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Regional variations in the carbon isotopic composition of phosphorite from the Early Cambrian Lower Tal Formation, Mussoorie Hills, India
Chemical Geology 175 Ž2001. 5–15 www.elsevier.comrlocaterchemgeo
Regional variations in the carbon isotopic composition of phosphorite from the Early Cambrian Lower Tal Formation, Mussoorie Hills, India A. Mazumdar ) , D.M. Banerjee Department of Geology, UniÕersity of Delhi, Delhi110007, India
Abstract Early Cambrian Lower Tal phosphorites of the Mussoorie Syncline have been analyzed for their carbon isotopic composition of apatite-bound carbonate and organic carbon. Two mine excavations at Maldeota and Durmala, located on two limbs of the syncline and ; 14-km apart were subjected to detailed sampling along selected stratigraphic profiles. Field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small-scale fluctuations in the sedimentation milieu. Isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at Durmala and Maldeota. In addition, a marked negative carbon isotope excursion has been recorded at the dolomite–phosphorite contact defined by the Krol–Tal stratigraphic boundary. It is thought to reflect the influx of 12 C- and P-rich waters from a deeper part of the sedimentary basin. Based on the isotopic composition of carbon in the phosphorite, sulfur in the associated pyrite and petrological data, it has been inferred that the Durmala phosphorite reflects deposition under a suboxic diagenetic environment and the Maldeota phosphorite formed within suboxic to anoxic sulfate-reducing conditions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Early Cambrian; Phosphorite; Apatite; Stable isotopes; Diagenetic environment
1. Introduction Sedimentary apatite is predominantly formed as a marine authigenic mineral ŽRiggs, 1979; Sheldon, 1981.. While some favor direct precipitation from the water column Že.g., Cook and Shergold, 1986b; Riggs, 1979; Sheldon, 1981., other have demonstrated experimentally that during early diagenesis amorphous phosphate gel recrystallizes into cryptocrystalline apatite mud and produces the mudstone )
Corresponding author. E-mail addresses: [email protected] ŽA. Mazumdar., [email protected] ŽD.M. Banerjee..
phosphorite ŽGulbrandsen et al., 1984.. The carbon and oxygen isotopic compositions of sedimentary apatite has been widely used to constrain its formation and, thus, elucidate the host rock’s depositional history ŽKolodny and Kaplan, 1970; Benmore et al., 1983; Piper and Kolodny, 1987; Shemesh et al., 1988.. More specifically, McArthur et al. Ž1980, 1986. utilized the carbon isotopic composition of the structural carbonate within the apatite to characterize the diagenetic conditions under which it was formed. Based on an extensive review of modern and ancient phosphorite deposits, McArthur et al. Ž1986. considered d13 C carb values between 0‰ to y6‰ as characteristic for apatite, which has been formed in
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A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
marine oxic–suboxic environments while those forming under anoxic conditions displayed more 12 C-depleted values. On the other hand, the oxygen isotopic composition of the marine apatite has primarily been used to estimate palaeoceanic andror burial temperatures ŽKarhu and Epstein, 1986; Shemesh et al., 1988; Kolodny and Luz, 1994.. In this paper, we discuss variations in d13 C and 18 d O measured for phosphorites from the Tommotian Lower Tal Formation with samples derived from two mine excavations located at Maldeota and Durmala in the Mussoorie syncline, India. Differences in the nature and extent of diagenetic alteration of these phosphorites have been elucidated by their stable
isotope values of the structural carbonate in the apatite lattice.
2. Geological setting The Neoproterozoic to Early Cambrian sequence of the Krol Belt in the Lesser Himalaya comprises sedimentary rocks of the Blaini, Krol and Tal Groups. This thick pile of sedimentary cover crops out in several discrete synclines ŽFig. 1. extending over a distance of more than 300 km. The Blaini Group consists of diamictite, arenite, shale and capcarbonate and is overlain by shale and siltstone of
Fig. 1. ŽA. Location and disposition of Mussoorie syncline with respect to Solan, Panchmunda, Nigalidhar, Garhwal and Nainital synclines in the Lesser Himalaya; ŽB. geological map of the western part of the Mussoorie Syncline showing the location of Maldeota and Durmala Mine sections; ŽC. lithologs of uppermost part of Krol-E and the Lower Tal Formation ŽChert-Phosphorite Member and Argillaceous Member.; MBF s Main Boundary Fault.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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Fig. 2. Typical hand specimen of Lower Tal rock displaying thick mudstone phosphorite layers, at times interbedded with apatite poor carbonate layers. Bar scale s 1 cm.
the Infrakrol Formation. The Infrakrol Formation is overlain by the Krol Group, which is divided into three formations: the Lower Krol ŽKrol-A Member; marlstone and sandstone., the Middle Krol ŽKrol-B Member; red shales and minor carbonates. and the Upper Krol comprising the Krol-C Žlimestone, dolomite and laminated algal dolomite., Krol-D
Žfenestral and stromatolitic dolomite with chert bands. and Krol-E Žargillaceous dolomite, laminated dolomite and grayish-black shales. Members. These rocks broadly reflect a supratidal–intertidal–subtidal depositional succession ŽSingh, 1979.. The Krol-E is unconformably overlain by the Chert-Phosphorite Member Žs Chert Member of Shanker, 1975. of the
Fig. 3. Cryptocrystalline apatite with recrystallized patches in microscopic vugs and irregularly dispersed organic matter. Bar scale s 60 mm.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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Lower Tal Formation, which represents the Tommotian stage of the Early Cambrian as constrained by abundant small shelly fossils and sponge spicules ŽBhatt et al., 1985; Brasier and Singh 1987; Mazumdar and Banerjee, 1998.. The Chert-Phosphorite Member consists of layers of bedded black chert alternating with blackrgrey shale and dark colored phosphorite. The latter lithology constitutes an important economic horizon ŽShanker, 1976, 1987. in the Krol belt with large phosphorite deposits in the Nigalidhar, Korgai, Mussoorie and Garhwal synclines ŽFig. 1.. Primary depositional features such as small-scale, low angle cross-bedding, ripple marks in grainstone phosphorites, intraclasts, ooids, microbial
laminites and small domal stromatolites ŽMazumdar, 1996, and references therein. are suggestive of an intertidal to shallow subtidal depositional setting for the Lower Tal Formation. Features indicating shallowing upward cycles can be demonstrated within 10-to 15-cm thick phosphorite beds, which clearly suggest rapid changes of water depths in a shallow marine basin. Phosphate intraclasts, rip-up clasts and fragmented stromatolitic columns further suggest deposition in a high-energy environment. Such gross textural features of a shallow marine facies are characteristic of Neoproterozoic and Early Cambrian phosphorite deposits worldwide ŽCook and Shergold, 1986a..
Table 1 Carbon and oxygen isotope values of Lower Tal phosphorite Sample no.
d 13 C carb Ž‰., PDB
d 18 Ocarb Ž‰., PDB
d 13 C org Ž‰., PDB
D d Ž‰., PDB
C org Ž%.
DQ14 DQ16 DQ17 DQ18 DU5 DU6 DU7 DU8 DU9 DU10 DU12 DUR18 DUR21 DUR24 DUR25 DUR28 DUR29 DUR31 DUR34 DUR35 SK1 SK2 SK3 SK4 SK5 SK6 SK7 SK8 MUS1 MUS4 MUS5 MUS7
y3.6 y4.5 y5.4 y7.1 y7.4 y8.0 y7.4 y6.4 y6.0 y7.1 y7.7 y5.6 y3.8 y4.5 y4.3 y8.4 y7.2 y4.4 y3.1 y4.5 y6.3 y11.4 y9.7 y17.8 y11.4 y10.0 y8.8 y8.2 y6.4 y9.9 y8.4 y8.5
y6.8 y7.9 y8.9 y8.1 y12.6 y13.4 y12.7 y10.5 y12.9 y12.3 y12.0 y11.6 y11.7 y14.3 y14.3 y10.5 y10.5 y11.2 y14.3 y11.8 y12.5 y11.0 y12.0 y11.6 y11.2 y12.1 y10.4 y11.5 y9.7 y10.9 y13.8 y
y35.2 y28.2 y32.3 y33.3 y28.9 y28.2 y32.6 y31.3 y30.2 y31.8 y28.6 y33.0 y32.0 y32.1 y32.2 y33.0 y32.4 y31.2 y32.0 y32.2 y32.8 y33.0 y33.8 y33.8 y33.9 y33.6 y34.5 y30.8 y33.0 y34.4 y33.6 y32.9
31.6 23.7 26.9 26.2 21.6 20.1 25.2 24.9 24.2 24.7 21.0 27.4 28.2 27.6 27.9 24.6 25.2 26.8 28.9 27.7 26.5 21.6 24.1 16.1 22.4 23.5 25.7 22.5 26.4 24.5 25.2 24.4
3.1 1.4 2.1 0.8 1.7 1.6 0.0 1.6 3.5 3.1 1.9 3.8 0.6 4.9 3.7 0.2 0.6 2.3 3.6 4.7 0.9 1.1 2.1 1.4 2.4 1.6 3.0 1.9 0.74 0.92 2.19 2.01
DUR, DQ, DU s DURMALA; SK, MUS s MALDEOTA.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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3. Sample selection
5. Results
Near pristine phosphate samples were selected on the basis of petrographic studies ŽMazumdar, 1996.. Megascopically visible dark mudstone phosphorite layers Žmicrosphorite of Riggs, 1979, see Fig. 2., more or less devoid of any carbonate veinlets and vugs were microsampled utilizing a tungsten carbide tipped dentist’s drill. However, some microinclusions of detrital carbonate derived from the underlying Krol-E dolomites could still be part of the samples. Based on previous studies of other phosphorite-rich samples, this does not result in a significant alteration of the carbon isotopic composition of the phosphate mud. All our analyses were confined to laminated mudstone phosphorite layers, consisting of micro- and cryptocrystalline apatite ŽFig. 3.. Results from SEM work acknowledge the presence of recrystallized patches consisting of columnar and radiating apatite crystals. The phosphate layers are dark to light brown depending upon organic content. Silt-sized detrital quartz, chert fragments, small mica flakes and pyrite are invariably associated with the phosphate matrix. At time, light colored detritus and carbonates form distinct layers in contrast to dark apatite beds. While dolomitization of the phosphate layers could be perceived at places, phosphatization of the carbonate minerals was generally not seen.
Carbon and oxygen isotopic compositions and organic carbon contents for the phosphorite samples from the Maldeota and Durmala sections are listed in Table 1 and presented in Figs. 4 and 5. Thereby, Fig. 4 provides the stratigraphic variations in carbonate and organic carbon and Fig. 5 displays various
4. Analytical methods For carbon and oxygen isotope analysis, powdered phosphorite samples were treated with phosphoric acid following the principles of McCrea Ž1950. and Craig Ž1953.. Subsequently, liberated CO 2 was measured on a VG Isogas Mass Spectrometer at the Max-Planck-Institut fur ¨ Chemie in Mainz, Germany. Details on analytical procedures, oxygen isotope correction factors and level of accuracy are similar to those provided by Banerjee et al. Ž1986.. A total of 32 phosphorite samples were selected from one profile at Maldeota and two profiles at Durmala. Apart from results obtained during this study, data described in Banerjee et al. Ž1986, 1997. will also be included in the present discussion.
Fig. 4. Ža and b. Isotopic variations along the vertical profiles near Maldeota Mines ŽSK samples. and Durmala Mines ŽDUR samples cf. Banerjee et al., 1997..
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A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
phosphorites define two distinct populations ŽFig. 5b.. The Durmala phosphorites display d13 C carb values in the range of y3.1‰ to y8.4‰ Žmean: y5.8‰., whereas carbonate carbon from the Maldeota phosphorites yielded distinctly more negative carbon isotope values between y6.3‰ and y11.4‰ including one sample as low as y17.7‰. In contrast, average d18 O values measured for samples from Durmala Žy11.4‰. and Maldeota Žy11.5‰. are nearly identical. However, despite a similar average oxygen isotopic composition, the Durmala phosphorites display a greater variability Žy6.7‰ to y14.3‰. as compared to the Maldeota samples Žy9.7‰ to y13.8‰.. No significant correlation between d13 C carb and d18 Ocarb is discernible for either sample population ŽFig. 5b.. Furthermore, the correlation between d13 C org and d13 C carb is rather poor in all profiles ŽFig. 5a.. No clear correlation exists between carbonate content and d13 C carb value nor between total organic carbon abundance ŽTOC. and isotopic composition. Although, samples with high TOC values seem to be characterized by more negative d13 C org values.
6. Discussion
Fig. 5. Ža. d13 C org – d13 C carb plots for Lower Tal phosphorite. Žb. d13 C carb – d18 O plots for Lower Tal phosphorite. Žc. D d – d13 C carb plots for Lower Tal phosphorite.
cross-plots of isotopic compositions. Despite some overlap, d13 C carb values of the Durmala and Maldeota
Carbonate fluorapatite ŽCFA., the prime component of the phosphorite is either an authigenic or early diagenetic mineral. The primary nature of mudstone phosphorite has been adequately demonstrated through petrographic studies from different parts of the world ŽRiggs, 1979; Sheldon, 1981; Ilyin and Ratnikova, 1981; Burnett, 1977; Swett and Crowder, 1982.. Petrographic features like localized replacement of early formed phosphate mud layers by dolomite, undisturbed primary apatitic layers, preservation of phosphatic small shelly fossils and microburrow type features suggest a primary or very early diagenetic formation. As such, the apatite derives its structural carbonate from the porewater in which the CO 2 or bicarbonate concentration is largely controlled through the amount of biodegradation in the sedimentary column. It has, thus, been argued ŽKolodny and Luz, 1994. that the d13 C carb composition of apatite can be utilized as a proxy for early diagenetic conditions in the sediment. Based on porewater data from organic-rich, modern marine
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
sediments and reflecting oxic–suboxic ŽFroelich et al., 1979; Emerson et al., 1982; Jahnke et al., 1982; McCorkle et al., 1985. or anoxic ŽLaZerte, 1981; Nissenbaum et al., 1972. conditions, McArthur et al. Ž1986. concluded that the d13 C of dissolved inorganic carbon varies within a range between 0‰ and y6‰ during oxic–suboxic diagenesis. In contrast, apatite precipitated in the zone of bacterial sulfate reduction inherits d13 C values, which are more 13 Cdepleted than y6‰. This is underlined by numerous studies of francolite from different ages. In the absence of any significant fractionation during precipitation, carbonate fluorapatite is thought to faithfully record these carbon isotope signals. Major phosphorite deposits of late Neoproterozoic to Early Cambrian time have been reported from China, Iran, Kazakhstan and Mongolia ŽCook and Shergold, 1986a.. In India, similar lithologies are present in the Chert-Phosphorite Member of the Lower Tal Formation, which is of Tommotian age. The stable isotope geochemistry of these phosphorite bearing successions ŽBrasier et al., 1990; Brasier, 1992; Banerjee et al., 1986, 1997; Kimura et al., 1997. have been utilized for interpreting the mechanism of phosphogenesis in the Early Cambrian sedimentary basins. Such studies have helped to establish relationships between geochemical anomalies and lithofacies variations observed in the late Neoproterozoic and across the Neoproterozoic–Cambrian boundary Žsee, e.g., Aharon et al., 1987; Narbonne et al., 1994; Magaritz et al., 1986, 1991; Brasier 1990, 1992; Brasier et al., 1990; Kirschvink et al., 1991; Ripperdan, 1994; Kaufman et al., 1993, 1996; Kimura et al., 1997; Bartley et al., 1998.. A sharp negative excursion in the temporal trend of d13 C carb relative to the underlying carbonate rocks appears to be characteristic for many Precambrian–Cambrian boundary sections around the world ŽAharon et al., 1987; Xu et al., 1989; Brasier et al., 1990; Kaufman et al., 1996; Banerjee et al., 1997 and many others.. Positive d13 C carb values in the carbonates and less 13 C-depleted organic carbon values in the Upper Krol Formation ŽKrol-E. have been reported by Banerjee et al. Ž1997., followed by a sharp negative excursion of ; 4–5‰ at the base of the Lower Tal Formation, which is of Tommotian age. The base of the Tommotian is defined by the first appearance and abundance of a small shelly fossil assemblage con-
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sisting of Anabarites sp., Protohertzina sp., Maldeotaia and Olivooides sp. ŽBhatt et al., 1985; Brasier and Singh, 1987; Kumar et al., 1987.. Although no diagnostic fauna has been identified so far in the underlying Krol-E Formation, it is tentatively correlated with the Nemakit-Daldynian Stage on the Siberian Platform. Due to the paucity of reliable radiometric dates, it would be premature to correlate the negative isotope excursions recorded in the Maldeota and Durmala sections either with the 544Ma-old ŽGrotzinger et al., 1995. base of NemakitDaldynian or with the 531-Ma-old ŽIsachsen et al., 1994. basal Tommotian. It has been suggested before Žsee, e.g., Narbonne et al., 1994. that the organic carbon isotopic composition would equally serve as a proxy signal for secular variations of the global carbon cycle, provided the signal has not been obliterated by post-depositional processes, notably extensive thermal alteration. With respect to the terminal Neoproterozoic and its transition into the Cambrian, the successful application of this approach has been shown, e.g., for respective sections in the Mackenzie Mountains ŽNarbonne et al., 1994. or the East European Platform ŽStrauss et al., 1997.. The latter succession did not contain any carbonate. The temporal record of organic carbon isotope values display a 13 C-depletion across the Precambrian–Cambrian transition, followed by an evolution to more 13 C-enriched values in the lower Cambrian. This negative d13 C-shift was attributed to a reduction in organic carbon burial as a consequence of a drop in productivity and the terminal Neoproterozoic extinction ŽStrauss et al., 1997.. A light brown colour of the kerogen, HrC values between 0.2 and 0.7 and illite crystallinity indices Ž D 0 2 u ) 0.42. ŽMazumdar, 1996; Mazumdar and Frank, in preparation. suggest that the organic matter in the Tal phosphorites has not suffered substantial thermal alteration. It is, thus, assumed, that the primary isotopic composition has not been altered substantially Žsee also Strauss et al., 1992. and that the negative d13 C org excursion of 5‰ Žy28‰ to y33‰. in the Lower Tal phosphorite relative to the underlying Krol-E dolomite, reflects a primary feature. Similarly, the covariance noted in the isotopic compositions of carbonate and organic carbon in the
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A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
Krol-E dolomites through time primarily reflects a secular change in the 13 Cr12 C of the oceanic C-reservoir and is not an artifact of secondary processes ŽSchidlowski et al., 1975; Veizer and Hoefs, 1976; Knoll et al., 1986; Narbonne et al., 1994; Narbonne and Aitken, 1995; Kimura et al., 1997.. The onset of phosphate deposition at the beginning of the Tal Formation is paralleled by a sharp negative excursion shift in the carbon isotopic composition. Characteristic isotope excursions of this type have also been recorded from the Early Cambrian organic-rich phosphaterblack shale horizons of South China and northern Iran ŽBrasier et al., 1990.. The marked decrease of the d13 C carb and d13 C org values within the Tal phosphorite can be related to the influx of 12 C- and P-rich deeper basinal waters to a platform setting during a contemporaneous ocean mixing event ŽDonnelly et al., 1990; Aharon and Liew, 1992; Banerjee et al., 1997; Mazumdar et al., 1999.. The spread in d13 C org data ŽTable 1. is thought to reflect variations in the rate of photosynthetic carbon fixation or in growth rate ŽHayes et al., 1989; Popp et al., 1997., which in turn is limited by the fluctuation in the nutrient concentration. Haddad et al. Ž1988. demonstrated that the rapid rates of carbon fixation associated with peak photosynthetic communities is accompanied by reduced discrimination against 13 C. It is therefore suggested that the overall negative carbon isotope excursion coincident with the Tommotian phosphogenic episode was superimposed by minor isotopic perturbations as a result of varying nutrient enriched ŽNEW. and nutrient depleted ŽNDW. water conditions ŽBrasier, 1992.. Marked differences in the nature of d13 C org plots for the Maldeota and the Durmala successions apparently reflect intrabasinal variations in the nutrient concentrations. In addition, variations in d13 C org could also reflect a varying input of heterotrophic biomass. This, however, is difficult to characterize, let alone quantify. It should be possible to unravel the effects of heterotrophic reworking only through compound-specific carbon isotope studies. It is obvious that the carbon isotopic composition of apatite reflects changes in the isotopic composition of the basinal water, with subsequent diagenetic modifications further affecting d13 C. Although an overall covariance of d13 C carb and d13 C org exists for samples from the Durmala Mine section ŽBanerjee et
al., 1997., data variability ŽFig. 5a. suggests some diagenetic modification of the carbonate carbon isotopic composition in the apatite. A most obvious source of alteration would be the incorporation of isotopically light carbon resulting from biodegradation of sedimentary organic matter in a suboxic environment. It has been suggested before Že.g., Kaufman et al., 1993. that the difference between the carbonate and organic carbon isotopic compositions Ž D d s d13 C carb y d13 C org . could be used to monitor a possible post-depositional alteration caused by the incorporation of organically derived carbon in the porewaters. In that respect, the D d values calculated in this study show a moderate to good positive correlation with d13 C carb for both, the Durmala Ž r s 0.77. and the Maldeota Ž r s 0.87. phosphorites ŽFig. 5c.. Jarvis et al. Ž1994. stressed that secular variations in the marine d13 C record with long-term and shortterm fluctuations should be taken into consideration when interpreting the diagenetic conditions of apatite precipitation. Despite some intrabasinal variation, the globally correlative negative carbon isotope excursion at the base of Tommotian is noteworthy. Assuming the carbon isotopic composition of organic matter in the Tal phosphorite as a proxy for an overall negative shift Ž4‰ to 5‰. of the dissolved inorganic carbon reservoir of the contemporary platformal water, the d13 C carb values of the apatite reflect the global signal as well as an additional negative shift resulting from incorporation of reworked carbon during variable diagenetic conditions. The two distinct clusters for samples from Maldeota and Durmala ŽFig. 5a. with a minor overlap indicate differences in the diagenetic conditions for both settings. It may be argued that the d13 C carb values for the Durmala phosphorite reflect a predominantly suboxic environment, while the Maldeota phosphorites formed in the suboxic to anoxic, sulfate reducing zone ŽMcArthur et al., 1986.. Phosphogenesis was apparently restricted to the shallow suboxic zone. In the anoxic sulfate reducing zone, apatite precipitation is expected to cease due to rapidly increasing carbonate alkalinity ŽJahnke, 1984; Tribble et al., 1995.. Considering organic-rich sediments of the Peru–Chile margins as a modern analog, Jarvis et al. Ž1994. concluded that apatite formation is Arestricted to within 5–20 cm of the sedi-
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
mentrwater interface because precipitation ceases at the high carbonate alkalinities, which develop rapidly in deeper sedimentsB. Extremely negative isotope values as low as y17‰ observed during this study could be a result of substantial incorporation of carbon derived from reworking of sedimentary organic carbon by bacterial sulfate reduction. It is, however, difficult to rule out upward diffusion of 13 C-depleted water from the anoxic sulfate reduction zone into the shallow suboxic zone. Sulfur isotope values of associated pyrites show a large spread Žy25‰ to q45‰. indicating pyrite formation in a sulfate reducing to methanogenic environment ŽMazumdar et al., 1999..
7. Conclusions Organic carbon and apatite-carbonate carbon isotopic compositions in phosphorites from the Early Cambrian Tal Formation have been studied to characterize different modes of formation prevalent in the Durmala and Maldeota sections, Mussoorie Hills, Lesser Himalaya, India. Results define two partially overlapping clusters in d13 C carb – d13 C org and d13 C carb – d18 Ocarb-diagrams. d13 C carb signatures determined in this study are interpreted to reflect fluctuations in the carbon isotopic composition of the water column partially amplified by subsequent diagenesis. The change in the isotopic composition of dissolved inorganic carbon in platformal water is thought to reflect the influx of P- and 12 C-rich waters from a deeper part of the basin. This shift in the isotopic composition can also be observed in the thermally largely unaltered organic carbon in the Tal phosphorites. The negative carbon isotopic excursion associated with the phosphogenic episode during basal Tommotian time was superimposed by minor isotopic perturbations caused by variations in the nutrient content of basinal waters. Absence of a significant positive correlation between d13 C org and d13 C carb points to an interpretation of the extremely negative shifts in the carbonate carbon isotopic composition as a result of the incorporation of 13 C-depleted CO 2 produced during bacterial degradation of the organic matter. At the site of phosphorite formation at Durmala, the process of biodegradation occurred essentially under suboxic diagenetic
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conditions, whereas the diagenetic environment at Maldeota was characterized by suboxic to anoxic conditions. Acknowledgements We thank Manfred Schidlowski for providing access to the mass-spectrometric facility during DMB’s tenure at the Max-Planck-Institut fur ¨ Chemie in Mainz, Germany, under the Alexander von Humboldt Foundation Revisit program. General laboratory facilities were drawn through a Project grant to DMB from the Department of Science and Technology, New Delhi. This paper is a contribution to the aims and objectives of IGCP Project No. 386. David McKirdy, Jay Kaufman and an anonymous reviewer helped us to revise the original text. We appreciate their help. References Aharon, P., Liew, T.C., 1992. An assessment of the Precambrian–Cambrian transition event on the basis of carbon isotope records. In: Schidlowski, M., Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ŽEds.., Early Organic Evolution — implications for Mineral and Energy Resources. Springer, pp. 212–224. Aharon, P., Schidlowski, M., Singh, I.B., 1987. Chronostratigraphic markers in the end-Precambrian carbon isotope record of the Lesser Himalaya. Nature 327, 699–701. Banerjee, D.M., Schidlowski, M., Arneth, J.D., 1986. Genesis of Upper Proterozoic–Cambrian phosphorite deposits of India: isotopic inferences from carbonate fluorapatite, carbonate and organic carbon. Precambrian Res. 33, 239–253. Banerjee, D.M., Schidlowski, M., Siebert, F., Brasier, M.D., 1997. Geochemical changes across the Proterozoic–Cambrian transition in the Durmala phosphorite mine section, Mussoorie Hills, Garhwal Himalaya, India. Paleogeogr. Paleoclimatol. Paleoecol. 132, 183–194. Bartley, J.K., Pope, M., Knoll, A.H., Semikhatov, M.A., Petrov, P.Yu., 1998. A Vendian–Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation. Geol. Mag. 135, 473–494. Benmore, R.A., Coleman, M.L., McArthur, J.M., 1983. Origin of sedimentary francolite from its sulphur and carbon isotope composition. Nature 302, 516–518. Bhatt, D.K., Mamgain, V.V., Misra, R.S., 1985. Small shelly fossils of Early Cambrian ŽTommotian. age from Chert-Phosphorite Member, Tal Formation, Mussoorie Syncline, Lesser Himalaya, India and their chronostratigraphic evaluation. J. Paleontol. Soc. India. 30, 92–102.
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Brasier, M.D., 1990. Phosphogenic events and skeletal preservation across the Precambrian–Cambrian boundary interval. In: Notholt, A.J.G., Jarvis, I. ŽEds.., Phosphorite Research and Development. Geol. Soc. Spec. Publ. ŽLondon. 52, pp. 289– 303. Brasier, M.D., 1992. Nutrient-enriched waters and early skeletal fossil record. J. Geol. Soc. ŽLondon. 149, 621–629. Brasier, M.D., Singh, P., 1987. Microfossils and Precambrian– Cambrian boundary stratigraphy at Maldeota, Lesser Himalaya. Geol. Mag. 124, 323–345. Brasier, M.D., Magaritz, M., Corfield, R., Luo, H., Wu, X., Ouyang, L., Jiang, Z., Hamdi, V., He, T., Fraser, A.G., 1990. The carbon and oxygen isotope record of the Precambrian– Cambrian boundary interval in China and Iran and their correlation. Geol. Mag. 127, 319–332. Burnett, W.C., 1977. Geochemistry and origin of phosphorite deposits from off Peru and Chile. Bull. Geol. Soc. Am. 88, 813–823. Cook, P.J., Shergold, J.H., 1986a. Phosphate Deposits of the World. Proterozoic and Cambrian Phosphorites vol. 1. Cambridge Univ. Press, Cambridge. Cook, P.J., Shergold, J.H., 1986b. Proterozoic and Cambrian phosphorites — nature and origin. In: Cook, P.J, Shergold, J.H. ŽEds.., Phosphate Deposits of the World. Proterozoic and Cambrian Phosphorites vol. 1. Cambridge University Press, pp. 369–390. Craig, H., 1953. The geochemistry of stable carbon isotopes. Geochim. Cosmochim. Acta 3, 53–92. Donnelly, T.H., Shergold, J.H., Southgate, P.N., Barnes, C.J., 1990. Events leading to global phosphogenesis around the ProterozoicrCambrian boundary. In: Notholt, A.J.G., Jarvis, I. ŽEds.., Phosphorite Research and Development. Geol. Soc. Spec. Publ. 52, pp. 273–287. Emerson, S., Grundmanis, V., Graham, D., 1982. Carbonate chemistry in marine pore waters: MNOP sites, C and S. Earth Planet. Sci. Lett. 61, 220–232. Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Ludtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta 43, 1075–1090. Grotzinger, J.P., Bowring, S.A., Saylor, B.Z., Kaufman, A.J., 1995. Biostratigraphic and geochronologic constraints on early animal evolution. Science 270, 598–604. Gulbrandsen, R.A., Roberson, C.E., Neil, S.T., 1984. Time and the crystallization of apatite in seawater. Geochim. Cosmochim. Acta 48, 213–218. Haddad, R.I., Des Marais, D.J., Nguyen, H., Bauer, J., Nguyen, M., 1988. Carbon isotope cycling in recent microbial mat. EOS 69, 1083–1084. Hayes, J.M., Popp, B.N., Takigiku, R., Johnson, M.W., 1989. An isotopic study of biogeochemical relationships between carbonates and organic carbon in the Greenhorn Formation. Geochim. Cosmochim. Acta 53, 2961–2972. Ilyin, A.V., Ratnikova, G.I., 1981. Primary, bedded, structureless phosphorite of the Khujbsugul Basin, Mongolia. J. Sediment. Petrol. 51, 1215–1222. Isachsen, C.E., Bowring, S.A., Landing, E., Samson, S.D., 1994.
New constraint on the division of Cambrian time. Geology 22, 496–498. Jahnke, R.A., 1984. The synthesis and solubility of carbonate fluoroapatite. Am. J. Sci. 284, 58–78. Jahnke, R.A., Heggie, D., Emerson, S.K., Grundmanis, V., 1982. Pore waters of the central Pacific ocean: nutrient results. Earth Planet. Sci. Lett. 61, 233–256. Jarvis, I., Burnett, W.C., Nathan, Y., Almabaydin, F.S.M., Attita, A.K.M., Castro, L.N., Flicoteaux, R., Hilmy, M.E., Husain, V., Qutawnah, A.A., Serjani, A., Zanin, Y.N., 1994. Phosphorite geochemistry: state-of-the-art and environmental concerns. Eclogae Geol. Helv. 87, 643–700. Kaufman, A.J., Jacobsen, S.B., Knoll, A.H., 1993. The Vendian record of Sr and C isotope variation in seawater: implications for tectonic and paleoclimate. Earth Planet. Sci. Lett. 120, 409–430. Kaufman, A.J., Knoll, A.H., Semikhatov, M.A., Grotzinger, J.P., Jacobsen, S.B., Adams, W., 1996. Integrated chemostratigraphy of Proterozoic–Cambrian boundary beds in the western Anabar region, northern Siberia. Geol. Mag. 133, 509–533. Karhu, J., Epstein, S., 1986. The implications of the oxygen isotope record in coexisting cherts and phosphates. Geochim. Cosmochim. Acta 50, 1745–1756. Kimura, H., Matsumoto, R., Kakuwa, Y., Hamdi, B., Zibaseresht, H., 1997. The Vendian–Cambrian d13 C record, North Iran: evidence for overturning of the ocean before the Cambrian explosion. Earth Planet. Sci. Lett. 147, 1–7. Kirschvink, J.L., Magaritz, M., Ripperdan, R.L., Zhuravlev, A.Yu., Rozanov, A.Yu., 1991. The PrecambrianrCambrian boundary: magnetostratigraphy and carbon isotopes resolve correlation problems between Siberia, Morocco, South China. GSA Today 1, 69–91. Knoll, A.H., Hayes, J.M., Kaufman, A.J., Swett, K., Lambert, I.B., 1986. Secular variation in carbon isotope ratios from Proterozoic succession of Svalbard and East Greenland. Nature 321, 832–838. Kolodny, Y., Kaplan, I.R., 1970. Carbon and oxygen isotopes in apatite CO 2 and co-existing calcite from sedimentary phosphorite. J. Sediment. Petrol. 40, 954–959. Kolodny, Y., Luz, B., 1994. Isotopic signature in phosphate deposits: formation and diagenetic history. In: Clauer, N., Chaudhuri, S. ŽEds.., Isotopic Signatures and Sedimentary Records. Springer, pp. 69–120. Kumar, G., Bhat, D.K., Raina, B.K., 1987. Skeletal microfauna of Meishucunian and Quingzhusian ŽPrecambrian–Cambrian Boundary. age from the Ganga valley, Lesser Himalaya, India. Geol. Mag. 124, 167–171. LaZerte, B.D., 1981. The relationship between total dissolved CO 2 and its stable isotope ratio in aquatic sediments. Geochim. Cosmochim. Acta 45, 647–656. Magaritz, M., Holser, W.T., Kirschvink, J.L., 1986. Carbon isotope events across the Precambrian–Cambrian boundary on the Siberian Platform. Nature 320, 258–259. Magaritz, M., Latham, A.J., Kirschvink, J.L., Zhuravlev, A.Yu., Rozanov, A.Yu., 1991. Precambrian–Cambrian boundary problem: carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco. Geology 9, 247–250. Mazumdar, A., 1996. Petrographic and geochemical characterisa-
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15 tion of the Neoproterozoic–Cambrian succession in a part of the Krol Belt, Lesser Himalaya. Unpublished PhD Thesis, University of Delhi, Delhi. Mazumdar, A., Banerjee, D.M., 1998. Siliceous sponge spicules in the Early Cambrian chert-phosphorite member of the Lower Tal Formation, Krol Belt, Lesser Himalaya. Geology 26, 899–902. Mazumdar, A., Banerjee, D.M., Schidlowski, M., Balram, V., 1999. Rare-earth elements and stable isotopic geochemistry of Early Cambrian, Lower Tal chert-phosphorite of Krol Belt, Lesser Himalaya, India. Chem. Geol. 156, 275–297. Mazumdar, A., Frank, W., in preparation. Illite crystallinity and Ar–Ar age dating of Neoproterozoic–Early Cambrian rocks of Krol Belt, Lesser Himalaya. McArthur, J.M., Coleman, M.L., Bremner, J.M., 1980. Carbon and oxygen isotopic composition of structural carbonate in sedimentary francolite. J. Geol. Soc. ŽLondon. 137, 669–673. McArthur, J.M., Benmore, R.A., Coleman, M.L., Soldi, C., Yeh, H.W., O’Brien, G.W., 1986. Stable isotopic characteristic of francolite formation. Earth Planet. Sci. Lett. 77, 20–34. McCorkle, D.C., Emerson, S.R., Quay, P.D., 1985. Stable carbon isotopes in marine pore waters. Earth Planet. Sci. Lett. 74, 13–26. McCrea, J.M., 1950. Isotopic chemistry of carbonates and a paleo-temperature scale. J. Chem. Phys. 18, 849–857. Narbonne, G.M., Aitken, J.D., 1995. Neoproterozoic of the Mackenzie Mountains, Northwestern Canada. Precambrian Res. 73, 101–121. Narbonne, G.M., Kaufman, A.J., Knoll, A.H., 1994. Integrated chemostratigraphy and biostratigraphy of the upper Windermere Supergroup ŽNeoproterozoic., Mackenzie Mountains, Northwestern Canada. Geol. Soc. Am. Bull. 106, 1281–1291. Nissenbaum, A., Presley, B.J., Kaplan, I.R., 1972. Early diagenesis in a reducing fjord, Sannich Inlet, British Columbia: I. Chemical and isotopic changes in major components of interstitial waters. Geochim. Cosmochim. Acta 36, 1007–1027. Piper, D.J., Kolodny, Y., 1987. The stable isotopic composition of a phosphorite deposit: d13 C, d34 S and d18 O. Deep-Sea Res. 34, 897–911. Popp, B.N., Parekh, P., Tillbrook, B., Bidigare, R.R., Laws, E.A., 1997. Organic carbon d13 C variations in sedimentary rocks as chemostratigraphic and paleoenvironmental tools. Paleogeogr., Paleoclimatol., Paleoecol. 132, 119–132. Riggs, R.R., 1979. Phosphorite sedimentation in Florida — a model phosphogenic system. Econ. Geol. 74, 285–314.
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Ripperdan, R.L., 1994. Global variations in carbon isotope composition during the latest Neoproterozoic and earliest Cambrian. Ann. Earth Planet. Sci. Lett. 22, 385–417. Schidlowski, M., Eichmann, R., Junge, C.E., 1975. Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Res. 2, 1–69. Shanker, R., 1975. Stratigraphic analysis of the Chert Member, Lower Tal Formation in Dehradun and Tehri districts. U.P. Rec. Geol. Surv. India 106, 54–74. Shanker, R., 1976. Maldeota phosphorite prospect Dehradun district. U.P. Geol. Surv. India Misc. Publ. 24, 465–575. Shanker, R., 1987. The Mussoorie Phosphate Basin, India. In: Notholt, A.J.G., Sheldon, R.P., Davidson, D.E. ŽEds.., Phosphate Deposits of the World vol. 2. Cambridge Univ. Press, pp. 443–448. Sheldon, R.P., 1981. Ancient marine phosphates. Annu. Rev. Earth Planet. Sci. 9, 251–284. Shemesh, A., Kolodny, Y., Luz, B., 1988. Isotope geochemistry of oxygen and carbon in phosphate and carbonate of phosphorite francolite. Geochim. Cosmochim. Acta 52, 2565–2572. Singh, I.B., 1979. Environment and age of the Tal formation of Mussoorie and Nilkanth areas of Garhwal Himalayas. J. Geol. Soc. India 20, 214–225. Strauss, H., Des Marais, D.J., Summons, R.E., Hayes, J.M., 1992. The carbon-isotopic record. In: Schopf, J.W., Klein, C. ŽEds.., The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge Univ. Press, pp. 117–127. Strauss, H., Vidal, G., Moczydlowska, M., Paczesna, J., 1997. Carbon isotope geochemistry and palaeontology of Neoproterozoic to Early Cambrian siliciclastic successions in the East European Platform, Poland. Geol. Mag. 134, 1–16. Swett, K., Crowder, R.K., 1982. Primary phosphatic oolites from the lower Cambrian of Spitsbergen. J. Sediment. Petrol. 52, 587–593. Tribble, J.S., Arvidson, R.S., LaneeIII, M., MacKenzie, F.T., 1995. Crystal chemistry, thermodynamics and kinetic properties of calcite, dolomite, apatite and biogenic silica: application to petrologic problems. Sediment. Geol. 95, 11–37. Veizer, J., Hoefs, J., 1976. The nature of 18 Or16 O and 13 Cr12 C secular trends in sedimentary carbonate rocks. Geochim. Cosmochim. Acta 40, 1387–1395. Xu, D.-Y., Yan, Z., Sun, Y.-Y., He, J.-W., Zhang, Q.-W., Chai, Z.-F., 1989. Astrogeological Events in China. Scottish Academic Press, Edinburgh, pp. 11–57.
Regional variations in the carbon isotopic composition of phosphorite from the Early Cambrian Lower Tal Formation, Mussoorie Hills, India A. Mazumdar ) , D.M. Banerjee Department of Geology, UniÕersity of Delhi, Delhi110007, India
Abstract Early Cambrian Lower Tal phosphorites of the Mussoorie Syncline have been analyzed for their carbon isotopic composition of apatite-bound carbonate and organic carbon. Two mine excavations at Maldeota and Durmala, located on two limbs of the syncline and ; 14-km apart were subjected to detailed sampling along selected stratigraphic profiles. Field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small-scale fluctuations in the sedimentation milieu. Isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at Durmala and Maldeota. In addition, a marked negative carbon isotope excursion has been recorded at the dolomite–phosphorite contact defined by the Krol–Tal stratigraphic boundary. It is thought to reflect the influx of 12 C- and P-rich waters from a deeper part of the sedimentary basin. Based on the isotopic composition of carbon in the phosphorite, sulfur in the associated pyrite and petrological data, it has been inferred that the Durmala phosphorite reflects deposition under a suboxic diagenetic environment and the Maldeota phosphorite formed within suboxic to anoxic sulfate-reducing conditions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Early Cambrian; Phosphorite; Apatite; Stable isotopes; Diagenetic environment
1. Introduction Sedimentary apatite is predominantly formed as a marine authigenic mineral ŽRiggs, 1979; Sheldon, 1981.. While some favor direct precipitation from the water column Že.g., Cook and Shergold, 1986b; Riggs, 1979; Sheldon, 1981., other have demonstrated experimentally that during early diagenesis amorphous phosphate gel recrystallizes into cryptocrystalline apatite mud and produces the mudstone )
Corresponding author. E-mail addresses: [email protected] ŽA. Mazumdar., [email protected] ŽD.M. Banerjee..
phosphorite ŽGulbrandsen et al., 1984.. The carbon and oxygen isotopic compositions of sedimentary apatite has been widely used to constrain its formation and, thus, elucidate the host rock’s depositional history ŽKolodny and Kaplan, 1970; Benmore et al., 1983; Piper and Kolodny, 1987; Shemesh et al., 1988.. More specifically, McArthur et al. Ž1980, 1986. utilized the carbon isotopic composition of the structural carbonate within the apatite to characterize the diagenetic conditions under which it was formed. Based on an extensive review of modern and ancient phosphorite deposits, McArthur et al. Ž1986. considered d13 C carb values between 0‰ to y6‰ as characteristic for apatite, which has been formed in
0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 3 6 0 - 0
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A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
marine oxic–suboxic environments while those forming under anoxic conditions displayed more 12 C-depleted values. On the other hand, the oxygen isotopic composition of the marine apatite has primarily been used to estimate palaeoceanic andror burial temperatures ŽKarhu and Epstein, 1986; Shemesh et al., 1988; Kolodny and Luz, 1994.. In this paper, we discuss variations in d13 C and 18 d O measured for phosphorites from the Tommotian Lower Tal Formation with samples derived from two mine excavations located at Maldeota and Durmala in the Mussoorie syncline, India. Differences in the nature and extent of diagenetic alteration of these phosphorites have been elucidated by their stable
isotope values of the structural carbonate in the apatite lattice.
2. Geological setting The Neoproterozoic to Early Cambrian sequence of the Krol Belt in the Lesser Himalaya comprises sedimentary rocks of the Blaini, Krol and Tal Groups. This thick pile of sedimentary cover crops out in several discrete synclines ŽFig. 1. extending over a distance of more than 300 km. The Blaini Group consists of diamictite, arenite, shale and capcarbonate and is overlain by shale and siltstone of
Fig. 1. ŽA. Location and disposition of Mussoorie syncline with respect to Solan, Panchmunda, Nigalidhar, Garhwal and Nainital synclines in the Lesser Himalaya; ŽB. geological map of the western part of the Mussoorie Syncline showing the location of Maldeota and Durmala Mine sections; ŽC. lithologs of uppermost part of Krol-E and the Lower Tal Formation ŽChert-Phosphorite Member and Argillaceous Member.; MBF s Main Boundary Fault.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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Fig. 2. Typical hand specimen of Lower Tal rock displaying thick mudstone phosphorite layers, at times interbedded with apatite poor carbonate layers. Bar scale s 1 cm.
the Infrakrol Formation. The Infrakrol Formation is overlain by the Krol Group, which is divided into three formations: the Lower Krol ŽKrol-A Member; marlstone and sandstone., the Middle Krol ŽKrol-B Member; red shales and minor carbonates. and the Upper Krol comprising the Krol-C Žlimestone, dolomite and laminated algal dolomite., Krol-D
Žfenestral and stromatolitic dolomite with chert bands. and Krol-E Žargillaceous dolomite, laminated dolomite and grayish-black shales. Members. These rocks broadly reflect a supratidal–intertidal–subtidal depositional succession ŽSingh, 1979.. The Krol-E is unconformably overlain by the Chert-Phosphorite Member Žs Chert Member of Shanker, 1975. of the
Fig. 3. Cryptocrystalline apatite with recrystallized patches in microscopic vugs and irregularly dispersed organic matter. Bar scale s 60 mm.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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Lower Tal Formation, which represents the Tommotian stage of the Early Cambrian as constrained by abundant small shelly fossils and sponge spicules ŽBhatt et al., 1985; Brasier and Singh 1987; Mazumdar and Banerjee, 1998.. The Chert-Phosphorite Member consists of layers of bedded black chert alternating with blackrgrey shale and dark colored phosphorite. The latter lithology constitutes an important economic horizon ŽShanker, 1976, 1987. in the Krol belt with large phosphorite deposits in the Nigalidhar, Korgai, Mussoorie and Garhwal synclines ŽFig. 1.. Primary depositional features such as small-scale, low angle cross-bedding, ripple marks in grainstone phosphorites, intraclasts, ooids, microbial
laminites and small domal stromatolites ŽMazumdar, 1996, and references therein. are suggestive of an intertidal to shallow subtidal depositional setting for the Lower Tal Formation. Features indicating shallowing upward cycles can be demonstrated within 10-to 15-cm thick phosphorite beds, which clearly suggest rapid changes of water depths in a shallow marine basin. Phosphate intraclasts, rip-up clasts and fragmented stromatolitic columns further suggest deposition in a high-energy environment. Such gross textural features of a shallow marine facies are characteristic of Neoproterozoic and Early Cambrian phosphorite deposits worldwide ŽCook and Shergold, 1986a..
Table 1 Carbon and oxygen isotope values of Lower Tal phosphorite Sample no.
d 13 C carb Ž‰., PDB
d 18 Ocarb Ž‰., PDB
d 13 C org Ž‰., PDB
D d Ž‰., PDB
C org Ž%.
DQ14 DQ16 DQ17 DQ18 DU5 DU6 DU7 DU8 DU9 DU10 DU12 DUR18 DUR21 DUR24 DUR25 DUR28 DUR29 DUR31 DUR34 DUR35 SK1 SK2 SK3 SK4 SK5 SK6 SK7 SK8 MUS1 MUS4 MUS5 MUS7
y3.6 y4.5 y5.4 y7.1 y7.4 y8.0 y7.4 y6.4 y6.0 y7.1 y7.7 y5.6 y3.8 y4.5 y4.3 y8.4 y7.2 y4.4 y3.1 y4.5 y6.3 y11.4 y9.7 y17.8 y11.4 y10.0 y8.8 y8.2 y6.4 y9.9 y8.4 y8.5
y6.8 y7.9 y8.9 y8.1 y12.6 y13.4 y12.7 y10.5 y12.9 y12.3 y12.0 y11.6 y11.7 y14.3 y14.3 y10.5 y10.5 y11.2 y14.3 y11.8 y12.5 y11.0 y12.0 y11.6 y11.2 y12.1 y10.4 y11.5 y9.7 y10.9 y13.8 y
y35.2 y28.2 y32.3 y33.3 y28.9 y28.2 y32.6 y31.3 y30.2 y31.8 y28.6 y33.0 y32.0 y32.1 y32.2 y33.0 y32.4 y31.2 y32.0 y32.2 y32.8 y33.0 y33.8 y33.8 y33.9 y33.6 y34.5 y30.8 y33.0 y34.4 y33.6 y32.9
31.6 23.7 26.9 26.2 21.6 20.1 25.2 24.9 24.2 24.7 21.0 27.4 28.2 27.6 27.9 24.6 25.2 26.8 28.9 27.7 26.5 21.6 24.1 16.1 22.4 23.5 25.7 22.5 26.4 24.5 25.2 24.4
3.1 1.4 2.1 0.8 1.7 1.6 0.0 1.6 3.5 3.1 1.9 3.8 0.6 4.9 3.7 0.2 0.6 2.3 3.6 4.7 0.9 1.1 2.1 1.4 2.4 1.6 3.0 1.9 0.74 0.92 2.19 2.01
DUR, DQ, DU s DURMALA; SK, MUS s MALDEOTA.
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
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3. Sample selection
5. Results
Near pristine phosphate samples were selected on the basis of petrographic studies ŽMazumdar, 1996.. Megascopically visible dark mudstone phosphorite layers Žmicrosphorite of Riggs, 1979, see Fig. 2., more or less devoid of any carbonate veinlets and vugs were microsampled utilizing a tungsten carbide tipped dentist’s drill. However, some microinclusions of detrital carbonate derived from the underlying Krol-E dolomites could still be part of the samples. Based on previous studies of other phosphorite-rich samples, this does not result in a significant alteration of the carbon isotopic composition of the phosphate mud. All our analyses were confined to laminated mudstone phosphorite layers, consisting of micro- and cryptocrystalline apatite ŽFig. 3.. Results from SEM work acknowledge the presence of recrystallized patches consisting of columnar and radiating apatite crystals. The phosphate layers are dark to light brown depending upon organic content. Silt-sized detrital quartz, chert fragments, small mica flakes and pyrite are invariably associated with the phosphate matrix. At time, light colored detritus and carbonates form distinct layers in contrast to dark apatite beds. While dolomitization of the phosphate layers could be perceived at places, phosphatization of the carbonate minerals was generally not seen.
Carbon and oxygen isotopic compositions and organic carbon contents for the phosphorite samples from the Maldeota and Durmala sections are listed in Table 1 and presented in Figs. 4 and 5. Thereby, Fig. 4 provides the stratigraphic variations in carbonate and organic carbon and Fig. 5 displays various
4. Analytical methods For carbon and oxygen isotope analysis, powdered phosphorite samples were treated with phosphoric acid following the principles of McCrea Ž1950. and Craig Ž1953.. Subsequently, liberated CO 2 was measured on a VG Isogas Mass Spectrometer at the Max-Planck-Institut fur ¨ Chemie in Mainz, Germany. Details on analytical procedures, oxygen isotope correction factors and level of accuracy are similar to those provided by Banerjee et al. Ž1986.. A total of 32 phosphorite samples were selected from one profile at Maldeota and two profiles at Durmala. Apart from results obtained during this study, data described in Banerjee et al. Ž1986, 1997. will also be included in the present discussion.
Fig. 4. Ža and b. Isotopic variations along the vertical profiles near Maldeota Mines ŽSK samples. and Durmala Mines ŽDUR samples cf. Banerjee et al., 1997..
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A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
phosphorites define two distinct populations ŽFig. 5b.. The Durmala phosphorites display d13 C carb values in the range of y3.1‰ to y8.4‰ Žmean: y5.8‰., whereas carbonate carbon from the Maldeota phosphorites yielded distinctly more negative carbon isotope values between y6.3‰ and y11.4‰ including one sample as low as y17.7‰. In contrast, average d18 O values measured for samples from Durmala Žy11.4‰. and Maldeota Žy11.5‰. are nearly identical. However, despite a similar average oxygen isotopic composition, the Durmala phosphorites display a greater variability Žy6.7‰ to y14.3‰. as compared to the Maldeota samples Žy9.7‰ to y13.8‰.. No significant correlation between d13 C carb and d18 Ocarb is discernible for either sample population ŽFig. 5b.. Furthermore, the correlation between d13 C org and d13 C carb is rather poor in all profiles ŽFig. 5a.. No clear correlation exists between carbonate content and d13 C carb value nor between total organic carbon abundance ŽTOC. and isotopic composition. Although, samples with high TOC values seem to be characterized by more negative d13 C org values.
6. Discussion
Fig. 5. Ža. d13 C org – d13 C carb plots for Lower Tal phosphorite. Žb. d13 C carb – d18 O plots for Lower Tal phosphorite. Žc. D d – d13 C carb plots for Lower Tal phosphorite.
cross-plots of isotopic compositions. Despite some overlap, d13 C carb values of the Durmala and Maldeota
Carbonate fluorapatite ŽCFA., the prime component of the phosphorite is either an authigenic or early diagenetic mineral. The primary nature of mudstone phosphorite has been adequately demonstrated through petrographic studies from different parts of the world ŽRiggs, 1979; Sheldon, 1981; Ilyin and Ratnikova, 1981; Burnett, 1977; Swett and Crowder, 1982.. Petrographic features like localized replacement of early formed phosphate mud layers by dolomite, undisturbed primary apatitic layers, preservation of phosphatic small shelly fossils and microburrow type features suggest a primary or very early diagenetic formation. As such, the apatite derives its structural carbonate from the porewater in which the CO 2 or bicarbonate concentration is largely controlled through the amount of biodegradation in the sedimentary column. It has, thus, been argued ŽKolodny and Luz, 1994. that the d13 C carb composition of apatite can be utilized as a proxy for early diagenetic conditions in the sediment. Based on porewater data from organic-rich, modern marine
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
sediments and reflecting oxic–suboxic ŽFroelich et al., 1979; Emerson et al., 1982; Jahnke et al., 1982; McCorkle et al., 1985. or anoxic ŽLaZerte, 1981; Nissenbaum et al., 1972. conditions, McArthur et al. Ž1986. concluded that the d13 C of dissolved inorganic carbon varies within a range between 0‰ and y6‰ during oxic–suboxic diagenesis. In contrast, apatite precipitated in the zone of bacterial sulfate reduction inherits d13 C values, which are more 13 Cdepleted than y6‰. This is underlined by numerous studies of francolite from different ages. In the absence of any significant fractionation during precipitation, carbonate fluorapatite is thought to faithfully record these carbon isotope signals. Major phosphorite deposits of late Neoproterozoic to Early Cambrian time have been reported from China, Iran, Kazakhstan and Mongolia ŽCook and Shergold, 1986a.. In India, similar lithologies are present in the Chert-Phosphorite Member of the Lower Tal Formation, which is of Tommotian age. The stable isotope geochemistry of these phosphorite bearing successions ŽBrasier et al., 1990; Brasier, 1992; Banerjee et al., 1986, 1997; Kimura et al., 1997. have been utilized for interpreting the mechanism of phosphogenesis in the Early Cambrian sedimentary basins. Such studies have helped to establish relationships between geochemical anomalies and lithofacies variations observed in the late Neoproterozoic and across the Neoproterozoic–Cambrian boundary Žsee, e.g., Aharon et al., 1987; Narbonne et al., 1994; Magaritz et al., 1986, 1991; Brasier 1990, 1992; Brasier et al., 1990; Kirschvink et al., 1991; Ripperdan, 1994; Kaufman et al., 1993, 1996; Kimura et al., 1997; Bartley et al., 1998.. A sharp negative excursion in the temporal trend of d13 C carb relative to the underlying carbonate rocks appears to be characteristic for many Precambrian–Cambrian boundary sections around the world ŽAharon et al., 1987; Xu et al., 1989; Brasier et al., 1990; Kaufman et al., 1996; Banerjee et al., 1997 and many others.. Positive d13 C carb values in the carbonates and less 13 C-depleted organic carbon values in the Upper Krol Formation ŽKrol-E. have been reported by Banerjee et al. Ž1997., followed by a sharp negative excursion of ; 4–5‰ at the base of the Lower Tal Formation, which is of Tommotian age. The base of the Tommotian is defined by the first appearance and abundance of a small shelly fossil assemblage con-
11
sisting of Anabarites sp., Protohertzina sp., Maldeotaia and Olivooides sp. ŽBhatt et al., 1985; Brasier and Singh, 1987; Kumar et al., 1987.. Although no diagnostic fauna has been identified so far in the underlying Krol-E Formation, it is tentatively correlated with the Nemakit-Daldynian Stage on the Siberian Platform. Due to the paucity of reliable radiometric dates, it would be premature to correlate the negative isotope excursions recorded in the Maldeota and Durmala sections either with the 544Ma-old ŽGrotzinger et al., 1995. base of NemakitDaldynian or with the 531-Ma-old ŽIsachsen et al., 1994. basal Tommotian. It has been suggested before Žsee, e.g., Narbonne et al., 1994. that the organic carbon isotopic composition would equally serve as a proxy signal for secular variations of the global carbon cycle, provided the signal has not been obliterated by post-depositional processes, notably extensive thermal alteration. With respect to the terminal Neoproterozoic and its transition into the Cambrian, the successful application of this approach has been shown, e.g., for respective sections in the Mackenzie Mountains ŽNarbonne et al., 1994. or the East European Platform ŽStrauss et al., 1997.. The latter succession did not contain any carbonate. The temporal record of organic carbon isotope values display a 13 C-depletion across the Precambrian–Cambrian transition, followed by an evolution to more 13 C-enriched values in the lower Cambrian. This negative d13 C-shift was attributed to a reduction in organic carbon burial as a consequence of a drop in productivity and the terminal Neoproterozoic extinction ŽStrauss et al., 1997.. A light brown colour of the kerogen, HrC values between 0.2 and 0.7 and illite crystallinity indices Ž D 0 2 u ) 0.42. ŽMazumdar, 1996; Mazumdar and Frank, in preparation. suggest that the organic matter in the Tal phosphorites has not suffered substantial thermal alteration. It is, thus, assumed, that the primary isotopic composition has not been altered substantially Žsee also Strauss et al., 1992. and that the negative d13 C org excursion of 5‰ Žy28‰ to y33‰. in the Lower Tal phosphorite relative to the underlying Krol-E dolomite, reflects a primary feature. Similarly, the covariance noted in the isotopic compositions of carbonate and organic carbon in the
12
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
Krol-E dolomites through time primarily reflects a secular change in the 13 Cr12 C of the oceanic C-reservoir and is not an artifact of secondary processes ŽSchidlowski et al., 1975; Veizer and Hoefs, 1976; Knoll et al., 1986; Narbonne et al., 1994; Narbonne and Aitken, 1995; Kimura et al., 1997.. The onset of phosphate deposition at the beginning of the Tal Formation is paralleled by a sharp negative excursion shift in the carbon isotopic composition. Characteristic isotope excursions of this type have also been recorded from the Early Cambrian organic-rich phosphaterblack shale horizons of South China and northern Iran ŽBrasier et al., 1990.. The marked decrease of the d13 C carb and d13 C org values within the Tal phosphorite can be related to the influx of 12 C- and P-rich deeper basinal waters to a platform setting during a contemporaneous ocean mixing event ŽDonnelly et al., 1990; Aharon and Liew, 1992; Banerjee et al., 1997; Mazumdar et al., 1999.. The spread in d13 C org data ŽTable 1. is thought to reflect variations in the rate of photosynthetic carbon fixation or in growth rate ŽHayes et al., 1989; Popp et al., 1997., which in turn is limited by the fluctuation in the nutrient concentration. Haddad et al. Ž1988. demonstrated that the rapid rates of carbon fixation associated with peak photosynthetic communities is accompanied by reduced discrimination against 13 C. It is therefore suggested that the overall negative carbon isotope excursion coincident with the Tommotian phosphogenic episode was superimposed by minor isotopic perturbations as a result of varying nutrient enriched ŽNEW. and nutrient depleted ŽNDW. water conditions ŽBrasier, 1992.. Marked differences in the nature of d13 C org plots for the Maldeota and the Durmala successions apparently reflect intrabasinal variations in the nutrient concentrations. In addition, variations in d13 C org could also reflect a varying input of heterotrophic biomass. This, however, is difficult to characterize, let alone quantify. It should be possible to unravel the effects of heterotrophic reworking only through compound-specific carbon isotope studies. It is obvious that the carbon isotopic composition of apatite reflects changes in the isotopic composition of the basinal water, with subsequent diagenetic modifications further affecting d13 C. Although an overall covariance of d13 C carb and d13 C org exists for samples from the Durmala Mine section ŽBanerjee et
al., 1997., data variability ŽFig. 5a. suggests some diagenetic modification of the carbonate carbon isotopic composition in the apatite. A most obvious source of alteration would be the incorporation of isotopically light carbon resulting from biodegradation of sedimentary organic matter in a suboxic environment. It has been suggested before Že.g., Kaufman et al., 1993. that the difference between the carbonate and organic carbon isotopic compositions Ž D d s d13 C carb y d13 C org . could be used to monitor a possible post-depositional alteration caused by the incorporation of organically derived carbon in the porewaters. In that respect, the D d values calculated in this study show a moderate to good positive correlation with d13 C carb for both, the Durmala Ž r s 0.77. and the Maldeota Ž r s 0.87. phosphorites ŽFig. 5c.. Jarvis et al. Ž1994. stressed that secular variations in the marine d13 C record with long-term and shortterm fluctuations should be taken into consideration when interpreting the diagenetic conditions of apatite precipitation. Despite some intrabasinal variation, the globally correlative negative carbon isotope excursion at the base of Tommotian is noteworthy. Assuming the carbon isotopic composition of organic matter in the Tal phosphorite as a proxy for an overall negative shift Ž4‰ to 5‰. of the dissolved inorganic carbon reservoir of the contemporary platformal water, the d13 C carb values of the apatite reflect the global signal as well as an additional negative shift resulting from incorporation of reworked carbon during variable diagenetic conditions. The two distinct clusters for samples from Maldeota and Durmala ŽFig. 5a. with a minor overlap indicate differences in the diagenetic conditions for both settings. It may be argued that the d13 C carb values for the Durmala phosphorite reflect a predominantly suboxic environment, while the Maldeota phosphorites formed in the suboxic to anoxic, sulfate reducing zone ŽMcArthur et al., 1986.. Phosphogenesis was apparently restricted to the shallow suboxic zone. In the anoxic sulfate reducing zone, apatite precipitation is expected to cease due to rapidly increasing carbonate alkalinity ŽJahnke, 1984; Tribble et al., 1995.. Considering organic-rich sediments of the Peru–Chile margins as a modern analog, Jarvis et al. Ž1994. concluded that apatite formation is Arestricted to within 5–20 cm of the sedi-
A. Mazumdar, D.M. Banerjeer Chemical Geology 175 (2001) 5–15
mentrwater interface because precipitation ceases at the high carbonate alkalinities, which develop rapidly in deeper sedimentsB. Extremely negative isotope values as low as y17‰ observed during this study could be a result of substantial incorporation of carbon derived from reworking of sedimentary organic carbon by bacterial sulfate reduction. It is, however, difficult to rule out upward diffusion of 13 C-depleted water from the anoxic sulfate reduction zone into the shallow suboxic zone. Sulfur isotope values of associated pyrites show a large spread Žy25‰ to q45‰. indicating pyrite formation in a sulfate reducing to methanogenic environment ŽMazumdar et al., 1999..
7. Conclusions Organic carbon and apatite-carbonate carbon isotopic compositions in phosphorites from the Early Cambrian Tal Formation have been studied to characterize different modes of formation prevalent in the Durmala and Maldeota sections, Mussoorie Hills, Lesser Himalaya, India. Results define two partially overlapping clusters in d13 C carb – d13 C org and d13 C carb – d18 Ocarb-diagrams. d13 C carb signatures determined in this study are interpreted to reflect fluctuations in the carbon isotopic composition of the water column partially amplified by subsequent diagenesis. The change in the isotopic composition of dissolved inorganic carbon in platformal water is thought to reflect the influx of P- and 12 C-rich waters from a deeper part of the basin. This shift in the isotopic composition can also be observed in the thermally largely unaltered organic carbon in the Tal phosphorites. The negative carbon isotopic excursion associated with the phosphogenic episode during basal Tommotian time was superimposed by minor isotopic perturbations caused by variations in the nutrient content of basinal waters. Absence of a significant positive correlation between d13 C org and d13 C carb points to an interpretation of the extremely negative shifts in the carbonate carbon isotopic composition as a result of the incorporation of 13 C-depleted CO 2 produced during bacterial degradation of the organic matter. At the site of phosphorite formation at Durmala, the process of biodegradation occurred essentially under suboxic diagenetic
13
conditions, whereas the diagenetic environment at Maldeota was characterized by suboxic to anoxic conditions. Acknowledgements We thank Manfred Schidlowski for providing access to the mass-spectrometric facility during DMB’s tenure at the Max-Planck-Institut fur ¨ Chemie in Mainz, Germany, under the Alexander von Humboldt Foundation Revisit program. General laboratory facilities were drawn through a Project grant to DMB from the Department of Science and Technology, New Delhi. This paper is a contribution to the aims and objectives of IGCP Project No. 386. David McKirdy, Jay Kaufman and an anonymous reviewer helped us to revise the original text. We appreciate their help. References Aharon, P., Liew, T.C., 1992. An assessment of the Precambrian–Cambrian transition event on the basis of carbon isotope records. In: Schidlowski, M., Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ŽEds.., Early Organic Evolution — implications for Mineral and Energy Resources. Springer, pp. 212–224. Aharon, P., Schidlowski, M., Singh, I.B., 1987. Chronostratigraphic markers in the end-Precambrian carbon isotope record of the Lesser Himalaya. Nature 327, 699–701. Banerjee, D.M., Schidlowski, M., Arneth, J.D., 1986. Genesis of Upper Proterozoic–Cambrian phosphorite deposits of India: isotopic inferences from carbonate fluorapatite, carbonate and organic carbon. Precambrian Res. 33, 239–253. Banerjee, D.M., Schidlowski, M., Siebert, F., Brasier, M.D., 1997. Geochemical changes across the Proterozoic–Cambrian transition in the Durmala phosphorite mine section, Mussoorie Hills, Garhwal Himalaya, India. Paleogeogr. Paleoclimatol. Paleoecol. 132, 183–194. Bartley, J.K., Pope, M., Knoll, A.H., Semikhatov, M.A., Petrov, P.Yu., 1998. A Vendian–Cambrian boundary succession from the northwestern margin of the Siberian Platform: stratigraphy, palaeontology, chemostratigraphy and correlation. Geol. Mag. 135, 473–494. Benmore, R.A., Coleman, M.L., McArthur, J.M., 1983. Origin of sedimentary francolite from its sulphur and carbon isotope composition. Nature 302, 516–518. Bhatt, D.K., Mamgain, V.V., Misra, R.S., 1985. Small shelly fossils of Early Cambrian ŽTommotian. age from Chert-Phosphorite Member, Tal Formation, Mussoorie Syncline, Lesser Himalaya, India and their chronostratigraphic evaluation. J. Paleontol. Soc. India. 30, 92–102.
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