given that modern sands from the region give an apparent age of 60 years. This implies that the samples were well-bleached and consequently the ages are robust. Radioactivity was measured using ZnS (Ag) thick source alpha counting and NaI (Tl) gamma spectrometry and radioactive equilibrium was assumed in the computation of the dose rates. In estimating the sample dose rates it was assumed that the soil carbonates precipitated soon after sedimentation of the sand, and that the dose rate at the mm-scale (beta range) was constant. This is plausible as the net dose rate is nearly the same as that from carbonate-poor sands. Samples for isotopic analysis were micro-drilled from the centre of individual nodules, which had been checked for potential contaminants (weathered rinds, spar-filled cracks, macro-organic matter). In two cases, four individual sub-samples were taken across the thickness of a single nodule to assess isotopic variability
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within a nodule. The isotopic chemistry procedures followed Andrews et al. (1993). Isotopic data are reported relative to the Vienna Pee Dee Belemnite (VPDB) scale (Coplen, 1994). The mass spectrometer was calibrated using the NBS 18 and NBS 19 standards. Measurements on random sample replicates indicated a precision better than ±0.06‰ for d13C and ±0.10‰ for d18O. 4. Litho-units and their morphology From the base upwards, seven litho-units, I–VII, were identified. These are described alongside the analytical data and optical ages in Fig. 2. Unit I comprises gravel with both clastic material and small round carbonate nodules in calcareous cement. In thin section these nodules contain pisoliths, and spar cement healed cracks. The unit is interpreted as the gravelly bed of an ephemeral stream with reworked calcrete nodules and clastic gravel derived from pre-existing deposits or bedrock. The six units above Unit I consist of aeolian sands. Unit II, 80 cm thick, contains abundant chalky nodules (see below); Unit III (1 m thick) has poorly sorted sand containing occasional nodules or chalky segregations. Unit IV, 4 m thick, comprises friable aeolian sands. Its basal part contains rhizoliths (see below) and carbonate-cemented lenticular features, while the middle part is punctuated by thin (5–20 cm thick), planar, carbonate-cemented bands that persist laterally for 100–150 m. The upper part of the
unit contains patchy nodular calcrete. Another distinguishing feature of Unit IV is the presence of faint, greyish-blue mottles suggesting alternating oxidizing and reducing conditions. Unit V is 1.2 m thick and is dominated by dense, hard nodular calcrete (20–25% of the unit mass) with nodules typically 1–2 cm in diameter. Sands between the nodules are calcareous with total carbonate content 20 wt% (Fig. 2). Another, laterally impersistent nodular calcrete, similar to Unit V was also recorded adjacent to the sampled section, above Unit V and below Unit VI. Unit VI is 2–3 m thick and comprises friable aeolian sand with its lower half containing a few chalky calcrete nodules, randomly distributed in the sandy matrix. Upwards, it graded into yellowish-brown sand, characterized by lower calcium carbonate content, decreased friability and with some evidence of ped formation. We interpret these features as evidence for partial pedogenesis of the sand. Unit VII has variable thickness and comprises highly friable, loose, lightyellowish-brown sand, typical of un-pedogenised sands. Litho-units II, III, IV, V and VI, all contain nodular carbonate concretions up to 2 cm across. These are pedogenic calcrete nodules with alpha fabrics (see Wright, 1990a). The hard nodules in Unit V comprise 85–90% CaCO3 and in thin sections these appear as a clotted white to light brown micritic ground mass, locally grading into micro-spar and spar calcite. This matrix occasionally contains shrinkage cracks filled by whitish micro-spar with clastic grains floating in the matrix. Some grains have pronounced spar coronas
Fig. 2. Field log and some analytical data of Chamu Section. Symbols used in the log: aeolian aggradations (fine dots), calcrete nodules (hollow circles), sampling site for dating (solid circle). The scale (in meters) appears on the left. Other features in the log are described in the text alongside. The CaCO3 profile shows its content in per cent in finely dispersed form (fine dots). The hollow circles in carbonate profile indicate nodules and other secondary carbonate. Also shown are IRSL ages of host sands.
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and most plagioclase grains have corroded margins (uncommon on quartz grains). Elemental data by electron microprobe analysis indicated that the whitish micrites and spar were almost pure calcite, whereas the browner patches were a mixture of clay with some amorphous silica. This suggests that some degradation of the host siliciclastic grains has occurred. Calcrete carbonates in Units II, III, IV and VI are soft and chalky; thin section and SEM observations reveal considerable porosity in the micrite groundmass. Some of these chalky nodules have an indurated core, somewhat like the hard nodules, that grade outwards into a chalky calcrete. The chalky nodules thus represent an early stage of nodule development, that over time developed into harder, well-cemented nodules that are petrologically more complex; however both types belong to the stage II classification of Gile et al. (1966) and Machette (1985). The calcite cemented lenses in the basal part of Unit IV contain void-filling spar and micro-spar cements suggesting the introduction of cement by groundwater. 5. Chronology The optical ages are stratigraphically consistent and the results show that about 160 ka of sedimentary record is preserved (Table 1 and Fig. 2). Unit III could not be dated but the next succeeding aggradation (Unit IV) was deposited 90 ka, probably over a period of 10–15 ka (see below). Unit V sands were deposited around 60 ka. The lower half of the Unit VI gave an age of about 27 ka, whereas sands in the upper part of this unit show progressively younger ages of about 17 and 14 ka, suggesting that this unit comprises at least two aggradation episodes. A diastem within Unit VI might have been expected to appear as a distinctive sedimentary surface (e.g. bedding plane or erosion surface) but this was not obvious in the field. Unit VII sands at the top (30 cm depth) of the section were dated to 60 years, confirming its very recent origin. These young ages also suggest that the sands in the region were well-bleached prior to deposition and will not cause significant errors on a millennial time scale. The multiple aliquot ages therefore are considered robust. In the case of litho-unit IV, the five values vary in a range of 105–81 ka. This variation in ages largely reflects undetected variability of dose rate and would need further investigation. The mean of these ages is 93 ka and we have for simplicity placed the time period for this phase of sand aggradation at 90 ka.
composition of the soil environment during nodule growth, or isotopic inhomogeneity caused by the presence of unseen patches of diagenetic spar beneath the sampled surface. Either way, due to this variability, the precision of the isotope data from the whole profile was taken to be ±0.25‰ for d13C and ±0.8‰ for d18O. Samples at 10 cm intervals were collected to document intra-unit variability (Fig. 3). The results (Fig. 3) show that d13C values cluster in a rather narrow range of +0.5 to 1.1‰ with a mean of 0.2‰ and standard deviation of 0.6‰. In each litho-unit, despite the narrow range in d13C values, a clear variability with depth exists. For example, in Unit VI the top sample has a d13C of 2.0‰, whereas the four samples below it have values close to 0‰, followed by samples with values around 1‰. The d18O values show a larger range of variation between 6.4 and 2.7‰. Besides significant variation within units, there is an overall trend toward less negative values upwards, particularly above the middle of Unit VI. The samples in this study, range in age from 90 ka to 14 ka, broadly corresponding to marine isotope stages (MIS) 5a, 4, 3 and 2. With the exception of the older samples in Unit IV (90 ka), most of the isotope data come from calcrete samples that formed after 62 ka and before 27 ka (Fig. 3). During this period, the d13C values varied little (between +0.5 to 1.1‰), consistent with calcrete d13C values from nearby Shergarh Trijunction (Andrews et al., 1998), and typical of the d13C from C4 dominated vegetation, particularly in Unit V (58 to 62 ka). The 1‰ variation in the d13C values could reflect minor fluctuations in the strength of the monsoon, with lower values indicating increased proportions of C3 grasses, or perhaps more likely an increase in soil respiration rate (see also Andrews et al., 1998). Thus, although calcrete formation is attributed to ameliorated climate interludes, the isotopic data suggest that the seasonality of rainfall and the high temperature regime, did not allow a sizable presence of C3 vegetation in the area. The d18O values of calcrete in Units IV and V ( 6.8‰ to 4.0‰) are typical of calcretes formed from slightly to moderately evaporated monsoon-sourced water (Andrews et al., 1998), the variability reflecting principally the degree of evaporation and small temperature changes. Thus, the rainfall regime was largely monsoonal (as it is today) during the periods when most calcrete development occurred. The tendency toward heavier d18O values in the upper part of Unit VI could reflect more intense evaporation and higher aridity, perhaps a switch toward weakened or non-monsoonal conditions (see also Andrews et al., 1998) and possibly the first signs of aridity associated with the Last Glacial Maximum (LGM).
6. Stable isotopes 7. Discussion The stable isotope composition of calcrete nodules in litho-units IV (top part 90 ka), V (60 ka) and VI (27–14 ka) were analysed (Table 2). The biggest intra-nodule variability is 1.6‰ for d18O and 0.5‰ for d13C. This variability could be due to changes in the isotopic
7.1. Aggradation episodes and periods of stability Sharp boundaries between sedimentary units and the presence of distinct calcrete in each litho-unit, suggest discrete sand aggra-
Table 1 Radioactivity, dose rate and IRSL ages of different litho-units. Litho-unit and depth (m)
Sample No.
U (ppm)
Th (ppm)
K (%)
Dose (Gy/ka)
De (Gy)
Age (ka)
VII 11.65 VI 10.30 VI 9.85 VI 8.70 VI 8.30 V 7.95 IV 5.95 IV 4.75 IV 4.30 IV 3.90 IV 3.10 IV 2.80 II 1.55
TR-CH1 TR-6 TR-7 TR-8 TR-9 TR-10 TR-12 TR-15 TR-16 TR-19 TR-CH21 TR-CH22 TR-152
2.15 ± 0.94 1.5 ± 0.43 1.5 ± 0.37 1 ± 0.65 2.50 ± 0.5 1.9 ± 0.50 1.2 ± 0.7 1.7 ± 0.41 1.24 ± 0.33 2.1 ± 0.70 3.3 ± 1.11 1.7 ± 0.7 0.12 ± 0.13
12.20 ± 3.2 6.4 ± 1.5 8.4 ± 1.3 9.6 ± 2.2 7 ± 1.75 4.8 ± 1.71 9.5 ± 2.3 5.90 ± 1.4 5.70 ± 1.18 9.7 ± 0.7 8.9 ± 3.8 8 ± 2.2 2.5 ± 0.47
1.69 ± 0.10 1.63 ± 0.10 1.4 ± 0.10 1.5 ± 0.10 1.43 ± 0.10 1.7 ± 0.10 1.4 ± 0.10 1.60 ± 0.10 1.9 ± 0.10 1.6 ± 0.10 1 ± 0.10 1.2 ± 0.10 1.6 ± 0.10
3.6 ± 0.3 3.0 ± 0.20 2.9 ± 0.18 3 ± 0.26 3 ± 0.22 3 ± 0.22 2.9 ± 0.27 2.93 ± 0.19 3.2 ± 0.18 3.3 ± 0.27 2.95 ± 0.40 2.7 ± 0.26 2.3 ± 0.14
0.20 ± 0.01 41 ± 8.0 50 ± 4.4 701 ± 2.8 83 ± 5.8 174 ± 11.2 290 ± 13.4 316 ± 10 287 ± 2.5 265 ± 26 295 ± 51 223 ± 21 351 ± 67
0.06 ± 0.01 13.7 ± 2.9 17.5 ± 1.9 27.0 ± 2.5 27.4 ± 2.7 58 ± 5.1 101 ± 10.4 105 ± 3.8 91 ± 8.8 81 ± 10.6 100 ± 22 82 ± 11 155 ± 31
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Table 2 Stable isotope data on carbonate nodules. Depth (m)
d13C
d18O
Litho-unit
Notes on nodule type and sampling
Unit VI
Fairly soft, v. slightly browny cream. Crushed Hard, pale grey Hard, pale grey Repeat sample Hard, creamy, avoided fractures Hard, creamy, avoided fractures Softer than above Soft, avoided inclusion, creamy Creamy Hard, creamy
2.03 0.22 0.18 0.18 0.15 0.35 1.14 1.10 0.73 0.54
6.08 3.40 3.54 3.58 2.69 4.29 5.12 4.82 4.17 5.53
7.90 7.90 7.90 7.90 7.80 7.70 7.60 7.50 7.40 7.30 7.20 7.10 7.00 7.00 6.90 6.80 6.60 6.60
Unit V
Hard, creamy. First of profile across nodule Hard, creamy. Second of profile across nodule Hard, creamy. Third of profile across nodule Hard, creamy. Fourth of profile across nodule Slightly softer, creamy Fairly hard Fairly hard Fairly hard Fairly hard soft edges (edges avoided) Fairly hard soft edges (edges avoided) Fairly hard Fairly hard Fairly hard Repeat Small hard nodules with hard edges Small hard nodules with hard edges Slightly softer, v. soft edges, small nodules Repeat
0.13 0.36 0.02 0.37 0.51 0.09 0.31 0.31 0.55 0.15 1.11 0.88 0.18 0.20 0.13 0.43 0.63 0.61
5.29 5.46 3.92 4.88 5.61 6.09 6.23 6.20 5.83 4.84 6.31 5.95 4.65 4.75 6.06 6.57 4.14 4.31
6.40 6.40 6.40 6.40 6.20 6.00
Unit IV
Hard. First of profile across nodule Hard. Second of profile across nodule Hard. Third of profile across nodule Hard. Fourth of profile across nodule Fairly hard soft edges (edges avoided) Hard, less grey. Crushed
0.06 0.12 0.04 0.14 0.08 0.06
6.69 6.84 5.96 5.25 6.10 5.12
Below profile surface 9.55 9.45 9.30 9.30 9.10 8.90 8.70 8.50 8.30 8.05
All samples were drilled from slabbed faces unless stated otherwise.
Fig. 3. Stable oxygen and carbon isotope data on calcrete nodules of litho-units IV, V and VI.
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dation events interspersed by periods of reduced sand flux and stability. The periods of stability facilitated pedogenesis and calcrete formation. The optical ages provide a time frame for these events and the date of 160 ka at the sequence base is one of the older ages for aeolian sand so far recorded in the Thar Desert. This extends the history of aeolian dynamism to the late Middle Pleistocene. Subsequent aggradation episodes are bracketed by the ages to the periods 90, 60, 27 and 17–14 ka. The 60 ka episode was probably a regional-scale event as it was also recorded in a sand covered piedmont section at Shergarh Tri Junction near Chamu (Andrews et al., 1998), and on the eastern and southern margins of the Thar (Chougaonkar et al., 1999; Juyal et al., 2003). Stable isotope data from Chamu calcretes are broadly similar to those from Shergarh Tri Junction and overall the d18O values do not imply strong aridity (Andrews et al., 1998). Aggradation episodes around 27 and 13 ka at Chamu have been reported from other parts of the Thar and its margin (Singhvi and Kar, 2004) suggesting that they were regional events. Between 60 and 27 ka (Units V/VI), the laterally impersistent strata are most likely to correspond to the 43–55 ka aggradation episode of Andrews et al. (1998), who suggested that this period was characterized by monsoonal climate. Several other terrestrial records also indicate that the period corresponding to MIS 3 was more humid (Tandon et al., 1997; Anonymous, 2000; Kar et al., 2001; Srivastava et al., 2003; Williams et al., 2006). In the basal part of Unit IV, the presence of groundwater-related cementation features accompanied by rhizoliths and mottled fabrics of alternating oxidizing and reducing conditions, suggest that after this phase of deposition at 90 ka, a locally raised groundwater table was established for some time. This presumably resulted from a period of higher than present rainfall, and/or lower temperatures with reduced evapotranspiration. Since the most pronounced and thick calcrete development occurred after 60 ka and before 27 ka, this period was probably also most conducive to the formation of these groundwater-related features. Amelioration of climate for this period has been reported from adjoining river basins by Tandon et al. (1997) and Juyal et al. (2006). The up-welling record from the Arabian Sea (Prell and Van Campo, 1986) also shows a strengthening of the monsoon during this period. Indeed indications of other climate ameliorations prior to, and after, this event in the Chamu record broadly follow fluctuations in monsoon strength as inferred from Arabian Sea records for the Late Quaternary (Prell and Van Campo, 1986) and from the Bay of Bengal (Kudrass et al., 2001). 7.2. Calcrete development and evidence of denudation Unit VI has pedogenically modified sands in the upper part, that grade gradually with depth into a weakly-developed calcrete. This is characteristic of the process of leaching of soil carbonate by rainwater from the upper part of a profile and its accumulation at the depth of the wetting front (Gile et al., 1966; Deutz et al., 2002). However, above the well-developed calcrete of Unit V there is no zone of leaching, only a calcrete-bearing stratum. As the optical ages of Units VI and V differ by 30–40 ka, the corresponding calcretes in these units were presumably related to separate episodes of aeolian aggradation. The upper part of the Unit V calcrete has patches of reworked nodules that clearly originated by surficial re-working of the nodules once the overlying aeolian sand in the upper part of the profile was removed. In general, aeolian sands are friable and erodible, unless strengthened by the development of calcrete. The calcretised surface of Unit V has a gentle slope and its setting suggests that it formed in the ‘nose’ of a fossil parabolic dune. The wide ‘nose’ between the two ‘arms’ of the dune is the zone that both accumulates the maximum amount of sand, and also discharges sand from the crest during downwind dune crest
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migration. Therefore, it is not unreasonable to suggest that such a donor zone was present but subsequently eroded. Such denudation of calcretised surfaces has been documented in coastal dunes of Western Australia (Arakel, 1982). Optical ages from the host sands potentially provide constraints on the time of their burial while pedogenesis and calcrete nodule formation must occur in time intervals shorter than those defined by the age of the host matrix. As the calcretes are related to aggradation units (see above), they allow assignation of an upper limit for the timing of calcrete formation. Thus, the hard nodular type II calcretes in Unit V must have taken <30 ka to form, during the period 60 to 30 ka (the age of overlying Unit VI). In this case, the impersistent aeolian aggradation event (adjacent to the sampled section – see above) probably correlates with the 43 ka event at Trijunction (Andrews et al., 1998). Hence the likely maximum duration of calcrete formation in Unit V can then be estimated as 17 ka. Similarly, the Unit VI calcrete with soft, chalky nodules must have formed between 30 ka and the start of LGM aridity (at 21 ka), suggesting a maximum duration of 10 ka for the formation of chalky nodules. Although both these types of nodules belong to calcrete stage II, the present study shows that the soft, chalky type can form in a much shorter time (<10 ka) than the hard and evolved type (15–20 ka). These estimates compare well with data from New Mexico (USA) where stage II calcretes were inferred to form over time periods of 15–8 ka (Gile, 1994) or 20–13 ka (Leeder, 1970; Wright, 1990b; Deutz et al., 2002). 8. Conclusions At the regional-scale, Late Quaternary aeolian sedimentary records from the Thar Desert broadly follow fluctuations in monsoon strength as inferred from the Arabian Sea and other regionally important palaeoclimatic records. Aeolian aggradation at Chamu was clearly episodic, with periods of low-sand accretion facilitating calcrete formation. Climatic amelioration between 50 and 30 ka was most pronounced which corroborates the earlier palaeoclimatic and oceanic records from the region. The calcrete profiles were repeatedly truncated by phases of denudation which mainly removed the solum, exposing a pavement of indurated calcrete nodules. This caused the formation of a ‘stacked’ calcrete profile. The temporal significance of stacked calcrete profiles is not yet well known. However, our combined sedimentological and optical dating approach confirms that stage II calcretes formed from chalky precursor nodules that evolved into harder more cemented nodules. The chalky nodules form in <10 ka, while the more indurated nodules probably take almost twice as long again to form (<20 ka). The formation of evolved, indurated, nodules indicates extended periods of climatic amelioration, effective enough to permit stability of land surfaces and extensive mobilization of carbonate from the mildly calcareous solum. Acknowledgements This work was done as a part of a research programme supported by the Department of Science and Technology (DST/ESS/CA-03/96). Sarah Dennis did the stable isotope analysis in the UEA stable isotope laboratory. We thank Dr. Y.C. Nagar for his patient help with the figures: Philip Judge (UEA) drafted the final version of Fig. 3. References Achyuthan, H., Quade, J., Roe, L., Placzek, C., 2007. Stable isotope composition of pedogenic carbonates from the eastern margin of the Thar Desert, Rajasthan, India. Quaternary International 162–163, 50–60. Allchin, B., Goudie, A.S., Hegde, K.M.T., 1978. The Pre-history and Palaeogeography of the Great Indian Desert. Academic Press, London.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Multiple episodes of aggradation and calcrete formation in Late Quaternary aeolian sands, Central Thar Desert, Rajasthan, India R.P. Dhir a, A.K. Singhvi b,*, J.E. Andrews c, A. Kar a, B.K. Sareen a,e, S.K. Tandon d, A. Kailath b, J.V. Thomas b a
Central Arid Zone Research Institute, Jodhpur 342003, India Physical Research Laboratory, Ahmedabad 380009, India c School of Environmental Sciences, University East Anglia, Norwich NR4 7TJ, UK d Department of Geology, University of Delhi, Delhi 110007, India e Geological Survey of India (WR), 15–16 Jhalana Doongri, Jaipur 302017, India b
a r t i c l e
i n f o
Article history: Received 7 August 2008 Received in revised form 10 June 2009 Accepted 2 July 2009
Keywords: Thar Desert Calcretes Aeolian sediments Palaeoclimate Luminescence dating
a b s t r a c t A 12 m thick section in a dune-sandy plain terrain of the Central Thar in Rajasthan, has provided a near continuous record of environmental change for the past 160 ka. The site presently receives 280 mm rainfall, almost entirely from the summer monsoon. The base of this section comprises a gravel bed of an ephemeral stream and the overlying six litho-units, each with discrete boundaries, comprise a succession of aeolian sands. Luminescence dating provided an estimate of the timing of the sand aggradation periods to 160, 90, 60, 27 and 17–14 ka and helped constrain the timing of calcrete formation periods. In each aggradation unit, discrete nodular calcretes formed by the leaching of carbonate from the overlying solum. This is analogous to present-day conditions in sandy plains during periods of increased rainfall and landscape stability. Several of these calcretes are, however, devoid of their donor solum, suggesting solum removal during a subsequent period of decreased rainfall and resultant surface instability. This is supported by the presence of reworked nodules on the surfaces of some calcretes. A prominent phase of calcrete development followed the aeolian sand aggradation at 60 ka, suggesting climate amelioration that also caused the formation of groundwater-related calcrete and mottling. The study suggests that stage II calcrete nodules form in a time frame of 10–20 ka, and confirms limited data on the duration and stage of calcrete development in the literature. The d13C values of calcrete carbonate lie in a narrow range (+0.5 to 1.1‰) suggesting formation under soils with C4 vegetation. This implies that even during phases of climatic amelioration, the high temperatures and increased seasonality of rainfall did not permit significant development of C3 plants in the Central Thar. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Sedimentary records from deserts and semi-deserts have been used to reconstruct climatic changes during the Quaternary (Glennie, 1987; Dhir et al., 2004; Singhvi and Kar, 2004; Lancaster, 2007; Singhvi and Porat, 2008). In these regions, climate amelioration leads to stabilization of aeolian sediments, soil profile development, activation of local fluvial systems, the formation of playas, and changes in the hydrology and geochemistry of lakes. Conversely, more arid episodes lead to desiccation, decreased vegetation cover and mobilization of sands. Soil carbonates (calcretes) in semi-deserts form during phases of climatic amelioration when sediment stabilization allows calcium carbonate to mobilize within * Corresponding author. Address: Physical Research Laboratory, Planatory and Geosciences, Navrangpura, Ahmedabad, Gujarat 380009, India. Tel.: +91 79 26314366; fax: +91 79 26301502. E-mail address: [email protected] (A.K. Singhvi). 1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2009.07.002
the solum (Wright, 2008). Calcretes are thus valuable palaeoclimatic indicators. So far, numerous ‘stacked’ calcrete profiles have been described from pre-Quaternary sediments (e.g. Watts, 1980; Tandon et al., 1995); however, these are often without chronometric controls. Consequently, the temporal significance of stacked calcrete profiles is largely undetermined. In this study, we have dated a stacked Quaternary calcrete profile from the Thar Desert (India) to provide chronometric constraints on the development of various calcrete types (Wright, 2008) and to understand their palaeoclimatic significance in both local, regional and global contexts. 2. Study area Modern mean annual rainfall in the Thar Desert ranges from 500 mm (eastern margin) to <100 mm (western margin) (Fig. 1). The potential evapotranspiration is 1650 mm (east) and 2000 mm (west), implying a high moisture deficit. Of the total
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Fig. 1. Sketch map of Thar Desert, India and location of Chamu Site (marked +). Also shown are rainfall isohyets (in mm).
rainfall, over 85% is received in a few discrete events, during the monsoon season (July–September). Although rainfall is sparse, high atmospheric humidity and moderate temperatures during the monsoon provide conditions favorable for seasonal plant growth and biological activity. Considerable diurnal and seasonal temperature variations occur during the year: mean maximum and minimum temperatures during summer (May–June) are 41.0 °C and 26.3 °C respectively, and during winter (January) are 24.8 °C and 10.2 °C respectively. The study site is in the Central Thar Desert at Chamu village (60 km northwest of Jodhpur), with a mean annual rainfall of 280 mm (Fig. 1). The terrain comprises dunes and sandy plains overlying Late Proterozoic sandstone. Groundwater in this sandstone aquifer is 50 m below surface. A few phreatophytic trees, shrubs and a variety of grasses constitute the modern flora and the area is seasonally cultivated today. The sand grains in the aeolian sediments are mainly 100– 150 lm size, dominated by quartz along with some orthoclase and traces of plagioclase. Silt and clay fractions constitute 2–4% and 5% respectively of the dune sands, and 8–10% of each of these fractions in the soils of the sandy plains. The sands are weakly calcareous with 2–4 wt% calcium carbonate. The carbonate can occur as sand-sized grains, but is commonly silt-sized material or as coatings on coarse grains (Courty and Fedoroff, 1986; Dhir et al., 2004). The Quaternary sedimentary record of the Thar shows that climatic conditions fluctuated through time (Anonymous, 2000; Singhvi and Kar, 2004). Large areas of alluvial sediments in the region suggest less arid climatic conditions in the past (Ghose, 1965; Kar, 1995; Jain and Tandon, 2003) and archaeological evidence supports this inference (Allchin et al., 1978; Misra et al., 1982). Studies on Didwana and other lakes (Singh et al., 1974; Wasson et al., 1984) have demonstrated accentuated aridity during the Last Glacial Maximum (LGM) that persisted to about 13 ka, the time when the SW monsoon re-established itself. These studies, and also those of Enzel et al. (1999) and Roy et al. (2008), further suggest a climate optimum phase around 7.5–5 ka. Singhvi and Kar (2004) provided a synthesis of chronological work on the aeolian sands. A peak in aeolian sand dune aggradation occurred during a narrow time-window 13 ka which coincided with re-establish-
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ment of the SW monsoon (Chawla et al., 1992; Sirockko et al., 1993; Thomas et al., 1999; Singhvi and Kar, 2004). The discovery of Middle and Lower Palaeolithic tools in a deep trench in an obstacle dune at Didwana indicates that dune formation episodes predated 100 ka (Misra et al., 1982). This is corroborated by similar luminescence ages from the eastern margin and the Lower Luni Basin, of the Thar (Chougaonkar et al., 1999; Kar et al., 2001). The new luminescence dates presented in this paper add to this limited chronometric data on the long-term aeolian dynamics and palaeoclimatic changes in the Thar. Andrews et al. (1998) combined stable isotope data from calcrete-bearing aeolian sediments with optical dating to suggest that periods of weakened monsoon occurred during the LGM and at around 60 ka, with a more intense monsoonal period between 50 and 30 ka. Similar inferences were also drawn from optically dated fluvial records on the southern margins of the Thar (Juyal et al., 2006). More recent work by Achyuthan et al. (2007) presents a calcrete stable isotope record apparently extending beyond 163 ka, which in the absence of methodological details on dating can only be regarded as preliminary. These authors suggest that the longer term d13C variation was controlled by expansion of C4 vegetation during warmer and wetter monsoon-dominated interglacials, rather than by variability in atmospheric pCO2 (as proposed by Andrews et al., 1998).
3. Materials and methods The landscape at Chamu comprises coalesced parabolic sand dunes and sandy plains. A 3.5 km long section, up to 12 m deep was exposed during excavation for a lift canal. The exposure was mapped for lithofacies distribution and a representative section was logged and sampled in detail. Sand samples were dated using the infrared stimulated luminescence (IRSL) of feldspar grains which offered the only possibility for dating due to their higher saturation doses, compared to quartz. Although the stability of the luminescence signals in feldspars has been debated (Huntley and Lamothe, 2001), our earlier work from a nearby Thar site indicated that fading of feldspars luminescence during burial was not a serious problem (Andrews et al., 1998; Kar et al., 2001). Feldspar grains were extracted by standard pre-treatment with 1 N HCl and 30% H2O2 followed by density separation. Etching of the grains was not carried out so as to avoid uneven breakdown of grains. A monolayer of grains was stuck on stainless steel discs using a silicone spray. The measurements were made in a Riso TL–OSL reader with stimulation at 880 nm and observation in the blue window defined by a Corning 7–59 + BG-39 filters coupled to an EMI 9235 QA photomultiplier tube. A pre-heat of 220 °C for 1 min was used. Multiple aliquot additive dose analysis with short shine normalization was carried out for the paleodose estimation. This was considered appropriate, given that modern sands from the region give an apparent age of 60 years. This implies that the samples were well-bleached and consequently the ages are robust. Radioactivity was measured using ZnS (Ag) thick source alpha counting and NaI (Tl) gamma spectrometry and radioactive equilibrium was assumed in the computation of the dose rates. In estimating the sample dose rates it was assumed that the soil carbonates precipitated soon after sedimentation of the sand, and that the dose rate at the mm-scale (beta range) was constant. This is plausible as the net dose rate is nearly the same as that from carbonate-poor sands. Samples for isotopic analysis were micro-drilled from the centre of individual nodules, which had been checked for potential contaminants (weathered rinds, spar-filled cracks, macro-organic matter). In two cases, four individual sub-samples were taken across the thickness of a single nodule to assess isotopic variability
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within a nodule. The isotopic chemistry procedures followed Andrews et al. (1993). Isotopic data are reported relative to the Vienna Pee Dee Belemnite (VPDB) scale (Coplen, 1994). The mass spectrometer was calibrated using the NBS 18 and NBS 19 standards. Measurements on random sample replicates indicated a precision better than ±0.06‰ for d13C and ±0.10‰ for d18O. 4. Litho-units and their morphology From the base upwards, seven litho-units, I–VII, were identified. These are described alongside the analytical data and optical ages in Fig. 2. Unit I comprises gravel with both clastic material and small round carbonate nodules in calcareous cement. In thin section these nodules contain pisoliths, and spar cement healed cracks. The unit is interpreted as the gravelly bed of an ephemeral stream with reworked calcrete nodules and clastic gravel derived from pre-existing deposits or bedrock. The six units above Unit I consist of aeolian sands. Unit II, 80 cm thick, contains abundant chalky nodules (see below); Unit III (1 m thick) has poorly sorted sand containing occasional nodules or chalky segregations. Unit IV, 4 m thick, comprises friable aeolian sands. Its basal part contains rhizoliths (see below) and carbonate-cemented lenticular features, while the middle part is punctuated by thin (5–20 cm thick), planar, carbonate-cemented bands that persist laterally for 100–150 m. The upper part of the
unit contains patchy nodular calcrete. Another distinguishing feature of Unit IV is the presence of faint, greyish-blue mottles suggesting alternating oxidizing and reducing conditions. Unit V is 1.2 m thick and is dominated by dense, hard nodular calcrete (20–25% of the unit mass) with nodules typically 1–2 cm in diameter. Sands between the nodules are calcareous with total carbonate content 20 wt% (Fig. 2). Another, laterally impersistent nodular calcrete, similar to Unit V was also recorded adjacent to the sampled section, above Unit V and below Unit VI. Unit VI is 2–3 m thick and comprises friable aeolian sand with its lower half containing a few chalky calcrete nodules, randomly distributed in the sandy matrix. Upwards, it graded into yellowish-brown sand, characterized by lower calcium carbonate content, decreased friability and with some evidence of ped formation. We interpret these features as evidence for partial pedogenesis of the sand. Unit VII has variable thickness and comprises highly friable, loose, lightyellowish-brown sand, typical of un-pedogenised sands. Litho-units II, III, IV, V and VI, all contain nodular carbonate concretions up to 2 cm across. These are pedogenic calcrete nodules with alpha fabrics (see Wright, 1990a). The hard nodules in Unit V comprise 85–90% CaCO3 and in thin sections these appear as a clotted white to light brown micritic ground mass, locally grading into micro-spar and spar calcite. This matrix occasionally contains shrinkage cracks filled by whitish micro-spar with clastic grains floating in the matrix. Some grains have pronounced spar coronas
Fig. 2. Field log and some analytical data of Chamu Section. Symbols used in the log: aeolian aggradations (fine dots), calcrete nodules (hollow circles), sampling site for dating (solid circle). The scale (in meters) appears on the left. Other features in the log are described in the text alongside. The CaCO3 profile shows its content in per cent in finely dispersed form (fine dots). The hollow circles in carbonate profile indicate nodules and other secondary carbonate. Also shown are IRSL ages of host sands.
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and most plagioclase grains have corroded margins (uncommon on quartz grains). Elemental data by electron microprobe analysis indicated that the whitish micrites and spar were almost pure calcite, whereas the browner patches were a mixture of clay with some amorphous silica. This suggests that some degradation of the host siliciclastic grains has occurred. Calcrete carbonates in Units II, III, IV and VI are soft and chalky; thin section and SEM observations reveal considerable porosity in the micrite groundmass. Some of these chalky nodules have an indurated core, somewhat like the hard nodules, that grade outwards into a chalky calcrete. The chalky nodules thus represent an early stage of nodule development, that over time developed into harder, well-cemented nodules that are petrologically more complex; however both types belong to the stage II classification of Gile et al. (1966) and Machette (1985). The calcite cemented lenses in the basal part of Unit IV contain void-filling spar and micro-spar cements suggesting the introduction of cement by groundwater. 5. Chronology The optical ages are stratigraphically consistent and the results show that about 160 ka of sedimentary record is preserved (Table 1 and Fig. 2). Unit III could not be dated but the next succeeding aggradation (Unit IV) was deposited 90 ka, probably over a period of 10–15 ka (see below). Unit V sands were deposited around 60 ka. The lower half of the Unit VI gave an age of about 27 ka, whereas sands in the upper part of this unit show progressively younger ages of about 17 and 14 ka, suggesting that this unit comprises at least two aggradation episodes. A diastem within Unit VI might have been expected to appear as a distinctive sedimentary surface (e.g. bedding plane or erosion surface) but this was not obvious in the field. Unit VII sands at the top (30 cm depth) of the section were dated to 60 years, confirming its very recent origin. These young ages also suggest that the sands in the region were well-bleached prior to deposition and will not cause significant errors on a millennial time scale. The multiple aliquot ages therefore are considered robust. In the case of litho-unit IV, the five values vary in a range of 105–81 ka. This variation in ages largely reflects undetected variability of dose rate and would need further investigation. The mean of these ages is 93 ka and we have for simplicity placed the time period for this phase of sand aggradation at 90 ka.
composition of the soil environment during nodule growth, or isotopic inhomogeneity caused by the presence of unseen patches of diagenetic spar beneath the sampled surface. Either way, due to this variability, the precision of the isotope data from the whole profile was taken to be ±0.25‰ for d13C and ±0.8‰ for d18O. Samples at 10 cm intervals were collected to document intra-unit variability (Fig. 3). The results (Fig. 3) show that d13C values cluster in a rather narrow range of +0.5 to 1.1‰ with a mean of 0.2‰ and standard deviation of 0.6‰. In each litho-unit, despite the narrow range in d13C values, a clear variability with depth exists. For example, in Unit VI the top sample has a d13C of 2.0‰, whereas the four samples below it have values close to 0‰, followed by samples with values around 1‰. The d18O values show a larger range of variation between 6.4 and 2.7‰. Besides significant variation within units, there is an overall trend toward less negative values upwards, particularly above the middle of Unit VI. The samples in this study, range in age from 90 ka to 14 ka, broadly corresponding to marine isotope stages (MIS) 5a, 4, 3 and 2. With the exception of the older samples in Unit IV (90 ka), most of the isotope data come from calcrete samples that formed after 62 ka and before 27 ka (Fig. 3). During this period, the d13C values varied little (between +0.5 to 1.1‰), consistent with calcrete d13C values from nearby Shergarh Trijunction (Andrews et al., 1998), and typical of the d13C from C4 dominated vegetation, particularly in Unit V (58 to 62 ka). The 1‰ variation in the d13C values could reflect minor fluctuations in the strength of the monsoon, with lower values indicating increased proportions of C3 grasses, or perhaps more likely an increase in soil respiration rate (see also Andrews et al., 1998). Thus, although calcrete formation is attributed to ameliorated climate interludes, the isotopic data suggest that the seasonality of rainfall and the high temperature regime, did not allow a sizable presence of C3 vegetation in the area. The d18O values of calcrete in Units IV and V ( 6.8‰ to 4.0‰) are typical of calcretes formed from slightly to moderately evaporated monsoon-sourced water (Andrews et al., 1998), the variability reflecting principally the degree of evaporation and small temperature changes. Thus, the rainfall regime was largely monsoonal (as it is today) during the periods when most calcrete development occurred. The tendency toward heavier d18O values in the upper part of Unit VI could reflect more intense evaporation and higher aridity, perhaps a switch toward weakened or non-monsoonal conditions (see also Andrews et al., 1998) and possibly the first signs of aridity associated with the Last Glacial Maximum (LGM).
6. Stable isotopes 7. Discussion The stable isotope composition of calcrete nodules in litho-units IV (top part 90 ka), V (60 ka) and VI (27–14 ka) were analysed (Table 2). The biggest intra-nodule variability is 1.6‰ for d18O and 0.5‰ for d13C. This variability could be due to changes in the isotopic
7.1. Aggradation episodes and periods of stability Sharp boundaries between sedimentary units and the presence of distinct calcrete in each litho-unit, suggest discrete sand aggra-
Table 1 Radioactivity, dose rate and IRSL ages of different litho-units. Litho-unit and depth (m)
Sample No.
U (ppm)
Th (ppm)
K (%)
Dose (Gy/ka)
De (Gy)
Age (ka)
VII 11.65 VI 10.30 VI 9.85 VI 8.70 VI 8.30 V 7.95 IV 5.95 IV 4.75 IV 4.30 IV 3.90 IV 3.10 IV 2.80 II 1.55
TR-CH1 TR-6 TR-7 TR-8 TR-9 TR-10 TR-12 TR-15 TR-16 TR-19 TR-CH21 TR-CH22 TR-152
2.15 ± 0.94 1.5 ± 0.43 1.5 ± 0.37 1 ± 0.65 2.50 ± 0.5 1.9 ± 0.50 1.2 ± 0.7 1.7 ± 0.41 1.24 ± 0.33 2.1 ± 0.70 3.3 ± 1.11 1.7 ± 0.7 0.12 ± 0.13
12.20 ± 3.2 6.4 ± 1.5 8.4 ± 1.3 9.6 ± 2.2 7 ± 1.75 4.8 ± 1.71 9.5 ± 2.3 5.90 ± 1.4 5.70 ± 1.18 9.7 ± 0.7 8.9 ± 3.8 8 ± 2.2 2.5 ± 0.47
1.69 ± 0.10 1.63 ± 0.10 1.4 ± 0.10 1.5 ± 0.10 1.43 ± 0.10 1.7 ± 0.10 1.4 ± 0.10 1.60 ± 0.10 1.9 ± 0.10 1.6 ± 0.10 1 ± 0.10 1.2 ± 0.10 1.6 ± 0.10
3.6 ± 0.3 3.0 ± 0.20 2.9 ± 0.18 3 ± 0.26 3 ± 0.22 3 ± 0.22 2.9 ± 0.27 2.93 ± 0.19 3.2 ± 0.18 3.3 ± 0.27 2.95 ± 0.40 2.7 ± 0.26 2.3 ± 0.14
0.20 ± 0.01 41 ± 8.0 50 ± 4.4 701 ± 2.8 83 ± 5.8 174 ± 11.2 290 ± 13.4 316 ± 10 287 ± 2.5 265 ± 26 295 ± 51 223 ± 21 351 ± 67
0.06 ± 0.01 13.7 ± 2.9 17.5 ± 1.9 27.0 ± 2.5 27.4 ± 2.7 58 ± 5.1 101 ± 10.4 105 ± 3.8 91 ± 8.8 81 ± 10.6 100 ± 22 82 ± 11 155 ± 31
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Table 2 Stable isotope data on carbonate nodules. Depth (m)
d13C
d18O
Litho-unit
Notes on nodule type and sampling
Unit VI
Fairly soft, v. slightly browny cream. Crushed Hard, pale grey Hard, pale grey Repeat sample Hard, creamy, avoided fractures Hard, creamy, avoided fractures Softer than above Soft, avoided inclusion, creamy Creamy Hard, creamy
2.03 0.22 0.18 0.18 0.15 0.35 1.14 1.10 0.73 0.54
6.08 3.40 3.54 3.58 2.69 4.29 5.12 4.82 4.17 5.53
7.90 7.90 7.90 7.90 7.80 7.70 7.60 7.50 7.40 7.30 7.20 7.10 7.00 7.00 6.90 6.80 6.60 6.60
Unit V
Hard, creamy. First of profile across nodule Hard, creamy. Second of profile across nodule Hard, creamy. Third of profile across nodule Hard, creamy. Fourth of profile across nodule Slightly softer, creamy Fairly hard Fairly hard Fairly hard Fairly hard soft edges (edges avoided) Fairly hard soft edges (edges avoided) Fairly hard Fairly hard Fairly hard Repeat Small hard nodules with hard edges Small hard nodules with hard edges Slightly softer, v. soft edges, small nodules Repeat
0.13 0.36 0.02 0.37 0.51 0.09 0.31 0.31 0.55 0.15 1.11 0.88 0.18 0.20 0.13 0.43 0.63 0.61
5.29 5.46 3.92 4.88 5.61 6.09 6.23 6.20 5.83 4.84 6.31 5.95 4.65 4.75 6.06 6.57 4.14 4.31
6.40 6.40 6.40 6.40 6.20 6.00
Unit IV
Hard. First of profile across nodule Hard. Second of profile across nodule Hard. Third of profile across nodule Hard. Fourth of profile across nodule Fairly hard soft edges (edges avoided) Hard, less grey. Crushed
0.06 0.12 0.04 0.14 0.08 0.06
6.69 6.84 5.96 5.25 6.10 5.12
Below profile surface 9.55 9.45 9.30 9.30 9.10 8.90 8.70 8.50 8.30 8.05
All samples were drilled from slabbed faces unless stated otherwise.
Fig. 3. Stable oxygen and carbon isotope data on calcrete nodules of litho-units IV, V and VI.
R.P. Dhir et al. / Journal of Asian Earth Sciences 37 (2010) 10–16
dation events interspersed by periods of reduced sand flux and stability. The periods of stability facilitated pedogenesis and calcrete formation. The optical ages provide a time frame for these events and the date of 160 ka at the sequence base is one of the older ages for aeolian sand so far recorded in the Thar Desert. This extends the history of aeolian dynamism to the late Middle Pleistocene. Subsequent aggradation episodes are bracketed by the ages to the periods 90, 60, 27 and 17–14 ka. The 60 ka episode was probably a regional-scale event as it was also recorded in a sand covered piedmont section at Shergarh Tri Junction near Chamu (Andrews et al., 1998), and on the eastern and southern margins of the Thar (Chougaonkar et al., 1999; Juyal et al., 2003). Stable isotope data from Chamu calcretes are broadly similar to those from Shergarh Tri Junction and overall the d18O values do not imply strong aridity (Andrews et al., 1998). Aggradation episodes around 27 and 13 ka at Chamu have been reported from other parts of the Thar and its margin (Singhvi and Kar, 2004) suggesting that they were regional events. Between 60 and 27 ka (Units V/VI), the laterally impersistent strata are most likely to correspond to the 43–55 ka aggradation episode of Andrews et al. (1998), who suggested that this period was characterized by monsoonal climate. Several other terrestrial records also indicate that the period corresponding to MIS 3 was more humid (Tandon et al., 1997; Anonymous, 2000; Kar et al., 2001; Srivastava et al., 2003; Williams et al., 2006). In the basal part of Unit IV, the presence of groundwater-related cementation features accompanied by rhizoliths and mottled fabrics of alternating oxidizing and reducing conditions, suggest that after this phase of deposition at 90 ka, a locally raised groundwater table was established for some time. This presumably resulted from a period of higher than present rainfall, and/or lower temperatures with reduced evapotranspiration. Since the most pronounced and thick calcrete development occurred after 60 ka and before 27 ka, this period was probably also most conducive to the formation of these groundwater-related features. Amelioration of climate for this period has been reported from adjoining river basins by Tandon et al. (1997) and Juyal et al. (2006). The up-welling record from the Arabian Sea (Prell and Van Campo, 1986) also shows a strengthening of the monsoon during this period. Indeed indications of other climate ameliorations prior to, and after, this event in the Chamu record broadly follow fluctuations in monsoon strength as inferred from Arabian Sea records for the Late Quaternary (Prell and Van Campo, 1986) and from the Bay of Bengal (Kudrass et al., 2001). 7.2. Calcrete development and evidence of denudation Unit VI has pedogenically modified sands in the upper part, that grade gradually with depth into a weakly-developed calcrete. This is characteristic of the process of leaching of soil carbonate by rainwater from the upper part of a profile and its accumulation at the depth of the wetting front (Gile et al., 1966; Deutz et al., 2002). However, above the well-developed calcrete of Unit V there is no zone of leaching, only a calcrete-bearing stratum. As the optical ages of Units VI and V differ by 30–40 ka, the corresponding calcretes in these units were presumably related to separate episodes of aeolian aggradation. The upper part of the Unit V calcrete has patches of reworked nodules that clearly originated by surficial re-working of the nodules once the overlying aeolian sand in the upper part of the profile was removed. In general, aeolian sands are friable and erodible, unless strengthened by the development of calcrete. The calcretised surface of Unit V has a gentle slope and its setting suggests that it formed in the ‘nose’ of a fossil parabolic dune. The wide ‘nose’ between the two ‘arms’ of the dune is the zone that both accumulates the maximum amount of sand, and also discharges sand from the crest during downwind dune crest
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migration. Therefore, it is not unreasonable to suggest that such a donor zone was present but subsequently eroded. Such denudation of calcretised surfaces has been documented in coastal dunes of Western Australia (Arakel, 1982). Optical ages from the host sands potentially provide constraints on the time of their burial while pedogenesis and calcrete nodule formation must occur in time intervals shorter than those defined by the age of the host matrix. As the calcretes are related to aggradation units (see above), they allow assignation of an upper limit for the timing of calcrete formation. Thus, the hard nodular type II calcretes in Unit V must have taken <30 ka to form, during the period 60 to 30 ka (the age of overlying Unit VI). In this case, the impersistent aeolian aggradation event (adjacent to the sampled section – see above) probably correlates with the 43 ka event at Trijunction (Andrews et al., 1998). Hence the likely maximum duration of calcrete formation in Unit V can then be estimated as 17 ka. Similarly, the Unit VI calcrete with soft, chalky nodules must have formed between 30 ka and the start of LGM aridity (at 21 ka), suggesting a maximum duration of 10 ka for the formation of chalky nodules. Although both these types of nodules belong to calcrete stage II, the present study shows that the soft, chalky type can form in a much shorter time (<10 ka) than the hard and evolved type (15–20 ka). These estimates compare well with data from New Mexico (USA) where stage II calcretes were inferred to form over time periods of 15–8 ka (Gile, 1994) or 20–13 ka (Leeder, 1970; Wright, 1990b; Deutz et al., 2002). 8. Conclusions At the regional-scale, Late Quaternary aeolian sedimentary records from the Thar Desert broadly follow fluctuations in monsoon strength as inferred from the Arabian Sea and other regionally important palaeoclimatic records. Aeolian aggradation at Chamu was clearly episodic, with periods of low-sand accretion facilitating calcrete formation. Climatic amelioration between 50 and 30 ka was most pronounced which corroborates the earlier palaeoclimatic and oceanic records from the region. The calcrete profiles were repeatedly truncated by phases of denudation which mainly removed the solum, exposing a pavement of indurated calcrete nodules. This caused the formation of a ‘stacked’ calcrete profile. The temporal significance of stacked calcrete profiles is not yet well known. However, our combined sedimentological and optical dating approach confirms that stage II calcretes formed from chalky precursor nodules that evolved into harder more cemented nodules. The chalky nodules form in <10 ka, while the more indurated nodules probably take almost twice as long again to form (<20 ka). The formation of evolved, indurated, nodules indicates extended periods of climatic amelioration, effective enough to permit stability of land surfaces and extensive mobilization of carbonate from the mildly calcareous solum. Acknowledgements This work was done as a part of a research programme supported by the Department of Science and Technology (DST/ESS/CA-03/96). Sarah Dennis did the stable isotope analysis in the UEA stable isotope laboratory. We thank Dr. Y.C. Nagar for his patient help with the figures: Philip Judge (UEA) drafted the final version of Fig. 3. References Achyuthan, H., Quade, J., Roe, L., Placzek, C., 2007. Stable isotope composition of pedogenic carbonates from the eastern margin of the Thar Desert, Rajasthan, India. Quaternary International 162–163, 50–60. Allchin, B., Goudie, A.S., Hegde, K.M.T., 1978. The Pre-history and Palaeogeography of the Great Indian Desert. Academic Press, London.
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