Strontium isotope systematics of conodonts: Middle Devonian, Eifel Mountains, Germany
Introduction
The goal of the present study, and of similar efforts elsewhere, is to develop a high resolution chemostratigraphy for the Paleozoic. This task is considerably more difficult than was the case, for example, for the Cenozoic, due to potentially higher diagenetic overprint and to poorer stratigraphic resolution. In addition, the availability of reliable materials that record the properties of sea water is limited. The principal samples used for delineation of the details of Sr isotopic composition of Paleozoic sea water are low-Mg calcitic shells of brachiopods (Popp et al., 1986; Brand, 1991; Gruszczynski et al., 1992; Diener et al., 1996). However, the stratigraphic resolution of brachiopods in the Paleozoic is mostly inferior to that of conodonts. The apparent advantages of the latter include the fact that they have high Sr contents (Pietzner et al., 1968; Wright et al., 1984, Wright et al., 1990; Kürschner et al., 1992; Bertram et al., 1992) and that their degree of post-depositional alteration can be reliably calibrated via the colour alteration index (CAI) (Epstein et al., 1977; Königshof, 1991). For these reasons, conodonts have been viewed as potentially the best material for studies of the Sr isotopic composition of Paleozoic sea water (Kovach, 1980, Kovach, 1981, Kovach, 1984; Wright et al., 1984; Keto and Jacobsen, 1987; Kovach and Miller, 1988), providing their CAI does not exceed a value of 2.5 (Bertram et al., 1992).
These presumed advantages are somewhat mitigated by the fact that conodonts are the remains of an extinct group of chordates (Aldridge et al., 1986; Conway-Morris, 1989) and Paleozoic samples cannot be directly compared to any extant counterparts. The average composition of conodont apatite, according to Pietzner et al. (1968), can be expressed as:
The OH− or H2O are not present in the lattice position, as in hydroxy-apatite, but substitute, together with carbonate, for phosphate. This is typical of francolite. Conodonts contain three chemically and morphologically different domains. The first shows in thin section a lamellar texture and contains enhanced contents of CO2−3 and a higher proportion of organic matter. The second domain, the `white matter', has no lamellae, less organic matter and less carbonate. This material is more frequent in the phylogenetically older, particularly Ordovician, conodonts and probably represents recrystallised portions of conodonts. The timing of such presumed recrystallisation, pre- or post-mortem, is open to discussion (Lindström and Ziegler, 1971). The third domain is the so-called basal filling of conodonts. In well preserved conodonts, it is frequently found attached to the conodont element and is composed of small apatite crystals which are enriched in REE and depleted in Sr (Pietzner et al., 1968).
In terms of trace elements, of particular importance for the present study is the behaviour of strontium. In sedimentary phosphates, Sr substitutes for Ca in the crystal lattice (McClellan, 1980; McArthur, 1985) and its concentration in francolite should be proportional to that in a parent solution (LeGeros et al., 1980; McArthur, 1985). If the mineralization of conodont francolite has not changed in the course of geologic history, its chemistry may have been comparable to that of recent fish teeth. The latter have a Sr content of 1–100 ppb (Darlet, 1984; Elderfield and Pagett, 1986). However, the Sr contents increase very rapidly, to 1000 ppm, during very early diagenesis (Schmitz et al., 1991). Fossil fish teeth have even higher Sr contents of 1500–5000 ppm (Staudigel et al., 1985; Grandjean et al., 1987; Schmitz et al., 1991), with the variability possibly related to differences in diagenetic alteration. Similarly, well preserved conodonts contain about 3000–7000 ppm Sr (Pietzner et al., 1968; Wright et al., 1984; Bertram et al., 1992; Kürschner et al., 1992; Holmden et al., 1994).
From studies of REE in fish teeth (Bernat, 1975; Elderfield and Pagett, 1986; Grandjean et al., 1987, Grandjean et al., 1988; Grandjean and Albarède, 1989), which show a similar post-mortal enrichment as Sr, it appears that these elements are scavenged from Fe–Mn oxyhydrates, pellets and organic debris that are decomposing on the ocean floor, producing reducing microenvironments and migration of such elements into francolite. This sequence of events is believed to be the reason for preservation of a marine Sr isotope signal, since the microenvironment is buffered by marine products and marine pore waters in terms of its 87Sr/86Sr ratio. Subsequently, diagenetic overprint at higher pressures and temperatures results in progressive transformation of the metastable francolite into fluorapatite, which is accompanied by the loss of Na, Sr and REE (McArthur, 1985; Burnett, 1988). Our studies of conodonts with CAI 1–5 (Kürschner et al., 1992) show an average Sr decline from 3500 to 2000 ppm, although the scatter is considerable. This scatter is probably due to the variable proportions of the `white matter' in the samples, probably a primary or early diagenetically formed fluorapatite. Its higher stability may result in greater retention of early diagenetic (primary?) chemistry at comparable P–T overprint. Ramiform conodonts usually contain more `white matter' and appear, therefore, more suitable for isotopic studies. As an additional precaution, when conodont elements had to be pooled for Sr isotope measurements, we combined two or three monogeneric types in order to minimize effects resulting from the variable content of the `white substance' in the samples. Weathering also appears to result in a loss of trace elements such as Sr and Na, as demonstrated for sedimentary phosphates by McArthur (1978)and McClellan (1980). Analyses by Shemesh (1990)on recent marine apatites and onshore phosphates revealed a decrease in Sr with increasing phosphate crystallinity. The processes of enrichment and depletion of trace elements may follow the same pattern, but not necessarily the magnitude, of trace element repartitioning typical for carbonates (cf. Veizer, 1983a,Veizer, 1983b).
According to Pietzner et al. (1968)and Wright et al. (1990), exceptionally well preserved conodonts contain about 1.8% carbonate. X-ray diffraction analysis of one conodont sample with a CAI of 3 (Kürschner et al., 1992) suggests that its carbonate content is only 0.8%. This would indicate that, even at this stage, conodonts suffer from diagenetic recrystallisation.
The complex post-depositional history of conodont elements, outlined above, casts sufficient doubt as to the preservation of the original Sr isotopic signature in phosphatic remains of pre-Mesozoic age to warrant more detailed study.
Section snippets
Geology and stratigraphy
Considering that a high-resolution 87Sr/86Sr curve, for any time interval, incorporates compound effects of primary oscillations of the signal, of superimposed post-depositional overprint, and of experimental limitations (and errors), any anticipated detailed study must rely on high-density sampling of a well studied area with well established stratigraphy and very low diagenetic overprint. We selected the Eifel Mountains, the Middle Devonian stratotype area, as the most promising test
Sample preparation and selection
For separation of conodonts, 2–6 kg of visually unaltered, washed and crushed rock was dissolved in 5% acetic acid (95% technical acetic acid plus distilled water). Remains were decanted at intervals of 2–3 days. The insoluble residues were sieved to obtain the 80 μm–2 mm fraction, washed carefully with distilled water, and dried at 50°C.
Conodonts which appeared to be clean were hand picked under binocular microscope and washed in distilled water and in ultrasound for as long as required to remove
Results
The general trend of the Middle Devonian Sr isotope curve based on conodonts (Fig. 3), resembles that of the brachiopods (Diener et al., 1996), albeit shifted to more radiogenic values. On average, conodonts are about 5×10−5 more radiogenic than brachiopods. This effect, however, is more pronounced for conodont samples that originate from argillaceous limestones or from some oolitic, iron-enriched, limestones that are typical of most of the Eifelian (Fig. 4a,b). The 87Sr/86Sr ratios for the
Discussion
Assuming that the brachiopods and conodonts contained identical initial 87Sr/86Sr ratios, the question arises at what stage, and by what processes, did the present day discrepancies arise. The following alternatives come to mind:
- 1.
post-depositional decay of Rb in conodonts;
- 2.
post-depositional incorporation of radiogenic Sr due to diagenetic recrystallisation in pore waters;
- 3.
post-depositional diffusion of Sr between conodonts and pore waters.
We shall discuss these three alternatives below.
Conclusions
Conodonts, because of their ubiquity, high stratigraphic resolution, apatite mineralogy, high Sr content, and the ease of evaluation of their preservation state, appear to be an adequate material for development of a 87Sr/86Sr sea water curve for the Paleozoic. High density sampling in the Eifel Mountains, the stratotype area for the Middle Devonian, yielded a secular isotopic curve that mimics that obtained from coeval brachiopods, but is shifted (by some 10−5 to 10−4) in the more radiogenic
Acknowledgements
The authors acknowledge donation of 13 conodont samples from the stratigraphic collections of K. Weddige, Forschungsinstitut Senckenberg, Frankfurt a.M. We thank M. Reß for preparation of the plate, and B. Razcek for sample preparation. We thank W. Kürschner for essential help and discussion of conodont alteration. This research was supported financially by the Deutsche Forschungsgemeinschaft (grant Ve 112/3-1).
References (80)
- et al.
The affinities of conodonts — new evidence from the Carboniferous of Edinburgh, Scotland
Lethaia
(1986) - et al.
Calculations of simultaneous isotopic and trace element variations during water–rock interaction with application to carbonate diagenesis
Geochim. Cosmochim. Acta
(1990) Les isotopes de l'uranium et du thorium et les terres rares dans l'environment marin
Cah. OSTROM Sér. Géol.
(1975)- et al.
87Sr/86Sr, 143Nd/144Nd and REEs in Silurian phosphatic fossils
Earth Planet. Sci. Lett.
(1992) - Birck, J.L., 1986. Precision K–Rb–Sr isotopic analysis: Application to Rb–Sr chronology. Chem. Geol. 56,...
Strontium isotope diagenesis of biogenic aragonite and low-Mg calcite
Geochim. Cosmochim. Acta
(1991)Physical and chemical changes in conodonts from contact-metamorphosed limestones
Irish J. Earth Sci.
(1988)- et al.
A study of strontium diffusion in apatite using Rutherford backscattering spectroscopy and ion implantation
Geochim. Cosmochim. Acta
(1993) Preliminary submission for Lower–Middle Devonian boundary stratotype in the Barrandian area. In: Ziegler, W., Werner, R. (Eds.), On Devonian Stratigraphy and Paleontology of the Ardenno–Rhenish Mountains and Related Devonian Matters
Cour. Forsch.-Inst. Senckenberg
(1982)- et al.
Reflection of possible global events in the Barrandian area, C.S.S.R
Lecture Notes Earth Sci.
(1986)
Das Nehden der Büdesheimer Teilmulde (Prümer Mulde/Eifel)
Fortschr. Geol. Rheinl. Westfalen
Conodont paleobiology: recent progress and unsolved problems
Terra Nova
Strontium isotope stratigraphy of the Middle Devonian: brachiopods and conodonts
Geochim. Cosmochim. Acta
REE in ichtyoliths: Variations with redox conditions and depositional environment
Sci. Total Environ.
Conodont colour alteration — an index to organic metamorphism
U.S. Geol. Surv. Prof. Pap.
Fazies-Gliederung und -Entwicklung im Mittel-Devon der Eifel (Rheinisches Schiefergebirge)
Mainzer Geowiss. Mitt.
Beziehungen zwischen morphologischen Merkmalen der Brachiopoden und Fazies, dargestellt an Beispielen des Mitteldevons der Eifel und Südmarokkos
Neues Jahrb. Geol. Paläontol. Abh.
Chronometric calibration of mid-Ordovician to Tournaisian conodont zones: a compilation from recent graphic-correlation and isotope studies
Geol. Mag.
Zur Bestimmung der Versenkungstemperatur im Ordovizium Thüringens und Skandinaviens mit Hilfe der Conodontenfarbe
Neues Jahrb. Geol. Paläontol. Monatsh.
Ion probe measurement of rare earth elements in biogenic phosphates
Geochim. Cosmochim. Acta
The assessment of REE patterns and 143Nd/144Nd ratios in fish remains
Earth Planet. Sci. Lett.
The REE and ϵNd of 40—70 Ma old fish debris from the West African platform
Geophys. Res. Lett.
Fossilinhalt and Biostratigraphie des Ooser Plattenkalkes (Frasnium Büdesheimer Mulde, Eifel, Deutschland)
Mainzer Geowiss. Mitt.
Seawater strontium isotopic perturbation at the Permian–Triassic boundary, West Spitzbergen, and its implications for the interpretation of strontium isotopic data
Geology
The Middle/Upper Devonian series boundary and decisions of the International Congress. In: Ziegler, W., Werner, R. (Eds.), On Devonian Stratigraphy and Paleontology of the Ardenno—Rhenish Mountains and related Devonian Matters
Cour. Forsch.-Inst. Senckenberg
Correlation of mid-Paleozoic ammonid evolutionary events with global sedimentary pertubation
Nature
Diffusion of rare earth elements in fish teeth from deep-sea sediments
Nature
Nd and Sr isotopic variations of Early Paleozoic oceans
Earth Planet. Sci. Lett.
Conodont colour alteration adjacent to a granitic intrusion, Harz mountains
Neues Jahrb. Geol. Paläontol. Monatsh.
Variations in the strontium isotopic composition of seawater during Paleozoic time determined by analysis of conodonts
Geol. Soc. Am. Abstr. Prog.
The strontium content of conodonts and possible use of the strontium concentrations and strontium isotopic composition of conodonts for correlation purposes
Geol. Soc. Am. Abstr. Prog.
The Paleozoic–early Mesozoic seawater 87Sr/86Sr curve based on analysis of biogenic phosphates
Int. Geol. Cong. Abstr. Prog.
Eustatic sea-level changes near the Cambrian–Ordovician boundary: Data from Sr isotope analysis of biogenic apatites
Geol. Soc. Am. Abstr. Prog.
Das Oberdevon der Prümer Mulde/Eifel unter Ausschluß der Dolomit-Fazies
Notitzbl. Hess. Landesamtes Bodenforsch.
Strontium isotopes of conodonts: Devonian/Carboniferous transition, the northern Rhenish Slate Mountains, Germany. In: Streel, M., Papproth, E., Sewastopolo, G.D. (Eds.), The Final Report of the International Working Group on the Devonian/Carboniferous Boundary
Ann. Soc. Geol. Belg.
Cited by (47)
A comparison of hydrothermal events and petroleum migration between Ediacaran and lower Cambrian carbonates, Central Sichuan Basin
2023, Marine and Petroleum GeologyCarbon isotope study of conodont elements: Applications and limitations
2023, Marine MicropaleontologyFault-controlled hydrothermal dolomitization of Middle Permian in southeastern Sichuan Basin, SW China, and its temporal relationship with the Emeishan Large Igneous Province: New insights from multi-geochemical proxies and carbonate U–Pb dating
2022, Sedimentary GeologyCitation Excerpt :About 100 mg of sample powder was dissolved in 5 ml of hydrochloric acid (2.5 mol/L) for >2 h at room temperature. The strontium was separated using Bio-Rad AG50W X8 cation exchange resin, following the procedure described by Diener et al. (1996) and Ebneth et al. (1997). The measured 87Sr/86Sr ratios were normalized to the NBS-987 standard averaged 0.710273 ± 0.000012, with a mean error (2σ) better than 0.000015.
Sr–Nd isotopic constraints on carbonates, conodonts, and brachiopods of Early-Middle Pennsylvanian Itaituba and Nova Olinda formations, Amazonas Basin, Brazil
2021, Journal of South American Earth SciencesGeologic variability of conodont strontium isotopic composition quantified by laser ablation multiple collection inductively coupled plasma mass spectrometry
2021, Palaeogeography, Palaeoclimatology, Palaeoecology