CARBON AND OXYGEN ISOTOPE PROFILES FROM THE TERMINAL PRECAMBRIAN MARWARSUPERGROUP, RAJASTHAN, INDIA

'Anil Maheshwari, 2A.N. Sial, and 3S.C. Mathur'Department of Geology, University of Rajasthan, Jaipur-t, India; Tel: +91(141)301534; anilm@datainfosys.net

WEG-LABISE, Dept. of Geology, CP, 7852, Federal University of Pernambuco, Recife, Brazil-50,732-9703Department of Geology, J.N V University, Jodhpur, India

ABSTRACT: Resultsofcarbonandoxygenisotope studiesfromearlyCambrianunfossiliferous carbonatesbelonging totheMarwarSupergroup,westernRajasthan, India are presented. The carbonatesare enrichedin "heavy" 0180 values (upto+ 6.50%0 PDB)and rules out the diageneticmodificationsofisotope signals. Themultiple, shorttermnegative (upto-1 0.31%0 PDB)andpositive(upto+2.80%0 PDB)carbonisotopeoscillationsobservedinMarwarcarbonateswellmatcheswiththelowerCambrian(Nemakit-Da1dynian) carbonates reportedworldwideandtheseoscillationshavebeenattributedtotheresultofclimaticoscillations eitherfromglaciations orfrom"coldhousephases"thatdidnot involvemajorglaciations.

INTRODUCTION

The Marine limestones and even dolostones faithfully recordthe carbon isotopic composition of the ocean water in whichthey formed (Scholle and Arthur 1980; Gao and Land 1991;Wang et al. 1996). However, many scientists have beenskeptical of interpretations based on carbon isotopestratigraphy, particularly when used as indicator of time inotherwise undatable Precambrian sedimentary sequences(e.g. Kaufinan and Knoll 1995). Saltzman et al. 1998investigated the carbon isotope stratigraphy of the well datedcarbonates of the Phanerozoic part of geological record andconfirmed that large perturbations in the carbon isotope ratiosof common carbonate rocks may be used as a precisemeasures of time.

Carbon isotope stratigraphy has also emerged as a major toolfor correlation of carbonate bearing rocks across thePrecambrian-Cambrian transition (Brasier and Sukhov 1997).About 10 carbon isotopic oscillations that range from -6.2 to+5.4%0 (PDB) have been documented through the EarlyCambrian (Ripperdan 1994; Brasier et al. 1994a, 1994b,1996) and an excursion of up to + 4.5%0 is now globallydemonstrated for the upper Cambrian (Saltzman et al. 1995).These carbon isotopic oscillations, where global, have beentaken to indicate changes in the rate of removal of the light12Corg isotope into biomass and (or) its burial in sediments(see Kaufinann and Knoll 1995). The highly oscillatory natureof Early Cambrian isotopic record contrast with carbonisotope values ranging between -1 and + 1%0 from MiddleCambrian formation from Australia (Donnelly et al. 1988) andin the Great Basin of the U.S.A. (Brasier 1992a). The aim ofthis paper is to provide a carbon isotopic curve obtained fromunfossiliferous carbonate sequence of Marwar Supergroup,western Rajasthan, India and to discuss its chronostratigraphicimplications.

GEOLOGY

The plains ofwestern Rajasthan, India is occupied by a numberof sedimentary basins ranging in age from the late Proterozoicto the Tertiary. The Marwar basin is the largest and the oldest

Carbonates and Evaporites, v. 18,no. 1,2003, p. 10-18.

basin of this region. The regional geology of this part ofsubcontinent has been shown in Fig. 1. Bounded by the earlyto middle Proterozoic Aravalli-Delhi orogen (> 1400 Ma) andthe late Proterozoic Malani Igneous Suite of rocks ( 780 to680 Ma), the Marwar basin is interpreted to be a typicalintracratonic sag basin. The Marwar basin comprises apackage of unfossiliferous siliciclastic, carbonate andevaporite facies, stratigraphically designated as the MarwarSupergroup. The rocks have formed mainly under shallowwater regime. The basin is largely floored by the MalaniIgneous Suite (780 to 680 Ma). The Marwar basin, beyondNagaur district, is overlain by the younger tertiary basin. It isbordered by the Jaisalmer and Barmer basin of the Mesozoic­Tertiary age.

The isochron ages of Malani Igneous Suite, the basement forthe deposition of Marwar Supergroup ranges between 780 to680 Ma (Rathore et al. 1999). Accordingly the Marwar basinhas come into existence later than 680 Ma. A number ofunfossiliferous sequences in the extra-peninsular region ofIndia have been assigned lower Cambrian age (Raha and Dass1989). Global Neoproterozoic glacial activity in the westernRajasthan is also reported and is represented by PokaranBoulder Bed, few kms away from study area. The PokaranBoulder Bed overlies the Malani Igneous Suite and is mainlyconsist of boulders and angular fragments of igneous rocksbelonging to the Malani Suite (Srivastava 1992) andaccordingly appear to be representing the Varangian (~620 to550 Ma) glacial episode. On the basis of seismic probing andlithological correlation the Marwar Supergroup ofrocks wereequated with the Saline Series of Pakistan, and were assignedCambrian age (Narayanan 1971; Pareek 1981, 1984). TheMarwar rocks were correlated with the Cambrian ofthe Salineseries in Salt Range of Pakistan on the basis of (1) occurrenceof salt pseudomorph shales (Srivastava and Srinivasan 1964),(2) evaporite sequence (Virendra Kumar 1995, 1999), (3)stromatolites (Barman 1980; Paliwal 1975) and (4) a doubtfulbrachiopod fossil (orthis) of the Cambrian age (Khan 1973).

The stratigraphic succession of Marwar Supergroup has beenprovided in Table 1. The package of Marwar Supergroup ofrocks comprises siliciclastic carbonate and evaporite

MAHESHWARI, SIAL, AND MATHUR

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00000000

v.,..",,'""'.

0 50 iCOI I I

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./(./0" .,1 0 0 0 0 ,. e.-

fo 0 0 0 0 l.Iooooo(

/ 0 0 0 0 0 -r~

.,' 0 0 0 0 0

",//" 0 0 0 0 0

{o 0 0 00000

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1/-\-'-1f

~6

I 0

70

o26-

o28-

o24-

o3()-

Figure 1. Regional geological map of western Rajasthan, India. (1) Banded Gneissic Complex, (2) Aravalli Supergroup,(3) Delhi Supergroup, (4) Proterozoic sediments of uncertain age, (5) Neoproterozoic igneous activity, (6) MarwarSupergroup, (7) Jalore and Siwana granite, (8) Mesozoic sediments, (9) Deccan Traps, (10) Tertiary sediments.

lithofacies having maximum thickness of 2000 metres.Stratigraphically these rocks are divided into lower JodhpurGroup consisting of Pokaran boulded bed and Jodhpursandstone with subordinate shales, the middle Bilara Groupcomprising stromatolitic dolomitic limestone and interbeddedlimestone and dolomite, and the upper Nagaur Group,containing shaley sandstone, pebbly to conglomeraticsandstone and evaporites (Pareek 1981, 1984). Some

investigatorsplace the evaporites as a separate group betweenBilara Group and Nagaur Group. It is designated as HanseranGroup which comprises interbedded sequence of clay,dolomite, anhydrite and halite (Das Gupta et al. 1988).

Bilara Group, the middle horizon of Marwar Basin isrepresented by a wide variety of carbonate lithofacies. Thereare stromatolitic dolostone and microbial calcitic dolostones.

11

Nagaur Group

Hanseran Group

CARBON AND OXYGEN ISOTOPE PROFILES OF THE MARWAR SUPERGROUP, RAJASTHAN, INDIA

Table 1. Stratigraphic succession of Marwar Supergroup (modified after Pareek 1984).

Tunklian Sandstone

Nagaur Sandstone

claystone, siltstone

dolostone, halite, anhydrite

Bilara Group Gottan Limestone

Dhanapa Dolomite

Jodhpur Group Girbhaker Sandstone

Sonia Sandstone

----------Unconformity----------

Pokaran Boulder Bed Glacial bed with boulder of

(Neoproterozoic) Malani Suite of rocks

----------Unconformity----------

Malani Igneous Suite

(780 to 680 Ma)

These biofacies demonstrate that microbial marshesdeveloped within the confines of supratidal to intertidalsettings. The microbial mats induced magnesian carbonateprecipitation and dolomitisation and developed into regularstromatolites, where protected sites existed; elsewhere themicrobial mats were destablised possibly by tidal currents andas a consequence structureless microbial calcitic dolostonesformed. The interbedded sequence of dolostones andlimestones is attributed to intermittent changes in waterchemistry as a consequence of periodic/episodic rise and fallin the water level ofthe basin (Chauhan 1999). The nature andorganic content of the associated spherulitiic limestone areanalysed. It is envisaged that saturated calcium carbonatesolution dispersed within oily droplets crystallized intohydrocarbon contaminated calcite spherulites. It is inferredthat the process of potential hydrocarbon formation in theIndian subcontinent first started in the Marwar basin (Chauhan1996). Nagaur Group, the youngest horizon of the Marwarbasin is represented by a well exposed sequence ofsiliciclastic facies. There are shaley sandstone, mediumgrained sandstone and pebbly to conglomeratic sandstones.These lithofacies contain a reasonable proportion ofimmature detritus of older shales and sandstones. Poorly tomoderately developed trough cross bedding is presentthroughout the sequence.

The sedimentary traits of the Marwar Supergroup of rockssuggest that the basin remained as a perfectly stable shallowwater basin, throughout its life span. Its siliciclastic facies

12

Basement rocks

developed predominantly under fluvial environment and lesscommonly under shallow marine beach environment. Furthertheir largely quartz arenitic nature with less than 2 percent ofbasement provenance suggest that the basin almostpeneplained prior to the formation of the Marwar basin. Thestromatolitic dolomitic limestone interbedded with lime­stones and dolomite representing a carbonate faciesdeveloped under the confmes of supratidal to shallowsupratidal settings provide complimentary evidence in thisregard. The evaporite facies consisting of inter beddedsequence of clay, dolomite, anhydrite and halite furtherprovide evidence that the sedimentary milieu of the Marwarbasin continued to be extremely shallow.

ANALYTICAL METHODS AND SAMPLING

Carbonates were sampled at close interval around Bilara(Barna phase I) and Dhanapa town of Marwar basin. Thethickness of study carbonate sequence is approximatelyaround 25 m. A total of23 carbonate samples were collected.CO

2was extracted from carbonates in a high vacuum line after

reaction with phosphoric acid at 25°C, and cryogenicallycleaned, according to the method described by Craig (1957).CO

2gas released by this method was analyzed for 0 and C

isotopes in a double inlet, triple collector V.G. ISOTECHmass spectrometer, using the reference gas BSC (BorboremaSkarn Calcite) that calibrated against NBS-18, NBS-19 andNBS-20, has a 8180 value of-l 1.28± 0.004 %0 PDB and 8l3C=-8.58 ± 0.02 %0 PDB. The results are expressed in the

MAHESHWARI, SIAL, AND MATHUR

notation %0 (per mil) in relation to international PDB scale.

STABLE ISOTOPIC RESULTS

Table 2. Stable isotope data from Marwar carbonates, westernRajasthan, India.

The carbon and oxygen isotope results of Bilara carbonateshave been provided in Table 2 and plotted in Fig. 2. The oxygenisotopes in Bilara carbonates range between -7.19 to + 6.50%0 PDB. The falling amplitude of 0]80 through Marwarcarbonates may easily be observed and its oscillations almostmatch with ol3C variation along the sequence (Fig. 2). Theoxygen isotope values are significantly heavier in most ofthese carbonates. These high 0]80 values have not beenreported earlier from elsewhere. The relatively 'Heavy' 0]80values are however, reported from eastern Siberia (e.g.,Brasier et al. 1993, 1994a, 1994b) and Budai et al. 1987, havealso reported heavy 0]80 values for Carboniferous dolomitesfrom Madison Group, Wyoming and Utah overthrust belt. Noexplanation is proposed for such high 0]80 enrichment in theMarwar carbonates however, the low temperature conditionsor cold house phases existed during the early Cambrian may beone ofthe reason for this enrichment. In general it is observedthat oxygen isotopes are relatively prone to isotope exchangewith meteoric and burial waters during diagenesis, since thesecontain more of the lighter isotope ]60 and are likely to havea higher temperature (e.g., Hudson 1977). However, forpresent studies it is presumed that diagenetic modifications ofisotope signals has been negligible in this sequence.

The carbon isotopic results are plotted against lithocolumn inFig. 2. The perusal of plot indicate presence of short termnegative and positive carbon isotopic oscillations in the studysequence. The Dhanapa carbonates, at the base of sequenceshow strongly negative values upto -10.31%0 PDB. TheGottan limestones overlying Dhanapa carbonates yieldmoderately positive 0]80 values up to + 1.43%0 PDB and arefollowed by the carbonates with negative o13C. The negativeol3C values are once again replaced by positive ol3C up to amaximum of +2.80 in younger carbonates which finallyfollowed by carbonates with negative ol3C values.

The ol3Crecords from Marwar carbonates may be comparedwith those obtained from type localities for the Nemakit­Daldynian and Tommotian in Siberia (Fig. 3). The negativecarbon isotopic signals seen at the base of Marwar carbonatescorrelate in position with negative anomaly W as shown in Fig.3, which is known from the basal Nemakit-Daldynian of thetransitional belt (Brasier et al. 1993; Brasier et al. 1994b), aswell as from the eastern belt (Kaufman et al. 1996),southwestern Mongolia (Brasier et al. 1996) and several otherlocalities worldwide (Brasier et al. 1997). The negativeanomaly W in Marwar carbonates is more pronouncedcompared to other Lower Cambrian sequences worldwide.The carbon isotope oscillations in Marwar carbonates aretherefore comparable with those reported worldwide fromcarbonate sequences of Nemakit-Daldynian period of lowerCambrian.

Sample 0180 poe 013c poe17 -2.140 -1.155

16 +1.293 -0.672

15 -1.581 -1.844

14 +0.323 -0.379

13 +1.945 +2.804

12 -1.044 +0.314

11 -3.311 -5.898

10 +0.826 -1.897

9 +1.716 -1.104 ,

8A -2.690 -2.192

8 -6.782 -2.774

7 +1.326 -0.848

6 -6.185 -2.889

5 -4.191 +0.514

4 -7.248 +0.394

3 +3.653 +1.256

2 -5.478 +0.727

1 -4.049 +1.436

05 +6.507 -0.185

04 +3.833 -2.287

03 -7.196 -10.313

02 +2.157 -1.325

01 -7.002 -9.247(1-17): Gottan limestone(01-05): Ohanapa dolostone

DISCUSSION

Carbon isotope stratigraphy has also emerged as a major toolfor correlation ofcarbonate-bearing rocks during the terminalProterozoic and several markedly positive ol3C excursionshave been identified in Neoproterozoic carbonates. Theseisotope excursions help in delineating global events indicatingperiods when organic matter comprised an exceptionally largeproportion of the C being sedimented from the oceans (Knollet al. 1986). A global positive 0]3C event during the interval590-550 Ma is accompanied by high 87Sr/86Sr values (Derry etal. 1992) which helps us to infer that this geochemical eventtriggered an enhanced rate of continental erosin which wassustained through the Pan-African uplift (Asmerom et al.1991). Consequently it is believed that the absolute rates of

13

CARBON AND OXYGEN ISOTOPE PROFILES OF THE MARWAR SUPERGROUP, RAJASTHAN, INDIA

Dark orey bedded strornotcrutclimestone with IIQht orey whitish siltstone

Dark orey massive limestone

Lioht orey whilish limestone withclays tone / siitstolill

Light yellowish claystone/siltstonewith dolomite

Light orey, white bedded Ilmeston!'

!:1ii'mi!iJ Litht yellowish claystone/siltstoneIi with dalomile

Dork OffY massive limestone withoccurrences of pockets of cloy and chert

Lioht brownish yellow cloystone/siltslone

Stromatolitic dolomite

Cherty uoiostone

513C(%. PDB)

I I +<5 (JI 0 (JI

\(

.....> ......-.:-- ---.... ,

"f'

fIIIII

~ '.,/

ti.._ ...~I\\,,

N\~

II

II

/_......-----~-- - --..:...,....------

Figure 2. Carbon and oxygen isotope profile of Marwar Supergroup carbonates.

organic sedimentation were accelerated during the closingphase of Neoproterozoic and the atmospheric oxygen levelsmarkedly increased (Kaufmann and Knoll 1995). The endNeoproterozoic isotope event accompanied the first welldemonstrated occurrence of multicellular life (Kaufmann andKnoll 1995; Brasier 1990). Both carbon isotopic andaccompanying biological events are consistent with anincrease in atmospheric O

2level (Knoll and Holland 1995;

Des Marais 1997).

The carbon isotopic records of Neoproterozoic-Cambriansections from China (Hsu et al. 1985; Brasier et al. 1990;Zhang et al. 1987; Kimura et al. 1997), Iran (Amini 1988;Brasier et al. 1990), Vindhyan Basin, Central India(Chakraborty and Friedman 1998; Friedman and Chakraborty1997; Friedman et al. 1996), Lesser Himalaya (Aharon et al.1987; Banerjee et al. 1986, 1996), Rajasthan (Banerjee et al.1998; Banerjee and Majumdar 1999; Kumar et al. 2002;Peters et al. 1995) and Oman (Bums and Matter 1993) and ageneral review by Ripperdan (1994) demonstrate an overallconsistency in the variation pattern. The terminalNeoproterozoic carbonates in the Atlas Mountain in North

14

Africa, the Yangtze Gorges and Sichuan sections of the SouthChina, Krol belt in the lesser Himalaya, the Aldan shield inSiberia and the Elburz Mountains in Northern Iran are largelycharacterised by positive 813C value. A gradual deflection ofcarbon isotope curve towards heavier values is followed by asharp change to lighter isotopic values generally at theTommotian stratigraphic level. The basal Cambrian (NemakitDaldynian) falls in the field showing progressive decline in the813C values. The evaporite dominated Neoproterozoic-EarlyCambrian Marwar succession in the subsurface bore holes ofwestern Rajasthan, surface and subsurface rock in the OmanMountains and the Huqf area in southern Arabia demonstrateprevalence of positive values in the basal carbonates whichsharply changes to negative at the base ofCambrian (Bums andMatter 1983; Banerjee et al. 1998). The C and Sr isotope dataofcarbonate rocks ofVindhyan Supergroup matches well withthe data available for Mesoproterozoic and Neoproterozoicperiod of deposition (Kumar et al. 2002) worldwide.

The negative carbon isotopic signals observed in the Dhanappadolomites, the lowermost part of Bilara carbonate sequencecorrelate in position with negative anomaly W (See Fig. 2 and

MAHESHWARI, SIAL, AND MATHUR

200m

z

Nemakit-Daldynian

II

IAtdabanianTommotian

I

VII VVIII

LOW E RCA t,A B Rift N

Botomisn-Toyonian mass extinction

Toyonian I Botomian

Aldan and Lenarivers,Siberia

O+--1~----4'ilI,--A----+---I----'

-4

-2-

4 !

-6

coao,o~o........

( MID. CAMS. !

'-~t.JJCC

...I-<Z.o-r--v:Z-< 0" C"):- ....-

to

Figure 3. Carbon isotope stratigraphy of the Transitional belt of the Siberian Platform and other sections of LowerCambrian (Figure modified after Brasier and Sukhov 1998 and from data summarized in Brasier et al. 1993, 1994a, 1994b,1996).

3), which is known from the basal Nemakit-Daldynian of thetransitional belt (Brasier et al. 1993; Brasier et al. 1994b), aswell as from the eastern belt (e.g. Kaufmann et al. 1996; Knollet al. 1995b) and several other localities worldwide (Brasieret al. 1997). The Z and I position ofcarbon isotopic oscillationof trasitional belt well matches with the Bilara carbonatesequence. The similarities of carbon isotope oscillation inBilara carbonate sequences and lower Cambrian carbonatesequences from different parts of the world indicate thatBilara carbonates were most likely deposited during earlylower Cambrian (Nemakit-Daldynian) period.

Eight major carbon isotopic oscillations have been reportedbetween 543 and 515 Ma from Lower Cambrian (Brasier andSukhov 1997). The falling amplitude in carbon isotopic valuesin Cambrian compared to Neoproterozoic is a part of a long­term pattern that is traceable back beyond 700 Ma. The first­order signal consists of a long-term decline in the maximum813C values, which fell from approx. +11%0 in the "post­Sturtian" to approx. +8%0 in the Ediacara and thence to approx.+5%0 in the cambrian (Brasier et al. 1996). The second-ordersignal comprises shorter term oscillations, which in theCambrian spanned about 4 Ma. Together, these two signalsresulted in a falling amplitude of carbon isotopic oscillationsthrough time. Brasier and Sukhov (1997) discussed factorsthat may have been involved in bringing these changes about:biosphere changes, sea-level rise and climate warming.

Major transformations were taking place in the biosphere,with the emergence of soft-bodied metazoans and trace fossilby 600 Ma and ofpenetrative bioturbation plus skeletal fossils.by 543 Ma (Brasier and McIlroy 1998). Current evidence

suggests that these trasformations of the biosphere wererelatively gradual (Brasier et al. 1996, 1997), so they couldwell have contributed to long term decline in 813C maximathrough the Neoproterozoic to Cambrian. The biologicalchanges have also contributed to the short term oscillationsand the each ofthe 813C peaks in the lower Cambrian coincidedwith the appearance of a major biomineralized fossil group(Brasier et al. 1994a).

A variety of evidence points to a major encroachment of thesea onto the continents between the Neoproterozoicglaciations and the end of the Cambrian. This evidenceincludes the presence of profound regional unconformity atthe base of the Lower or Middle Cambrian in many regions.Crustal and sea-level changes may together have brought abouta shift in the loci of hydrocarbon source rocks between 730and 490 Ma. Mackenzie and Piggot (1981) have noted thattimes of high sea level tend to be associated with intervals oflower 813C values in the Phanerozoic. Hydrocarbon sourcerocks of the terminal Neoproterozoic to Atdabanian aretypically associated with evaporitic basins of cratonicinterior, as for example in Siberia (Kontorovitch et al. 1990),Oman and Iran (Husseini and Husseini 1990; Looseveld 1996)and Pakistan (Dolan 1990). The process of potentialhydrocarbon formation in the Indian subcontinent first startedin the Marwar basin (Soni 1993; Chauhan 1996).

The highly oscillatory carbon isotopic record of the laterNeoproterozoic is known to have coincided with major glacialto interglacial fluctuations. Negative 813C excursions trackedthe transition from glacial to postglacial facies on perhaps asmany as four separate occasions between 730 and 550 Ma

15

CARBON AND OXYGEN ISOTOPE PROFILES OF THE MARWAR SUPERGROUP, RAJASTHAN, INDIA

(Brasier et al. 1996; Kaufmann et al. 1997). These negativeexcursions have been explained as the result of overturn instratified water masses during times of climatic cooling,leading to the upward advection of isotopically light CO

2and

HCO)-into surface waters (Knoll et al. 1996).

Evidences of terminal Neoproterozoic glaciation have beenreported from this part of Indian subcontinent. The rocks ofMarwar Supergroup are underlain by "Pokaran Boulder Bed"a product of terminal Neoproterozoic glaciation. Noglaciations are known from the Cambrian, which is generallythought to have enjoyed "greenhouse" conditions (Berner1990; Tucker 1992). It is possible that the negative excursionofthe Early Cambrian reflect a transitional regime, ofclimaticcooling without major glaciation. The falling amplitude ofcarbon isotopic values between 730 and 500 Ma maytherefore have been associated in some way with thistransformation from glacial "icehouse" condition in theCryogenian to "greenhouse" conditions in the MiddleCambrian. The carbonates which cap the glacial depositsplunge to depleted 813C values (Hoffinan et al. 1998). Carbonisotopic oscillations in the Early Cambrian would then be theresult of climatic oscillations either from glaciations that havenot yet been found (Riperdan 1994) or from "coldhousephases" that did not involve major glaciations (Tucker 1992;Brasier 1992c). The similarities in short term carbon isotopeoscillations observed in Marwar carbonates and early lowercambrian carbonate sequences worldwide points to theirformation under similar climatic oscillations.

ACKNOWLEDGEMENTS

AM is thankful to CNPq for financial assistance to visit toNEG-LABISE, Recife, Brazil. AM and ANS wish to expresstheir gratitude to Gilsa M. de Santana and Vilma S. Bezerra forthe assistance with the carbon isotope analyses at the LABISE,Federal University of Pernambuco, Brazil. SCM acknow­ledges the financial assistance received from DST, India.

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