Original ArticlesThe strontium isotopic composition of Ordovician and Silurian brachiopods and conodonts: relationships to geological events and implications for coeval seawater
Introduction
Strontium isotopic composition of marine carbonate minerals, if not altered during diagenesis, reflects the isotopic composition of seawater at the time of deposition, and such data indicate that 87Sr/86Sr ratio has varied systematically during the Phanerozoic (Burke et al., 1982). These variations result mostly from waxing and waning of the following major fluxes: radiogenic 87Sr/86Sr input due to subaerial weathering of continental crust, delivered principally via dissolved load of rivers; nonradiogenic 87Sr/86Sr flux that results from water/rock interactions at midoceanic ridges and from submarine alteration of basalts; and removal of Sr via precipitation and sedimentation, mostly by marine carbonates (e.g., Faure, 1986). Secular variations in the 87Sr/86Sr of seawater during geologic history can, therefore, provide valuable information for understanding the dynamics of the Earth system and can serve as an important baseline in the course of investigation of stratigraphy, petrology, diagenesis, and genesis of mineral deposits in sedimentary basins (Veizer, 1989).
The long oceanic residence time of Sr (∼4 m.y.) and the rapid mixing rate of the oceans (103 yr) have caused the strontium isotope ratio of seawater to be globally homogeneous at any given time, as documented by identical 87Sr/86Sr ratios for coeval marine carbonates (e.g., Burke et al., 1982) and by measurements of modern seawater (Elderfield, 1986). This enables utilization of the seawater strontium isotopic curve for dating and correlation purposes, particularly for sparsely fossiliferous sequences. For example, Ludwig et al. (1988) were able to show that the sharp rises in 87Sr/86Sr at Enewetak atoll correlate with disconformities caused by subaerial erosion, whereas intervals with little change correspond to times of rapid accumulation of shallow water carbonates. This technique is most useful for dating Cenozoic sediments, where the curve is steep and unidirectional (De Paolo, 1986), but similar steep slopes have been documented also for earlier geologic history (cf. Burke et al., 1982).
Detailed knowledge of 87Sr/86Sr of ancient seawater is also crucial for deconvolution of the sedimentary and diagenetic histories of sedimentary basins. Such information, in turn, can help to interpret geologic events, such as dolomitization and mineralization in sedimentary basins (e.g., Banner et al 1988, Mountjoy et al 1992, Saller 1984, Swart et al 1987). Strontium isotopes can also serve as tracers of subsurface fluid movement and as indicators of connectivity of conduit systems, aquifers, or hydrocarbon reservoirs Connolly et al 1990, Qing and Mountjoy 1992, Qing and Mountjo 1994. Furthermore, strontium isotopes have also been utilized to investigate the origin and timing of gangue minerals in ore deposits (e.g., Kessen et al 1981, Kesler et al 1983, Kesler et al 1988, Barbieri et al 1987, Ruiz et al 1988).
The earlier systematic studies of Paleozoic sedimentary carbonates succeeded in delineation of the secular patterns for strontium isotopes of Phanerozoic seawater that had a resolution of ∼107 yr (e.g., Peterman et al 1970, Veizer and Compston 1974, Burke et al 1982). For Ordovician and Silurian, the subject of the present study, the earlier published data were based primarily on whole rock samples Burke et al 1982, Gao and Land 1991, Gao et al 1996, Denison et al 1997, although some measurements have been obtained from conodonts Bertram et al 1992, Ruppel et al 1996, Holmden et al 1996. Since whole rocks inevitably contain not only the primary marine components precipitated from ambient seawater, but also diagenetic cements that may have precipitated from fluids of different strontium isotopic compositions (see James and Choquette 1990, Choquette and James 1990), the resulting strontium isotope values are only averages for the constituting phases (e.g., Banner et al 1988, Gao and Land 1991). The main objectives of this paper are, therefore, (1) to refine the strontium isotope curve for the Ordovician and Silurian seawater, utilizing monomineralic low-Mg calcitic brachiopod shells, marine calcite cements, and conodonts, (2) to compare these results with the previously published data, and (3) to search for possible causes that could have generated the observed isotopic patterns.
Section snippets
Materials used for reconstruction of the strontium isotope age curve
The bulk of the samples in this study comprises articulate brachiopods and conodonts. These samples were collected from localities in Laurentia, at paleolatitudes less than 30° (Fig. 1). Their locations, stratigraphic assignment, and generic identification are summarized in Appendix 1, Appendix 2. For the purposes of this paper, the standard British stage/series nomenclature is used even though many of the samples come from North America. The absolute chronology is based on Harland et al.
Sample preparation and analytical procedures
The collected brachiopod shells were small (0.5–2 cm) with thin shells (0.1–1 mm), particularly the early Ordovician ones. In order to avoid contamination from the matrix and/or from altered shell material by “dental drill” technique, we have used the preparation technique for brachiopod samples that was developed by the research group at the Ruhr University in Bochum (see Bruckschen et al 1995, Diener et al 1996, Veizer et al 1997). The selected brachiopod shells were examined under
Textural evaluation
In order to select the “best” preserved brachiopod shells, thin sections of brachiopods were examined under transmitted light and subsequently by cathodoluminescence (CL). As a rule, the unaltered shells consist of nonluminescent fibrous layers with no visible dissolution and cementation features (cf. Popp et al., 1986). Scanning electron microscope (SEM) studies confirmed the generally good preservation of the original brachiopod fabric, but dissolution features have been observed even in some
Temporal trend
The overall temporal trend for strontium isotopes based on Ordovician brachiopods and marine cements and on Ordovician and Silurian conodonts is presented in Fig. 6. Together, they define a curve with far less scatter than that developed by Burke et al. (1982) using whole rock samples. For the reasons outlined by Veizer and Compston (1974) we utilize the least radiogenic values to constrain the strontium isotope trends for the Ordovician and Silurian seawater. The presently defined strontium
Discussion
The strontium isotopic composition of seawater is determined primarily by the balance between radiogenic Sr delivered to the oceans via continental weathering (global average riverine 87Sr/86Sr ∼0.7119; Palmer and Edmond, 1989) and relatively unradiogenic Sr released by mid-ocean ridge hydrothermal systems (87Sr/86Sr about 0.7035; Palmer and Elderfield, 1985). An increase in continental erosion rate would, theoretically, increase the 87Sr/86Sr ratio of seawater, whereas an increase in
Conclusions
156 Ordovician and Silurian brachiopod shells, marine calcite cements, and conodonts were analyzed for strontium isotopes in order to refine the strontium isotope curve for the Ordovician and Silurian seawater. Preservation of the brachiopod shell material was examined by petrographic and geochemical criteria and only the well preserved internal secondary layer of the shells were utilized for strontium isotope measurements.
The measurements define secular trend of decreasing strontium isotope
Acknowledgements
We acknowledge donation of samples by T. E. Bolton (Geological Survey of Canada, Ottawa), H. J. Hoffman (University of Montreal, Montreal), E. Landing (New York Geological Survey, Albany), J. F. Miller (S.W. Missouri State University, Springfield) and J. B. Waddington (Royal Ontario Museum, Toronto). This study was financially supported by the Natural Sciences and Engineering Research Council of Canada, the Deutsche Forochungsgemeinschaft, and Royal Holloway University of London. We appreciate
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2023, Journal of Petroleum Science and EngineeringA reassessment on the timing and potential drivers of the major seawater <sup>87</sup>Sr/<sup>86</sup>Sr drop in the Ordovician Period: New evidence from conodonts in China
2022, Chemical GeologyCitation Excerpt :The seawater 87Sr/86Sr ratios have been increasingly utilized as a proxy for high-resolution inter-continental stratigraphic correlations (Burke et al., 1982; Veizer et al., 1999; McArthur et al., 2020) and, in combination with other isotopic data (e.g., δ13Ccarb, δ18O, and δ44/40Ca), to identify and quantitatively interpret major paleo-oceanic events and fundamental geologic processes (e.g., tectonic cycles, climate change, and life evolution) through Earth history (Jones and Jenkyns, 2001; Jacobson et al., 2002; Chen et al., 2005; Song et al., 2015; Dudás et al., 2017; Kristall et al., 2017; Montañez et al., 2018; Zhang et al., 2020; He et al., 2021; Paytan et al., 2021; Wang et al., 2021a; Wang et al., 2021b; Chen et al., 2022). In particular, the Ordovician Period was characterized by a major strontium isotope perturbation featuring a large (~ − 0.001 in magnitude), rapid (~5.0–10.0 × 10−5/Myr), and long-lasting (~10 Myr) seawater 87Sr/86Sr negative excursion straddling the Middle-Upper Ordovician boundary (i.e., late Darriwilian to Sandbian ages) (Qing et al., 1998; Shields et al., 2003; Young et al., 2009; Saltzman et al., 2014; Edwards et al., 2015), which was the most rapid change of this magnitude in the entire Phanerozoic (McArthur et al., 2020), and regarded as a major “isotopic event” in the geologic past (Shields et al., 2003). The long-term monotonous decrease pattern of seawater 87Sr/86Sr ratios through the Ordovician Period has been well documented by analyses of different types of samples, such as brachiopod low-magnesium calcite (LMC) (Veizer et al., 1999; Shields et al., 2003; Rasmussen et al., 2016), bulk carbonate rock (Jiang et al., 2001; Young et al., 2009; Edwards et al., 2015; Pokrovsky et al., 2018), and conodont apatite (Saltzman et al., 2014; Kozik et al., 2019).
The evolution and initial rise of pelagic caryocaridids in the Ordovician
2022, Earth-Science Reviews
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Present address: Department of Geology, Royal Holloway, University of London, Egham Surrey TW20 0EX, England ([email protected]).