High-resolution strontium isotope stratigraphy across the Cambrian-Ordovician transition

doi:10.1016/S0016-7037(01)00580-4

Geochimica et Cosmochimica Acta

Volume 65, Issue 14, 1 July 2001, Pages 2273-2292

https://doi.org/10.1016/S0016-7037(01)00580-4 Get rights and content

Abstract

We have analyzed 214 fossil apatite samples from nine stratigraphic sections worldwide that cover the lower Cambrian to lower Ordovician for their strontium isotope ratios. Of these samples, 180 from six sections that cover the Cambrian-Ordovician transition (<5 Ma) reveal how the extent of isotopic alteration can differ greatly according to lithology and sample type. From three limestone sections, 126 euconodont samples yield the most consistent ⁸⁷Sr/⁸⁶Sr ratios and are used in this article to constrain seawater ⁸⁷Sr/⁸⁶Sr across the Cambrian-Ordovician boundary. By contrast, protoconodonts, paraconodonts, and inarticulate brachiopods are far less likely to retain a primary ⁸⁷Sr/⁸⁶Sr signature and appear to be of only limited value for chemostratigraphy. Seawater ⁸⁷Sr/⁸⁶Sr fell from maximum values for the Phanerozoic of more than 0.7092 during the early Late Cambrian to 0.7090 by the earliest Ordovician. High-resolution sampling has permitted the recognition of sinusoidal variations with a wavelength on the order of <1 Ma that are superimposed on the overall fall in ⁸⁷Sr/⁸⁶Sr. These variations may be caused by high-order cycles in seawater ⁸⁷Sr/⁸⁶Sr or diagenetic alteration.

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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Citation Excerpt :
For instance, the first appearance datum of small shelly fossils (SSFs) is globally accompanied by a negative δ13C excursion and a declining trend in seawater 87Sr/86Sr ratios (Cowie and Glaessner, 1975; Landing, 1989; Zhu et al., 2006; Li et al., 2013; Zhu et al., 2019), although there are still debates about the exact boundary definition between the Ediacaran and the Cambrian (Geyer and Landing, 2017). Secular variations in seawater 87Sr/86Sr ratios have been widely applied to reflect continental uplift, silicate weathering and variations in atmospheric carbon dioxide concentration (Burke et al., 1982; Keto and Jacobsen, 1987; Kaufman et al., 1993; Derry et al., 1994; Montañez et al., 1996; Denison et al., 1998; Montañez and Banner, 2000; Ebneth et al., 2001; Thomas et al., 2001; Melezhik et al., 2009; Sawaki et al., 2010). The 87Sr/86Sr ratio of seawater is controlled by a balance between continental weathering-derived Sr, hydrothermal Sr input and seafloor weathering (Palmer and Edmond, 1989).
The radiation of early animals during the Cambrian explosion was accompanied by widespread perturbations in oceanic and atmospheric conditions. However, the cause-effect relationships between evolutionary innovations, carbon (C) cycling, and continental weathering are still a matter of debate. Further paleoenvironmental reconstructions of poorly studied paleo-continents covering the Precambrian/Cambrian (PC/C) transition may improve the correlation of the events and their impact on the Earth system on a global scale. The Tarim Basin was presumably located proximal to the well-studied South China craton during the Ediacaran-Cambrian boundary and is expected to contribute equally to our paleoenvironmental reconstructions. Here we apply carbon and strontium (Sr) isotope records (δ¹³C and ⁸⁷Sr/⁸⁶Sr) obtained from well-preserved marine carbonates spanning the late Ediacaran to the late Cambrian from the Keping region in the northwestern Tarim Basin, NW China. By comparison to other carbonate successions from various paleo-continents, our obtained seawater ⁸⁷Sr/⁸⁶Sr curve in this study shows a stepwise first-order increase to more radiogenic values superimposed by second-order presumably regional variations. Our obtained δ¹³C variations exhibit four prominent negative and three positive excursions identical to the global δ¹³C record. Based on existing paleontological evidence and our new Tarim δ¹³C and ⁸⁷Sr/⁸⁶Sr curves, we propose a stratigraphic correlation from the late Ediacaran to the late Cambrian between the Tarim Basin and other paleo-continents. Besides, our paleoenvironmental reconstruction reveals both regional and global sea-level changes that may have controlled the influx of radiogenic Sr and presumably the influx of nutrients controlling primary production and the rate of organic carbon burial in the Tarim Basin at the time of deposition. These feedbacks may have ultimately influenced the rapid diversification of metazoans and the oxygenation of the Cambrian ocean and atmosphere approaching-present pO₂ levels.
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Tracing the sources of sedimentary Cu and Mn ores in the Cambrian Timna Formation, Israel using Pb and Sr isotopes
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Diagenesis and epigenetic remobilization frequently obscure the original depositional environment of sediments that have been affected by Cu- Mn- mineralization. Such is the case for the sedimentary Cu and Mn ores of the Cambrian Timna Formation in Southern Israel, where different interpretations of field, petrographic and geochemical data have led to conflicting genetic models. In this study we show how Sr and Pb isotopic data can help unravel the origins and complex processes involved in the sedimentary-hosted Cu- Mn- mineralization. Applying Pb and Sr isotopic systems to the various host rock types, two petrographical and chemically distinct types of Mn nodules (referred to as A and B) and reduced and oxidized Cu minerals of the Timna Formation indicates that two major solution types were involved. The first solution is depleted in Th and enriched with U, with a constant ²⁰⁸Pb/²⁰⁴Pb ratio and single U source as reflected by the correlation between ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb. The second solution has higher Th and lower U concentrations; thus ²⁰⁸Pb/²⁰⁴Pb ratios significantly vary whereas the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios remain relatively constant. The low Th/high U solution is associated with the deposition of Type A Mn nodules, whose ⁸⁷Sr/⁸⁶Sr ratio of 0.7093 ± 0.0005 matches that of the Cambrian Sea. Thus, in keeping with previous petrographic and mineralogical evidence, the isotope data are consistent with primary Mn nodule formation under oxidizing conditions during early diagenesis. In contrast, the second high Th/low U solution is associated with the transformation of primary Type A to epigenetic Type B Mn nodules. During these later processes, U was leached from Type A Mn nodules to form secondary Pb enriched Type B nodules. The Pb isotope composition of these nodules clearly shows that the epigenetic solutions were significantly different from those of a major phase of Oligocene-Miocene Fe- mineralization that affected the bedrocks adjacent to the Dead Sea Transform. Cu- Mn- mineralization exhibits more complex isotopic signatures. Cu-sulfides (mainly covellite) deposited during early diagenesis under reducing conditions, and although their ⁸⁷Sr/⁸⁶Sr ratio in places is 0.709, as a whole, their Sr-isotope ratios reflect their location rather than the Cambrian sea water signature, as clearly evident from their Pb isotope compositions. During later epigenetic stages, copper sulfides were altered to form malachite and paratacamite, whose Pb isotope compositions show that the oxidizing solutions underwent fluid-rock interaction with their host rocks, particularly during the transport of Pb in lower pH chloride solutions.
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High-resolution strontium isotope stratigraphy across the Cambrian-Ordovician transition

Abstract

Introduction

Section snippets

Definition and age of the cambrian-ordovician boundary

Sample material

Geologic setting and sample selection

Analytical techniques

Results

Discussion

Conclusions

Acknowledgements

Earth Planet. Sci. Lett.

Chem. Geol.

Sediment. Geol.

Chem. Geol.

Chem. Geol.

Geochim. Cosmochim. Acta

Palaeogeog., Palaeoclim., Palaeoecol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Chem. Geol.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Chem. Geol.

Silurian strontium isotope stratigraphy

Geol. Soc. Am. Bull.

The proposed Cambrian-Ordovician global boundary stratotype and point (GSSP) in western Newfoundland, Canada

Geol. Mag.

Integrated chemo- and biostratigraphic calibration of early animal evolution of southwest Mongolia

Geol. Mag.

Variations of seawater 87Sr/86Sr through Phanerozoic time

Geology

The recommended Cambrian-Ordovician global boundary stratotype of the Xiaoyangqiao section (Dayangcha, Jilin Province, China)

Geol. Mag.

Ion-probe zircon dating of a mid-Early Cambrian tuff in South Australia

J. Geol. Soc. (London)

The Ordovician time scale calibration of graptolite and conodont zones

Acta Universitatis Carolinae

New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary

Geol. Mag.

Anomalous geochemical signals from phosphatic Middle Cambrian rocks in the southern Georgia Basin, Australia

Sedimentology

Cambrian-Ordovician conodonts from the Burke River structural belt, Queensland

BMR Bull. Austral. Geol. Geophys.

Conodont color alteration—an index to organic metamorphism

USGS Professional Paper

Principles of isotope geology

Variations of seawater ⁸⁷Sr/⁸⁶Sr through Phanerozoic time