High-resolution strontium isotope stratigraphy across the Cambrian-Ordovician transition
Introduction
Marine authigenic minerals, if unaltered, retain the strontium isotopic composition of the seawater in which they precipitated. By analyzing stratigraphically well-constrained marine carbonates, phosphates, and barites for their 87Sr/86Sr ratios, we can reconstruct temporal variations in seawater Sr isotopic composition. Such 87Sr/86Sr curves can be used for global chemostratigraphic correlation as well as global tectonic interpretation (Veizer, 1989), provided that the oceans have always been isotopically homogeneous with respect to strontium, which appears likely due to the long residence time of Sr in seawater (Holland, 1984). The roots of modern Sr isotope stratigraphy can be found with Peterman et al. (1970), who were the first to demonstrate unequivocally that seawater 87Sr/86Sr did not increase unidirectionally with time (Wickman, 1948) but has instead varied around a mean value of 0.7080 since the Cambrian. Veizer and Compston (1974) supplied additional constraints on these fluctuations before a concerted effort by researchers at Mobil led to the construction of the first Phanerozoic 87Sr/86Sr curve (Burke et al., 1982). This curve, although comprehensive for most of the Phanerozoic, did not cover the entire Cambrian period, and related publications provide no biostratigraphic information for the lower Paleozoic parts of the curve (e.g., Denison et al., 1998). Subsequent studies, only some of which incorporate biostratigraphy Keto and Jacobsen 1987, Donnelly et al 1988, Donnelly et al 1990, Gao and Land 1991, Montañez et al 1996, Montañez et al 2000, Saltzman et al 1995, have constrained seawater 87Sr/86Sr to ≥0.7090 during the Middle and Late Cambrian. As part of a much wider chemostratigraphic study covering the entire pre-Cenozoic Phanerozoic (Veizer et al., 1999), the present contribution seeks to constrain seawater 87Sr/86Sr during the Cambrian-Ordovician transition using well-preserved fossil apatite. In addition, this study aims to assess the suitability for Sr isotope stratigraphy of various groups of skeletal phosphate (euconodonts, paraconodonts, protoconodonts, and inarticulate brachiopods). To achieve adequate coverage, samples from nine carefully selected sections from around the world were analyzed (Fig. 1). Six of these sections cover the Cambrian-Ordovician boundary interval only (Fig. 2).
Section snippets
Definition and age of the cambrian-ordovician boundary
Fossiliferous sedimentary rocks of this age are widespread, and global stratigraphic correlation is made easier by the combination of three fossil groups of high stratigraphic potential, the trilobites, graptolites, and conodonts, the latter two being particularly cosmopolitan in nature. This allowed Norford (1988) to write on behalf of the working group on the Cambrian-Ordovician boundary that “sequences can be correlated with one another with considerable precision.” As a result of a plenary
Sample material
Previous studies have demonstrated that low-Mg calcite is the most likely of the common marine precipitates to preserve the 87Sr/86Sr ratio of seawater over geologic time scales (Veizer et al., 1999). Low-Mg calcite fossil tests, if well preserved, can be used to reconstruct variations in seawater 87Sr/86Sr, with foraminifera (Cretaceous until Recent), belemnites (Mesozoic), and articulate brachiopods (Ordovician until Recent) having been widely used for this purpose in recent years. However,
Geologic setting and sample selection
The samples for this study were selected from diverse depositional settings, mostly shallow marine carbonate shelves. Paleogeographic reconstructions (Fig. 1) place these sedimentary basins at equatorial to mid latitudes on five different paleocontinents. On the basis of conodont CAI, which we required to be lower than 2 and which was generally lower than 1.5, nine sections were selected for Sr isotope analysis. These were (1) The Black Mountain (Unbunmaroo) section (Fig. 3) of the eastern
Analytical techniques
Bulk samples from which fossils were later separated were washed thoroughly, with weathered crusts removed where present. Areas with clearly identifiable fractures or veining were generally not considered for analysis. Further crushing of the samples was followed by dissolution in 5% acetic acid. Every 2 to 3 days, the sample was decanted, the fraction 80 μm to 2 mm being retained, rinsed, and dried at 50°C. Fossils were hand picked under a binocular microscope without any further chemical
Results
Black Mountain, western Queensland, Australia: 87Sr/86Sr decreases from 0.709120 ± 0.000010 in the Hispidodontus resimus and Hispidodontus appressus Zones of the Upper Cambrian to 0.708990 ± 0.000010 by the Cordylodus angulatus Zone of the Lower Ordovician (Fig. 3). High-resolution features with amplitudes ≤50 × 10−6 are discernible, especially close to the Payntonian-Datsonian Stage boundary and across the Hirsutodontus simplex-Cordylodus prolindstromi Zone boundary, which also marks a
Discussion
Apatite fossils are often used only reluctantly to reconstruct seawater 87Sr/86Sr because of a general inconsistency of results compared with low Mg-calcite fossils, such as articulate brachiopods. For example, Ebneth et al. (1997) demonstrated that conodont 87Sr/86Sr was systematically more radiogenic, by up to 0.0001, than coeval brachiopods from the same section (Diener et al., 1996). On the other hand, some studies report little deviation between the two fossil-types (e.g., Qing et al.,
Conclusions
Our study shows that limestone-hosted euconodonts with low CAI (<2) can be used to pinpoint seawater 87Sr/86Sr during the early Paleozoic. Such material is generally better for this purpose than carbonate rock components and compares favorably with well-preserved calcitic brachiopods. Other types of fossil apatite, such as protoconodonts, paraconodonts, and inarticulate brachiopods, are less likely to retain primary Sr isotope signatures even when otherwise apparently unaltered. Seawater 87Sr/86
Acknowledgements
This study was made possible through the generously provided expertise and samples of several specialists to whom we are extremely grateful: Galina Abaimova (SNIIGGMS, Novosibirsk); Per Ahlberg (Lund University); Alexei Fedorov (SNIIGGIMS); Dieter Buhl, Rolf Neuser (Ruhr University, Bochum); Jun-yuan Chen (Institute of Paleontology and Geology, Academia Sinica Nanjing); Svetlana Dubinina (Paleontological Institute, Russian Academy of Sciences); Klaus J. Müller (Bonn University); Leonid Popov
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