Precambrian Research 106 (2001) 277–290

Positive d13C excursion in carbonate and organic fractionsfrom the Paleoproterozoic Aravalli Supergroup,

Northwestern India

B. Sreenivas a,*, S. Das Sharma a, B. Kumar a, D.J. Patil a, A.B. Roy b,R. Srinivasan a

a National Geophysical Research Institute, Hyderabad 500007, Indiab M.L. Sukhadia Uni6ersity, Udaipur 313003, India

Received 3 May 1999; accepted 21 July 2000

Abstract

Carbon and oxygen isotopic investigations have been carried out on the Archean and Paleoproterozoic carbonaterocks of the Udaipur region in the Aravalli Mountain Belt, northwestern India. The study has led to the interestingfinding of 13C enrichment in the carbonate rocks of the Jhamarkotra Formation (d13Ccarb up to 11.1‰ V-PDB)belonging to the �2200–1900 Ma Paleoproterozoic Aravalli Supergroup. Further, it is observed that the organiccarbon from phosphorite bearing stromatolitic dolomites of the same formation are also enriched in 13C (d13Corg upto −11.1‰). However, it must be emphasized that the 13C enriched carbonates and the organic fractions are fromdifferent stratigraphic levels although in the same formation, the former being at a lower stratigraphic level. A criticalanalysis of the field geological, petrological as well as the isotopic data indicates that the observed d13C excursion inthe carbonate carbon is not due to local causes such as methanogenesis or evaporitic conditions. The shallow waterenvironment of deposition of these carbonates as well as the immediately overlying formations rules out the stratifiedocean model for the observed excursion. The model involving high sedimentation rates and organic carbon burial hasbeen preferred to explain the d13C excursion in the carbonates of the Jhamarkotra Formation. The 13C enrichmentin organic fraction occurred in localized regions of the Aravalli sedimentary basin, where high productivity wassupported by phosphorous supply indicating that the diffusion-limited pathways of organic carbon fixation wereoperative in the Paleoproterozoic itself. The 13C enriched carbonate rocks of the Jhamarkotra Formation serve as theIndian example for the Paleoproterozoic global d13C excursion. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Paleoproterozoic; Aravalli Supergroup; India; Carbonates; Organic carbon; 13-C excursion

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1. Introduction

The long-term variation in carbon cycle as re-vealed by the d13C record of the carbonate rocksis very important in understanding the geological

* Corresponding author. Tel.: +91-40-7170141; fax: +91-40-7171564.

E-mail address: postmast@csngri.ren.nic.in (B. Sreenivas).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00131 -5

B. Sreeni6as et al. / Precambrian Research 106 (2001) 277–290278

controls on the evolution of atmosphere, hydro-sphere and biosphere. The Precambrian is charac-terized by incidences of 13C excursions in thecarbonate carbon especially during the Pale-oproterozoic (2500–1600 Ma) and Neoprotero-zoic periods (1000–540 Ma), deviating from theglobal mean of 092‰ (Knoll et al., 1986; Karhuand Holland, 1996). A positive excursion in Pale-oproterozoic d13C values (up to 12‰) was firstobserved in the carbonate rocks from theFennoscandian shield of Russian Karelia (Gal-imov et al., 1975; Schidlowski et al., 1975) andfrom the Lomagundi province of Zimbabwe(Schidlowski et al., 1976). These anomalous iso-topic records initially were considered to be oflocal significance. However, it has been found inthe last few years, that 13C enrichment is morecommon in carbonate rocks of 2200–2000 Ma oldsequences from northern Europe (Baker andFallick, 1989a,b; Karhu, 1993; Tikhomirova andMakarakhin, 1993; Melezhik and Fallick, 1996),North America (Schidlowski et al., 1983; Mirotaand Veizer, 1994; Melezhik et al., 1997), SouthAfrica (Gauthier-Lafaye and Weber, 1989; Buicket al., 1998) and Australia (McNaughton andWilson, 1983; Russell, 1992 as quoted in Tabakhet al., 1999). This finding in several continentspoints to the fact that the Paleoproterozoic d13Cexcursion is perhaps global in nature. It musthave originated due to a major disturbance in thegeochemical cycle of carbon and therefore, mayhave profound influence on the evolution of at-mosphere, hydrosphere and biosphere during thePaleoproterozoic. Between 2430 and 1940 Ma, atleast three discrete positive d13C excursions havebeen suggested by Buick et al. (1998) in theTransvaal and Olifantshoek Supergroups ofSouth Africa. They seem to bracket at least threediscrete glacial deposits of the Paleoproterozoic.The pattern of d13C excursions together withglacial events is strikingly similar to that in theNeoproterozoic carbonate sequences depositedbetween 850 and 550 Ma (Knoll et al., 1986;Kaufman and Knoll, 1995). This suggests that thecarbon cycle, probably played a major role inbringing about long-term climatic variations, atleast since last 2 billion years (cf. Kaufman, 1997).

The geological controls leading to disturbance inthe carbon cycle and resultant d13C excursionsduring the Paleoproterozoic are topics of activeresearch.

Sreenivas et al. (1998, 1999) for the first timerecorded 13C enrichment in Paleoproterozoic car-bonate rocks of the Jhamarkotra Formation ofthe Aravalli Supergroup, northwestern India. Inan independent study, Maheswari et al. (1999)have also observed this feature. The interestingaspect of the C isotopic composition of theJhamarkotra Formation is that, the formationconsists of mineable deposits of phosphorites inthe stromatolitic horizon. The organic carbonfrom these phosphatic stromatolites is also en-riched in 13C (Banerjee et al., 1986). Occurrence of13C enrichment in both organic and carbonatecarbon in a single stratigraphic formation is virtu-ally unknown from Paleoproterozoic carbonatesequences.

In this communication; (i) new carbon andoxygen isotopic data on carbonate rocks and co-existing organic carbon of the Jhamarkotra For-mation are provided; (ii) C and O isotopic datafor the carbonate rocks of the formations under-lying and overlying the Jhamarkotra Formationare given; (iii) the problem of enrichment of 13C inboth organic and inorganic fractions in a singlestratigraphic formation is addressed; and (iv) sig-nificance of the present find in the context ofglobal Paleoproterozoic 13C excursion event(s) isdiscussed.

2. Geological setting

The study area is around Udaipur city(24°34%48¦ and 73°40%20¦) in the Aravalli Moun-tain Belt of Rajasthan, Northwestern India (Fig.1). Here, the rock formations of the AravalliSupergroup unconformably overlie the weatheredArchean gneiss-migmatite basement. The base-ment complex has been designated as the MewarGneiss Complex (MGC, Roy, 1988; Roy andKroner, 1996). Geochronological study has shownthat the MGC evolved from 3300 to 2500 Ma ago(Gopalan et al., 1990; Wiedenbeck and Goswami,1994; Roy and Kroner, 1996; Wiedenbeck et al.,

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1996) and was stabilized around 2500 Ma(Wiedenbeck et al., 1996). MGC has large en-claves of metamorphosed quartz arenites, calciticmarbles, metabasaltic amphibolites and rarelybanded iron formations. The sedimentary pro-toliths are suggested to have been deposited in riftbasins, which witnessed synsedimentational vol-canism. The assemblage is similar to cratonicassociation of Condie (1989). The rocks have beenpolyphase deformed, metamorphosed and migma-

tized under amphibolite facies. These Archeanrocks are weathered to give rise to Al2O3 — richand Fe — poor paleosol — now a sericite schist(Roy and Paliwal, 1981; Banerjee, 1996; Sreenivaset al., 1999), which separates the basement rocksfrom the overlying Aravalli Supergroup.

The classification of the Aravalli Supergroupafter Roy et al. (1993) is shown in Fig. 1. TheDelwara Formation at the base of the AravalliSupergroup is composed of quartz arenites, rarely

Fig. 1. Geological map of the region around Udaipur showing the areas from where carbonate samples belonging to differentstratigraphic formations of the Aravalli Supergroup and cacitic marbles of the MGC have been collected. The lithostratigraphicsuccession of the Aravalli Supergroup (after Roy et al., 1993) is also depicted.

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Fig. 2. Karauli–Ghasiar and Jhamarkotra sections illustratingthe position of phosphorite marker horizon. The massivedolomites of the Karauli and Ghasiar underlie the phosphoritemarker.

are considered to have accumulated on shallowshelf zones (Roy et al., 1988). The JhamarkotraFormation is overlain by greywackes of theUdaipur Formation, which is succeeded by theZawar Formation. The carbonate rocks of theZawar Formation host syngenetic Pb-Zn sulfidedeposits. The model Pb isotopic ages of galenasfrom the Zawar Formation (Deb et al., 1989) andDelwara Formation (Deb, 1999) are reported tobe around 1700 Ma and 2000–2200 Ma, respec-tively. These ages estimated on the basis ofmodified ‘Wisconsin model’ designated as‘Proterozoic model’ (which uses the parametersT0=3810 Ma, A0=10.844, B0=12.632 and C0=31.1; Deb et al., 1989) bracket the approximatetime of deposition of the Jhamarkotra Formationbetween 2200 and 1700 Ma. The Rb-Sr age of theDarwal Granite that intrudes the Aravalli Super-group is 1900980 Ma (Chaudhary et al., 1984).While confirming the Paleoproterozoic affinity forthe Jhamarkotra Formation, this age narrowsdown the lower and upper age limits for theJhamarkotra Formation to be between 2200 and1900 Ma.

There are also other formations overlying theZawar Formation, some of which consist of car-bonate rocks (e.g. the Kabita dolomite) that havenot been examined in this study. The rock forma-tions of the Aravalli Supergroup around Udaipurare deformed and have been metamorphosed un-der green schist to lower amphibolite facies. Themetamorphic reconstitution is highly variable.The rocks in certain low strain zones virtually aredevoid of metamorphic recrystallization. In suchsectors as for example in the inner hinge zones offolds, detrital textures in clastic rocks and micritictextures in carbonate rocks are well preserved.

3. Petrographic description of samples

Locations of samples for this study are shownin the geological map of the area around Udaipur(Fig. 1). The samples include marbles from theArchean basement and dolomites from the Del-wara, Jhamarkotra and Zawar Formations. Abrief description of samples is given below:

associated with cherty dolomites both of whichoccurring interbedded with metabasaltic flows.The Delwara Formation is succeeded by theJhamarkotra Formation, which is essentially com-posed of stromatolitic and massive (non-stroma-tolitic) dolomites. The stromatolitic dolomitesnear Jhamarkotra and Kanpur, south of Udaipurare rich in phosphorite (fluorapatite-francolite,Banerjee et al., 1986). In the sections north ofUdaipur such as around Karauli, a large thicknessof massive dolomite has been noted below a thinphosphorite-rich horizon (Kaul et al., 1989).While the stromatolitic dolomites are believed tohave been deposited in land-locked epicontinentalshelf regions, the massive dolomites have beeninterpreted as carbonate shelf-bank deposits (Royand Paliwal, 1981). The two sections north andsouth of Udaipur, can be correlated, taking thephosphoritic horizon as a marker (Fig. 2). Fromthis it is seen that, in the Jhamarkotra Formation,the massive dolomite horizon is succeeded by thestromatolitic horizon that is rich in phosphorite.The Delwara and the Jhamarkotra Formations

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3.1. Darauli marbles from the MGC

Four samples BS/94/61, BS/94/61A, KA6 andKA10 were collected from this Archean unit.These are white colored, coarse-grained marblescomprising largely crystalline grains of calcite.These amphibolite facies marbles consist of a fewgrains of hornblende and locally fine bands ofgraphite. Intrusions of �2500 Ma granites andpegmatites have been noticed in them.

3.2. Dolomitic carbonates from the Ara6alliSupergroup

3.2.1. Delwara FormationThe samples were collected from two localities,

one sample (BS/94/38) from Nagaria and twosamples (BS/94/24A and BS/94/25A) to the northof Sarara ‘inlier’. The samples are of dolomiticmarble, containing crystalline grains of dolomitewith a few grains of tremolite and zoisite.

3.2.2. Jhamarkotra FormationThis is the principal carbonate formation of the

Aravalli Supergroup. Samples from this forma-tion were collected from four areas keeping inview the low strain zones. Twelve samples fromJhamarkotra mine area (3/2A to R) and one fromKanpur (BS/94/51) are from stromatolitic phos-phorite horizon. Those of Jhamarkotra are fromwithin the phosphorite-rich zone, the Kanpursample represents the dolomite which immediatelyunderlies the phosphatic horizon. The stroma-tolitic carbonates have dissemination of carbona-ceous matter. From the massive dolomite horizon,well below the stromatolitic phosphorite horizon,14 samples from Ghasiar (BS/94/19, 19A to C;BS/95/7, 7A to J) and one from Karauli (BS/94/49) were collected. There are thin interlayers ofchert in the dolomite of Ghasiar. Petrographicobservations show that the stromatoliticdolomites of Jhamarkotra and massive dolomitesof Ghasiar preserve the micritic nature of carbon-ate, not having been affected by recrystallization.

3.2.3. Zawar FormationTwo samples (BS/94/69B and BS/94/71A) were

collected from the Zawar Formation. These car-

bonate rocks host syngenetic deposits of lead andzinc. Framboids of pyrite are noticed in thisformation.

4. Experimental procedure

Samples weighing approximately 2 kg were re-duced to pebble-size using a steel mortar andpestle and were thoroughly ultrasonicated in MQwater. The chips were carefully examined andthose with silica veins were discarded and the restwere powdered using an agate mortar and pestle.The sample powders used for carbon and oxygenisotope analyses were treated with 100% phospho-ric acid at 25°C for 48 h following the method ofMcCrea (1950). The CO2 liberated from the sam-ples were analyzed using VG 903 mass spectrome-ter. NBS-19 analyzed (n=4) during the course ofthe analyses of samples of present study yieldedd13C=1.9590.1 and d18O= −2.290.1‰ (V-PDB). Repeated measurements on the same sam-ple are always within 90.2‰. The total organiccarbon (TOC) was separated from the dolomitesand Phosphatic Stromatolites of the JhamarkotraFormation. For this purpose, powders of vein-freesamples were repeatedly treated with hot and coldHCl. The cleaned and dried residue was heated upto 1000°C in the presence of spec pure V2O5 for30 min under vacuum (10−5 torr). The resultingCO2 was measured mass spectrometrically for thecarbon isotope composition. NBS-21 standardyielded d13C of −28.290.1‰ during the analysisand a spec pure graphite (internal standard) gavereproducible values with a precision of 90.2‰(n=3).

5. Results

A total of 37 new C (d13Ccarb) and O (d18Ocarb)isotope compositions of carbonate rocks havebeen presented in this work (Table 1). Organiccarbon fractions separated from 20 samples (nine,massive dolomites; 11, phosphorite bearing stro-matolitic dolomites) are also presented in Table 1.d13C and d18O are reported with respect to V-PDB. The variations in 13C and 18O compositionswith reference to stratigraphy are shown in Fig. 3.

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Table 1C and O isotopic composition of carbonate rocks of the Archean MGC and various formations of the Aravalli Supergroupa

Sample c d13Ccarb d13Corg d18Ocarb DCSample description and location� (‰, V-PDB)�

Archean MGC−9.3BS/94/61 Marble enclave, Darauli −1.1

−12.90.9BS/94/61A ----------------do--------------------------------do---------------- −0.2 −8.8KA6

−8.5−0.7KA10 ----------------do----------------

Delwara Formation−18.1BS/94/38 Dolomite, Nagaria −1.6−18.7b−6.7bDolomites, SararaBS/94/24A

−2.2---------do------------ −17.2BS/94/25A

Jhamarkotra Formation−23.0 −9.3b10.7b 33.7Micritic dolomite, GhasiarBS/94/19−27.1b −10.3BS/94/19A 35.6----------------do----------------- 8.5

−7.4b11.1bBS/94/19B ----------------do-----------------10.6----------------do----------------- −23.8b −9.6 34.4BS/94/19C

−25.3 −10.89.5 34.8----------------do-----------------BS/95/78.2----------------do----------------- −29.5 −11.9 37.6BS/95/7A8.8b----------------do----------------- −28.6 −10.3b 37.4BS/95/7B

−26.790.2c −10.29.3 36.1BS/95/7C ----------------do-----------------5.4----------------do----------------- −22.4 −11.8 27.8BS/95/7D

−10.4BS/95/7E ----------------do----------------- 8.3−8.39.2----------------do-----------------BS/95/7G

9.2b----------------do----------------- −9.6bBS/95/7HBS/95/7I 9.6 −25.1b −9.4 34.7----------------do-----------------

−11.090.2c9.390.2cBS/95/7J ----------------do-----------------−8.4BS/94/49 Dolomite, Karauli 5.1

−22.6 −11.6−0.1 22.53/2A Dolomite hosting phosphaticStromatolite, Jhamarkotra

−15.890.2c −11.1−0.2 15.6----------------do-----------------3/2B−0.4----------------do----------------- −17.6 −11.1 17.23/2C−0.2b----------------do----------------- −22.5 −11.2b 22.33/2D

−11.4 −10.1−0.5 10.93/2E Whole-rock phosphorite, Jhamarkotra−0.7----------------do----------------- −11.190.2c −12.0 10.43/2F

−11.9 −10.83/2G 11.6----------------do----------------- −0.3−11.1 −11.390.2c−0.790.1c 10.4----------------do-----------------3/2H

−0.2----------------do----------------- −12.3b −11.0 12.13/2I3/2O −10.3----------------do----------------- −0.6

−23.3 −7.10.2 23.5Dolomite underlying phosphatic3/2PStromatolite, Jhamarkotra

−22.9 −6.50.6 23.53/2R ----------------do-----------------0.1bDolomite underlying phosphatic −7.6bBS/94/51

Stromatolite, Kanpur−0.790.7Dolomites, Udaipur & Jhabua −15.192.7 −12.192.0 14.42.1–2.12d

(n=12) (n=10)

Zawar FormationBS/94/69B 1.5 −7.8Dolomite, ZawarBS/94/71A -----------------do--------------- −10.81.1

a d13Corg of dolomites from Ghasiar and Jhamarkotra are also given.b Average of duplicate analyses.c Average of triplicate analyses with standard deviation.d Data from Banerjee et al. (1986).

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Fig. 3. Variation in d13Ccarb, d18Ocarb and d13Corg of carbonate rocks of the Aravalli Supergroup and the marbles of the ArcheanMGC shown along side the stratigraphic column. A is the Karauli–Ghasiar section; B is the Jhamarkotra section. Note positiveexcursion in d13Ccarb of massive dolomite of Ghasiar (open circle) belonging to the Jhamarkotra Formation. The stromatoliticphosphorites of this formation from Jhamarkotra mine area (open square; data from this study and Banerjee et al., 1986) show 13Cenrichment in the organic fraction, while their d13Ccarb has remained close to normal marine value. Depletions in C and O isotopiccompositions of the impure dolomites of the Delwara Formation are a result of metamorphic decarbonation. Carbonate rocks ofrest of the formations are characterized by d18Ocarb values typical of Late Archean and Paleoproterozoic marine carbonates.

5.1. Carbon isotopic composition

The d13Ccarb values of Archean calcitic marbles(−1.1–0.9‰) are in the typical marine carbonaterange. Small negative (up to −6.7‰) and largepositive (up to 11.1‰) excursions in d13Ccarb valuesof carbonate rocks of the Delwara and Jhamarko-tra Formations, respectively, can be seen. Thesamples of the Zawar Formation are characterizedby typical marine carbonate values of 1.1 and 1.5‰.

The most significant feature of the carbon iso-topic compositional data is the 13C enrichment upto 11.1‰ (range 5.1–11.1‰) observed in the un-metamorphosed micritic massive dolomites of theJhamarkotra Formation collected from nearGhasiar and Karauli. The compositions of thecoexisting organic carbon (d13Corg) separated fromGhasiar samples vary from −29.5 to −22.4‰. Bycontrast, it is interesting to note that the carbonatecarbon of the phosphorite bearing stromatoliticdolomites of the Jhamarkotra Formation fromJhamrkotra mine area is in the normal marinecarbonate range of −0.7 to 0.1‰. But the organic

fraction in these samples are enriched in 13C, d13Corg

varying from −22.6 to −11.1‰. Our new data onphosphatic stromatolites are in consonance withthe earlier published data on these rocks fromUdaipur and Jhabua reported by Banerjee et al.(1986).

5.2. Oxygen isotope composition

Archean marbles of the MGC are characterizedby d18O values of −12.9 to −8.5‰, typical of lateArchean marine carbonate rocks (Veizer et al.,1990). The d18O values of impure dolomites of theDelwara Formation are depleted (varying from−18.7 to −17.2‰) when compared to the compo-sitions of Palaeoproterpzoic carbonates (Veizer etal., 1992a,b). Depletion in carbon and oxygenisotopic compositions, coupled with petrographicobservations which show presence of calc-silicateminerals such as tremolites in these impuredolomites, clearly points out that their isotopiccompositions were altered during metamorphicdecarbonation reaction (cf. Bottinga, 1969).

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The d18O values of the 13C enriched massivedolomites of the Jhamarkotra Formation fromGhasiar and Karauli vary between −11.9 and−7.4‰. The dolomites hosting phosphatic stro-matolites of Jhamarkotra area range from −12to −10.1‰. These data of Jhamarkotra Forma-tion are discussed in the subsequent section in thecontext of evaluation of post-depositional alter-ation. Two samples from Jhamarkotra mine areaand one sample from Kanpur, representingdolomites underlying stromatolites yield d18O val-ues in the range of −7.6 to −6.5‰. Two sam-ples of the Zawar Formation are characterized byd18O values of −7.8 and −10.8‰, which areagain in the range of the Paleoproterozoic marinecarbonate rocks.

6. Discussion

The most important finding of the 13C composi-tions of carbonate rocks of the Aravalli Super-group is the enriched d13C value for both thecarbonate and organic fractions of a single strati-graphic formation, namely the Jhamarkotra For-mation. Before evaluation of the significance ofthese results, it is first necessary to ascertain thatthese values are not artifacts of post-depositionalalteration processes.

6.1. E6aluation of the isotopic data forpost-depositional alteration

The d13Ccarb and d18Ocarb of micritic dolomitesof Ghasiar and Jhamarkotra areas are plotted inFig. 4. Burdett et al. (1990) have shown that theisotopic ratios can be altered either by evaporativeprocess or burial temperature/exposure to mete-oric water. While the evaporative process resultsin enrichment of d18O with variable d13C, theburial temperatures and exposure to meteoric wa-ter cause depletion in both C and O isotopecompositions (see inset of Fig. 4). It can be seenthat, although some of the samples of massivedolomites from Ghasiar have been slightly af-fected by the latter process, majority of the sam-ples show restricted ranges of d13C (8.5–11.1‰)and d18O (−11.9 to −7.4‰, average −10.19

1.3‰) indicating that the depletion in C and Oisotopic compositions due post-depositional alter-ation was insignificant. As high d13C compositionscannot be a result of either of the above processes,these carbonates are inferred to have been precip-itated from seawater bicarbonate enriched in 13C.The d13Ccarb and d18Ocarb values of dolomiteshosting phosphatic stromatolites from Jhamarko-tra also fall in a narrow range (see Fig. 4), againsuggesting no evidence for post-depositional alter-ation effect.

The carbon isotopic compositions of organiccarbon from Ghasiar area are characterized bynormal biogenic range (average d13Corg= −25.792.5‰) and are comparable to values re-ported from several unmetamorphosedPrecambrian sedimentary successions of the world(Schidlowski, 1988). However, the high d13C val-

Fig. 4. Relation between d13C and d18O values in carbonaterocks of the Jhamarkotra Formation; massive dolomites ofGhasiar (open circle) and phosphorite bearing stromatoliticdolomites (filled square, this study; open square, Banerjee etal., 1986). Both these carbonates show narrow range in 13Cand 18O compositions. No evidences for high temperatureburial or evaporitic trends can be noticed indicating that theeffect of secondary processes is negligible. Inset shows twoalteration trends (after Burdett et al., 1990); (a) evaporitic; and(b) high temperature burial and/or exposure to meteoric water.

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ues up to −11.1‰ of organic fractions obtainedon the phosphatic stromatolite samples fromJhamarkotra are deviated by about 10–15‰ withrespect to normal biogenic carbon and hence needcritical evaluation for the effect of any secondaryprocesses. It has been demonstrated that consider-able shift towards heavy values in d13Corg takesplace in response to high temperature exchange of13C/12C between coexisting organic and carbonatecarbon (Valley and O’Neil, 1981). However, toproduce the isotopic shifts of the order of 15‰ inthe organic carbon as noticed in the JhamarkotraFormation, high temperature equilibration in ex-cess of 500°C is required (ibid). As pointed outearlier, the carbonate samples of the JhamarkotraFormation have not been affected by such hightemperature metamorphism as indicated by theirtextural characteristics (see also Banerjee et al.,1986). In addition, subtle differences in d13Corg canbe noticed between phosphorite-rich samples anddolomites hosting them and there is no evidence forisotopic homogenisation as expected in a metamor-phic mileu. For example, the dolomite samples(c3/2A to 3/2D) are characterized by values of−22.6 to −15.8‰. By contrast, the phosphoritewhole-rock samples (c3/2E to 3/2I and 3/2O),which contain higher content of phosphorite arecharacterized by more enriched d13Corg values (−12.3 to −11.1‰). Gas chromatographic study onthe organic carbon fractions of intercolumnardolomites and phosphorite-rich stromatoliticcolumns also show that subtle differences in then-alkane distribution patterns are preserved(Banerjee and Klemm, 1985). These observations,therefore, suggest that the 13C enriched composi-tions of organic carbon do not have artifacts of anysecondary processes and reflect primary isotopicsignatures. Having ascertained that the isotopiccompositions of the carbonate as well as theorganic fractions of the carbonate rocks of theJhamarkotra Formation are primary, we proceedto explore the cause for positive 13C excursion.

6.2. Causes for 13C excursion in the JhamarkotraFormation

It may be recalled that, in the JhamarkotraFormation, the massive dolomites occurring in the

lower stratigraphic level have carbonate carbonenriched in 13C, whereas the stromatoliticdolomites with phosphorites occurring at a higherstratigraphic level, show 13C enrichment in theorganic fraction (Fig. 3). From this observation, itmay be suggested that the enrichment in 13C in thecarbonate and organic fractions were not coevalevents.

6.2.1. Causes for 13C enrichment in carbonatecarbon

On a local scale, 13C enrichment in carbonatecarbon could result by methanogenesis or evapora-tive conditions. On a larger scale it is consideredthat the increase in the organic carbon burial eitherdue to ocean stratification or high sedimentationrates induces such 13C enrichment. We examinewhich of these processes may have given rise to d13Cexcursion in the Jhamarkotra carbonate rocks.

The late Archean and Paleoproterozoic antiquityof methanogens as important contributors to thecarbon geochemical cycle is now well established(Winter and Knauth, 1992; Hayes, 1994; Dix et al.,1995). The degradation of organic matter in thebacterial fermentation zone producing 13C enrichedCO2 will lead to the precipitation of heavy carboncarbonates due to preferential enrichment of 12C inmethane (Irwin et al., 1977). This possibility is ruledout for the 13C enrichment in the Ghasiar dolomitesdue to following reasons: (i) Methanogenesis pro-duces large variation in d13C between −20 and+15‰ in the composition of the resultant carbon-ates because the rate of CO2 released and d13C ofdiagenetically introduced CO2 will vary with depth(Irwin et al., 1977). By contrast the Ghasiar carbon-ates, which have been collected across the strike,show only a narrow d13C range. (ii) Methanogene-sis results in formation of secondary carbonates dueto bacterial fermentation (cf. Dix et al., 1995). Nosuch secondary carbonate minerals are present inthe Ghasiar dolomites. Further, it is very difficultto envisage the source of carbon in these dolomitesof large thickness to methanogenesis alone.

Hypersaline environments can lead to 13C enrich-ment (Friedman, 1970; Stiller et al., 1985). Absenceof any evaporitic minerals or their pseu-

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domorphs in the Ghasiar dolomites and lack ofevaporitic trend in d18O versus d13C plot (Fig. 4)rules out this environment.

The foregoing discussion shows that the localscale processes such as methanogenesis or hypersa-line environment cannot be responsible for theobserved isotopic excursion. These open up thepossibility, that it may be a feature associated withincreased burial of organic carbon, caused either bystratification of ocean in terms of dissolved oxygenor high sedimentation rates. If the ocean becomesstratified with respect to dissolved oxygen, so thatthe deep ocean water remains anoxic, it wouldfavor accumulation of organic carbon in reducedform in the deeper parts of the ocean, leading to13C enrichment in the shallow water bicarbonate(Des Marais et al., 1992; Knoll et al., 1996). Sucha paleooceanographic model has been proposed toexplain the Neoproterozoic d13C excursions andattendant climatic variation (Knoll et al., 1996).Shields (1997) has opined that the extension ofstratified ocean model for the Paleoproterozoic,can explain not only the 13C enrichment in thecarbonates, but also several other associated fea-tures. According to stratified ocean model, the deepwater CO2 should be highly depleted in 13C becauseof organic matter metabolism resulting in theprecipitation of carbonates with highly negatived13C, similar to the mesopelagic limestones of thePermian Phosphoria Formation (Keith and Weber,1964). Main limitation of this model for the Pale-oproterozoic (2200–2000 Ma) d13C excursion,however, is that the deep-water carbonates of thisperiod are also 13C enriched. Evidence for thiscomes from Lofoten-Vesteralen, Norway (Bakerand Fallick, 1989b); Francevillian, Gabon (Gau-thier-Lafaye and Weber, 1989) and Uve Formationof the Labrador Trough, Canada (Melezhik et al.,1997). Therefore, for the Paleoproterozoic d13Cexcursion this model does not appear to be valid(see also Melezhik and Fallick, 1996, 1997). In anycase, the shallow water depositional environmentfor the massive dolomites of the JhamarkotraFormation and the immediately preceding Delwaraand succeeding Udaipur Formations is not consis-tent with the ocean stratification scenario.

Large-scale organic carbon burial can also beaccomplished by high sedimentation rates. High

sedimentation rate induces increased productivitybecause of enhanced nutrient supply. The burialthat accompanies sedimentation in such an envi-ronment should deposit carbonate rocks withgreater organic carbon content. Higher rates ofsedimentation on platforms evolving over a pene-plained basement can be due to denudation undergreenhouse conditions. Assuming that greenhousecondition (which were extant during the develop-ment of paleosols at the base of the AravalliSupergroup, and during the rift volcanism associ-ated with clastic sedimentation in the DelwaraFormation) prevailed through the Jhamarkotratimes, we envisage higher rate of sedimentation forthe Jhamarkotra carbonate rocks. While this pro-cess is our preferred interpretation for the observed13C enrichment in the Ghasiar dolomites, it mustbe admitted that the organic carbon content ofthese carbonate rocks is low. It is possible that someof the organic carbon might have been remineral-ized (sensu Des Marais et al., 1992) or may havemigrated out of the system.

6.2.2. Enrichment in organic carbon ofJhamarkotra stromatolitic phosphorites

The 13C enrichment in the organic carbon coex-isting with the dolomites hosting the stromatoliticphosphorites is an intriguing feature. While thed13Ccarb of these dolomites fall in the range ofnormal marine carbonate, the d13Corg values show\10‰ positive shift from the normal biogeniccarbon. Occurrence of 13C enriched carbonate andorganic carbon is a common feature among theNeoproterozoic carbonate sequences (Knoll et al.,1986; Iyer et al., 1995; Kaufman and Knoll, 1995;Hayes et al., 1999), so that their DC (d13Ccarb−d13Corg) remain constant at �28.5‰. However, inthe Jhamarkotra Formation, the average DC valuesof massive dolomites and phosphatic stromatolitesare 15 and 34‰, respectively (Fig. 5). These valuesare substantially deviated from the global mean of25‰. Hence, the d13C composition of carbonaterocks of the Jhamarkotra Formation represents aunique situation. Such a peculiarity is hithertounknown among other Paleoproterozoic highd13Ccarb sequences, although it is true that thed13Corg database is still scanty for the Pale-

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Fig. 5. Cross-plot between d13Ccarb and d13Corg of massive andphosphorite bearing stromatolitic dolomites of the Jhamarko-tra Formation showing two distinct fields as expressed by theirDC values. Note positive d13C excursion in carbonate carbonin the case of massive dolomites of Ghasiar and 13C enrich-ment in the organic fractions of the phosphorite bearingstromatolitic dolomites of Jhamarkotra. Symbols are same asin Fig. 4.

during organic carbon fixation. Hence, it is in-ferred that the high d13Corg values in the phos-phorites of Jhamarkotra Formation may be aresult of high organic productivity in localizedregions. The deposition of stromatolites andphosphorites in land-locked epicontinental settingcorroborate this inference. While prolific develop-ment of stromatolites indicate high organic activ-ity, the phosphorites point to enhancement ofessential nutrient phosphorous (Redfield, 1958)during the deposition of these carbonate sedi-ments. Further, the evidence from the carbonaterocks of Jhamarkotra Formation indicates thatthe diffusion-limited pathways associated withhigh productivity may have been operative sincethe Paleoproterozoic times.

6.3. Significance of 13C enrichment in theJhamarkotra Formation in the context ofPaleoproterozoic d13Ccarb excursion

The deposition of the high d13C Ghasiardolomites of the Jhamarkotra Formation can bebracketed between 2200 and 1900 Ma as evi-denced from the ages of galena from the DelwaraFormation and the Darwal Granite intrusive intothe Aravalli Supergroup, respectively (see Section2). A perusal of existing data on the 13C enrichedcarbonate sequences of the Paleoproterozoic fromseveral continents reveals three observations: (i)these carbonate sequences range in age between2430 and 1930 Ma; (ii) the majority of 2220–2060 Ma carbonate sequences are characterizedby d13Ccarb between 8 and 12‰; and (iii) theavailable d13Corg values of many of these carbon-ate sequences range between −30 and −25‰similar to normal biogenic carbon. The d13Ccarb

and d13Corg values of massive dolomites fromGhasiar with averages of 9.2‰ and −25.2 aresimilar to the data obtained on carbonate se-quences of 2200–2000 Ma. This points out to thepossibility that the 13C excursion in Ghasiardolomites serves as an Indian example of Pale-oproterozoic d13C excursion.

The increase in organic carbon burial as acause for the 13C excursion has a direct linkage tothe global scale oxygenation event, because it can

oproterozoic carbonate rocks. Furthermore, thedata obtained on the Jhamarkotra Formation isdifferent from those of Neoproterozoic sequences,in the sense that, 13C enrichment in carbonateand organic carbon has occurred at differentstratigraphic levels of a single formation resultingin distinctly different DC values.

Banerjee et al. (1986) suggested that the d13Corg

values of stromatolitic phosphorites are primaryand attributed them to the operation ofcyanobacteria in a diffusion-limited assimilatorypathway similar to autotrophic microorganismsor aquatic plants in which the degree of enrich-ment in 13C is directly correlatable with the salin-ity of their habitat. Earlier, it has been suggestedthat the heavy carbon biomass will result fromCO2 diffusion, limited by salinity and tempera-ture (Schidlowski et al., 1984). However, modernocean data as well as recent experimental evi-dences suggest that the heavy carbon biomassresults due to enhancement in productivity (Rauet al., 1992; Schidlowski et al., 1992) and is prin-cipally controlled by diffusion-limited pathways

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cause the accumulation of free-molecular oxygenin the atmosphere-hydrosphere system (Rye andHolland, 1998). Based on chemistry of paleosols ithas been concluded that the atmospheric PO2 rosefrom 58×10−4 atm to ]0.03 atm between2200 and 2000 Ma (ibid). Thus it appears that amajor disturbance in carbon cycle during the Pa-leoproterozoic led to the first major oxygenationevent of the earth history.

7. Conclusions

1. The massive dolomites of the JhamarkotraFormation belonging to the Aravalli Super-group deposited between 2100 and 1900 Maexhibit d13Ccarb values between 5.1 and 11.1‰and d13Corg −29.5 to −22.4‰. The 13C en-richment is suggested to be a result of increasein burial of organic carbon due to enhancedsedimentation rates. The Ghasiar dolomitesrepresent Indian example of the global Pale-oproterozoic d13C excurison.

2. A distinctive feature of the Jhamarkotra For-mation is that the dolomites hosting the phos-phatic stromatolites at a younger stratigraphiclevel overlying 13C enriched carbonates show13C enrichment in the organic fraction, whichis interpreted to be the result of diffusion-lim-ited pathways of organic carbon fixation underhigh productivity condition. Such high pro-ductivity appears to be a consequence of in-crease in the input of phosphorous nutrient.

Acknowledgements

This work forms a part of the Research Projecton Recognition of Archean-Proterozoic Boundaryfunded by the Department of Science and Tech-nology, India. The work has been carried outduring the tenure of Fellowship awarded to BS bythe Council of Scientific and Industrial Research.Authors are thankful to Dr Harsh K. Gupta,Director, National Geophysical Research Institutefor his permission to publish this work. Criticalcomments by Drs J.A. Karhu and B. Jahn and

suggestions by Dr K. Eriksson helped in improv-ing the paper. The authors are grateful to them.

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