a positive δ 13 c carb anomaly in paleoproterozoic carbonates of the aravalli craton western india support for a global isotopic excursion abstract carbon isotope measurements carried out on carbonate samples from different localities of the early proterozoic aravalli craton western india yield positive 13 c values up to 11 20 pdb this widespread isotopically anomalous carbonate province reported from this subcontinent coincides with the appearance of stromatolites and the development of red beds these observations support a global paleoenvironment change during the paleoproterozoic as reported from other parts of gondwanaland including the kapvaal craton africa and the sao francisco craton brazil

global nature of the paleoproterozoic lomagundi carbon isotope excursion a review of occurrences in brazil india and uruguay abstract positive carbon isotope excursion is reported from paleoproterozoic carbonates of the aravalli supergroup northwestern india the minas supergroup brazil and new sections of the paso severino formation uruguay the 2 42 ga gandarela formation minas gerais brazil contains red carbonate facies bif grading into dolostones and limestones and yielding 13 c values ranging from 1 6 to 0 4 v pdb the positive c isotope excursion up to 11 v pdb in marine shallow water carbonates in india and brazil jhamarkotra formation in northwestern india and cercadinho and fecho do funil formations in minas gerais state brazil is comparable to that observed in 2 22 2 1 ga carbonate successions worldwide that were deposited during the lomagundi excursion in uruguay 13 c values up to 11 6 v pdb in the deep water paso severino formation of the piedra alta terrane are compatible with deposition at ca 2 15 ga as indicated by the 2146 7 ma u pb age of dacites occurring at the top of the unit negative 13 c values are also present in carbonates of the paso severino formation but an origin related to organic matter remineralization cannot be ruled out thin carbonate beds in the rio itapicuru greenstone belt bahia state brazil are associated as in the paso severino formation with deep water black shales and have carbon isotope values up to 9 v pdb high metamorphic grade carbonates of the jacurici terrane in the medrado ipueira area bahia brazil have carbon isotope values up to 6 9 v pdb consistent with their minimum age of 2085 5 ma inferred from the intrusive contact with and the age of the medrado norite no evidence was found in india brazil or uruguay for paleoproterozoic glacial events recognized in the 2 45 2 22 ga sedimentary successions worldwide unconformities between the gandarela and cercadinho formations in brazil and the banded gneissic complex and the lower aravalli supergroup in india might explain the absence of glacial record compositional and isotopic data presented here for studied paleoproterozoic carbonate successions allow their integration into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age the study highlights the global nature of the lomagundi excursion furthermore it indicates that the lomagundi excursion is recorded in both shallow water aravalli and minas supergroups and deep water carbonates paso severino formation and rio itapicuru greenstone belt negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow water open marine and isolated basins

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

genesis of upper proterozoic cambrian phosphorite deposits of india isotopic inferences from carbonate fluorapatite carbonate and organic carbon carbon and oxygen isotope analyses performed on sedimentary carbonates organic carbon kerogen and structurally bound co2 3 groups of carbonate fluorapatite francolite from four widely separated indian phosphorite provinces have shown that each of the deposits is characterized by specific basically facies dependent distribution patterns of 13ccarb 13ocarb 13corg and 13capat comparison of the carbon isotope composition of apatite co2 with that of the coexisting sedimentary carbonate has served to further constrain the genetic type of the individual phosphorite deposit investigated augmenting relevant geological evidence by an additional geochemical discriminator as a whole the results confirm the importance notably of carbon isotope work for elucidating the processes involved in sedimentary phosphorite formation

stabilization of the aravalli craton of northwestern india at 2 5 ga an ion microprobe zircon study abstract in order to establish the stabilization age of the aravalli craton of northwestern india we have used an ion microprobe to measure 207 pb 206 pb ages of zircons from six samples collected from the southern aravalli mountains our analyses gave precise results for four granitoids whose minimum crystallization ages ranged between 2562 6 and 2440 8 ma 1 a single sample of gneiss gave an age of 2502 4 ma which is unexpectedly younger than an age of 3281 ma which we previously obtained from another gneissic sample we interpret the 2502 ma age as the time of emplacement of the sample s igneous protolith suggesting that the mewar gneiss contains multiple metamorphic components of differing ages based on our data we conclude that the southern segment of the aravalli craton had broadly stabilized by 2 5 ga this stabilized landmass formed the basement on which the younger aravalli supergroup was unconformably deposited this study represents the first application of a small geometry ion microprobe for routine geochronology

transvaal supergroup carbonates implications for paleoproterozoic δ18o and δ13c records abstract the paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean atmosphere system in this study carbonates from the transvaal group the silverton lucknow rooinekke and particularly the duitschland formations were analyzed for isotopes of carbon oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution although the trace element data showed variable alteration of the carbonate samples selected samples with a lesser degree of alteration yielded interpretable results the best measured 87sr 86sr isotope value of 0 7031 at 2 35 ga ago fits well with the published curve for precambrian seawater confirming that paleoproterozoic budget of sr was dominated by input from continental sources during the paleoproterozoic times carbonates with 13c values of about 10 are known to have been present worldwide and they have been studied also during this research the observation that in the duitschland formation such high isotope values and normal marine values were both present within a single formation indeed within its single beds suggested that the large spread of 13c values reflects local or regional rather than global phenomena the oxygen isotope data show a large spread of values from 3 to 16 while their corresponding 13c signals are almost unchanged over this entire range with the lucknow and silverton formations clustering around 10 and the rooinekke formation around 2 the duitschland formation has a bimodal distribution with both 13c populations present the last formation also contains samples with 18o as low as 20 particularly in the vicinity of the bushveld intrusive body these highly 18o depleted samples show homogenization of 13c values towards 5 modelling suggests that the trends could have resulted from post depositional alteration by basinal fluids at near surface to about 50 c temperatures in a rock buffered system the duitschland formation samples particularly at duitschland farm the locality closest to the bushveld complex were then further altered at temperatures up to about 80 c in response to this intrusion

high δ13c paleoproterozoic carbonates from the transvaal supergroup south africa marbles from the transvaal supergroup kaapvaal craton south africa record two positive carbon isotope excursions to average 13 c carbonate values of 6 to 8 relative to v pdb separated by carbonates with normal marine 13 c values 0 2 these elevated values have been preserved through metamorphic devolatilization together with an already documented high 13 c excursion in overlying rocks three separate positive carbon isotope excursions occurred on the kaapvaal craton in the interval 2 43 to 1 93 ga it is not yet clear how many of these excursions reflect global rather than local processes

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

systematic decrease of high δ13c values with burial in late archaean 2 8 ga diagenetic dolomite evidence for methanogenesis from the crixas greenstone belt brazil abstract remnants of fe poor 2700 4400 ppm dolomite crystals crystal aggregates and microconcretions are cryptically distributed within stratiform bodies tens of metres across of metamorphic fe rich 3 6 wt fe dolomite that occur in graphitic schists of the 2 8 ga ribeira o das antas formation in the crixas greenstone belt brazil fe poor dolomite is a diagenetic carbonate formed during burial of organic bearing muddy sediments using cathodoluminescence microscopy a consistent sequence of dolomite growth stages is well defined within individual crystals crystal aggregates and microconcretions from comparison with similar textures in phanerozoic carbonates we suggest that the early dolomite formed during progressive burial aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction the 13cdolomite values of crystals and concretion cores are very high 12 14 pdb and a negative shift of about 7 occurs over millimetres in subsequent dolomite generations early diagenetic fe poor dolomite and late stage fe rich dolomite show overlapping ranges of initial 87sr86sr ratios between 0 7080 and 0 7164 and 18o values 14 to 16 pdb these similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids yet the well preserved growth zonation of the fe poor dolomite suggests that such exchange did not follow a process of recrystallization the decrease in the 13c of dolomite is correlated with crystal and concretion growth during burial this behaviour is interpreted to indicate a change in the 13c of pore water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis the crixas dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early precambrian the pattern of 13c values found for the crixas dolomite is also documented in other early to middle precambrian dolomites and limestones previous interpretations have considered positive 13c values to be the result of primary oceanographic controls whereas variability in 13c is commonly linked to metamorphic alteration we suggest that the role played by organic diagenesis in controlling the 13c values of diagenetic carbonates has been underestimated

modeling of kashmir aftershock decay based on static coulomb stress changes and laboratory derived rate and state dependent friction law we model the spatial and temporal evolution of october 8 2005 kashmir earthquake s aftershock activity using the rate and state dependent friction model incorporating uncertainties in computed coseismic stress perturbations we estimated the best possible value for frictional resistance a n background seismicity rate r and coefficient of stress variation cv using maximum log likelihood method for the whole kashmir earthquake sequence we measure a frictional resistance a n 0 0185 mpa r 20 m3 7 events year and cv 0 94 0 01 the spatial and temporal forecasted seismicity rate of modeled aftershocks fits well with the spatial and temporal distribution of observed aftershocks that occurred in the regions with positive static stress changes as well as in the apparent stress shadow region to quantify the effect of secondary aftershock triggering we have re run the estimations for 100 stochastically declustered catalogs showing that the effect of aftershock induced secondary stress changes is obviously minor compared to the overall uncertainties and that the stress variability related to uncertain slip model inversions and receiver mechanisms remains the major factor to provide a reasonable data fit

carbon isotope stratigraphy of upper cambrian steptoean stage sequences of the eastern great basin record of a global oceanographic event a large global positive excursion in 13 c from 0 5 to 4 5 during the late cambrian pterocephaliid biomere steptoean stage aphelaspis elvinia zones is documented at high stratigraphic resolution in three sections in the eastern great basin the excursion which we refer to as the steptoean positive carbon isotope excursion or spice excursion began coincident with a world wide extinction event the 13 c data from the great basin reflect global seawater values in a wide range of lithologies including oolitic grainstones wackestones thrombolitic boundstones and flat pebble conglomerates we use a section at shingle pass in the southern egan range to divide the pterocephaliid biomere into eight isotope steps that represent equal divisions of the 13 c rise and fall 4 this provides a basis for recognition of a revised chronostratigraphic framework for the pterocephaliid biomere steptoean stage strata deposited during the beginning of the spice excursion record a major change in the pattern of sedimentation in the eastern great basin this is reflected in a siliciclastic carbonate transition at shingle pass nevada and a carbonate siliciclastic transition at the house range and lawson cove sections in utah a regional siliciclastic influx recognized throughout the great basin occurs near the peak of the spice excursion carbon isotope analyses from cratonal sections in wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the spice excursion the correlated changes in 13 c relative sea level and the marine biota during the spice excursion provide remarkably detailed records of a major paleoceanographic event we speculate that changes in sea level climate or tectonics may have triggered the spice excursion and coeval extinction event subsequent burial of organic carbon caused the increase in 13 c and may have led to an interval of global cooling the results of this study lend confidence to carbon isotopic studies of pre mesozoic rocks

evolution of lower proterozoic epicontinental deposits stromatolite bearing aravalli rocks of udaipur rajasthan india abstract the aravalli rocks 2060 ma old which crop out around udaipur western india comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement known as the banded gneissic complex ca 2585 ma old the rocks have undergone a very low grade of metamorphism and display a complex structure resulting from two major and several minor episodes of folding there are two distinctly different facies sequences in the aravalli rocks indicating deep sea and nearshore shelf environments the stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs altered to greenschists and quartizites with arkosic conglomerate in the next sequence carbonates predominate in association with orthoquartzites carbonaceous phyllites phyllites and stromatolitic rock phosphate the carbonate sequence passes upward into greywacke phyllite lithic arenite in the distal parts and conglomerate arkose orthoquartzite in proximal areas the youngest sequence comprises orthoquartzite with silty arenite the distribution of different facies particularly that of dolomite with stromatolitic rock phosphate is controlled by sea floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault controlled troughs along craton margins terrigenous debris was deposited in newly developed troughs with contemporaneous volcanicity along trough margins with the erasing of the ephemeral relief in the provenance carbonate deposition was initiated the environment encouraged algal growth and formation of stromatolitic rock phosphate carbonaceous phyllites developed in areas of restricted circulation rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke phyllite and lithic arenite in the deeper parts of the epicontinental sea and conglomerate arkose orthoquartizite in the marginal areas the final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling the nature of the terrigenous clasts indicates a predominantly granitic source of sediments supply of sediment was mainly from the continent to the east and partly from a landmass to the west the cycle of sedimentation noted in the epicontinental aravalli sea is broadly similar to the model of tectonic stages suggested by krynine 1942

two billion year old isotopically heavy carbon evidence from the labrador trough canada forty eight samples of bulk carbonates were studied from the dunphy portage alder uve denault and abner formations and the undivided pistolet subgroup in the labrador trough quebec canada the carbonate units occur as a number of thick beds up to 300 m within 100 600 m thick formations they are interbedded with siliciclastic sediments and are composed of crystalline sparry micritic allochemical and stromatolitic dolostones the carbonate rocks are recrystallized and metamorphosed up to amphibolite grade they are chemically dolostones and include chemical or biochemical laminated and massive dolostones dolorudites dolarenites and dololitites which were deposited in shallow water dunphy denault and abner formations to relatively deep water uve formation marine environments reef facies are present in the denault and abner formations the dunphy portage alder and uve formations deposited between 2 17 and 2 14 ga yield isotopically heavy values of 13c pdb ranging from 5 3 to

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

single zircon evaporation ages constraining the growth of the archaean aravalli craton northwestern indian shield single grain 207 pb 206 pb zircon ages were determined for granitoids and gneisses which constitute the archaean basement rocks of the aravalli craton of rajasthan northwestern indian shield the protolith ages for two gneisses collected from east of udaipur are 3230 ma and 2887 ma respectively the granitoids display an intrusive relationship with the gneisses and yielded ages ranging between 2666 ma and 2620 ma these ages provide the basis for a geochronological model of evolution of the oldest basement of the aravalli craton

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

formation of spinels in cosmic objects during atmospheric entry a clue to the cretaceous tertiary boundary event abstract magnetic spinels produced by oxidation of extraterrestrial objects in the atmosphere have a composition distinct from terrestrial spinels they are characterized by a high iron oxidation state arising from crystallization under high oxygen fugacities and a high nickel concentration due to the relatively high abundance of this element in extraterrestrial material the iron oxidation state increases from micrometeorites to meteoroid ablation material and to impact generated products this reflects a progressive increase of the oxygen fugacity corresponding to decreasing altitudes of crystallization spinels found at the cretaceous tertiary boundary are similar to those that crystallized from meteoroid ablation material and impact generated products supporting the view that a collisional event did occur at the end of the cretaceous

geochemistry of precambrian carbonates vi aphebian albanel formations quebec canada abstract the 1 85 0 15 ga old albanel formations of northern quebec canada an example of paleoproterozoic carbonate buildups are lithologically composed of cherty dolostones subordinate limestones are encountered only near the base of the sequence coincident with maximal transgression the chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases such as chert quartz plagioclase illite k spar pyrite and fe mn oxides all abundant particularly in the basal part of the section the chemistry of the carbonate portion of limestones is strongly reset by post depositional phenomena but the relict high sr contents suggest that aragonite may have been an original precursor phase geochemically the carbonate portion of dolostones can be subdivided into two populations reflecting the stratigraphie bottom and top of the sequence trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures but in a more open system the latter reflects the greater lithological variability and permeability of the younger strata the 13c varies up section from about 3 to 1 perhaps a reflection of progressively more open marine circulation the associated four generations of post depositional pyrites have overlapping 34s from 7 1 to 54 with a mean of 24 a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and or replenishment the best measured 87 sr 86 sr of 0 70600 although possibly not a direct reflection of the coeval seawater may still be a near original value as indicated by its coherency with the roughly co stratigraphic paleo proterozoic sequences evaluation of the existing data indicates that strontium isotopic composition of the precambrian seawater was characterized by oscillations similar to those of the phanerozoic the interval of 0 8 1 5 ga ago being dominated by relatively nonradiogenic values followed by a radiogenic period at 1 6 2 2 ga ago near mantle values during the late archean and possibly another radiogenic peak at 3 5 ga ago

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

fluid rock interaction during low pressure polymetamorphism of the reynolds range group central australia marbles and metapelites from the reynolds range group central australia were regionally metamorphosed at low pressure during m at 6 go m ranged in grade from gnenschist to granulite fades along the length of the reynolds range and overprinted l 78 ga granites and their contact aureoles in the reynolds range group metasediments at all m 2 grades the marbles and metapelites have highly variable oxygen isotope ratios marbles 5i8o carb 14 20 o metapelites b180 6 14 o similarly l 78 ga granites have highly variable oxygen isotope ratios 8i8o 5 13 o with the lowest values occurring at the granite margins in all rock types the lowest oxygen isotope values are consistent with the infiltration of channelled magmatic and or meteoric fluids the variable lowering of oxygen isotope values resulted from pre mi2 contact metamorphism and fluid rock interaction around the l 78 ga granites in contrast mineral assemblages in the marbles define a trend of increasing xco with increasing grade from 0 05 gnenschist fades to 0 71 0 granulite fades this together with the lack of regionally systematic resetting of oxygen isotope ratios implies that there was little fluid rock interaction during prograde regional metamorphism key word law pressure polymetamorphism fluids stable isotopes petrology

high primary productivity and nitrogen cycling after the paleoproterozoic phosphogenic event in the aravalli supergroup india the causes of atmospheric oxygenation in the paleoproterozoic are multifaceted yet poorly understood an increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere a likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate which would have resulted in a fundamental change from the archean nitrogen cycle in this study we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the paleoproterozoic aravalli supergroup india the shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2 0 ga shales in the phosphate domain of the jhamarkotra formation from the lower aravalli group have distinct characteristics from coeval shales in the non phosphate domain carbonaceous shales of the phosphate domain contain relatively high v and u th typical of oxygen deficient environments these shales also have a high organic content of up to 14 wt generally homogeneous 13 corg values around 29 c nwr up to 360 and a large range of 15