Strontium isotope stratigraphy of the Middle Devonian: Brachiopods and conodonts

Geochtmicn

et Cwmochtmica

Pergamon

Acta, Vol. 60, No. 4, pp. 639-652, 1996 Copyright C 1996 Elsevier Science Ltd Printed in the USA. All rights reserved

Oil I h-7037/96

$ I S.00 + .OU

0016-7037( 95)00409-2

Strontium

isotope stratigraphy

of the Middle Devonian:

Brachiopods

and conodonts

ANDREAS DIENER, ’ STEFAN EBNETH, ’ JAN VEIZER, ‘J and DIETER BUHL ’ ‘Institut fiir Geologie, Ruhr Universitit, 44780 Bochum, Germany ‘Derry/Rust Research Unit, Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada (Received

January

19,

1995; accepted in revisedfmm

November

I, 1995)

Abstract-A set of 145 strontium isotope ratios, based on skeletal components of Emsian to Frasnian age, reveals a detailed structure of the seawater X7Sr/XhSr curve that can ultimately serve as a tool for “high resolution isotope stratigraphy” in the Devonian. Brachiopod and conodont samples were collected in the Eifel region of Germany, the area around the global stratotype for the Middle Devonian, with an apparent average temporal resolution in the lo5 y range. Preservation of the brachiopod shell material has been assessed by optical microscopy, SEM, and cathodoluminescence and only the better preserved internal ( “secondary”) layer of the shell has been utilized for strontium isotope measurements. The X7Sr/XhSr curve shows a short decline from 0.7081 to 0.7078 through the late Emsian, followed by short-term fluctuations with an amplitude of up to 10m4 around a mean of -0.7078 during the Eifelian and Givetian, and a rise from late Givetian into the Late Devonian. The above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records, due to erosional and nondepositional events, but a direct confirmation for such a scenario can be claimed only for the latest Eifelian oromari-event. Insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes. Conodont skeletal elements, even if well preserved at CA1 < 2.5, yielded results that are mostly enriched in radiogenic ‘%r, if compared to coeval brachiopods. Although this enrichment is usually within the lo-’ range, conodonts, despite their superior stratigraphic resolution, should be utilized only as a material of second choice in cases where brachiopods are rare or absent. 1. INTRODUCTION

simultaneous odonts.

The strontium isotopic composition of Phanerozoic seawater (Peterman et al., 1970; Veizer and Compston, 1974; Burke et al., 1982) is being increasingly utilized as a dating and correlation tool. The reliability of such dating and correlation depends on the fineness of the biostratigraphic dating for samples that define the primary calibrating curve and on the ability to select well preserved and uncontaminated samples. Since these requirements are best met in geologically young materials, it is not surprising that most ‘chronological applications have been restricted to the Cretaceous-Cenozoic, which was also an interval showing a steep increase in the “Sr/?3r of seawater (cf. the reviews of De Paolo, 1986; Elderfield, 1986; Veizer 1989; Smalley et al., 1994; McArthur, 1994). Comparably steep slopes in the seawater strontium isotope curve also exist for other intervals of the Phanerozoic (Burke et al., 1982) and theoretically, these could also be exploited for correlation purposes. The constraining factors are: ( 1) the large uncertainty in assigning absolute ages (cf. Harland et al., 1990) to particular horizons and the difficulty of stratigraphic correlation based on paleontological criteria and (2) the availability of suitable samples for construction of the reference curve. Brachiopod shells and conodonts are believed to represent the most suitable study material, particularly for the Paleozoic era, because of their abundance, stratigraphic utility, and low-Mg calcitic and phosphatic shells. In order to attain the best possible biostratigraphic framework, we have selected the Eifel region of Germany, the stratotype for the Middle Devonian, for a detailed study of the strontium isotopic composition of seawater. Apart from its high biostratigraphic resolution, this sequence enables also

high density sampling of brachiopods

1.1. Geology and Stratigraphy

and con-

of the Eifel Mountains

The Eifel Mountains, spanning the German/Belgian borderlands, contain the classical stratotypes of the Middle Devonian. These sections are particularly suitable for studies of the isotopic evolution of coeval seawater, due to a long-standing research effort that has resulted in a relatively well defined bio- and lithostratigraphy and due to abundance of brachiopods and conodonts. The modem synthesis of geology and stratigraphy was first published by Krijmmelbein et al. ( 1955) and later revised by Struve (1961, 1976, 1978, 1992). The studied region, about 80 km SSW of Cologne, covers the northeastern section of the Eifel and is characterized by alternation of northeast-southwest (NE-SW) trending saddles and troughs (Fig. 1 ) The former comprise mostly Lower Devonian sandstones and mudstones, while the troughs contain mostly Middle to Upper Devonian carbonates. In addition, the NE-SW trending mid-Eifel ridge represents a structural high separating the northern from the southern troughs. The details of stratigraphy, geology, and sampling, the latter confined to the Dollendorf, Ahrdorf, Hillesheim, Priim, and Gerolstein throughs, are described in Diener ( 1991), Ebneth ( 1991), and Pawellek ( 1991) and listed also in the Appendix. The above are available on request from the Lehrs“Diplomarbeiten” tuhl fiir Sediment- und Isotopengeologie der Ruhr-Universittit Bochum for reproduction and mailing costs. Paleogeographically, the Eifel represents a shelf region of the Devonian Old Red Continent which was situated to the NW of the studied area. This continent served as the source for the Lower Devonian elastic sediments (Figs. 1, 2). Commencing with the 639

A. Diener et al

640

trinite reflection index (R,,, values of 0.6- 1.53), the coalification index and the illite crystalinity index, all suggest a maximal temperature of about 60°C (Teichmiiller and Teichmiiller, 1952, 1979). A composite profile for the Devonian stratigraphy and lithology, based on the thicknesses and centered in the Hillesheim and Priim throughs, is presented in Fig. 2. The local sections (Fig. 1) can be correlated up to a member, and frequently a set, within this composite profile. The superposition of samples therefore represents a correct relative temporal progression. The onset and terminations of the Eifelian and Givetian stages were set by Harland et al. ( 1990), somewhat arbitrarily, at 386 (+5), 382 (+9/- 14) and 377 (ClO.5) Ma, respectively. Theoretically, the sampling density represents, therefore, an average resolution of about 100,000 years.

1.2. SkeletalElementsof Brachiopodsand Conodonts

facies facies

A 6

facies C

sampling points carbonates arenaceous-argiliaceous

rocks

FIG. 1. The studied region in the Eifel Limestone Synclinorium: (1) SGtenich trough; (2) Blankenheim trough; (3) Rohr trough; (4) Dollendorf trough; (5) Ahrdorf trough; (6) Schneifel trough; (7) Hillesheim trough; (8) Gerolstein trough; (9) Priim trough; (10) Salmerwald trough. The facies subdivision A-C for the Middle Devonian is that of Faber et al. (1977) and Faber (1980). The facies type A is dominated by detrital sediments eroded from the Old Red Continent to the NW. The type B consists of platform carbonates with widespread biostromal reefs that have formed at times of periodic uplift at the eastern margin of the studied area. The type C is composed predominantly of marls, carbonates and elastics, with decreasing grain size and increasing carbonate content in the SE direction.

Heisdorf Formation of the uppermost Emsian, carbonates became the dominant lithology. During the terminal Eifelian the reefal growth on the NE-SW trending mid-Eifel ridge was interrupted, resulting in a hiatus, the so-called “Great Gap” of Struve (1982b, 1988). This hiatus may represent about 10% of the entire Middle Devonian sequence in the northern troughs, but was rather short-term in the eastern part of the Hillesheim trough. The subsequent, Givetian sedimentation was characterized by the growth of reefs with their peripheral sediments. In the latest Givetian to early Frasnian these limestones were frequently replaced by early diagenetic dolostones (Richter, 1974). The reefal sedimentation in the Eifel was terminated in the Frasnian. The sediments of the region were subjected to only a minor thermal postdepositional overprint. The vi-

Brachiopods, marine organisms with shells of two valves, are known from about 3000 fossil genera, but only some 100 recent species (Clarkson, 1993 ) The shell of articulate brachiopods is usually composed of low-Mg calcite, but some inarticulate ones have also a phosphatic shell. Recent brachiopod shells contain 0.5-7 mol% MgCO, (Lowenstam, 1961; Morrison and Brand, 1986; Carpenter and Lohmann, 1995). Ancient counterparts were likely of comparable mineralogy, as indicated by Mg-poor shell fragments enclosed in authigenie quartz (Richter, 1972) or by low-Mg calcitic shells that coexist with aragonitic rugose corals in the Carboniferous Kendrick fauna (Brand, 198 1) The inorganic part of the shell for articulate brachiopods consists of the outer “primary” and the inner “secondary” layer. The primary layer is finely granular and has a distinct lineation that is oriented perpendicular to the shell surface. In contrast, the distinct secondary layer consists of elongated calcitic fibers (MacKinnon, 1974). The brachiopod shells are of three types. The impunctate and pseudopunctate shells have no perforations and the latter group frequently has irregularly developed secondary layers. The endopunctate shells, on the other hand, are perforated-in 0.05-0.1 mm intervals-by “channels” that are oriented perpendicular to the shell surface (Fiichtbauer, 1988). These channels can be filled by secondary minerals and such samples are thus susceptible to postdepositional contamination. In addition, some species of articulate brachiopods have hollow spines that perforate the entire shell. In this work, we have studied, therefore, only the shells of the impuctate type. Conodonts are phosphatic remains of an extinct group of chordates (Aldridge et al., 1986; Conway-Morris, 1989) that yield the most detailed stratigraphic resolution of any Paleozoic fossil group. Because of this, they could potentially represent the best material for generation of high resolution strontium isotope curves for Paleozoic seawater. Furthermore, conodont elements are composed of apatite with Sr contents of several thousands ppm (Pietzner et al., 1968; Wright et al., 1984, 1990; Kiirschner et al., 1992) and their original “Sr/ “‘Sr is, thus, relatively difficult to contaminate by extraneous Sr. In addition, some authors (e.g., Kolodny and Epstein, 1976; Karhu and Epstein, 1986) propose that apatite is more stable and more resistant to postdepositional alteration than calcite. Finally, the reconnaissance work on conodonts by

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SAMPLES

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SUBFORMATION

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FIG. 2. A composite standard profile of the Middle Devonian in the Hillesheim trough (after Struve, 1982a). The late Lower Devonian and the Upper Devonian portions are based on the research of Werner (1969, 1973), Krebs (1962), and Clausen (1968) in the Primmer trough. Fm. = Formation; Sbfm. = Subformation; Mb. = Member; Sbmb. = Submember; (Be) Berle-Fm.; (Wi) Wiltz-Fm.; (We) Wetteldorf-Fm; (H) Heisdorf-Fm.; (La) Lauch-Fm.; (Wf) Wolfenbach-Mb.; (Ds) Dorsel-Mb.; (No) Nohn-Fm.; (Zi) Zilsdorf-Sbfm.; (Ki) Kirberg-Mb.; (Ah) Ahiitte-Mb.; (St) Stroheich-Sbfm.; (Dk) Dankerath-Mb.; (Hd) Hundsdell-Mb.; (Ad) Ahrdorf-Fm.; (Bb) Betterberg’Sbfm.; (Bi) Bildstock-Mb.; (K6) KGll-Mb.; (F) Flesten-Mb.; (W) Wasen-Mb.; (Nd) Niederehe-Sbfm.; (Jb) JunkerbergFm.; (Hz) Heinzelt-Sbfm.; (Kb) Klausbach-Mb.; (MS) Mussel-Mb.; (Hb) Hiinselberg-Mb.; (R) Rechert-Mb.; (Gr) Grauberg-Sbfm.; (Ni) Nims-Mb.; (Gs) Giesdorf-Mb.; (FI) Freilingen-Fm.; (E) Eilenberg-Mb.; (Bn) Bohnert-Mb.; (Ab) Ahbach-Fm.; (Mw) Maiweiler-Sbfm.; (HI) Hallert-Mb.; (Lr) Lahr-Mb.; (Mii) Mtillert-Sbfm.; (01) Olifant-Mb.; (Zb) Zerberus-Mb.; (Lg) Loogh-Fm.; (Eo) Eowotan-Mb.; (Wt) Wotan-Mb.; (Rh) Rech-Mb,; (Cii) Ciirten-Fm.; (Fb) Felschbach-Mb.; (Eg) Epger-Sbmb.; (Pg) Perger-Sbmb.; (Mm) Marmorwand-Mb.; (F) Forstberg-Mb.; (M) Meerbiisch-Mb.; (Dm) Dreimiihlen-Fm.; (B) Binz-Mb.; (L) Ley-Mb.; (Gb) Galgenberg-Mb.; (Rd) Rodert-Fm.; (Hs) Hessenhaus-Mb.; (Qu) Quadram-Mb.; (Ci) Cistercienses-Mb.; (Fi) Finiroden-Mb.; (Kp) KerpenFm.; (Bc) Belcor-Mb.; (Rb) Rossberg-Mb.; (Bd) Bolsdorf-Fm.; (Pb) Primibol-Mb.; (Rb) Ramobol-Mb.: (Wh) Wallersheim-Fm.; (0) Oos-Fm.; (Bh) Biidesheim-Fm.; (UK) Upper Kellwasser Horizon; (N) Neuoos-Fm.

100 m

0

LOWER DEVONIAN EMSIAN Be Wi We

1

I

642

A. Diener

Kovach ( 1980, 1981), Keto and Jacobsen ( 1987), and Bertram et al. ( 1992) and the detailed study of Martin and Macdougal ( 1995) support their utility for anticipated research.

2. SAMPLE

PREPARATION, ANALYTICAL AND SELECTION CRITERIA

t

/I

TT

V

,

I

0.7078 0.7080 87Sr/ 86Sr brachiopods “old preparation” FIG. 3. Comparison of *‘Sr/?Sr measurements shells prepared by the “new” as well as the “old” rorbars represent ?20.

0.7088

,

TECHNIQUES,

Samples collected at outcrops were lo-30 cm in size, clear of weathered rinds and macro-fissures. These samples were cut to 15 x 15 x 38 mm blocks that were utilized for preparation of polished sections for cathodoluminescence as well as for chemical and isotopic studies. Initially, the shell material was drilled from the blocks in a manner similar to that described by Popp et al. ( 1986). This, however, resulted in poor-reproducibility and radiogenic strontium isotope values (Fig. 3), caused either by contamination from matrix (Fig. 4) and/or from inclusion of altered shell material. This “old preparation” technique was therefore abandoned. The “new preparation” technique was based on smashing l-2 blocks into 2-3 mm size pieces, followed by cleaning in 250 mL of distilled water, decantation of the fine pat&dates, and drying for 3 h at 50°C. The cleanest shell splinters of the secondary layer, clearly recognizable by their structure of fine fibers, were then picked manually under binocular microscope. All splinters with recognizable micro-fissures were rejected. The selected shell fragments were several times ultrasonically cleansed in distilled water until its cloudiness disappeared. These samples were subsequently utilized for optical and SEM studies. The aliquot for strontium isotopic studies was ultrasonically cleansed in p.a. acetone prior to chemical treatment. The samples utilized for construction of the *‘S#Sr reference curve were very well preserved, judging from optical, cathodoluminescence, and SEM studies (Diener, 1991; Pawellek, 1991). In no case did we observe recrystallization of the shell fibers. Note, however, that this comment relates only to the relatively coarse fibers of the secondary layer. The primary layer, because it was frequently altered and bccause it adhered to the matrix was not sampled in this study. Bruckschen et al. (1995a,b) and Veizer et al. (1996) provide additional documentation of our selection procedures and criteria. For separation of conodonts, 2-6 kg of visually unaltered, washed, and crushed sample has been dissolved in 5% acetic acid. Remains were washed with distilled water and decanted at intervals of 2-3 days. ‘Ihe sieved 80 pm-2 mm fraction was dried at 50°C. Clean conodonts with alteration index (CAI: Epstein et al., 1977; Kijnigshof, 1991) of 1.5-2.5 were handpicked under binocular microscope. For strontium isotope measurements, one to three large, or up to ten small conodonts, all of the same genera were washed in distilled water by ultrasound and dissolved in 2 mL of 2.5 N HCl, in preparation for the ion exchange columns.

0.7082

et al.

0.708641

I

0.7082

for brachiopod technique. Er-

I/0.7077

0.7079 0.7081 a7Sr/ @%r matrix

FIG. 4. Comparison of *?+$‘Sr shells and their enclosing carbonate than the size of the dots.

measurements rock matrix.

0.7088

from brachiopod Error bars are less

For strontium isotopes, the experimental procedure was as follows: 0.5-2 mg of samples were dissolved in 5 mL of 2.5 N supra pure HCI for at least 2 h at room temperature. After evaporation, Sr was extracted via 4.5 mL quartz glass exchange columns filled with Bio Rad AG50WX8 ion-exchange resin and eluted with 2.5 N supra pure HCl. The eluant containing most of the Sr cut was dried down and approximately 150-250 ng of Sr was loaded on single Re filaments using a TaZ05-HN04-HF-H#04 solution (Birck, 1986). Samples were analyzed on a Finnigan MAT 262 multicollector mass-spectrometer. Two standards, the NBS 987 (75 ng load) and the USGS EN-l (CaCO? from a recent TridacImo shell, Enewetak Lagoon, Marshall Island) were measured with every set of eleven samples. The mean of 550 NBS 987 measurements over a 4’/2 year interval was 0.710231 2 3.8 X lo-’ (1 standard deviation; s.d.). 130 measurements of EN-l yielded a value of 0.709145 ? 3.2 X lo-’ (1 s.d.). Note that the error values listed here are not the usually quoted 2u (man, (equals 2 standard error, s.e.). The latter, due to the large number of standard measurements, would result in 1.7 x 1O-6 and 2.8 x 10m6 for the NBS 987 and the EN-l, respectively. The *‘Sr/ ?Sr ratios were not corrected for Rb, since a realistic *7Rb/SSRb fractionation during a mass spectrometer run cannot be calculated. The upper limit for 85Rb was 5 x lo-’ V at a 3-5 V “Sr signal. ?3r/ ?Sr ratios were normalized to “Sr/88Sr of 0.1194. The 2u,,.., error for a single sample was within 6-40 X 10m6 range, with an average of +9 X 10m6. Changes in the efficiency of the Faraday cups, which led to their eventual replacement, resulted in a long-term drift to lower 87Sr/*6Sr ratios for the NBS 987 standard. To compensate for this drift, we introduced a correction term. All samples from a single carousel were reported relative to a NBS 987 value representing an average of three standard measurements, the standard in the set itself and in the preceding and the successive sets. The average error for these three standard measurements was 22.7 X lo-’ (1 s.d.) Each carousel set was then normalized to our long-term mean for the NBS 987 of 0.710231. Previous work on conodonts with variable CA1 (Bertram et al., 1992; Ktirschner et al., 1992) showed that coeval samples with CA1 higher than 2.5 had considerably greater dispersions in their strontium isotope values (l-2 X 10m4) than their better-preserved counterparts with CA1 of l-2 ( 2 X 1O-’ ) In addition, samples with high CA1 usually had more radiogenic values. Taking this into account, we utilized only concdonts with CA1 of 1.5-2.5. Nonetheless, a cross-plot of correlated samples (Fig. 5) shows that even these conodonts are mostly more radiogenic than brachiopods. In general, diagenetic overprint tends to enhance the proportion of radiogenic 87Sr in the samples (cf. Veizer and Compston, 1974). This, the relatively radiogenic nature of the ma&ix (Fig. 4), the small size, and the high surface/volume ratio of conodonts, all indicate that it is the conodonts that suffered a greater degree of resetting of 87Sr/86Sr values. Consequently, the “secondary” layer of brachiopod shells represents material that is more suitable for studies of past seawater composition than the conodonts. We opted therefore for exclusion of all fiftyseven conodont values (Appendix 1) from construction of the seawater reference curve (Fig. 6). Conodonts can still be utilized as a

Sr isotope. ratios of Paleozoic

brachiopods

g 0.7081 $

mented

8 b 2 0.7079 2 cn 6

0.7079 0.7081 87Sr186Sr conodonts

FIG. 5. X’Sr/“Sr of brachiopods and conodonts from the same conodont zone. Most conodonts gave more radiogenic values than the secondary layer of brachiopod shells. Error bars represent the ranges, dots the means of “‘Sr/d%r values from a given conodont zone. IIIX relates to conodont zones listed in Fig. 6.

supplementary material for time intervals with brachiopods rare or absent, but with a proviso that the results may be. altered within the IO-’ range (see Ebneth et al., 1996, for further discussion). The entire set of results is listed in the Appendix I, but, with the exception of Fig. 7, only the measurements of “Sr/‘%r from the secondary layer of brachiopods prepared by the “new technique” are discussed in the subsequent text. Note that our stringent selection approach resulted in reduction of the original set of 300 samples to 145 measured brachiopods and conodonts, but only sixty-seven brachiopods are utilized to define the Middle Devonian reference curve. 3. THE

643

reflect variations in the global strontium isotopic composition of coeval seawater. Alternatively, they could be simply a postdepositional effect due to diagenesis or to contamination by matrix material. Several of the oscillations, such as the one associated with one limb of the otomari-event, are docu-

g

0.7077

and conodonts

MIDDLE DEVONIAN ISOTOPE CURVE

STRONTIUM

The time span studied here includes the uppermost Emsian to basal Frasnian. The Middle Devonian time interval, from 386 ? 5 to 377 ? 10.5 Ma (Harland et al., 1990) spans eight conodont zones (Fig. 6) and represents approximately 9 Ma. Consequently, the average duration of a biostratigraphic zone is about 1 million years. The brachiopod strontium isotope curve indicates a declining trend through the Emsian, a gently declining trend during the Eifelian-Givetian, and a rise from the Late Devonian into the Early Carboniferous (cf. also the conodont data of Kiirschner et al., 1992). Higher order variations, with an amplitude of up to 138 X 10mh and a frequency of -lo’lo6 years, appear to be superimposed on the gently declining Middle Devonian part of the trend. This newly defined curve shows considerable discrepancies with the Middle Devonian trend of Burke et al. ( 1982), but a direct comparison is complicated by the fact that their experimental data are not as yet publicly available and only interpolations from graphical presentations is possible. Nevertheless, the 87Sr/86Sr minimum of -0.7079 in the Burke et al. ( 1982) curve appears to be at the Emsian/Eifelian transition and most of their values are more radiogenic than those obtained in our sample set. We believe that the above discrepancies arise principally from the problems of stratigraphic correlation and resolution and from alteration effects, particularly for the Burke et al. ( 1982) sample set. The question arises whether the observed higher order oscillations of Fig. 6 are significant and, if so, whether they

by multiple

and

internally

consistent

measurements

(Figs. 6, 7) and display an amplitude considerably in excess of analytical uncertainty. We therefore believe that at least some of the higher-order oscillations are a real feature of the isotopic record. Unfortunately, not all can be documented by multiple measurements, due to the fact that suitable shell material is not freely available throughout the studied profiles. Note also that we have sampled a fossil-rich stratotype. It is therefore unlikely that sampling elsewhere will yield better stratigraphic resolution and/or higher density of suitable samples. A recourse to whole rock data in order to fill the available gaps is, in our view, a retrograde step (see Fig. 4). The apparent higher order oscillations (Fig. 6) could result from ( 1) analytical errors, (2) postdepositional alteration of the original seawater signal, and (3) temporal variations in 87Sr/86Sr composition of Devonian seawater. The contribution of single sample analytical error to dispersion of the 87Sr/86Sr values is relatively minor, at -1 x 10e5 (Fig. 7; Fauville, 1995). We have tried to minimize factor 2 by thorough selection and preparation of the best available shell material. Nevertheless, we cannot entirely discount the possibility that the studied shells may still harbour slightly altered isotopic signals, although the comparable carbonate lithology of the matrix through most of the sections is not conducive to variable offsets. The role that factor 3 may, theoretically, play in the generation of such higher order oscillations will be discussed in the subsequent text. In reality, the limitations of the geological record may prove insurmountable for quantification of the relative contributions of factors 2 and 3 to the observed higher order 87Sr/86Sr signal. Detailed studies of multiple samples for a single profile across the Heisdorf and Lauch Formations at the Emsian/Eifelian transition (Figs. 2, 6 ) by Fauville ( 1995 ) and of corellative sections up to 15 km apart by Golks ( 1995) show that the trends and the R7Sr/8hSr values for the well preserved samples are reproducible to within 53.9 x 10e5. Utilizing strictly lithological correlations, it is possible to reduce this value to k3.0 X 10e5, comparable to our “carousel” correction of k2.7 X 10m5. Nevertheless, taking into account all uncertainties that beset correlations, regardless whether based on biostratigraphy, thickness, or lithology, we opted for the larger value of 23.9 x 10e5 as the realistic estimate of geological reproducibility (Fig. 6). The subsequent discussion will therefore concentrate only on spikes in excess of this magnitude. 4. DISCUSSION

Accepting the possibility of the higher order secular oscillations in the strontium isotopic composition of the Middle Devonian seawater, the question arises whether such fluctuations are theoretically feasible. Taking, as an example, a residence time of Sr in the ocean as 2.1 Ma, a riverine 87SrlX6Sr input of 0.711 (Wadleigh et al., 1985; Palmer and Edmond, 1989) and the 87Sr/86Sr of modem seawater as 0.7091, an oscillation of a magnitude of -lo-“ in seawater isotopic com-

t

200 m

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CONODONT ZONES

LOCAL

AGE [Ma]

STANDARD ZONATION

Sr isotope ratios of Paleozoic brachiopods and conodonts

back-reef _-__-__-__-__--_.-__-~~-~~--~--~--~~-~~-~~-~.-~.-~~-~~block-reef

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shallow wate

a-*, . !l!llhb 10 30 5orr

ofomari

events

645

“Great Gap”

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z

0

0.70785

0.70780

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brachiopod “old preparation

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conodont zones ;tandard

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FIG. 7. Details of the “‘Sr/%r seawater curve across the oromari-event (Walliser, 1982) and the “Great Gap Event” (Struve, 1982b. 1988). The curve of relative sea level variations is based on Struve (1988). The position of the ofomarievent in the Eifel after Weddige (1988). (Jb) Junkerberg-Fm.; (Ni) Nims-Mb.; (Gs) Giesdorf-Mb.; (Fl) Freilingen-Fm.; (E) Eilenberg-Mb.; (Bn) Bohnert-Mb.; (Ab) Ahbach-Fm.; (HI) Hallert-Mb.; (Lr) Lahr-Mb. position can perhaps be generated in about lo’-10’ years, principally by radically changing the isotopic composition of the riverine Sr flux (Fig. 8). Yet, the “otomuri drop” of 15 X 10m5, documented by eight measurements in the Eilenberg and Bohnert Members, is almost instantaneous (Fig. 7). A linear extrapolation of the Middle Devonian timescale, based on thickness estimates, mitigates against allocation of the required 105- 10” years time span to this interval. On the other

hand, the shape of the strontium isotope signal appears to reflect the sea level curve, as derived from geological record, with radiogenetic peak coincident with low sea level stand (Fig. 7). Furthermore, the rapid decline in X7Sr/XhSr appears at the time of the previously discussed “Great Gap” erosional event postulated by Struve (1982b, 1988). For all these reasons, we believe that the time interval represented by the sudden s’Sr/%r drop is much longer than is indicated by the

FIG. 6. The “‘Sr/%r seawater curve for the Middle Devonian based on the brachiopod shells from the Eifel Mountains. The bar represents 22~ for a single measurement (Appendix I). Standard conodont zones after Weddige (1977, 1988) and Grimm and Rothausen (1992): I. inversuslaticostatus; II. serotinus; III. pat&s; IV. partitus; V. costatus costatus; VI. kockelianus australis; VII. kockelianus kockelianus: VIII. ensensis (with arkonensis); VIIYa. ensensis-obliquimarginatus; VIIyb. ensensis-bipenatus: IX. hemiansatus; X. varcus: XI. hemanni-cristatus; XII. disparijis; XIII. falsiovalis; XIV. transitam: XV. punctata; XVI. hassi; XVII. jamieae; XVIII. rhenana; XIX. linguijormis; XX. triangular-is; XXI. crepida. The position and extent of the “gap” in the record of marine Middle Devonian after Struve (1982a). This gap either lacks lithic record or may be represented by lagoonal sediments, evaporites, and/or red beds. Note that in the Eifel the gap may vary between zero and its maxima1 extent that is denoted by dark shading. The global bioevents and transgression/regression (T-R) cycles for North America and Europe (Krebs, 1979; Johnson et al., 1985; Talent et al., 1993) are based on biostratigraphic criteria, particularly conodont biozonation (Walliser, 1985; Weddige, 1988). For the Eifelian interval, the following events have been postulated: the jug/en’ (Chlup%, 1982; ChlupiE and Kukal, 1986; Walliser, 1985), the “Ohle shale deepening event” (Krebs, 1979; Johnson et al,, 1985), and the otomari (also rouvillei or KaCak) event (Walliser, 1982; House, 1982, 1985). Because of their uncertain stratigraphic position the Givetian events, particularly the pumiUio event (Lottman et al., 1986) and the Pharciceras (Walliser, 1982, 1984) or Tonganic (House, 1985) event, are not plotted in this figure.

646

A. Dienet

0.7100 -----_ -

( B7Sr/86Sr)R,= J&

= 2.73

a-’

f 0.5 x 1O"g

L 0.7096 cn 22

‘s

0.711 f 0.003

,/

/

:

:

:

:

,/ :

2’ ,/

/’

___________.. ---------- ---*----....___ --.. --..

(no.7092 k

k

- -.,

%. ‘. ‘. ‘.

om70881 10 000

100 000 time [ya]

FIG. x xX

1000 000

8. The length of time required to change the strontium isotopic composition of present-day seawater by I X IO-“, assuming an instantaneous departure in the river flux of Sr equal to kO.5 X JO” ga-’ from its present-day value of 2.73 JO” ga-’ (solid lines). The second calculation (dashed lines) assumes an instantaneous departure in isotopic composition of 3 1 O-‘, from a value of 0.7 11. for the constant riverrine flux of 2.73 IO” ga-‘. The calculations are based on parameters, variables, and algorithms in Goldstein and Jacobsen (1987).

et al.

thickness of the corresponding strata. If so, the EilenbergBohnert sedimentary package must encompass hiatus(es) of unspecified duration. It is possible that other comparable sharp isotopic excursions (Fig. 6) are of similar origin, but the correlations with transgressions/regressions (“sea level” stands) and with bioevents are as yet equivocal. This is because the latter were proposed from geographically distant domains, with stratigraphic correlations for the Paleozoic reliable up to a biozone, that is to about 1 Ma. Since the sedimentary record, at any one profile, is discrete and not continuous, the juxtaposition of phenomena that occur at frequencies of < 10” years is beset by difficulties. We do not discount entirely the validity of such “high resolution global” correlations, but only wish to point out that the approach poses a danger of circular reasoning (cf. also Miall, 1992). We therefore suggest that the application of chemostratigraphy should be confined, at least in its nascent stages, to basinal scales, where controls by lateral reckoning are still possible. On such scales, the isotope stratigraphy may potentially surpass even the best biostratigraphy. For interbasinal to global scales, on the other hand, the biostratigraphy dictates the limits of resolution and correlation capabilities and thus the width of the X7Sr/X6Sr reference curve (Fig. 9). The correlation potential of the latter is thus considerably reduced. 5. CONCLUSIONS Theoretical considerations suggest that variations in the strontium isotopic composition of Phanerozoic seawater can

0.7081 CD clo

0.7080

0.7078 number of measurements

10 !5

Conodont zone

Age [MaI

Fordham 1992

i

FIG. 9. The means and ranges of X7St/XhSt values for brachiopods zones listed in Fig. 6.

from a single conodont

zone. I-XIX

are conodont

Sr isotope ratios of Paleozoic brachiopods and conodonts be potentially utilized as a high resolution dating and correlation tool, providing the s’Sr/?Sr seawater curve can be constrained into a sufficiently narrow band. The major limiting problems are ( 1) the availability of suitable material that reflects the “Sr/%r composition of coeval seawater and preserves it during its postdepositional history and (2) the uncertainties in time resolution and correlation that are imposed by the existing geological record. For the Paleozoic, the lowMg calcitic secondary layer of brachiopods can serve as a suitable study material. In order to overcome the problems of correlations, we have densely sampled a composite profile of the Middle Devonian at its stratotype in the Eifel Mountains. The resulting s’Sr/“%r curve shows a decline from -0.7081 to -0.7078 throughout the late Emsian, fluctuations around the latter value throughout the Eifelian and Givetian, and a rise in the radiogenic component from the late Givetian into the Late Devonian (and Early Carboniferous) The EifelianGivetian isotope oscillations have an amplitude of up to 10 m4 range. Locally, as for example across the terminal Eifelian “Great Gap” event, these oscillations coincide with “eustatic sea level” changes, suggesting that at least some spikes may result from the condensation of sedimentary record by erosional and non- or low-depositional events. This may be the case also for the other short-term isotopic spikes, but the corroborative geological evidence is equivocal due to correlation problems at temporal resolutions of < I Ma. Acknowledgmenfs-This work has been supported financially by the Deutsche Forschungsgemeinschaft (Grant Ve 112/l-3). We acknowledge technical support of B. Razcek and R. Neuser, logistic support in the field by F. Pawellek, discussions with W. Kiirschner and P. Copper, review by E. E. Martin, and donation of thirteen conodont samples from the stratigraphic collections of the Forschungsinstitut Senckenberg in Frankfurt by K. Weddige. A. Fauville, C. Golks, and H. Strauss permitted quotation from their recent studies of geological reproducibility. Editorial

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F. A. Podosek REFERENCES

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Sr isotope

ratios of Paleozoic

brachiopods

and conodonts

649

APPENDIX Description of samples and analytical results. For details of stratigraphic sections and localities (throughs) see Figs. 1 and 2. Conodont zones I-XXI as in Fig. 6. Material abbreviations are the following: bo-brachiopod: old preparation; bn-brachiopod: new preparation; c-conodonts; cS-conodonts donated by K. Weddige, Senckenbergische Naturforschende Gesellschaft, Frankfurt a. M., m-matrix, and f-fish teeth. Age assignment is based on linear thickness interpolation and on Fordham ( 1992) timescale. Sample 145

No.

Location

Localstratigraphy Wiltz

Fm.

R2529060/H5561800

Trough

Conodontzone

Assigned age [Ma]

Prom

inversus-la~jcosfatus

387.83

bn 0.708057il3 ho 0.708141

387.73 387.63

bo 0.712734i22

387.53

bn 0.708044ilO

387.18 0

bn 0.707890 bn 0.70787OilO

108a

,I

108b

,!

147 148

,!

41

,!

Heisdorf

Fm

Ahrdorf ,,

41

!I II

41 41 41 We541/6-22 DP?6-17 45

Wolfenbach

Mb. II

45 M45

R25498001H5585440 II

1

Wolfenbach

Mb.

Priim I, II

44 Dorsel ,!

I@

112 Weilersbach

Set II

57 55 Schleit

Set !,

36 56

R*556930/H5578120 I,

59 37

SchmitzbachSet

60

Erdel

Set

R25558201H5575310 I,

!I

60 60 61 62 P 719-728/D P 719.728lD

Hunnertsberg Dankerath

Set Mb. " !I

R25568521H5578105 R25568401H5578310 R25322101H5556870 0

bo 0.707847iO9 0.708086il7

c

0.708066i17

385.50 ,!

bn 0.707777*11 bo 0.707866ilO bo 0.707883i23

385.50

bo 0.707843iO8 bo 0.707808i14 bo 0 707933il6 bo 0 708641il2 bo 070814O-tlO CS 0.707875iO9

385.20

bn 0 707872ili

Gerolstein

385.25

bn 0.707839i23

Hillesheim I! !I

385.07

bn 0.707825fll c 0.708022i.6

II

384.93

II

4, !I

i 11

385.70

3, 4,

c

0 707992i25

m

0.708316il2

bo 0.707863izl3 bo 0.707865il6 c 0.707953*20

,I II

M36

f 09

cs 0.707949 bn 0.707833i18 0.708711i13

I,

M57

cs 0.707990

c

II

Mb.

m

0.708143*10

384.82 II

bn 0.707856i15 bo 0.708875zt.9

384.70

bo 0.707869*09

(I

c

0.707917*24

,I

c

0.707976zt15 0.707986i41

,,

384.64

c

I!

384.52

bn 0.707817*14

Prom ,I

iO8

0.708159i18 0.708224i22

m

I,

44

36

385.90 II

0

1,

44

30

CS 0.708009i14

,! I,

44 44

We601 43

,I

Priim II

44

44 44

c c

Dollendorf ,I II

fl5

bn 0.708031iO8

II

Ahrdorf

106

“Sr18%r * 2omdX10*)

bo 0.707882ilO

Priim 3,

DP27

5 i 2

CS 0.707804*15 cs 0.707909

f 13

A. Diener

650

Appendix

Sample

No.

Localstratigraphy

Location

(Continued)

Trough

46

Dankerath

Mb.

R2554580/H5583150

Ahrdorf

52 52

Hundsdell

Mb.

R25545001H"580640 II

Hillesheim ,,

87

Bildstock

Mb. 11

20

Rz555390/Hs575360 II

20

II

20 20

II II

F20 11

1,

92 13

Rz5543501H=74790 Kbll

13 47

Mb. II

R=54530lH=74720 II

II

II

Flesten

12 12

Mb.

ID Wasen

27

Niederehe Niederehe

Klausbach

Mb. II II

48 Mussel

Mb.

Hdnselberg

Mb.

IO

H=74755

R2551160/H5576200 II R'=543801H5=74780

Recherl

Mb. II

21 21a 21a

Nims

Set1

82D 109

Nims

Set2

Nims

Set3

82E 76

Nims

Set5

143

Eilenberg

Mb.

costafuus

11

I australis 0

[MaI

384.08 II

bn 0.707873il

II

8, II

II

383.99

II I,

383.82 !, II 383.61

41

383.59 0,

11 8, I,

11 383.54

II

383.41 I,

1, 1,

383.22 II

II I!

II

81

382.83

bo 0.7082OOi13 c 0.707880*12 c

0.707992*24

f c

0.708720*17 0.707957*16

bn 0.707784il bo 0.707928iO8 c 0.707968*15 bn 0.707887i21 bn 0.707859*12 bo 0.707878*13 c 0.707911 bn 0.707841 bo 0.707786+10 c 0.707891 bn 0.707820*19 c

0.707928*17

bn 0.70785OilO

Hillesheim

382.38

bn 0.707821iO8

R2532450/H5561090

Priim Hillesheim II

382.05 I,

bn 0.707828i13 c 0.707863*13

41 II

381.85 ,I

II

I,

f c

0.708480f13 0.707885*13

c

0.707892

c

0.707836*20

II

II

381.78 II

41

II

381.71

c 0.707843i.19 bn 0.707821

R2532480/H5561080

Priim

381.61

bo 0.707816*12

R2555310/H5576400

Hillesheim II

381.15 381.05

c 0.707851 bn 0.707818iO8

II II

kockelianus ensensis ,I

I ensensis

380.88

,I

II II

II

11

I,

142

1,

II

380.85 I,

141

II

II

11

13

f 09

c 0.707928+13 bo 0.707871i12

R=54720/H=75440

R=50960/H=75660 8,

f IO *07

CS 0.707914i16

II

Set1

bo 0.708016*09

382.74

97

Bohnerl

0.708018i25

Ahrdorf

32 32 96

c

R2555180/H5582020

R25553101H5576400 II II

kockelianus

87Sr/%r

* 2~,.,WOd)

bn 0.707885*09 bn 0.707925*09

II

australis II

=

384.38 384.18 II

II I, I, II

z 2

Hillesheim II

R=54495/H5=75575

Mbg.3444

II

Assigned age

R25546501H5578110 II

R2554020/H5574700 II

FIO 21

R2=54460/

R25551101H"575780 II

II

7b/l-2 75 110

Set2 II

88E 88E 35

Set1 II

27 89

Mb.

costatus II

II

R2555830/H5582140

12 90

I,

Conodontzone

II

R=54480/H=74730 II

I,

et al.

380.86 II $8

f IO

*15 k22

bn 0.707941

*12

CS 0.707881

*I5

bn 0.707920*20 bo 0.707909*10 bo 0.707933i18 bn 0.707847i12 bn 0.707858+12 bn 0.707827*13

Sr isotope

ratios

of Paleozoic Appendix

Sample

No.

144

Local

stratigraphy

Bohnert

14

Eohneri

M34 34

Bohnert

83

Location

Set 2

R%0960

Mb

Lahr

84 39

Mb.

Olifant

11

66 65

Zerberus

25

Eowotanium

Wotan

P242-238/D

115

R=54830

Wotan 62/D

Wotan Wotan Rech

113 18

Felschbach

74

Mannorwand

380.34

bo 0.707916

f 11

II I#

bo 0.707837 m 0.707913

,I

380.30

c

f 11 f 11 f 37

01 ,I

It 380.27

c 0.707903 bn 0.707837

II

380.25

bn 0.707822

II II

380.08 380.00

bn 0.707772 bo 0.707787

!I

II

bo 0.707792

f 10 f 13 f 11

ID

!I

c

II

379.94

I H=76590

Hillesheim

top

R’“32320

I H=57250

Priim

Set 8 Mb.

R=54830 R==54870

/ H=76590 I H=‘57730

R*r53220

I H=74950 11

Hillesheim Hillesheim 8,

Mb.

I H=72820

R=52770

I HE574450

R=51900

/ H=73130 II

17 17 Ml7 Cur 6-8

II 1, II I, ,I

70

Quadram

114

R=53970

/ H=76020 II

Mb. II

70 6 7

Mb.

Cistercienses Belcor

Mb. Mb.

R2553200

0.707812

f 12

c

0.707829

f 12

bn 0.707751

f 11

bo 0.707850 bo 0.707817 m

II 378.90

f 14

378.70

bn 0.707787

f 19

I# 378.60

c 0.707852 bn 0.707794

f 14 f 08

I, 1,

bo 0.707851 bo 0.707828

f 12 f 10

II

378.50

bn 0.707800

f 09

n

378.19

bn 0.707794

f 13

I,

378.14 II

bn 0.707778 c 0.707850

Hillesheim 11 I,

Hessenhaus

f 33

bn

379.04

f 15

I H=‘76130 !I ,,

69

0.707887 0.707879

0.707844

Prum 8,

! H”575930

f 13

bn 0.707793 bn 0.707892

c

I H=65590 11

R=54020

f 12

CS 0.707877

378.78

R’r35060

Mb.

0.707825

c

II

Set

Ley

f 10

bn

0.707871

Niesenberg

3

0.707949

cS 0.707871

105 105

2

f 22

cs

bo

I,

I Hc574310

II

f 18

1,

II ID

R=51650

R%4200

0.707832

bn 0.707819

#I

II / H=74310

f 36 f 09 k 12

f 14 f 13 f 16

Set

Mb.

varcus 0,

0.707882

f 13 f 15 f 08

9,

Entenbach

Binz

hemiansatus I,

379.65 379.55

8,

16

73 2

379.85

,I 01

R=51650

0.708010

II II

R%4830

R=48645

m

II

Set 3

Mb.

f 30

f 12 f 12

Priim

19

f 11

0.707910

II

I H=57210

380.72

bn 0.707826 c

f 19

R=32335

II

19

1 H=76590 II II #I

Set 1

Wotan

85 86

Mb. II

85E 134

PI 72-l

I, II

Set !I

380.84

87Sr/ “Sr f 2om..“,w 0’)

c 0.707917 bn 0.707613

I H=77940 I!

64

=

bn 0.707948

R”54920

01 #I

WI

380.61 380.46

/ H=76490

Set 2

z 2

380.69

R=55060

II

ensensis ensensis I hemiansatus hemiansatus 8, ID

Assigned age

f 10 f 12

11 I,

II

zone

bo 0.707814

I H=78480 II

M39

Conodont

,e

0

39

25

Hillesheim

!,

R=55090

II

63 63

Trough

Set 7

!I

651

and conodonts

(Continued)

Mb

Hallert

23

! H=75660

brachiopods

,I

II

ID

II

II

0.707857

f 13 f 10 f 12

I H=75520 I!

II

377.86

bn 0.707751

f 18

II

377.66

bo 0.707736

f 07

c

652

A. Diener Appendix

Sample

No.

Local

stratigraphy

Location

et al. (Continued)

Trough

Conodont

zone

Assigned age

3 5

WI 7

Belcor

79

Roi3berg

79

Mb.

Rz553200

Mb.

R=53120

II

GA 1

01

Wallersheim

29E F29E

00s

I H=75520 I H-76660

Fm. Fm.

R2541 000

I H5565000

R2540730

I H=66940

varcus I,

377.66 377.53

c c

0.707839 0.707882

f 16

II

It

8,

c

0.707854

CS 0.707969

f 15 f 15

371.81

c

0.708052

f 32

II 369.56

f 0.708628 bn 0.708092

f 18

Priim II

disparilis

- punctata !!

104

103 102

00s Bijdesheim II Up.Kellwasserkalk

Fm.

87Sr/86Sr

* 2fJ,,..,w 03

Hillesheim 1,

104 Stbr. 103

a

,I ,I

c

0.708050

cS 0.708118i

Rz540420

I Hs564600 ,,

” II

linguiformis II

367.61 ,a

c c

0.708142 0.708019

R2541880

/ Hs566440

”

triangularis

367.45

c

0.708207

f 23

f 12 i 27 12 f 18 f 16 f 26