Carbon and Oxygen Isotope Ratios in Rona Limestone,
Romania. The carbon and oxygen isotopic compositions of
limestones provide criteria for the evaluation of the
depositional environment. For Jurassic and younger samples,
the best discrimination between marine and fresh-water
limestones is given by Z parameter, calculated as a linear
correlation between 13C and 18O (0/00 PDB). Rona Limestone
(Upper Paleocene Lower Eocene), outcropping on a small area
in NW Transylvania (Mese area) is a local lacustrine facies.
There, it divides Jibou Formation into the Lower Red Member
and the Upper Variegated Member, respectively. --
STUDIA UNIVERSITATIS BABE -BOLYAI, GEOLOGIA, XLVI, 1, 2001 CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA STELA CUNA 1 DANA POP 2 ALEXANDRU HOSU2 ABSTRACT. Carbon and Oxygen Isotope Ratios in Rona Limestone, Romania. The carbon and oxygen isotopic compositions of limestones provide criteria fo r the evaluation of the depositional environment. For Jurassic and younger samples, the best discrimination between marine and fresh-water limestones is given by Z parameter, calculated as a linear correlation between 13C and 18O ( PDB). Rona Limestone (Upper Paleocene Lower Eocene), outcropping on a small area in NW Transylvania (Mese area) is a local lacustrine facies. There, it divides Jibou Formation into the Lower Red Member and the Upper Variegated Member, respectively. Recently, a s equence containing a marine nannoplankton assemblage was identified in the base of Rona deposits. The main goal of our study was to characterize, based on the isotopic record, the primary environment of formation of the deposit, as well as that in which some di agenetic processes (the formation of dol omite and of green clay around the siliceous chert nodules) took place. Ten samples representing limestones, dolomitic limest one, marls and the green carbonate-rich clay were studied from petrogr aphical and mineralogical points of view, and the carbon and oxygen isotopic ratios from the car bonate (calcite) component were measured. In conclusion, it was found that the procedure of extraction of CO2 we used enabled the discrimination between the isotopic pr ints of calcite vs. dolomite. This pleads for considering our results as a primary isotopic pattern in the bulk rock. The oxygen and carbon isotope data indicate a freshwater depositional environment with Z<120. The 13C mean value (-4.96 PDB) is, generally, represent ative for fresh-water car bonates of the Tertiary period. The same environment characterized also the formation of carbonates within the green clay. KEY WORDS : mass spectrometry, XRD, carbon and oxygen isotopic compositions, carbonate rocks, pal eoenvironment, lacustrine green clay, Rona Limestone, Transylvania, Paleogene Introduction The carbon and oxygen isotopic compositions (the variations of the C13/C12, and O18/O16 ratios) of limestones provide criteria for the evaluation of the depositional environment for samples of a wide range of geological age. Especially the carbon isotopes are the most significant, as tracers for 1 National Institute of Research and Devel opment for Isotopic and Molecular Technology, D onath Str., 65-103, POBox 700, RO-400 Clu j-Napoca, Fax: 40 64 420 042, E-mail: firstname.lastname@example.org 2 Babe -Bolyai University, Dept. of Mineralogy, Kog lniceanu Str., 1, RO-3400 Cluj-N apoca, Fax: 40 64 191 906, E-mail: email@example.com firstname.lastname@example.org
STELA CUNA, DANA POP, ALEXANDRU HOSU 140 the origin of carbon, while oxygen isotopes store paleotemperature information (Clark & Fritz, 1997). The isotopic composition of carbon and oxygen is expressed in terms of the (delta) value (given in per mil, ), defined as follows: RR R xsamplesdard sdard ()(tan) (tan)1000 where R represents the isotope ratio: R C C 13 12 for carbon and R O O 18 16 for oxygen. If A B it means that A is enriched in the rare isotope, or heavier than B. For various elements, a convenient "working standard" is used in each laboratory. However, in the literature all measured values are compared with an isotopic standard. To convert -value from one standard to another, the following equation may be used: XA BAXB 10 1 10 111033 3 where X represents the sample, while A and B various standards. The universally adopted standard for carbon and oxygen stable isotope variation in carbonates was the so-called PDB 3 while in natural waters was the so-called SMOW 4 The main mineral components of carbonate rocks are calcite and dolomite. Dolomite typically shows higher 13C and 18O values as compared to calcite, due to the enrichment in 13C and 18O occuring during dolomitecalcite fractionation. In addition, diagenetic processes lead to a relative enrichment in 13C in both minerals (Clark & Fritz, 1997). The 13C (PDB) values for marine carbonate rocks are usually constant and close to 0. On the other hand, the fresh-water limestones are usually enriched in 12C as a result of the organic influences (plant respiration, oxidation of organic rests etc.), and the 13C (PDB) values are more scattered (Faure, 1977). In the references, the oxygen isotopic ratio for carbonates was compared to both PDB and SMOW. The 18O values in the case of freshwater carbonate rocks are usually lower, due to a depletion in 18O as compared to sea waters (Faure, 1977) and may vary within larger limits (Table 1). Table 1 3 PDB originates from the CaCO3 of the rostrum of a Cretaceous belemnite ( Belemnitella americana ) collected in the Peedee Formation of S outh Carolina, USA. It has absolute ratios 13C/12C=11237.2 2.9 and 18O/16O = 2067.1 2.1 4 SMOW (Standard Mean Ocean Water) corresponds to a hypothetical water sample having both oxygen and hydrogen isotopic ratios equal to the mean isotopic ratios of ocean water, which have been evaluated as 18O/16O = (1993.4 2.5)x10-6 and D/H = (158 2)x10-6
CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA 141 Carbon and oxygen isotope ratios in carbonate rocks, according to previous references Reference Marine Fresh-water 13C ( PDB) 1 mean value: +0.56 mean value: -4.93 2 shallow deposits: (+2 +4.5) deep sea deposits: (0 +2) limits of variation: (-17 -2) 3 mean values: (+0.56 +1.55) limits of variation: (-0.99 +2.11) limits of variation: (-7.68 2.18) 18O ( PDB) 1 mean value: -5.25 mean value: -8.66 2 shallow deposits (-0.5 +1.5) deep sea deposits (+1 +3) limits of variation: (-10 +4) References: 1 Keith & Weber (1964) (selected group of samples) 2 Veizer (1983), according to the diagram of Milliman (1974) 3 Faure (1977) For Jurassic and younger samples (assumed to be nonrecrystallized rocks) the best discrimination between marine and freshwater limestones is given by the following equation (Keith & Weber, 1964): Z = 2.048(13C + 50) + 0.498 ( 18O + 50) where both 13C and 18O are expressed as PDB. Limestone samples with a Z value above 120 would be classified as marine, while those with Z below 120 as fresh-water ones. Geologically older specimens can probably be better classified into marine and fresh-water categories using the carbon isotope ratio alone. Geological background Rona Limestone is a lens-shaped body located in NW Transylvania, having a local development. The classic outcrop is located westwards from Jibou (S laj district), on the right bank of Some river, near Rona village. In the neighborhoods of Jibou and Rona the deposit has the maximum thickness of about 300 m; towards NE it gradually becomes thinner, until it disappears in the area of Caselor and Gard Brooks (Popescu, 1984).
STELA CUNA, DANA POP, ALEXANDRU HOSU 142 Fig. 1. Synthetic sedimentological l og of Jibou Formation in Rona area and location of samples under study (R1-R46 ) Rona Limestone outcrops only in Mese area which is the median sector of deposition of Paleogene formations in the NWrn part of the Transylvanian Depression. Stratigraphically, it divides Jibou Formation into the Lower Red Member and the Upper Variegated Member, respectively (Popescu, 1984). The deposits consist of limestone with organic content interlayered with dolomite beds, and grayish-greenish mudstones or clayey sands (Popescu, 1984). Due to the content of mollusk lumaschelles (limneids and planorbis), ostracods and Chara algae, the deposit was being described for a long time as fresh-water limestone (Hauer & Stache, 1863; Koch, 1894). It was considered to be one of the thickest formation of this type in the European Eocene (Popescu, 1984). The age of Rona Limest one is Late Paleocene Early Eocene (Gheerbrant et al., 1999). Even if the lacustrinepalustrine environment of formation is now widely accepted, there were
CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA 143 other opinions, as well: Szdeczky (1915, in Pauc 1977) considered Rona Limestone as a desert-type formation; Pauc (1977) named it "the most extended continental evaporite from Romania". More recently (Mszaros, 1995), deposits containing a marine nannoplankton assemblage belonging to the NP 14 zone (Middle Lutetian) were identified "below the green illite level", but within "the green illite nests" in the base of Rona lacustrine deposits. There is no detailed location al ong Rona profile given for this marine level and some uncertainty still persists. In conclusion, the author includes a thick (more than 80 m) marine (littoral) sequence between the Lower Jibou Formation and the Rona Beds, which he makes responsible for the formation of the hydrocarbon deposits from Brsa. From mineralogical and genetic points of view, the presence of chert nodules surrounded by green clay of a diagenetic nature as a unique paragenesis in the Romanian references is particularly worthy to mention. In the lower part of the succession, at about 10 m from the base of the classic Rona profile, these chert nodules ("septaria") of spherical to ellipsoidal morphologies are hosted by a 30-40 cm thick level of carbonate rock (Ghiurc & Tudoran, 1997). The chert nodules are of gemological interest (Ghiurc & Pop, 1994; Ghiurc 2000). They are always surr ounded by an intensely colored green clay coating, 0.5-1 cm thick (mentioned as nests of green marls by Bombi & Baltre 1986). The above relatively extended geological information was meant to explain the premises and our reasons for undertaking the study of the carbon and oxygen isotopic composition: 1. to check, by isotopic geochemistry means, if a certain environmental print can be noticed; 2. to clarify if there is any marine influence in the samples under study, as suggested by Mszaros (1995); and 3. to define the environment of formation of the green clay and of the hostrock at the level of the chert nodules. Samples and analytical methods Ten samples, among which nine of carbonate rocks and one (R28V) of the green clay surrounding the chert nodules were collected from the classic outcrop near Rona village. Sample R28 represents the carbonate rock that hosts the chert nodules. The location of the studied samples along Jibou Formation profile is presented in Fig. 1. The samples were mineralogically and petrographically analyzed, by means of optical microscopy on thin sections, and X-ray diffraction. Sedimentological observations were recorded in the field, as well as under microscope. The
STELA CUNA, DANA POP, ALEXANDRU HOSU 144 X-ray study was carried out using the DRON-3 diffractometer 5 with Cu anticathode, on both powder mounts and on oriented samples (air-dried and glycolated). For carbon and oxygen isotope measurements, samples were crushed to 100 m mesh size and heated at 400 C for about 30 minutes in order to drive off volatile organic compounds. The CO2 for mass spectrometer analysis was obtained from the reaction of carbonate with 100 % phosphoric acid, at 25 C (Mc Crea, 1950). The gas was collected, purified and analyzed in the ATLAS 86 mass spectrometer with double collector 6 Our "working standard" was Carrara marble, with and Measured data were corrected using Craigs formula (Craig, 1957). The random analytical error is less than 0.1 Recorded values are the mean values of replicate runs. 96.113 C 96.118 O Due to the presence in the analyzed samples of both calcite and dolomite in various ratios, some details on the analytical procedure are worthy to mention. The methods of investigation and calculation of the isotopic ratios are the same for both minerals, but the procedure of extraction of CO2 is slightly different. The CO2 released during the first hour of reaction has essentially the same values as that from pure calcite; as the reaction follows, CO2 collected between 4 to 72 h has the same values as that from pure dolomite (Degens & Epstein, 1964). In our investigation, the acidic attack was performed for 1 h, in order to prevent the contribution of the carbon and oxygen isotopes from dolomite in the resulting CO2. Thus, we considered that the isotopic compositions were indicative only for the syngenetic processes, which lead to the formation of calcite, and were probably not influenced by the diagenetic environment that transformed part of the calcite into dolomite. Results According to the petrographical and sedimentological features, the carbonate facies are stacked like transgressive-regressive cycles, two main lithofacies associations being recognizable in a sequence (Hosu & Pop, 1995): 1. Lacustrine association with lime mudstone to packstone with scarce mollusks and charophyte debris, interbedded with black-greenish shales sometimes organic-rich. The lack of bioturbations at some levels indicates the presence of water stratification and anoxia. 5 at the Department of Mineralogy, B abes-Bolyai University in Cluj-Napoca 6 at the National Institute of Research and Development for Isotopic and Molecular Tec hnology, Cluj-Napoca
CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA 145 2. Palustrine association with pellet and intraclast grainstones or packstones, the intraclasts representing reworked fragments of brecciated sediments exposed at the lake margins. Pedogenetically modified limestones are characterized by the development of calcareous soils, exhibiting features as brecciation, root tubules, microkarst, various cavities or iron oxide mottling. The processes of dolomitization that were noticed in several carbonate sequences plead for an advanced degree of diagenesis. The diffractograms of the nine carbonate-rich samples indicate the dominance (60-90 %) of calcite and dolomite in various ratios, besides which clay minerals (5-30 %) and quartz (< 5 %) are also present (Table 2). Thus, the nine samples can be defined as limestones (R20, R25, R28, R35), dolomitic limestone (R6) and marls (R1, R16, R32, R46). Table 2 Relative participation of the main mineralogical components in the analyzed samples (semi-quantitative) Sample CARBONATES (relative frequency, % among carbonate minerals) NON-CARBONATE MINERALS (*=relative frequency of each component) Calcite Dolomite Quartz Smectite Illite R1 55 45 * R6 30 70 * R16 65 35 * R20 70 30 * R25 70 30 * R28 95 5 * R28V 85 15 ** *** R32 95 5 ** * R35 95 5 * R46 80 20 ** The clay minerals assemblage is dominated by smectite, which is present in all the analyzed samples. Besides it, illitic material is present in some carbonate levels, in both the calcite-rich (R20, R25, R32, and R35) and the dolomite-rich (R6) ones. Some 10-14M interstratifications having variable degrees of ordering can also be mentioned, being subordinated in the mixture. As referred to a sequence, the amount of illite-type material increases upwards, while the smectites decline. Thus the maximum illite participation is located in the palustrine association, suggesting a pedogenic illitization process. Sample R28V represents a carbonate-rich clay (30 % carbonate minerals, very poorly crystallized) with a relatively high content of quartz (10 %). The clay assemblage is clear ly dominated by the illite-type phase, besides which smectite is also present. Some previous investigations (Pop
STELA CUNA, DANA POP, ALEXANDRU HOSU 146 et al., 1995) based on Mssbauer spectroscopy and XRF showed that the green clay has a relatively high amount of Fe. Due to the lack of chemical data, the mineralogical nature of the illitic phase was not clearly defined: the authors were assuming either "Fe-illite" or a "glauconitic mineral". As a preliminary result, it was found that the 10 phase from the green crusts is diffractometrically very similar with the illitic material present along the sampled profile. The carbon and oxygen isotope data are reported as ( PDB) in Table 3. Table 3 Carbon and oxygen isotope ratios and calculated Z parameter Sample 13C ( PDB) 18O ( PDB) Z R1 -4.02 -14.13 112.03 R6 -5.11 -13.72 110.00 R16 -4.15 -14.78 111.44 R20 -5.40 -11.33 110.60 R25 -6.07 -13.39 108.20 R28 -4.91 -15.68 109.44 R28V -6.78 -14.60 106.14 R32 -5.49 -8.62 111.76 R35 -4.56 -19.20 108.40 R46 -3.12 -9.41 116.22 Discussion Lacustrine calcite records the stable isotope fractionation patterns of several overlapping genetic processes involving bedrock geology, vegetation of the lake and its surroundings, soil formation, hydrogeology and possibly, environmental change (Hammarlund et al., 1997). Besides, diagenetic factors should be added. Without comparative data on carbon and oxygen isotopic compositions and carbon distribution in several types of materials, such as mollusk shells, ostracode valves and possibly relic Chara encrustations, a refined analysis is difficult to be done, and beyond our purpose. Based on the current results, some aspects are still relevant for the general environment of formation of Rona Limestone. The first topic under discussion is whether the isotopic compositions reflect only the environment of deposition of the carbonates from Rona, or if diagenesis also influenced the carbon and oxygen ratios? In principle, the discussion should be reduced to the different isotopic contributions which calcite vs. dolomite can bring to the final 13C and 18O values. Our cautions were firstly taken concerning the extraction procedure, as already mentioned. More, having a closer look at the various samples and their isotopic data, supplementary control can be obtained. Thus, R6 contains the highest
CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA 147 amount of dolomite among the studied samples, while R28, R32 and R35 show the highest amounts of calcite (samples R32 and R28 have also significant amounts of clay minerals). In spite of this fact, both the 13C and 18O values for R6 fall within the limits of variation of the other three samples. Based on these arguments, we believe it is realistic to assume that dolomite thus diagenesis, did not significantly overprint the carbon and oxygen isotope measured values. Concerning the carbon isotopes, the 13C (PDB) values are typical for fresh-water carbonates (they range between -3.12 -6.78 ), all being below -3 (Table 3). These values are consistent with the isotopic record ( 1 ) on the organic-rich units of a Holocene lacustrine carbonate sequence (Hammarlund et al., 1997). Along the profile, some trends can be noticed: in the lower part there is a systematic variation of the 13C (PDB) values; theoretically this means a relatively enrichment in 13C in samples R1, R16, and enrichment in 12C (of an organic origin) in samples R6, R20, and R25. The differences are relatively small, but they are above the random analytical error. The geological meaning of this fact could be related to rhythmical changes between lacustrine and palustrine environments. in the upper part of the analyzed profile an increasing trend of the 13C values is noticeable from samples R32 to R46. This is due to a gradual enrichment in 13C, which could be explained by progressive restriction of palustrine environments. when comparing samples R28 (carbonate host-rock) with R28V (green clay), quite different carbon isotopic ratios can be noticed. In fact, sample R28V registers the lowest 13C value (-6.78 ) in the samples under study, while sample R28 shows a value (-4.91 ) close to the average one for the carbonate succession along the profile. This fact could be the result of various causes, among which: the different mineralogical nature of the samples (R28V is a clay-dominated sample, whilst R28 is calcite-rich) might have lead to different isotopic fractionation patterns; the different environmental conditions of formation a primary genesis is assumed for the host-rock while the green clay probably formed during diagenesis. The relative 13C-enrichment of some biogenic calcite-rich levels (leading to relatively higher 13C values) may be correlated with the carbon isotope separation pattern of Chara algae, which were previously identified in Rona Limestone. Hammarlund et al. (1997) proposed a kinetic carbon isotope fractionation model of photosynthetic assimilation of dissolved
STELA CUNA, DANA POP, ALEXANDRU HOSU 148 inorganic carbon by means of proton pumping that can explain some of the variations of the 13C values. For oxygen isotopes, the limits of variation of the 18O (PDB) values range between (-8.62 -19.20) (Table 3). A somehow reversed trend as compared to the 13C values can be noticed in the lower part of the profile. No clear trend is obvious in the upper part. When comparing the values of samples R28V vs. R28, a slight enrichment in 18O can be noticed in the calcite from the green clay, as compared to the carbonate host-rock. An interesting feature is worthy to mention when comparing our data with previous results in the literature, plotted in Fig. 2. In general, there is a clear separation between the fresh-water limestones and the marine (both shallow-water and deep-sea) ones. The samples from Rona plot in a distinctive field, which partly overlaps the fresh-water limestones. Fig. 2. Distribution of 18O vs. 13C ( PDB) values in various types of carbonate rocks, including this study (after Milliman, 1974, in Veizer, 1983)
CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIA 149 At the same time, the relatively low 18O (PDB) ratios (< -10 ) are unusual when compared with other data in the references. These values suggest a 18O depletion in most of our samples (excepting R32, and R46). A relationship between the mineralogical composition and the 18O values could not be identified. The cause of this fractionation pattern cannot be evidenced starting from the current data. As working hypotheses, the influence of climaterelated changes, and/or changes in the terrestrial vegetation of the lake catchment could be used, as suggested by Hammarlund & Edwards (1998) in the case of Holocene lacustrine carbonates. Their samples of fine-grained calcite showed values up to 13.5 The synthetic Z parameter based on both 13C and 18O shows values characterizing the fresh-water carbonates (between 106.14 116.22) for all the analyzed samples (Table 3). Concerning the mineralogical nature of the green clay around the chert nodules, our study does not bring new results. Due to the 13C value which is representative for the fresh-water environments, one can only presume that the green clay is more likely to be a Fe-illite than a glauconitic mineral the latter one being typically of marine origin. But even in nonmarine conditions, a large mineralogical variety of green clays can be formed, including glauconite (Porrenga, 1968). Anyhow, the genetic criterion is not valid when defining the mineral species, according to AIPEA classification (Bailey, 1980); only crystal chemistry should be taken into consideration. For that, more detailed analytical data are needed. Conclusions 1. The procedure of extraction of CO2 we used enabled the discrimination between the carbon and oxygen isotopic patterns of calcite vs. dolomite; in our case, the primary environment of formation could thus be recorded without the diagenetic overprint. 2. The oxygen and carbon data on samples from Rona Limestone indicate a fresh-water depositional environment, with Z<120. The mean carbon isotopic composition (-4.96 ) fits the values for fresh-water carbonates of the Tertiary period, in general. 3. The 13C and 18O values obtained for the carbonate minerals in the green clay surrounding the chert nodules also suggest a fresh-water carbonate paleoenvironment. Acknowledgement The authors wish to thank the revi ewers (B. Onac, C. Ionescu, and L. Ghergari) for some of their critical comments, which helped to improve the manuscript.
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