Palaeohydrologic situation in the Thar Desert

The different recharge rates of the shallow and deep groundwaters in the study area may be explained by means of the palaeohydrologic situation. Reliable temporal information exists from the eastern part of the Thar Desert in India. Sedimentological and geochemical evidence as well as 14C dates of salt-lake deposits of Sambhar (Thar Desert), Lunkaransar (India) and Didwana Lake (SE Cholistan) revealed that pluvial conditions existed between 13 000 and about 4000 years BP (Singh et al. 1972, 1974; Wasson et al. 1984; Fig. 5). Two pluvial periods might have existed which lasted from 13 000 to 8000 years BP and from 7000 to 4000 years BP. However, groundwater dating was necessary to gather palaeoclimatic information of our study area. It was done on the basis of

-1300 years

Hamad

dry period

/

_

Cholistan i i r dry period

reservoir reservoir correction

eu 4

eu 4

14 000 12 000 10 000

Fig. 5. Histograms of C dates reflecting the deterioration of the pluvial conditions within the desert belt between North Africa and the Thar Desert at about 4000 years bp. (Bottom) 14C dates from organic sediments from ancient salt lakes in the Indian desert (Singh et al. 1972, 1974). (Middle) DIC 14C dates for groundwater samples collected in Cholistan (Geyh & Ploethner 1995). (Top) DIC 14C dates of groundwater samples from the Hamad Basin (Iraq, Jordan, Syria and Saudi Arabia). The reservoir corrections for the Cholistan and Hamad groundwater samples of —3000 and —1300 years, respectively, were empirically estimated (Geyh et al. 1985).

14 000 12 000 10 000

8000 6000 4000 2000 0

conventional 14C age (years bp)

Fig. 5. Histograms of C dates reflecting the deterioration of the pluvial conditions within the desert belt between North Africa and the Thar Desert at about 4000 years bp. (Bottom) 14C dates from organic sediments from ancient salt lakes in the Indian desert (Singh et al. 1972, 1974). (Middle) DIC 14C dates for groundwater samples collected in Cholistan (Geyh & Ploethner 1995). (Top) DIC 14C dates of groundwater samples from the Hamad Basin (Iraq, Jordan, Syria and Saudi Arabia). The reservoir corrections for the Cholistan and Hamad groundwater samples of —3000 and —1300 years, respectively, were empirically estimated (Geyh et al. 1985).

the radioactive decay of radiocarbon in DIC (= HCO— + CO2) of the discovered groundwater body. Carbon-14 in biogenic carbon dioxide is involved during the process of groundwater recharge. A 14C age range of about 40 000 years is covered. The participation of fossil or almost fossil soil lime necessitates a reservoir correction of the conventional 14C ages of DIC. From this, the time scale of the groundwater is transferred to calendar years. The precision of the 14C dates may not be better than about +500 years. DIC was precipitated from 60 litres of groundwater with a saturated barium hydroxide solution in the field. The 14C measurements were done on acetylene as counting gas prepared via decomposition of DIC to CO2, transformation to lithium carbide and C2H2 with tritium-free water. The 14C measurements were carried out with proportional counters of different size. The DIC 1 C age of 16 fresh to saline groundwater samples covers a time span of 7700 to 15 900 years BP (Table 1).

The reliability of the uncorrected DIC 14C ages was verified by a geohydraulic estimate. The trend of 14C ages from east, at tube well T/W-9, to west, at tube well T/W-8, over a distance of 18 500 m allowed estimation of the tracer velocity Vtracer. We obtained 6m/a or 0.016 m/day based on the 14C age of 3000 years. It follows according to Darcy's law:

Using the regional hydraulic gradient 1 = 0.0003 and the effective porosity ne = 0.15, a value of Vtracer = 6m/a is obtained which leads to the hydraulic conductivity K = 8 m/day. This is the lowest K established by aquifer testing. Applying the total porosity (40%) as appropriate for mass transport calculations with 14C (Geyh et al. 1984; Maloszewski & Zuber 1985) the K value is around 20 m/day which is the mean K found by aquifer testing. This means that the tracer velocity reliably reflects the regional geohydraulic situation.

In order to reconstruct the palaeoclimate of the study region, the reservoir correction had to be determined. The limnologic time marks suggest a reservoir correction of 23000 years resulting in a range of the actual water age from 12 900 to 4700 years BP.

A methodically independent estimate of the reservoir correction was possible, with the 14C age of 2100 years BP (= 76.8 pMC) of the shallow groundwater sample taken from the test well

T/H-26 beside an artificial pond. In Cholistan slightly calcareous soils prevail. In 1990 the atmospheric and biogenic CO2 had a specific 14C activity of about 114 pMC. The lowering of the 14C value to 77.4 pMC in DIC of recent groundwater, or 32%, is due to dissolution of soil lime with a 14C activity of 0 pMC. It corresponds to a reservoir correction of —3100 years. This value is comparable with that mentioned above. The same process might have been effective during recharge of the discovered fossil groundwater body as the pedologic conditions did not change during the last 15 000 years.

Hydrochemical corrections of the 14C values were not necessary as the ranges of the bicarbonate concentration and the d13C values of 200 to 500 mg/l and — 4.9 to — 5.8%o, respectively, are small. The uncertainty of the 14C ages resulting from these ranges is already inherent in the uncertainty of + 500 years of the reservoir-corrected 14C groundwater dates.

The validity of the methodically differently determined ranges of the pluvial periods applying — 3000 years as reservoir correction is supported by the obvious synchronism of the groundwater recharge period in the sub-Indian continent (e.g. Cholistan) and in the Middle East (e.g. Hamad Basin; Geyh et al. 1985; Geyh 1994; Fig. 5). The time span lasting from 13 000 to about 4000 years BP includes the Neolithic Pluvial (9000 to 4500 years BP), geomorphologically proven by Kaiser (1973) and Kaiser et al. (1973). A similar palaeoclimatic scenario prevails in the Eastern Sahara where in Western Nubia the widespread distribution of stable freshwater lakes is documented by intense lacustrine sedimentation lasting from 11 500 to 5500 years BP. The deterioration of the palaeohydrological conditions between about 5000 and 4000 years BP within the desert belt extending from North Africa (Claussen et al. 1999; Pachur & Hoelzmann 2000) to the Thar Desert supports the concept that the Old Hakra River most likely disappeared due to the shift of the monsoonal belt southwards, as concluded by Wilhelmy (1969). Consequently, seepage of water from the Old Hakra River during periods of inundation is considered as the source of the fresh groundwater resource in Cholistan. According to Radhakrishna & Merh (1999) tectonic events also played a role. It has been assumed (Singh et al. 1972, 1974; Wasson et al. 1984) that the end of this pluvial period coincided with the decline of the Harappan Culture around 4000 years ago. But Enzel et al. (1999), while establishing a more reliable chronology, came to the conclusion that the end of the lake phase preceded the early and mature Harappan Culture by more than 800-1000 years.

The scatter of the 14C water ages along the fresh groundwater body may be explained by meandering

Table 1. Isotope results from Cholistan, Pakistan

Location

Well

Depth (m)

818O (%<,)

82H (%<,)

dex (%«)

3H (TU)

813C (%<,)

14C (pMC)

14C age (year BP)

14C age corrected

Mojgarh Forest

T/W-0

42-91

-5.08

-40.8

-0.2

<3.1

-4.9

24.9

+ 0.6

1116G

+ 200

8200

Mojgarh Forest

T/W-0

42-91

2 4.89

-40.2

-1.1

-4.9

25.1

+ 0.6

11080

+ 190

8100

S of Mirgarh Fort

T/W-4

61-12G

-5.38

- 41.8

+ 1.3

<1.9

-3.4

22.6

+ 0.4

11965

+ 155

9000

N of Wakarwala Toba

T/W-5

52-146

-4.79

- 40.1

-1.8

<2.1

- 4.3

23.1

+ 0.6

1176G

+ 200

8800

S of Januwali

T/W-6

51-105

-5.25

- 40.9

+ 1.7

<2.6

- 5.8

38.0

+ 0.7

7780

+ 145

4800

W of Mojgarh

T/W-8

56-99

-5.11

- 39.8

+ 1.1

<4.6

- 5.0

22.6

+ 0.4

1196G

+ 13G

9000

SSW of Chapuwala Toba

T/W-9

58-86

-4.94

-38.2

+ 1.3

- 5.1

32.7

+ 0.4

8965

+ 105

6000

N of Mojgarh

T/W-1G

50-104

-3.37

-39.6

<2.4

- 5.4

38.6

+ 0.7

7655

+ 145

4700

S of Chapuwala Toba

T/W-11

45-99

- 2.68

- 37.7

-12.6

- 5.0

32.6

+ 0.5

8990

+ 115

6000

Pirwala Toba

T/W-12

55-108

- 4.78

- 40.4

- 16.3

- 4.4

27.6

+ 0.6

10360

+ 175

7400

Karawala Tibba

T/W-14

42-79

- 5.40

- 42.9

- 2.2

-3.8

28.5

+ 0.5

10100

+ 135

7100

NE of Haiderwala Toba

T/H-26

21-24

- 5.25

- 42.0

+ 0.3

<2.1

-1.5

76.8

+ 0.7

2125

+ 75

0

NE of Haiderwala Toba

T/H-26

101-104

- 5.35

- 40.9

+ 1.9

<1.3

- 4.9

29.0

+ 0.3

9930

+ 75

6900

NE of Haiderwala Toba

T/H-26

137-14G

-5.18

- 39.4

+ 2.G

<2.0

- 4.9

21.6

+ 0.3

12325

+ 105

9300

NE of Haiderwala Toba

T/H-26

155-158

- 5.23

- 41.7

+ 0.1

<1.8

- 5.8

14.2

+ 0.2

15665

+ 115

12700

Toba

T/H-26

198 - 201

- 5.34

- 41.4

+ 1.3

<2.3

- 5.7

13.9

+ 0.5

15850

+ 315

12900

NE of Haiderwala Mojgarh

H/P-200

35-38

- 5.28

- 40.2

+ 2.G

<2.1

- 4.5

21.8

+ 0.5

12230

+ 175

9200

Mansura/Ranger's post

H/P-162

37-4G

- 4.32

- 34.3

+ 0.3

4.2 + 0.7

- 4.8

68.4

+ 0.9

3060

+ 105

0

Chak-315 at Mosque

H/P no

19-2G

- 7.66

- 47.6

+ 13.7

-11.6

96.8

+ 1.G

255

+ 85

Hakra Left at RD 47

Canal

0

- 7.44

- 46.8

+ 12.7

- 5.0

95.9

+ 1.2

335

+ 10G

Minor L-3

Canal

0

- 7.79

- 51.6

+ 10.7

- 5.0

89.8

+ 1.2

865

+ 105

Sutlej River / South. Bank

River

0

- 6.55

- 44.3

+ 8.1

16.4 +1.3

94.9

+ 1.4

415

+ 12G

215 m N at Sutlej River

Irr.-Well

15-64

- 8.02

- 54.6

+ 9.6

9.0 +1.2

-8.7

94.4

+ 0.7

465

+ 65

565 m N at Sutlej River

Handpump

7-8

- 8.42

- 56.3

+ 11.1

47.5 +1.4

-13.3

97.2

+ 0.7

230

+ 60

620 m N at Sutlej River

Irr.-Well

15-64

-8.11

- 56.4

+ 8.5

40.5 +1.2

93.4

+ 0.7

545

+ 55

ATER

Ui of the Old Hakra River. As the seepage could have occurred within decades, the ages reflect the time when river water seeped into the subsurface. Moreover, one has to keep in mind that the sampled wells might pump young groundwater from the aquifer below the old bed of the Old Hakra River rather than the youngest. This may explain some deviation from the historical records of its perennial behaviour.

An alternative interpretation is, however, also feasible. The youngest groundwater occurs around Khirsar village in the west where agricultural irrigation was applied. The corrected water ages here range from 4700 to 6000 years BP coinciding with low 813C values.

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