In contrast to a supposedly stable chemical composition of seawater during the Phanerozoic, the isotope ratios of C, O, and Sr show secular variations by far too large to be described as a steady state system (e.g., Veizer, 1989; Jones, 1994a,b). Sources for such variations include (1) the river water runoff, (2) mid-ocean ridge interactions, and (3) the sedimentary burial of components (Holland, 1984). Therefore, high-resolution isotope curves may be useful tools for global stratigraphy (Williams et al., 1988). Any high-resolution curve demands high quality sample material reflecting the primary isotopic composition (e.g., Qing and Veizer, 1994). This requires a firm assessment of the diagenetic overprint, high quality age assignments for these samples and well defined and stable analytical parameters. The Jurassic and Cretaceous systems offer a variety of materials that can be utilized: oysters, brachiopods, foraminifers and belemnites (Jones et al., 1994 a,b). We selected belemnites as the source for high-resolution isotope stratigraphy, applying a well controlled sampling technique.
250 belemnite rostra from the late Jurassic and the early Cretaceous were analyzed for their trace element contents (Mg, Sr, Fe, Mn), d18O, d13C, and 87Sr/86Sr isotope ratios. The internal structure and its diagenetic alteration were monitored using cathodoluminescence and SEM techniques. Details concerning preparation are given in Podlaha (1995) and Diener et al. (1996).
Our own results, supplemented by ~250 previous measurements (Jones et al., 1994a,b), represent a reliable time series for providing data on Sr isotope stratigraphy or paleotemperature studies on the base of d18O measurements.
The 87Sr/86Sr ratios are significantly lower compared to previously published data. A distinct excursion in the Kimmeridgian suggests a new shape of the Sr isotope curve for the late Jurassic. Moreover, the error defining the curve-width shrinks to a range inbetween ±6·10-6 ±1.2·10-5. This increase in data quality was obtained by a better control of diagenetic alteration and the new microdrilling sampling technique. The application of a high-resolution 87Sr/86Sr curve for dating-purposes (e.g., Williams et al., 1988) requires a steep gradient in order to obtain unequivocal age assignement. Due to the shape of the curve during this time interval, even the new database does not improve the quality of age data when compared to biostratigraphy. Nevertheless, the lowest 87Sr/86Sr ratio for the Phanerozoic was observed at the Oxfordian/Kimmeridgian boundary, followed by a steady increase towards a local maximum in the Barremian.
In the d18O and d13C curves no offset between the earlier published and the actual data was observed. Despite of this general coherence, the individual structure of the sampled belemnites suggest an individual alteration of old and new samples.
Future work should concentrate on the improvement of sampling techniques for biogenic material in combination with a better control of diagenetic alteration.
Diener, A., Veizer, J., Buhl, D. & Ebneth, S., Geochim. Cosmochim. Acta, in press (1996).
Holland, H.D., The Chemical Evolution of the Atmosphere and Oceans (Princeton Univ. Press, 1984).
Jones, C.E., Jenkyns, H.C. & Hesselbo, S.P., Geochim. Cosmochim. Acta 58, 1285-1301 (1994a).
Jones, C.E., Jenkyns, H.C. & Hesselbo, S.P. Geochim. Cosmochim. Acta 58, 3061-3074 (1994b).
Podlaha, O.G., unpubl. PhD thesis, Ruhr-Universität Bochum (1995).
Qing, H. & Veizer, J., Geochim. Cosmochim. Acta 58, 4429-4442 (1994).
Veizer, J., Ann. Rev. Earth Planet. Sci. 17, 141-167 (1989).
Williams, D.F., Lerchie, I. & Full, W.E., Isotope Chronostratigraphy: Theory and Methods (Acad. Press, 1988).