- Email: [email protected]
Devonian boundary stratotype
Geobios 35 (2002) 21–28 www.elsevier.com/locate/geobios
Seawater strontium isotope curve at the Silurian/Devonian boundary: a study of the global Silurian/Devonian boundary stratotype Courbe des isotopes du strontium de l’eau de mer à la limite Silurien/Dévonien : une étude du stratotype global de la limite Silurien/Dévonien Jirˇí Fry´da a,*, Jindrˇich Hladil b, Karel Vokurka a b
a Czech Geological Survey, Klarov 3/131, 118 21 Prague 1, Czech Republic Institute of Geology AS CR, Rozvojová 135, 165 02 Prague 6, Czech Republic
Received 14 January 2001; accepted 15 June 2001
Abstract Present application of 87Sr/86Sr chemostratigraphy to detailed stratigraphical tasks is limited by inaccurate calibration of the general seawater strontium curve to absolute as well as to relative time scales. For this reason, refinement of the general seawater strontium curve has been suggested, using mainly clearly defined global boundary stratotype sections. This study reports the first 87Sr/86Sr data from the global Silurian/Devonian boundary stratotype section and fills an existing 1-Ma gap in available data. Generally, the data from the stratotype fit the range interpolated from published 87Sr/86Sr data of the general curve, but the slight differences may suggest an existence of a high-order oscillation near the Silurian/Devonian boundary. Higher 87Sr/86Sr values in the Devonian part of boundary bed 20 (20-beta) may indicate an exotic material influx of recycled sediment. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Résumé L’application de la chemostratigraphie du 87Sr/86Sr en vue d’analyses stratigraphiques détaillées est limitée par la calibration de la courbe générale des isotopes du strontium aussi bien pour des échelles de temps absolues que relatives. Pour cette raison, l’amélioration de la courbe générale du strontium de l’eau de mer a été proposée, utilisant des sections clairement définies tels que des stratotypes de limite globale. Cette étude présente les premières données du 87Sr/86Sr de la région stratotypique pour la limite Silurien/Dévonien et comble une lacune de 1 Ma dans les données disponibles. Généralement, les données de cette coupe stratotypique corrobore l’intervalle interpolé d’après les données publiées de la courbe générale du 87Sr/86Sr mais de légères différences peuvent suggérer l’existence d’une oscillation de premier ordre près de la limite S/D. Les hautes valeurs du 87Sr/86Sr dans la partie dévonienne du banc 20 (20-beta) pourraient indiquer l’apport de matériel exotique de sédiments recyclés. © 2002 E´ditions scientifiques et médicales Elsevier SAS. Tous droits réservés. Keywords: Strontium isotopes; Marine carbonates; Silurian/Devonian GSSP; Barrandian area; Czech Republic; Central Europe Mots clés: Isotopes du strontium; Carbonates marins; Stratotype du Silurien/Dévonien; Aire Barrandienne; République Tchèque; Europe centrale
* Corresponding author. E-mail address: [email protected] (J. Fry´da).
© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 0 1 6 - 6 9 9 5 ( 0 2 ) 0 0 0 0 6 - 2
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1. Introduction In recent years several papers with rich datasets regarding strontium (Sr) isotopic compositions of past seawater have been published (Peterman et al., 1970; Veizer and Compston, 1974; Burke et al., 1982; Asmerom et al., 1991; Kaufman et al., 1993; Smalley et al., 1994; Veizer et al., 1999). At present, the general configuration of the Sr isotopic curve throughout the Phanerozoic is well known [see Veizer et al. (1999) and reference therein]. In addition, the existence of high-order oscillations in seawater Sr isotopic composition, with a magnitude of 0.1–1 Ma, was observed. These observations have revealed the high potential of Sr isotopic stratigraphy as a geochronologic tool (Smalley et al., 1994; Ruppel et al., 1996; Diener et al., 1996; Azmy et al., 1999). However, insufficient stratigraphic resolution has complicated, or even precluded, the use of 87Sr/86Sr chemostratigraphy as a high-resolution stratigraphic method (Diener et al., 1996; Veizer et al., 1999). Study of 87Sr/86Sr isotopic composition of marine carbonates from the global stratotypes or parastratotypes, as well as from boundary intervals of two adjacent, geographically well distributed biozones could considerably refine a time scale of the known seawater Sr isotope curve. However, sedimentary layers with well-known stratigraphic position, like those from many stratotypes, generally did not yield sufficient shell material for Sr isotopic study. For this reason, the outcrops with well-preserved shell material, but
commonly less-known biostratigraphy, have been preferred for such studies. In this paper, we report 87Sr/86Sr analyses of marine carbonates collected from the global Silurian/Devonian boundary stratotype. Data derived from them illustrate both the potential to use whole-rock samples for Sr isotope study, and solves some problems of the global Silurian/Devonian boundary stratotype.
2. Materials and methods All whole-rock samples come from the global Silurian/Devonian boundary stratotype: the Klonk outcrop near the village of Suchomasty (Prague Basin, Czech Republic). Stratigraphy, paleontology, lithology, and chemostratigraphy of the section are described in papers of Chlupácˇ et al. (1972), Chlupácˇ and Kukal (1988), Hladil (1992), Hladiková et al. (1997), Chlupácˇ and Hladil (2000), and references therein. Polished sections from each field sample were observed under a light microscope and only samples lacking any traces of post-depositional alterations (like a presence of microfactures, dolomitization, recrystalization or silicification) were used for further study. Stratigraphic position of the studied samples is shown in Fig. 1. Detailed description of sample lithologies together with a microfacies data may be found in papers of Hladil (1992), Hladiková et al. (1997), and Chlupácˇ and Hladil (2000). Whole-rock powders were prepared from field samples for determination of Si, Ti, Al, Fe, Mn, Mg, Ca, Sr, Rb, Li, Na,
Fig. 1. The Silurian/Devonian boundary interval at Klonk (GSSP), showing occurrences of selected fossils (biostratigraphical markers), distribution of principal microfacies, and magnetosusceptibility stratigraphy zones. Black arrows show a stratigraphic position of samples used for 87Sr/86Sr study. Limite Silurien–Dévonien à Klonk (GSSP) montrant la présence de fossiles sélectionnés (marqueurs stratigraphiques), distribution des principaux microfaciès et zones stratigraphiques d’après les magnétosusceptibilités. Les flèches noires montrent la position stratigraphique des échantillons utilisés pour les analyses de 87Sr/86Sr.
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K, P, CO2, C, H2O+, H2O–, F, S, REE, and Y concentrations (wet methods, ICP, AAS; Chemical laboratory of the Czech Geological Survey having the Czech Quality Accreditation). In cases of repeat measurements (for both chemical and isotope analyses) different slabs of rock from the same limestone bed were used. The majority of the samples are identical with those used by Hladiková et al. (1997) for carbon and oxygen isotope studies. More detailed sampling was focused on the Silurian/Devonian boundary bed (no. 20) from which four Silurian and one Devonian samples with different stratigraphic positions were studied. For Sr isotope studies fine-grained carbonate aggregates were obtained from polished sections of the whole-rock samples. Material was microsampled from the polished sections under a binocular microscope by smashing the suitable fine-grained carbonate aggregates with a stainless steel dental pick. The grains (fragments) were cleaned in an ultrasonic bath. The powders from these aggregates were leached in ultra clean, weak (0.5 M) acetic acid to avoid the dissolution of possible clastic components. After leaching, solutions were decanted, dried and re-dissolved in 1.5 M HCl. Two aliquots were later weighed from each of these solutions; first for the determination of Ca, Mg, Mn, Fe, Sr, and Rb concentrations, and second for determination of Sr isotope compositions. Only samples having Sr and Mn contents of > 750 ppm and < 350 ppm, respectively, were selected for Sr isotope study using trace element criteria (Brand and Veizer, 1980; Veizer, 1983; Denison et al., 1994). Strontium was separated from the selected solutions by standard ion exchange techniques and its isotopic composition determined on a Finnigan MAT 262 at the Czech Geological Survey, Prague. The NBS 987 standard analysed during this work yields average value of 0.710247 with a (2σ) precision calculated from five measurements of ± 0.000012. The latter value is close to the recommended value of the NBS 987 standard [0.710249; see, for example, Azmy et al. (1999)]. A long-term reproducibility may be characterized by an average of 75 measurements of the NBS 987 standard, which gives a value of 0.710255 ± 0.000022.
3. Silurian/Devonian boundary seawater Sr isotope curve Veizer et al. (1999) developed a very large dataset of Sr isotopes from marine carbonates throughout the Phanerozoic time, and have re-evaluated previously published data. There is broad consensus concerning the general configuration of the Silurian and Devonian 87Sr/86Sr seawater curve (Ruppel et al., 1996; Diener et al., 1996; Denison et al., 1997; Azmy et al., 1999; Veizer et al., 1999; Fig. 1) in that it forms a flattop peak with maximum values close to the Silurian/Devonian boundary. However, a more detailed overview of available data for the Silurian/Devonian boundary interval (see www.science.uottawa.ca/geology/ isotope_data) shows only 10 strontium isotopic analyses of
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Devonian brachiopod shells collected from the woschmidti biozone from the Dniestr River outcrop (Ukraine) and analysis of only one Silurian brachiopod shell from transgrediens biozone of Podolia. Recently Azmy et al. (1999) published two additional measurements from the transgrediens biozone—one from Podolia and the second from Latvia. All available data come from the same palaeocontinent, which is however different from that where the stratotype area has been situated (Perunica microplate). This fact suggests possible uncertainties in relative age determination of the samples from these remote sections. On the other hand, the samples from the global standard section and point (GSSP) lack such uncertainties. In addition, according to the time calibration made by Veizer et al. (1999) (see www.science.uottawa.ca/geology/isotope data) there is a gap of about 1 Ma adjacent to the Silurian/Devonian boundary. Veizer et al. (1999) discussed time parameter problems of the strontium isotopic seawater curve and suggested that the highest resolution could be achieved with biostratigraphy. Nevertheless, resolution limit for the latter method is typically slightly worse than 0.5 Ma (Veizer et al., 1999). Uncertainty in time parameter may be even higher when geological sections far from stratotype areas are used. Poor time determination of studied samples considerably complicates studies of the higher-order oscillations in the seawater Sr isotopic composition. On the other hand, studying samples from the stratotype section eliminates determination problems of relative time parameter. Similarly, if samples from the boundary interval of two adjacent biozones are used, the uncertainty of relative time parameter may be lowered. Such methodology is complicated due to the rarity of suitable shell material for Sr isotopic study.
4. Estimation of absolute time distances of analysed samples The latest radiometric data based on U-Pb (Tucker et al., 1998) have changed the previously used boundary age estimates of the Silurian/Devonian from 408 Ma to 418 Ma and the duration of the Lochkovian stage is currently estimated to be 4.5 Ma. However, the Lochkovian carbonate rhythmites of the Barrandian area contain unusually high number (450) of bedding couplets, which are characterised by gradually increasing and decreasing carbonate contents (Chlupácˇ , 2000). Accepting that these lithological/faunistic couplets are a result of climatic oscillations (Chlupácˇ , 2000), this may indicate longer duration for the Lochkovian stage. Tucker’s duration of Lochkovian (4.5 Ma), if divided by this number of couplets (450), gives unusually highfrequency oscillations of about 10 ka duration. This average duration is 5–7 Ma shorter than that assumed in previous studies dealing with the Klonk GSSP (Hladil, 1991; Chlupácˇ , 2000). In contrast, the magnetosusceptibility (MS) data on this section (Crick et al., 2001) indicate cyclic
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patterns of longer duration than that derived from the fine-couplet lithological alternations in the outcrop. The MS cycles correspond most closely with the recalculated Devonian periodicity (38 848 a) of the modern 54 000-a obliquity cycle and in total may correspond well to present radiometric constraints for the duration of the Lochkovian. Although the relationship between these two periodicities is unknown in detail, deposition of calciturbidites, or sediments derived from them, may surely increase the number of lithological couplets (Hladil, 1991). Considering this uncertainty in absolute ages, the time range of analysed beds at Klonk is about 0.5–0.7 Ma. In comparison with all other data from distant and only roughly correlated sections, we know the exact stratigraphical position of studied samples, which are without doubt related to the GSSP (Silurian/Devonian boundary). In addition, our data fill the existing gap (about 1 Ma) in the world seawater Sr isotope curve (Veizer et al., 1999).
5. Sr isotope curve from the global Silurian/Devonian boundary stratotype Data from the global Silurian/Devonian boundary stratotype fit quite well with the estimated seawater Sr isotopic composition for the Silurian/Devonian boundary (Figs. 2 and 3) based on interpolation of known data (Veizer et al., 1999). Average value for the Silurian/Devonian boundary interval from the global Silurian/Devonian boundary stratotype is 0.70867. The latter value is slightly lower than the range (0.70869–0.70874) linearly interpolated for the Silurian/Devonian boundary interval from brachiopod (Veizer et al., 1999) and whole-rock (Denison et al., 1997) analyses, or range (0.70869–0.70875) extrapolated from the latest Silurian conodont analyses of Ruppel et al. (1996) and brachiopod analyses of Azmy et al. (1999). This difference could result from (1) high-order oscillation of the seawater Sr isotopic curve, or (2) isotopic contamination of analysed marine carbonates by clastic material. At present, it is impossible to compare our data with other datasets because of the about 1-Ma gap in known data just around the Silurian/Devonian boundary. The observed difference between our data from the stratotype and that estimated from linear interpolation and extrapolation of earlier measured values is about 5 × 10–5 of 87Sr/86Sr ratio. The latter value seems to be significantly higher than analytical errors (i.e., error of measurements as well as interlaboratory offsets; see Materials and methods). As shown by Diener et al. (1996), such a small difference may originate during 0.1–1 Ma. Azmy et al. (1999) described a change of about 8 × 10–5 of 87Sr/86Sr ratio originated during about 0.5 Ma for samples from the Pridolian sections of Latvia [see Azmy et al. (1999); Fig. 4]. Thus, the time gap in hitherto known seawater strontium isotopic curve at the Silurian/Devonian boundary (Veizer et al., 1999) is long enough for the origin of the observed difference.
Fig. 2. Relationships of the global strontium isotope data (Veizer et al. 1999) and new data from the Silurian–Devonian GSSP at Klonk (present paper). Relations entre les données globales des isotopes du strontium (Veizer et al.,1999) et les nouvelles données obtenues à la limite Silurien/Dévonien du GSSP (global stratotype section point) à Klonk (ce papier).
The second explanation of the slightly lower 87Sr/86Sr values from the Silurian/Devonian boundary stratotype is isotopic contamination of the marine carbonates by basaltic material. Presence of different amounts of basaltic material from submarine volcanism is a typical feature of carbonate beds at the Silurian/Devonian boundary stratotype (Hladil, 1992). However, a lack of any correlation among the strontium isotopic compositions and the amount of volcanic material of each sample (estimated from the whole-rock chemical compositions) testifies against such an explanation. On the other hand, good linear correlation of all 87 Sr/86Sr values and their stratigraphic positions (Fig. 3) seems to be consistent with the first explanation. Thus, slightly lower 87Sr/86Sr values for the global Silurian/Devonian boundary stratotype indicates a nonlinear configuration of the seawater 87Sr/86Sr curve in the Pridolian–Lochkovian interval caused by presence of a
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sition of whole Silurian part of the boundary bed (no. 20) may be considered to be constant. However, repeated measurements of strontium isotopic composition of marine carbonates from the first Devonian layer (uppermost part of the bed no. 20 with Monograptus uniformis) yield a much higher radiogenic value [0.70881(1)] that differs significantly from the approximate linear course of all 87Sr/86Sr values coming from the global boundary stratotype (Fig. 3). Repeated measurements on two rock samples from the first Devonian layer gave identical 87Sr/86Sr values: 0.708806(13) and 0.708811(16) (NBS 987 value was 0.710244 ± 0.000007). For this reason we believe that the latter values were measured correctly and represent a strontium isotopic composition of carbonates from the uppermost part of the bed no. 20 with M. uniformis. 6.2. Chemical composition Fig. 3. Evolution of the δ18O and δ13C isotope values [‰ PDB; data by Hladiková et al. (1997)] compared with the new 87Sr/86Sr data. Error bars for 87Sr/86Sr refer to ± 2σ values. Note the sublinear distribution of measured samples. The prominent spike in the Devonian part of the S/D boundary bed (20-beta) has been confirmed by repetitive measurements. Évolution des valeurs des isotopes de l’oxygène 18 et du carbone 13 [% PDB, données de Hladiková et al.(1997)] comparée avec les nouvelles données du rapport 87Sr/86Sr. Les barres d’erreurs correspondent à 2σ. Noter la distribution sublinéaire des échantillons traités. Le pic principal dans la partie dévonienne du banc de la limite S/D (20-beta) a été confirmé par des mesures répétées.
high-order oscillation. However, additional investigation of this feature, as well as comparison with datasets from other outcrops is needed.
6. The first Devonian layer 6.1.
87
Sr/86Sr composition
Detailed description of the Silurian/Devonian boundary bed (no. 20) including its lithology as well as microfacies data may be found in papers of Hladil (1992). Four rock slabs for polished sections were selected from the Silurian part of the bed. Their stratigraphic distances were approximately identical. One rock slab was selected from the Devonian part of the boundary bed. Later, a second rock slab from the Devonian part of the same bed was selected from a different field sample and used for repeated 87Sr/86Sr analysis. Strontium isotopic and chemical compositions of the samples from the Silurian/Devonian boundary bed (no. 20) are plotted in Fig. 4A. Four of these samples, belonging to the uppermost Silurian, show a very small scatter in isotopic compositions (from 0.70866 to 0.70868) with an average value of 0.70867(1) (Fry´ da and Vokurka, 1996). Thus, on the basis of statistical tests the 87Sr/86Sr compo-
Chemical analyses of dissolved marine carbonates used for 87Sr/86Sr analyses and leached from the fine-grained carbonate aggregates (see Materials and methods) of the Silurian/Devonian boundary bed yielded relatively uniform results (Fig. 4B). The molar contents range from 97.6 to 98.1% of CaCO3, from 0.55 to 0.58% of FeCO3, and from 0.078 to 0.088% of MnCO3. Only the MgCO3 content varies from 1.00 to 1.37 molar percent and its values form a nearly symmetrical pattern with the highest values at the marginal part of boundary bed and the lowest in the middle of bed (Fig. 4B). The strontium content ranges from 0.110 to 0.121 wt.% Sr (i.e., from 0.274 to 0.302 mol % of SrCO3) within the Silurian part of bed and it is slightly higher (0.144 wt.% Sr or 0.361 mol % of SrCO3) in the first Devonian layer. The rubidium content in all analysed solutions was below the limit of detectability (< 2 ppm, ICP). The chemical composition of analysed carbonates from the Silurian/Devonian boundary bed fits with known ranges of chemical composition in the marine carbonates (Brand and Veizer, 1980; Veizer, 1983). In addition, the chemistry of these carbonates is in close agreement with sample selection criteria for 87Sr/86Sr analysis (i.e., contents of Sr > 750 ppm and Mn < 350 ppm). There is no significant difference in the chemical composition of leached carbonates from the Silurian/Devonian boundary bed that could explain the increase of radiogenic value of the first Devonian layer. The whole-rock chemical composition of five samples from the Silurian/Devonian boundary bed (no. 20) shows a nearly symmetrical pattern (Fig. 4C–E) reflecting its carbonate content. The Silurian samples of the boundary bed have different whole-rock compositions but constant strontium isotopic composition. On the other hand, the Devonian sample from the uppermost part of the same bed has a whole-rock composition similar to the lowermost Silurian sample of the boundary bed, but much greater radiogenic 87 Sr/86Sr ratio.
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Fig. 4. Isotope and chemical compositions of the S/D boundary bed 20 at the Klonk GSSP. A. 87Sr/86Sr values of analysed marine carbonate; error bars for 87 Sr/86Sr refer to ± 2σ values; B. Chemical compositions of marine carbonate used for 87Sr/86Sr analyses; carbonates were microsampled from polished sections and leached in ultra clean, weak (0.5 M) acetic acid; C–E. Chemical compositions of whole rock (WR) samples from which analysed marine carbonates were microsampled; analytical errors equal to or are less than symbol sizes; C. Rare earth elements (µg/g); D. Minor elements (wt.%); E. Major elements of the rocks (wt.%). Compositions isotopiques et chimiques du banc 20 de la limite S/D à Klonk (GSSP). A. Valeurs de 87Sr/86Sr des carbonates marins analysés. Les barres d’erreurs du 87Sr/86Sr correspondent à 2σ. B. Composition chimique des carbonates marins utilisés pour les analyses du 87Sr/86Sr, les carbonates ont été microéchantillonnés d’après les sections polies et filtrées avec de l’acide acétique. C–E. Composition chimique de la roche totale dans laquelle les carbonates marins ont été microéchantillonnés. Les erreurs analytiques sont équivalentes à la taille des symboles. C: terres rares (µg/g) ; D: éléments mineurs (wt.%) ; E: éléments majeurs des roches (wt.%).
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7. Discussion Different 87Sr/86Sr values of the analysed carbonates from the Silurian and Devonian parts of the boundary bed no. 20 could result from (1) post-depositional alteration, (2) different chemical or/and mineral compositions of the first Devonian layer, and (3) higher order oscillation in 87Sr/86Sr composition of seawater. However, petrological, mineralogical, chemical, and stable isotope data of samples from the Silurian/Devonian boundary bed (Hladil, 1992; Hladiková et al., 1997, and data therein) did not suggest different post-depositional alterations of the Devonian and Silurian parts of the boundary bed. In addition, our detailed microscopic and chemical investigation of the studied samples as well as stable isotopic compositions of the identical samples [data by Hladiková et al. (1997)] do not reveal any trace of the different postdepositional alterations of the Devonian and Silurian parts of the boundary bed. Nevertheless, such a possibility can never be definitely excluded. Much greater radiogenic value of 87Sr/86Sr (0.70881) of the first Devonian layer could be a product of a primary higher Rb/Sr ratio. A difference at least of about 0.025 in the Rb/Sr ratio between Silurian and Devonian samples is needed for time of 400 Ma (i.e., from beginning of the Devonian to Recent), in order to explain the observed difference in 87Sr/86Sr values as a product of different Rb/Sr values. Moreover, we have to presume that all 87Sr from Rb-bearing phase(s) was dissolved into the analysed solution in the Devonian sample, but none was dissolved in Silurian samples. Whole-rock Rb2O/SrO ratio of these samples ranges from 0.120 to 0.131 for the Silurian part of the boundary bed and it is 0.126 in the first Devonian sample. Thus, there is no significant difference in the Rb2O/SrO ratio among Silurian and Devonian whole-rock composition. We also analysed rubidium content of all solutions containing dissolved carbonates for analysis of 87 Sr/86Sr composition and the Rb content of none of them was above the detection limit of (< 2 ppm, ICP). Thus, results suggest a very limited dissolution of Rb-bearing phases in the analysed solutions. Because of the similar mineralogy and whole-rock composition of all samples from the boundary bed, it is difficult to consider a quite different dissolution mechanism for these samples. The strontium isotopic compositions of the Silurian samples fit well with the expected seawater composition (see above). These data suggest no or very low influence of noncarbonate phases on the measured 87Sr/86Sr values. Another possible explanation for the much greater radiogenic value of 87Sr/86Sr (0.70881) in the first Devonian layer is a deposition of clastic material with a primary higher value of 87Sr/86Sr that could equilibrate during diagenesis with the carbonates of the first Devonian layer. Such hypotheses are difficult to test, but there are several microfacies observations that could support it. As shown by Hladil (1992), the Devonian base with the first M. uniformis
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corresponds to a semi-lithified surface, a short break in deposition, and a change in direction of bottom currents. The higher radiogenic spike of 87Sr/86Sr at the first Devonian layer may also represent a product of a primary higher order oscillation in 87Sr/86Sr composition of seawater. Such oscillations commonly occur on the Silurian and Devonian 87Sr/86Sr seawater curve [e.g. Ruppel et al. (1996); Diener et al. (1996); Azmy et al. (1999)]. However, as shown by Diener et al. (1996) the length of time required to change the strontium isotopic composition to that of present-day seawater in magnitude of ∼10–4 is about 105–106 years. Accepting the possibility that the first Devonian spike in the 87Sr/86Sr ratio represents the higher order oscillation in the strontium isotopic composition, we have to consider that the break in sedimentation (i.e., any event of non-deposition or partial erosion) between last Silurian and the first Devonian layers was in the magnitude of 105–106 years. To consider a shorter time for such a sedimentation break, we would have to presume a much higher riverine 87 Sr/86Sr input or a much greater difference in the 87Sr/86Sr ratio between marine and riverine waters during Silurian/Devonian time. Both of the latter assumptions seem to be improbable. Moreover, Hladil (1992) estimated a break in sedimentation between last Silurian and the first Devonian layers at the global boundary stratotype in the order of 102–103 years. Such a time interval is too short for the change of the seawater strontium isotopic composition from the uppermost Silurian value of 0.70867 to the value of 0.70881 for the first Devonian sample. If we accept the possibility that the first Devonian spike in 87Sr/86Sr ratio represents a higher order oscillation in the strontium isotopic composition of seawater, we have to accept that the sedimentation break between last Silurian and the first Devonian layers at the global boundary stratotype exceeded for more than 102–103 years.
8. Conclusions We encourage the methodology of measuring 87Sr/86Sr compositions of the marine carbonates from stratotypes as well as from the boundary intervals of the adjacent biozones for refinement of the time scale derived from the known seawater Sr isotope curve (Veizer et al., 1999). The first 87 Sr/86Sr analyses of samples from the global Silurian/Devonian boundary stratotype illustrate the potential to use the marine carbonates from the whole-rock samples for such studies. Our data also have revealed partial timing and composition problems of the global Silurian/Devonian boundary stratotype. The greater radiogenic 87Sr/86Sr value of the carbonates from the first Devonian layer (uppermost part of bed no. 20) may be explained by (1) a deposition of material with a primary greater value of 87Sr/86Sr that during diagenesis reequilibrated with marine carbonates, or (2) by a primary higher-order oscillation in 87Sr/86Sr composition of seawa
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ter. The presence of a break in sedimentation (hiatus or maybe partial erosion) between last Silurian and the first Devonian layers as well as a change in direction of bottom currents during sedimentation the first Devonian layer observed by Hladil (1992) are not in conflict with any of these explanations. The first of these explanations seems to be the more probable. However, the acceptance of such hypothesis (i.e., deposition of foreign material with more radiogenic 87Sr/86Sr ) would indicate that at least part of the material composing the first Devonian layer (bed 20-beta) at the global Silurian/Devonian boundary stratotype is recycled sediment of unknown age and provenance. Thus, it may be slightly older than the underlying uppermost Silurian layers. For this reason, possible separation of allochthonous and pelagic components in the 20-beta bed will be useful for any future high-resolution correlation studies based on the chemistry or micropaleontology of the global stratotype point.
Acknowledgements The research was made possible by grants 205/98/1454 and 205/01/0143 from the Grant Agency of the Czech Republic. The structural research and development program CEZ Z 3-013-912 supported the geological studies. We also wish to acknowledge W. Oschmann (Frankfurt am Main), A. Eisenhauer and F. Böhm (Kiel) for their helpful, critical reviews of this paper.
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Crick, R.E., Ellwood, B.B., Hladil, J., El Hassani, A., Hrouda, F., Chlupácˇ , I., 2001. Magnetostratigraphy susceptibility of the Pridolian–Lochkovian (Silurian–Devonian) GSSP (Klonk, Czech Republic) and a coeval sequence in Anti-Atlas, Morocco. Palaeogeography Palaeoclimatology Palaeoecology 167 (1-2), 73–100. Denison, R.E., Koepnick, R.B., Fletcher, A., Howell, M.W., Calloway, W.S., 1994. Criteria for the retention of original seawater 87 Sr/86Sr ancient shelf limestones. Chemical Geology 112, 131–143. Denison, R.E., Koepnick, R.B., Burke, W.H., Hetherington, E.A., Fletcher, A., 1997. Construction of the Silurian and Devonian seawater 87Sr/86Sr curve. Chemical Geology 140, 109–121. Diener, A., Ebneth, S., Veizer, J., Buhl, D., 1996. Strontium isotope stratigraphy of the Middle Devonian: brachiopods and conodonts. Geochimica et Cosmochimica Acta 60, 639–652. Fry´ da, J., Vokurka, K., 1997. Strontium isotopic composition of Silurian and Devonian seawater in the Prague Basin (Barrandian): the study of Silurian–Devonian boundary stratotype (Klonk near Suchomasty). Geoscience Research Reports for 1996 Czech Geological Survey, 106–108. Hladil, J., 1991. Evaluation of the sedimentary record in the Silurian/Devonian boundary stratotype at Klonk (Barrandian area, Czechoslovakia). Newsletters on Stratigraphy 25, 115–125. Hladil, J., 1992. Are there turbidites in the Silurian/Devonian boundary stratotype? (Klonk near Suchomasty, Barrandian, Czechoslovakia). Facies 26, 35–54. Hladiková, J., Hladil, J., Krˇibek, B., 1997. Carbon and oxygen isotope record across Pridoli to Givetian stage boundaries in the Barrandian Basin (Czech Republic). Palaeogeography Palaeoclimatology Palaeoecology 132, 225–241. Kaufman, A.J., Jacobsen, S.B., Knoll, A.H., 1993. The Vendian record of Sr and C isotopic variations in seawater: implications for tectonics and paleoclimate. Earth and Planetary Science Letters 120, 409–430. Peterman, Z.E., Hedge, C.E., Tourtelot, H.A., 1970. Isotopic composition of strontium in seawater throughout phanerozoic time. Geochimica et Cosmochimica Acta 34, 105–120. Ruppel, S.C., James, E.W., Barrick, J.E., Nowlan, G., Uyeno, T.T., 1996. High-resolution 87Sr/86Sr chemostratigraphy of the Silurian: implications for event correlation and strontium flux. Geology 24, 831–834. Smalley, P.C., Higgins, A.C., Howarth, R.J., Nicholson, H., Jones, C.E., Swinburne, N.H.M., Bessa, J., 1994. Seawater Sr isotope variations through time: a procedure for constructing a reference curve to date and correlate marine sedimentary rocks. Geology 22, 431–434. Tucker, R.D., Bradley, D.C., Ver Straeten, C.A., Harris, A.G., Ebert, J.R., McCutcheon, S.R., 1998. New U-Pb zircon ages and the duration and division of the Devonian time. Earth and Planetary Science Letters 158, 175–186. Veizer, J., 1983. Chemical diagenesis of carbonates: theory and application of trace element technique. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable isotopes in sedimentary geology, 10, Society of Economic Paleontologists and Mineralogists, Short Course Notes. pp. 1–100. Veizer, J., Compston, W., 1974. 87Sr/86Sr composition of seawater during the Phanerozoic. Geochimica et Cosmochimica Acta 38, 1461–1484. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Bruhn, F., Buhl, D., Carden, G., Diener, A., Ebneth, S., Goddris, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, δ13C And δ18O evolution of Phanerozoic seawater. Chemical Geology 161, 59–88.
Seawater strontium isotope curve at the Silurian/Devonian boundary: a study of the global Silurian/Devonian boundary stratotype Courbe des isotopes du strontium de l’eau de mer à la limite Silurien/Dévonien : une étude du stratotype global de la limite Silurien/Dévonien Jirˇí Fry´da a,*, Jindrˇich Hladil b, Karel Vokurka a b
a Czech Geological Survey, Klarov 3/131, 118 21 Prague 1, Czech Republic Institute of Geology AS CR, Rozvojová 135, 165 02 Prague 6, Czech Republic
Received 14 January 2001; accepted 15 June 2001
Abstract Present application of 87Sr/86Sr chemostratigraphy to detailed stratigraphical tasks is limited by inaccurate calibration of the general seawater strontium curve to absolute as well as to relative time scales. For this reason, refinement of the general seawater strontium curve has been suggested, using mainly clearly defined global boundary stratotype sections. This study reports the first 87Sr/86Sr data from the global Silurian/Devonian boundary stratotype section and fills an existing 1-Ma gap in available data. Generally, the data from the stratotype fit the range interpolated from published 87Sr/86Sr data of the general curve, but the slight differences may suggest an existence of a high-order oscillation near the Silurian/Devonian boundary. Higher 87Sr/86Sr values in the Devonian part of boundary bed 20 (20-beta) may indicate an exotic material influx of recycled sediment. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Résumé L’application de la chemostratigraphie du 87Sr/86Sr en vue d’analyses stratigraphiques détaillées est limitée par la calibration de la courbe générale des isotopes du strontium aussi bien pour des échelles de temps absolues que relatives. Pour cette raison, l’amélioration de la courbe générale du strontium de l’eau de mer a été proposée, utilisant des sections clairement définies tels que des stratotypes de limite globale. Cette étude présente les premières données du 87Sr/86Sr de la région stratotypique pour la limite Silurien/Dévonien et comble une lacune de 1 Ma dans les données disponibles. Généralement, les données de cette coupe stratotypique corrobore l’intervalle interpolé d’après les données publiées de la courbe générale du 87Sr/86Sr mais de légères différences peuvent suggérer l’existence d’une oscillation de premier ordre près de la limite S/D. Les hautes valeurs du 87Sr/86Sr dans la partie dévonienne du banc 20 (20-beta) pourraient indiquer l’apport de matériel exotique de sédiments recyclés. © 2002 E´ditions scientifiques et médicales Elsevier SAS. Tous droits réservés. Keywords: Strontium isotopes; Marine carbonates; Silurian/Devonian GSSP; Barrandian area; Czech Republic; Central Europe Mots clés: Isotopes du strontium; Carbonates marins; Stratotype du Silurien/Dévonien; Aire Barrandienne; République Tchèque; Europe centrale
* Corresponding author. E-mail address: [email protected] (J. Fry´da).
© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 0 1 6 - 6 9 9 5 ( 0 2 ) 0 0 0 0 6 - 2
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1. Introduction In recent years several papers with rich datasets regarding strontium (Sr) isotopic compositions of past seawater have been published (Peterman et al., 1970; Veizer and Compston, 1974; Burke et al., 1982; Asmerom et al., 1991; Kaufman et al., 1993; Smalley et al., 1994; Veizer et al., 1999). At present, the general configuration of the Sr isotopic curve throughout the Phanerozoic is well known [see Veizer et al. (1999) and reference therein]. In addition, the existence of high-order oscillations in seawater Sr isotopic composition, with a magnitude of 0.1–1 Ma, was observed. These observations have revealed the high potential of Sr isotopic stratigraphy as a geochronologic tool (Smalley et al., 1994; Ruppel et al., 1996; Diener et al., 1996; Azmy et al., 1999). However, insufficient stratigraphic resolution has complicated, or even precluded, the use of 87Sr/86Sr chemostratigraphy as a high-resolution stratigraphic method (Diener et al., 1996; Veizer et al., 1999). Study of 87Sr/86Sr isotopic composition of marine carbonates from the global stratotypes or parastratotypes, as well as from boundary intervals of two adjacent, geographically well distributed biozones could considerably refine a time scale of the known seawater Sr isotope curve. However, sedimentary layers with well-known stratigraphic position, like those from many stratotypes, generally did not yield sufficient shell material for Sr isotopic study. For this reason, the outcrops with well-preserved shell material, but
commonly less-known biostratigraphy, have been preferred for such studies. In this paper, we report 87Sr/86Sr analyses of marine carbonates collected from the global Silurian/Devonian boundary stratotype. Data derived from them illustrate both the potential to use whole-rock samples for Sr isotope study, and solves some problems of the global Silurian/Devonian boundary stratotype.
2. Materials and methods All whole-rock samples come from the global Silurian/Devonian boundary stratotype: the Klonk outcrop near the village of Suchomasty (Prague Basin, Czech Republic). Stratigraphy, paleontology, lithology, and chemostratigraphy of the section are described in papers of Chlupácˇ et al. (1972), Chlupácˇ and Kukal (1988), Hladil (1992), Hladiková et al. (1997), Chlupácˇ and Hladil (2000), and references therein. Polished sections from each field sample were observed under a light microscope and only samples lacking any traces of post-depositional alterations (like a presence of microfactures, dolomitization, recrystalization or silicification) were used for further study. Stratigraphic position of the studied samples is shown in Fig. 1. Detailed description of sample lithologies together with a microfacies data may be found in papers of Hladil (1992), Hladiková et al. (1997), and Chlupácˇ and Hladil (2000). Whole-rock powders were prepared from field samples for determination of Si, Ti, Al, Fe, Mn, Mg, Ca, Sr, Rb, Li, Na,
Fig. 1. The Silurian/Devonian boundary interval at Klonk (GSSP), showing occurrences of selected fossils (biostratigraphical markers), distribution of principal microfacies, and magnetosusceptibility stratigraphy zones. Black arrows show a stratigraphic position of samples used for 87Sr/86Sr study. Limite Silurien–Dévonien à Klonk (GSSP) montrant la présence de fossiles sélectionnés (marqueurs stratigraphiques), distribution des principaux microfaciès et zones stratigraphiques d’après les magnétosusceptibilités. Les flèches noires montrent la position stratigraphique des échantillons utilisés pour les analyses de 87Sr/86Sr.
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K, P, CO2, C, H2O+, H2O–, F, S, REE, and Y concentrations (wet methods, ICP, AAS; Chemical laboratory of the Czech Geological Survey having the Czech Quality Accreditation). In cases of repeat measurements (for both chemical and isotope analyses) different slabs of rock from the same limestone bed were used. The majority of the samples are identical with those used by Hladiková et al. (1997) for carbon and oxygen isotope studies. More detailed sampling was focused on the Silurian/Devonian boundary bed (no. 20) from which four Silurian and one Devonian samples with different stratigraphic positions were studied. For Sr isotope studies fine-grained carbonate aggregates were obtained from polished sections of the whole-rock samples. Material was microsampled from the polished sections under a binocular microscope by smashing the suitable fine-grained carbonate aggregates with a stainless steel dental pick. The grains (fragments) were cleaned in an ultrasonic bath. The powders from these aggregates were leached in ultra clean, weak (0.5 M) acetic acid to avoid the dissolution of possible clastic components. After leaching, solutions were decanted, dried and re-dissolved in 1.5 M HCl. Two aliquots were later weighed from each of these solutions; first for the determination of Ca, Mg, Mn, Fe, Sr, and Rb concentrations, and second for determination of Sr isotope compositions. Only samples having Sr and Mn contents of > 750 ppm and < 350 ppm, respectively, were selected for Sr isotope study using trace element criteria (Brand and Veizer, 1980; Veizer, 1983; Denison et al., 1994). Strontium was separated from the selected solutions by standard ion exchange techniques and its isotopic composition determined on a Finnigan MAT 262 at the Czech Geological Survey, Prague. The NBS 987 standard analysed during this work yields average value of 0.710247 with a (2σ) precision calculated from five measurements of ± 0.000012. The latter value is close to the recommended value of the NBS 987 standard [0.710249; see, for example, Azmy et al. (1999)]. A long-term reproducibility may be characterized by an average of 75 measurements of the NBS 987 standard, which gives a value of 0.710255 ± 0.000022.
3. Silurian/Devonian boundary seawater Sr isotope curve Veizer et al. (1999) developed a very large dataset of Sr isotopes from marine carbonates throughout the Phanerozoic time, and have re-evaluated previously published data. There is broad consensus concerning the general configuration of the Silurian and Devonian 87Sr/86Sr seawater curve (Ruppel et al., 1996; Diener et al., 1996; Denison et al., 1997; Azmy et al., 1999; Veizer et al., 1999; Fig. 1) in that it forms a flattop peak with maximum values close to the Silurian/Devonian boundary. However, a more detailed overview of available data for the Silurian/Devonian boundary interval (see www.science.uottawa.ca/geology/ isotope_data) shows only 10 strontium isotopic analyses of
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Devonian brachiopod shells collected from the woschmidti biozone from the Dniestr River outcrop (Ukraine) and analysis of only one Silurian brachiopod shell from transgrediens biozone of Podolia. Recently Azmy et al. (1999) published two additional measurements from the transgrediens biozone—one from Podolia and the second from Latvia. All available data come from the same palaeocontinent, which is however different from that where the stratotype area has been situated (Perunica microplate). This fact suggests possible uncertainties in relative age determination of the samples from these remote sections. On the other hand, the samples from the global standard section and point (GSSP) lack such uncertainties. In addition, according to the time calibration made by Veizer et al. (1999) (see www.science.uottawa.ca/geology/isotope data) there is a gap of about 1 Ma adjacent to the Silurian/Devonian boundary. Veizer et al. (1999) discussed time parameter problems of the strontium isotopic seawater curve and suggested that the highest resolution could be achieved with biostratigraphy. Nevertheless, resolution limit for the latter method is typically slightly worse than 0.5 Ma (Veizer et al., 1999). Uncertainty in time parameter may be even higher when geological sections far from stratotype areas are used. Poor time determination of studied samples considerably complicates studies of the higher-order oscillations in the seawater Sr isotopic composition. On the other hand, studying samples from the stratotype section eliminates determination problems of relative time parameter. Similarly, if samples from the boundary interval of two adjacent biozones are used, the uncertainty of relative time parameter may be lowered. Such methodology is complicated due to the rarity of suitable shell material for Sr isotopic study.
4. Estimation of absolute time distances of analysed samples The latest radiometric data based on U-Pb (Tucker et al., 1998) have changed the previously used boundary age estimates of the Silurian/Devonian from 408 Ma to 418 Ma and the duration of the Lochkovian stage is currently estimated to be 4.5 Ma. However, the Lochkovian carbonate rhythmites of the Barrandian area contain unusually high number (450) of bedding couplets, which are characterised by gradually increasing and decreasing carbonate contents (Chlupácˇ , 2000). Accepting that these lithological/faunistic couplets are a result of climatic oscillations (Chlupácˇ , 2000), this may indicate longer duration for the Lochkovian stage. Tucker’s duration of Lochkovian (4.5 Ma), if divided by this number of couplets (450), gives unusually highfrequency oscillations of about 10 ka duration. This average duration is 5–7 Ma shorter than that assumed in previous studies dealing with the Klonk GSSP (Hladil, 1991; Chlupácˇ , 2000). In contrast, the magnetosusceptibility (MS) data on this section (Crick et al., 2001) indicate cyclic
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patterns of longer duration than that derived from the fine-couplet lithological alternations in the outcrop. The MS cycles correspond most closely with the recalculated Devonian periodicity (38 848 a) of the modern 54 000-a obliquity cycle and in total may correspond well to present radiometric constraints for the duration of the Lochkovian. Although the relationship between these two periodicities is unknown in detail, deposition of calciturbidites, or sediments derived from them, may surely increase the number of lithological couplets (Hladil, 1991). Considering this uncertainty in absolute ages, the time range of analysed beds at Klonk is about 0.5–0.7 Ma. In comparison with all other data from distant and only roughly correlated sections, we know the exact stratigraphical position of studied samples, which are without doubt related to the GSSP (Silurian/Devonian boundary). In addition, our data fill the existing gap (about 1 Ma) in the world seawater Sr isotope curve (Veizer et al., 1999).
5. Sr isotope curve from the global Silurian/Devonian boundary stratotype Data from the global Silurian/Devonian boundary stratotype fit quite well with the estimated seawater Sr isotopic composition for the Silurian/Devonian boundary (Figs. 2 and 3) based on interpolation of known data (Veizer et al., 1999). Average value for the Silurian/Devonian boundary interval from the global Silurian/Devonian boundary stratotype is 0.70867. The latter value is slightly lower than the range (0.70869–0.70874) linearly interpolated for the Silurian/Devonian boundary interval from brachiopod (Veizer et al., 1999) and whole-rock (Denison et al., 1997) analyses, or range (0.70869–0.70875) extrapolated from the latest Silurian conodont analyses of Ruppel et al. (1996) and brachiopod analyses of Azmy et al. (1999). This difference could result from (1) high-order oscillation of the seawater Sr isotopic curve, or (2) isotopic contamination of analysed marine carbonates by clastic material. At present, it is impossible to compare our data with other datasets because of the about 1-Ma gap in known data just around the Silurian/Devonian boundary. The observed difference between our data from the stratotype and that estimated from linear interpolation and extrapolation of earlier measured values is about 5 × 10–5 of 87Sr/86Sr ratio. The latter value seems to be significantly higher than analytical errors (i.e., error of measurements as well as interlaboratory offsets; see Materials and methods). As shown by Diener et al. (1996), such a small difference may originate during 0.1–1 Ma. Azmy et al. (1999) described a change of about 8 × 10–5 of 87Sr/86Sr ratio originated during about 0.5 Ma for samples from the Pridolian sections of Latvia [see Azmy et al. (1999); Fig. 4]. Thus, the time gap in hitherto known seawater strontium isotopic curve at the Silurian/Devonian boundary (Veizer et al., 1999) is long enough for the origin of the observed difference.
Fig. 2. Relationships of the global strontium isotope data (Veizer et al. 1999) and new data from the Silurian–Devonian GSSP at Klonk (present paper). Relations entre les données globales des isotopes du strontium (Veizer et al.,1999) et les nouvelles données obtenues à la limite Silurien/Dévonien du GSSP (global stratotype section point) à Klonk (ce papier).
The second explanation of the slightly lower 87Sr/86Sr values from the Silurian/Devonian boundary stratotype is isotopic contamination of the marine carbonates by basaltic material. Presence of different amounts of basaltic material from submarine volcanism is a typical feature of carbonate beds at the Silurian/Devonian boundary stratotype (Hladil, 1992). However, a lack of any correlation among the strontium isotopic compositions and the amount of volcanic material of each sample (estimated from the whole-rock chemical compositions) testifies against such an explanation. On the other hand, good linear correlation of all 87 Sr/86Sr values and their stratigraphic positions (Fig. 3) seems to be consistent with the first explanation. Thus, slightly lower 87Sr/86Sr values for the global Silurian/Devonian boundary stratotype indicates a nonlinear configuration of the seawater 87Sr/86Sr curve in the Pridolian–Lochkovian interval caused by presence of a
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sition of whole Silurian part of the boundary bed (no. 20) may be considered to be constant. However, repeated measurements of strontium isotopic composition of marine carbonates from the first Devonian layer (uppermost part of the bed no. 20 with Monograptus uniformis) yield a much higher radiogenic value [0.70881(1)] that differs significantly from the approximate linear course of all 87Sr/86Sr values coming from the global boundary stratotype (Fig. 3). Repeated measurements on two rock samples from the first Devonian layer gave identical 87Sr/86Sr values: 0.708806(13) and 0.708811(16) (NBS 987 value was 0.710244 ± 0.000007). For this reason we believe that the latter values were measured correctly and represent a strontium isotopic composition of carbonates from the uppermost part of the bed no. 20 with M. uniformis. 6.2. Chemical composition Fig. 3. Evolution of the δ18O and δ13C isotope values [‰ PDB; data by Hladiková et al. (1997)] compared with the new 87Sr/86Sr data. Error bars for 87Sr/86Sr refer to ± 2σ values. Note the sublinear distribution of measured samples. The prominent spike in the Devonian part of the S/D boundary bed (20-beta) has been confirmed by repetitive measurements. Évolution des valeurs des isotopes de l’oxygène 18 et du carbone 13 [% PDB, données de Hladiková et al.(1997)] comparée avec les nouvelles données du rapport 87Sr/86Sr. Les barres d’erreurs correspondent à 2σ. Noter la distribution sublinéaire des échantillons traités. Le pic principal dans la partie dévonienne du banc de la limite S/D (20-beta) a été confirmé par des mesures répétées.
high-order oscillation. However, additional investigation of this feature, as well as comparison with datasets from other outcrops is needed.
6. The first Devonian layer 6.1.
87
Sr/86Sr composition
Detailed description of the Silurian/Devonian boundary bed (no. 20) including its lithology as well as microfacies data may be found in papers of Hladil (1992). Four rock slabs for polished sections were selected from the Silurian part of the bed. Their stratigraphic distances were approximately identical. One rock slab was selected from the Devonian part of the boundary bed. Later, a second rock slab from the Devonian part of the same bed was selected from a different field sample and used for repeated 87Sr/86Sr analysis. Strontium isotopic and chemical compositions of the samples from the Silurian/Devonian boundary bed (no. 20) are plotted in Fig. 4A. Four of these samples, belonging to the uppermost Silurian, show a very small scatter in isotopic compositions (from 0.70866 to 0.70868) with an average value of 0.70867(1) (Fry´ da and Vokurka, 1996). Thus, on the basis of statistical tests the 87Sr/86Sr compo-
Chemical analyses of dissolved marine carbonates used for 87Sr/86Sr analyses and leached from the fine-grained carbonate aggregates (see Materials and methods) of the Silurian/Devonian boundary bed yielded relatively uniform results (Fig. 4B). The molar contents range from 97.6 to 98.1% of CaCO3, from 0.55 to 0.58% of FeCO3, and from 0.078 to 0.088% of MnCO3. Only the MgCO3 content varies from 1.00 to 1.37 molar percent and its values form a nearly symmetrical pattern with the highest values at the marginal part of boundary bed and the lowest in the middle of bed (Fig. 4B). The strontium content ranges from 0.110 to 0.121 wt.% Sr (i.e., from 0.274 to 0.302 mol % of SrCO3) within the Silurian part of bed and it is slightly higher (0.144 wt.% Sr or 0.361 mol % of SrCO3) in the first Devonian layer. The rubidium content in all analysed solutions was below the limit of detectability (< 2 ppm, ICP). The chemical composition of analysed carbonates from the Silurian/Devonian boundary bed fits with known ranges of chemical composition in the marine carbonates (Brand and Veizer, 1980; Veizer, 1983). In addition, the chemistry of these carbonates is in close agreement with sample selection criteria for 87Sr/86Sr analysis (i.e., contents of Sr > 750 ppm and Mn < 350 ppm). There is no significant difference in the chemical composition of leached carbonates from the Silurian/Devonian boundary bed that could explain the increase of radiogenic value of the first Devonian layer. The whole-rock chemical composition of five samples from the Silurian/Devonian boundary bed (no. 20) shows a nearly symmetrical pattern (Fig. 4C–E) reflecting its carbonate content. The Silurian samples of the boundary bed have different whole-rock compositions but constant strontium isotopic composition. On the other hand, the Devonian sample from the uppermost part of the same bed has a whole-rock composition similar to the lowermost Silurian sample of the boundary bed, but much greater radiogenic 87 Sr/86Sr ratio.
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Fig. 4. Isotope and chemical compositions of the S/D boundary bed 20 at the Klonk GSSP. A. 87Sr/86Sr values of analysed marine carbonate; error bars for 87 Sr/86Sr refer to ± 2σ values; B. Chemical compositions of marine carbonate used for 87Sr/86Sr analyses; carbonates were microsampled from polished sections and leached in ultra clean, weak (0.5 M) acetic acid; C–E. Chemical compositions of whole rock (WR) samples from which analysed marine carbonates were microsampled; analytical errors equal to or are less than symbol sizes; C. Rare earth elements (µg/g); D. Minor elements (wt.%); E. Major elements of the rocks (wt.%). Compositions isotopiques et chimiques du banc 20 de la limite S/D à Klonk (GSSP). A. Valeurs de 87Sr/86Sr des carbonates marins analysés. Les barres d’erreurs du 87Sr/86Sr correspondent à 2σ. B. Composition chimique des carbonates marins utilisés pour les analyses du 87Sr/86Sr, les carbonates ont été microéchantillonnés d’après les sections polies et filtrées avec de l’acide acétique. C–E. Composition chimique de la roche totale dans laquelle les carbonates marins ont été microéchantillonnés. Les erreurs analytiques sont équivalentes à la taille des symboles. C: terres rares (µg/g) ; D: éléments mineurs (wt.%) ; E: éléments majeurs des roches (wt.%).
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7. Discussion Different 87Sr/86Sr values of the analysed carbonates from the Silurian and Devonian parts of the boundary bed no. 20 could result from (1) post-depositional alteration, (2) different chemical or/and mineral compositions of the first Devonian layer, and (3) higher order oscillation in 87Sr/86Sr composition of seawater. However, petrological, mineralogical, chemical, and stable isotope data of samples from the Silurian/Devonian boundary bed (Hladil, 1992; Hladiková et al., 1997, and data therein) did not suggest different post-depositional alterations of the Devonian and Silurian parts of the boundary bed. In addition, our detailed microscopic and chemical investigation of the studied samples as well as stable isotopic compositions of the identical samples [data by Hladiková et al. (1997)] do not reveal any trace of the different postdepositional alterations of the Devonian and Silurian parts of the boundary bed. Nevertheless, such a possibility can never be definitely excluded. Much greater radiogenic value of 87Sr/86Sr (0.70881) of the first Devonian layer could be a product of a primary higher Rb/Sr ratio. A difference at least of about 0.025 in the Rb/Sr ratio between Silurian and Devonian samples is needed for time of 400 Ma (i.e., from beginning of the Devonian to Recent), in order to explain the observed difference in 87Sr/86Sr values as a product of different Rb/Sr values. Moreover, we have to presume that all 87Sr from Rb-bearing phase(s) was dissolved into the analysed solution in the Devonian sample, but none was dissolved in Silurian samples. Whole-rock Rb2O/SrO ratio of these samples ranges from 0.120 to 0.131 for the Silurian part of the boundary bed and it is 0.126 in the first Devonian sample. Thus, there is no significant difference in the Rb2O/SrO ratio among Silurian and Devonian whole-rock composition. We also analysed rubidium content of all solutions containing dissolved carbonates for analysis of 87 Sr/86Sr composition and the Rb content of none of them was above the detection limit of (< 2 ppm, ICP). Thus, results suggest a very limited dissolution of Rb-bearing phases in the analysed solutions. Because of the similar mineralogy and whole-rock composition of all samples from the boundary bed, it is difficult to consider a quite different dissolution mechanism for these samples. The strontium isotopic compositions of the Silurian samples fit well with the expected seawater composition (see above). These data suggest no or very low influence of noncarbonate phases on the measured 87Sr/86Sr values. Another possible explanation for the much greater radiogenic value of 87Sr/86Sr (0.70881) in the first Devonian layer is a deposition of clastic material with a primary higher value of 87Sr/86Sr that could equilibrate during diagenesis with the carbonates of the first Devonian layer. Such hypotheses are difficult to test, but there are several microfacies observations that could support it. As shown by Hladil (1992), the Devonian base with the first M. uniformis
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corresponds to a semi-lithified surface, a short break in deposition, and a change in direction of bottom currents. The higher radiogenic spike of 87Sr/86Sr at the first Devonian layer may also represent a product of a primary higher order oscillation in 87Sr/86Sr composition of seawater. Such oscillations commonly occur on the Silurian and Devonian 87Sr/86Sr seawater curve [e.g. Ruppel et al. (1996); Diener et al. (1996); Azmy et al. (1999)]. However, as shown by Diener et al. (1996) the length of time required to change the strontium isotopic composition to that of present-day seawater in magnitude of ∼10–4 is about 105–106 years. Accepting the possibility that the first Devonian spike in the 87Sr/86Sr ratio represents the higher order oscillation in the strontium isotopic composition, we have to consider that the break in sedimentation (i.e., any event of non-deposition or partial erosion) between last Silurian and the first Devonian layers was in the magnitude of 105–106 years. To consider a shorter time for such a sedimentation break, we would have to presume a much higher riverine 87 Sr/86Sr input or a much greater difference in the 87Sr/86Sr ratio between marine and riverine waters during Silurian/Devonian time. Both of the latter assumptions seem to be improbable. Moreover, Hladil (1992) estimated a break in sedimentation between last Silurian and the first Devonian layers at the global boundary stratotype in the order of 102–103 years. Such a time interval is too short for the change of the seawater strontium isotopic composition from the uppermost Silurian value of 0.70867 to the value of 0.70881 for the first Devonian sample. If we accept the possibility that the first Devonian spike in 87Sr/86Sr ratio represents a higher order oscillation in the strontium isotopic composition of seawater, we have to accept that the sedimentation break between last Silurian and the first Devonian layers at the global boundary stratotype exceeded for more than 102–103 years.
8. Conclusions We encourage the methodology of measuring 87Sr/86Sr compositions of the marine carbonates from stratotypes as well as from the boundary intervals of the adjacent biozones for refinement of the time scale derived from the known seawater Sr isotope curve (Veizer et al., 1999). The first 87 Sr/86Sr analyses of samples from the global Silurian/Devonian boundary stratotype illustrate the potential to use the marine carbonates from the whole-rock samples for such studies. Our data also have revealed partial timing and composition problems of the global Silurian/Devonian boundary stratotype. The greater radiogenic 87Sr/86Sr value of the carbonates from the first Devonian layer (uppermost part of bed no. 20) may be explained by (1) a deposition of material with a primary greater value of 87Sr/86Sr that during diagenesis reequilibrated with marine carbonates, or (2) by a primary higher-order oscillation in 87Sr/86Sr composition of seawa
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ter. The presence of a break in sedimentation (hiatus or maybe partial erosion) between last Silurian and the first Devonian layers as well as a change in direction of bottom currents during sedimentation the first Devonian layer observed by Hladil (1992) are not in conflict with any of these explanations. The first of these explanations seems to be the more probable. However, the acceptance of such hypothesis (i.e., deposition of foreign material with more radiogenic 87Sr/86Sr ) would indicate that at least part of the material composing the first Devonian layer (bed 20-beta) at the global Silurian/Devonian boundary stratotype is recycled sediment of unknown age and provenance. Thus, it may be slightly older than the underlying uppermost Silurian layers. For this reason, possible separation of allochthonous and pelagic components in the 20-beta bed will be useful for any future high-resolution correlation studies based on the chemistry or micropaleontology of the global stratotype point.
Acknowledgements The research was made possible by grants 205/98/1454 and 205/01/0143 from the Grant Agency of the Czech Republic. The structural research and development program CEZ Z 3-013-912 supported the geological studies. We also wish to acknowledge W. Oschmann (Frankfurt am Main), A. Eisenhauer and F. Böhm (Kiel) for their helpful, critical reviews of this paper.
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