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High-Resolution Holocene Environmental Changes in the Thar Desert, Northwestern India

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Science  02 Apr 1999:
Vol. 284, Issue 5411, pp. 125-128
DOI: 10.1126/science.284.5411.125

Abstract

Sediments from Lunkaransar dry lake in northwestern India reveal regional water table and lake level fluctuations over decades to centuries during the Holocene that are attributed to changes in the southwestern Indian monsoon rains. The lake levels were very shallow and fluctuated often in the early Holocene and then rose abruptly around 6300 carbon-14 years before the present (14C yr B.P.). The lake completely desiccated around 4800 14C yr B.P. The end of this 1500-year wet period coincided with a period of intense dune destabilization. The major Harrapan-Indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase.

The southwestern Indian monsoon is critical for understanding past global and regional monsoon variations (1–3). The few records of Holocene monsoon variations from the areas that border the Arabian Sea in southern Asia have been based on pollen assemblages associated with the deposits of Lunkaransar, Didwana, and Sambhar paleolakes from northwestern India (4). These records, although based on limited dating, have been used extensively in regional compilations, in analysis of relations between summer insolation and the monsoon, and in paleoclimatic models (2–7) as well as to determine the relations between paleoclimate and Indus Valley civilizations (8, 9). Here, we present more detailed Holocene chronology of Lunkaransar based on analyses of the lacustrine laminated deposits, age dating, and geochemical analyses.

Lunkaransar (10) is a small, closed, dry basin surrounded by dunes at the northeastern margin of the Thar Desert (Fig. 1). The basin receives input from groundwater and direct rain and no input from streams. The water table is currently 2.4 m below the dry lake bed, and this water is saline with a composition that includes Na, Ca, Mg, Cl, SO4, and HCO3. Incoming sediments are only eolian sand from local dunes and eolian clay silt dust (11). Normally, the lake basin is totally dry, but heavy rainfall can form a temporary pool of water that evaporates during the dry season.

Figure 1

Location map of Lunkaransar (L) and Didwana (D) dry lakes in the Thar Desert (shaded area) showing 250- and 500-mm/year isohyets.

Trenches were excavated into the lake bed down to a thick, hard, carbonate layer at a depth of 3 m. The sedimentology of the upper 240 cm above the water table (Fig. 2) was documented at submillimeter-to-millimeter scales in both the field exposures and in the continuous, overlapping box cores in the laboratory. We obtained 15 radiocarbon dates (Table 1). The sequence was divided into four zones (Fig. 2) on the basis of characteristics of the deposits. Zone 4, dated at 4800 14C yr B.P. to recent, has no primary laminar structure and contains mud cracks, silt, and sand; it is interpreted as a dry lake basin that episodically was inundated by ephemeral lakes. Zones 1 to 3 (Figs. 2 and 3) are composed of two types of thin beds: (i) silt- and clay-rich detritus laminae with carbonate and in some cases thin gypsum laminae at boundaries, and (ii) gypsum laminae with some thin silt and clay laminae. We separated the entire sequence into four sedimentary facies (II to V) according to the dominant type and the thickness of the various beds (Fig. 3E) and inferred relative water depths

Figure 2

Four stratigraphic zones of the Holocene deposits of Lake Lunkaransar. Detailed documentation of the cores is available atwww.sciencemag.org/feature/data/985056.shl. There is not much difference between our dates and Singh's (4) three dates. The use of depositional rates led him and others to a different chronology.

Figure 3

δ13C of organic matter (A) and of the carbonates (B). Grains coarser than 125 μm are shown as a fraction of the total sample (C). (D) Percent CaCO3. Note the increase and decrease in the sand fraction and CaCO3, respectively, during 5500 to 4800 yr B.P. Occurrence of the various sedimentary facies along the core (E) can be interpreted as a relative lake depth. The only zone without gypsum in Fig. 2 is reflected here in a continuous facies V. Zone 4 was not analyzed because it represents present-day playa conditions and destroyed primary sedimentary structures.

Table 1

Lunkaransar radiocarbon analyses: s, sediment; c, charcoal. Calibration was done according to (22). PDB, Pee Dee belemnite standards.

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Field observations show that the maximum lake stage did not reach 5 to 7 m. The ability of the basin to sustain even this level (facies IV and V) through successive years varied significantly throughout the Holocene (Fig. 3E).

Thin-section and x-ray diffraction analyses indicate that the clastics are allochthonous silicate minerals (quartz, plagioclase, potassium feldspar, clays such as chlorite and mica, and some hornblende), carbonate minerals, and some gypsum. Gypsum is the only evaporite mineral detected. The clay fraction (<2 μm) includes illite-smectite, illite, chlorite, and palygorskite. The palygorskite is authigenic and is indicative of intense evaporation episodes (12) at pH ≈ 8.5.

The concentration of clastic grains in separate laminae and the parallel orientation of the platy and elongated minerals indicate that the clastic grains were derived from dust storms (11) and they settled in water. The gypsum laminae contain fine authigenic crystals and abraded, sand-sized gypsum crystals. The abraded gypsum grains are most common in facies II and were probably blown in from drying mudflats at the margins (13) of Lunkaransar lake during periods of low lake levels.

The rise and fall of the water table at Lunkaransar reflect the regional precipitation over the basin. A lake is formed in Lunkaransar when the water table rises above the surface. Our data show that between about 10,000 and 4800 14C yr B.P. the lake did not dry, and therefore the water table was always above the basin floor. Since 4800 14C yr B.P., the water table has been below the surface. Because of deposition in the basin, the elevation of the present-day water table at 2.4 m below the surface can be identical to the elevation of its levels during the low stands of the early Holocene lake. The early Holocene monsoon rains were not able to maintain a stable lake in this area for more than a few decades at a time, although there is no evidence of complete desiccation at any time before 4815 ± 75 14C yr B.P. (Fig. 3E). The only period when a sustained high-lake stand is evident in this basin during the Holocene was from 6300 to 4800 14C yr B.P. (zone 3) (Fig. 3E). Few to no evaporite layers exist between the silt laminae in the deposits from this period, indicating that the relatively high lake levels persisted year round and did not decline annually during the winter dry season, as is evident in other parts of the section where gypsum laminae alternate with each of the submillimeter clastic dust layers.

The values of δ13C of organic carbon (14) in the lake show two dramatic shifts during the Holocene (Fig. 3A). δ13Corg decreased from −14 per mil to −20 per mil around 6300 14C yr B.P. and returned abruptly at about 5500 to 5000 14C yr B.P. (Fig. 3A). The sudden drop could reflect an abrupt increase in the relative abundance of C3 versus C4 vegetation transported into the lake during a period of increased precipitation. Such an explanation, however, implies an extremely rapid change in the regional environmental conditions of the Thar desert.

A more plausible explanation is that the δ13Corg within the lake itself changed. During its earlier history (before 6300 B.P.) the lake was likely a shallow pan covered by extensive microbial (algal) mats. Remnants of mats are evident in all parts of the Lunkaransar cores. Productive mats can deplete a shallow water column of its dissolved inorganic carbon (DIC) (15). This depletion in turn drives an influx of atmospheric CO2, which yields DIC with a negative (typically −6 per mil) δ13C value (15,16). In deep water, the mat photosynthesis will not be able to deplete the DIC and the δ13CDIC will rise. The commonly used proxy, the isotopic composition of CaCO313Ccarb), is misleading in this case (Fig. 3B) because it represents mainly wind-blown carbonates, whereas δ13Corg represents genuine autochthonous organic matter (17). In this model, the time of maximum insolation of the early Holocene (<630014C yr B.P.) is associated in Lunkaransar with rather high δ13Corg, representing mat growth in extremely shallow water (10 to 40 cm); the noticeable shift in δ13Corg at about 6300 14C yr B.P. indicates, according to this model (18), a rise in lake level to >50 cm. The absence of gypsum at this zone (Fig. 2) supports the suggested depth increase and the dilution of the water.

The final decline of the lake high stand was associated with many minor lake-level fluctuations and an increase in eolian sand coarser than 125 μm (Fig. 3C) and containing subangular quartz grains derived from the surrounding dunes. This increase in sand coincides with a decrease in the CaCO3 percentage and the presence of broken, reworked calcic concretions from soil horizons developed in eolian sands. The increased transport of material from the dunes indicates destabilization of the surfaces of the dunes. No similar input of sand into the lake basin occurred during the previous 5000 years. This sudden pulse of eolian sand implied that there was a major environmental change in the surrounding dune field that is unprecedented in the earlier Holocene.

Our data indicate that the environment and climate of the past 500014C years were similar to those of the present. This long period of relatively low water table was punctuated by a few episodes of inundation of the dry bed of the playa lake as shown by the thin beds of weakly laminated fine silt observed in zone 4; two of theses beds are dated at 3785 ± 75 and 2325 ± 65 14C yr B.P. None of these beds are lacustrine deposits, which may indicate short episodes of heavy rains during a few consecutive years.

The lack of evidence for secondary playa processes that destroyed the laminated deposits and the dates from Lunkaransar indicate that a middle Holocene lake existed from 6300 to 4800 14C yr B.P., and that the highest lake stand probably ended before 5000 B.P. and not 3500 B.P., as previously suggested (4). The record implies that Lunkaransar lake rose abruptly around 6300 14C yr B.P. (5000 B.C.), persisted with minor fluctuations for the following 1000 calendar years, fell abruptly to the range of 10 to 40 cm of water at about 5500 14C yr B.P. (4200 B.C.), and dried completely by 4800 14C yr B.P. (3500 B.C.). A zone of ceramics and associated charcoal indicates that humans occupied the dry lake bed by 4230 ± 55 14C yr B.P. (2894 to 2643 B.C.) (Fig. 2). Therefore, the final drying occurred earlier than 4200 B.P. Extrapolation from the deposition rates of the lacustrine phase to the boundary between zone 3 and zone 4 indicates that the final drop of the water table below the surface occurred around 4600 B.P.

This drying phase precedes by 800 to 1000 years the rise of the Early and Mature Harappan phases of the Indus civilization from 4100 to 3500 14C yr B.P. (2600 to 2000 B.C.) (19) (Fig. 3). This contradicts the climate-culture hypothesis for northwestern India and Pakistan (8, 9). Improved climatic conditions did not lead to the rise of this major urban civilization, as has been suggested (8, 9). The collapse of the Indus culture in 3400 to 3300 14C yr B.P. (1700 to 1900 B.C.) has been attributed to a change to a more arid climate at the end of the middle Holocene wet period (4, 8, 9). Our chronology indicates that there is no relation between the proposed drought that caused the desiccation of the lakes and the collapse of the Indus culture, as the lakes dried out >1500 years earlier. The wet climate–Indus civilization relationship was previously challenged (20), but it remains a prime example of a climate-civilization relationship [p. 208 in (9)]. The Indus civilizations flourished mainly along rivers (20) during times when northwestern India experienced semiarid climatic conditions that are similar to those at present.

A few paleoclimate records from the Arabian Sea indicate an increase in the southwestern monsoon activity 10,000 to 9500 years ago. These records also show the decade- to century-scale variations observed in Lunkaransar and that the monsoon weakened about 5500 B.P. (1, 2). The Lunkaransar lacustrine record shows a simultaneous weakening of the monsoon on the Indian subcontinent, which is atmospherically downstream of the Arabian Sea during the southwestern monsoon. However, the contrast in the nature of the early Holocene and the middle Holocene lacustrine phases in Lunkaransar indicates that the hydrological conditions, and therefore the rainfall input, were different then. These differences are supported by records from southeastern Arabia (21). Two observations support the idea that an additional source of water beyond the summer monsoon precipitation is required to produce a perennial lake in Lunkaransar. First, the supposedly maximum summer monsoon rains (7) of the early Holocene were not able to maintain a perennial lake in Lunkaransar, such as existed 6300 to 4800 14C yr B.P. Second, none of the lake basins of northwestern India that currently experience 450 to 550 mm of summer monsoon rains sustain perennial lakes; according to pollen analyses (6), this was the amount of precipitation that prevailed in Lunkaransar during the lacustrine phases. Therefore, we postulate that an additional source of rainfall must be identified for explaining the lake rise and stabilization during the middle Holocene. We propose that winter precipitation, which currently accounts for only 20% of total precipitation (10), is a potential source (3–7). Winter precipitation may have a much larger effect on percolation to the subsurface hydrology that feeds the lake than increased monsoon rains alone. The additional winter rains made the critical difference between the early and middle Holocene hydrologic conditions. They eliminated the high-frequency level changes and the drop of water to gypsum deposition range and allowed for a perennial lake during the middle Holocene.

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