Strontium isotope stratigraphy of the Early Silurian (Llandovery): Implications for tectonics and weathering

1. Strontium isotope stratigraphy of the Early Silurian (Llandovery): Implications for tectonics and weathering Jeremy C. Gouldey, Dr. Matthew R. Saltzman, Dr. Kenneth A. Foland, Jeffrey S. LinderThe Ohio State University, School of Earth Sciences, Columbus, OH 43210 Abstract # 127751 Geologic Background An abundant series of K-bentonite beds has been well documented in the British Isles and Baltoscandanavian region, and recently in the Appalachian region of North America. These ash beds may correlate between Llandovery sections in North America and Estonia (Bergstrom et al., 1992; Bergstrom et al, 1998; Lehnert et al., 1999), and may also help to explain changes in Sr ratios. A high resolution, more complete Llandovery Sr record may aid in this correlative process, which would place the Sr changes in a more detailed geologic context to address tectonic and climate interactions. In the Llandovery, there have been several stratigraphic and biostratigraphic studies conducted, including conodont biozonation in Anicosti Island (Zhang et al., 2002), marine sediment and conodont biozonation in Scandanavia (Dahlqvist et al., 2005), and graptolite, conodont, and chitinozoan biozonation in the Baltic region (Loydell et al., 2003; Kaljo et al., 2000). These studies can be used to assist in regional correlation. Previous Llandovery Sr records are based essentially on biostratigrapically correlated brachiopods, and record only a few data points. The Sr data in this study is based upon whole rock carbonate samples collected from a well dated and stratigraphically known Estonian drill core (Ikla core), and provides a much more complete Sr record for the early Llandovery due to a higher sampling resolution. Conclusions -87/86Sr ratios rapidly increase starting in the mid Aeronian through to the end of the Llandovery. -Weathering from Silurian continent-continent collisions and from large scale felsic volcanics could explain the trend to more radiogenic Sr ratios. -Data from other Llandovery deposits (i.e. Pancake range, Nevada) could help to assess the probability of global volcanics affecting the Sr record. Abstract A high resolution Sr isotope data set, generated from a fossiliferous drill core through the Llandovery in Estonia, shows that the 87/86Sr ratio reaches a minimum in the early Llandovery, and then trends to more radiogenic ratios in the mid to late Llandovery. The range of values is in general agreement with previous sample sets of calcitic brachipods and conodonts recovered from localities in North America and Europe that record a rising trend in the 87/86Sr ratio throughout the Silurian from approximately 0.70787 to 0.70835. Our data, however, show a general decreasing trend in the 87/86Sr ratio from the end of the Ashgillian/beginning of the Rhuddanian until the early Aeronian, with values ranging from 0.70834 to 0.70804. During the Aeronian, 87/86Sr ratios slowly trend towards more radiogenic ratios. Starting in the late Aeronian, the isotope record shows a rapid shift to radiogenic ratios, ranging from 0.70807 to 0.70844 through the remainder of the Aeronian and the Telychian. Increases in the 87/86Sr ratio during the late Llandovery may be due to increased riverine flux of radiogenic Sr into the oceans due to weathering of non-volcanic continental silicate rocks that were uplifted during early Silurian continent-continent collisions. Alternatively, or in addition to non-volcanic weathering, a radiogenic Sr flux from exposed felsic volcanics in the Balto-Scandanavian region is also consistent with the presence of K-bentonites in late Aeronian and early Telychian strata. Ikla Core Introduction Changes in the 87/86Sr isotopic record of seawater represent one of the most useful paleoceanographic tools in the study of ancient climates. These Sr isotope trends are accurately recorded in marine carbonate rocks and phosphatic shells back at least a half billion years, particularly in research done in recent years (Veizer et al., 1999, Azmy et al., 1999; Ruppel et al., 1996; Shields et al., 2003). The two sources of marine Sr are from the weathering of continental silicates and hydrothermal exchange of seawater at mid-ocean ridges. These two sources differ greatly in their 87/86Sr ratio and may thus affect the ocean composition. In the case of continental silicate rocks, either an increase in the rate of weathering or a change in the 87/86Sr ratio of the rocks being weathered can change the riverine flux of Sr into the oceans (Azmy et al., 1999; Veizer et al., 1999; Ruppel et al, 1996). Tectonic uplift resultant from continental collisions may allow for older, highly radiogenic silicate rocks to be exposed, which can cause a rising trend in Sr. In addition, if overall silicate weathering rates were to increase, global temperatures would be expected to drop due to uptake of atmospheric CO2. This enhanced weathering has been proposed for the Himalayan uplift, which may have caused cooling that led ultimately to the Late Cenozoic Ice Age (Raymo et al., 1988). Figure 2. Map of Baltic region where Ikla Estonian core was extracted (after Kaljo et al., 2000). Figure 4. Stratigraphy and lithology of Estonian Ikla core (after Kaljo et al., 2000). Graptolite zones: 1, C. insectus-O. spiralis; 2, Mcl. crenulata-Mcl. griestoniensis; 3, Str. crispus-Spir. guerichi; 4, St. sedgwickii; 5, D. convolutus; 6, M. argenteus; 7, D. pectinatus-D. triangulatus; 8, Cor. cyphus; 9, Cys. vesiculosus; 10, Par. acuminatus. 87/86Sr and inorganic 13C curves, respectively, through Ikla core. Blue lines indicate unconformities. Telychian crispus turriculatus maximus sedgwicki Llandovery convolutus Aeronian References Azmy et al., 1999, Silurian strontium isotope stratigraphy, GSA Bulletin, v. 111, no. 4, p. 475-483. Bergstrom et al., 1992, Silurian K-bentonites in the Iapetus Region: A preliminary event-stratigraphic and tectonomagmatic assesment: Geologiska Foreningens I Stockholm Forhandlingar, v. 114, pt. 3, p. 327-334. Bergstrom et al., 1998, The Lower Silurian Osmundsberg K-bentonite. Part I: stratigraphic position, distribution, and palaeogeographic significance: Geological Magazine v. 135 (1), p. 1-13. Berner, R. A., 2006, Inclusion of the Weathering of Volcanic Rocks in the GEOCARBSULF Model: American Journal of Science, v. 306, p. 295-302. Dahlqvist et al., 2005, The lowermost Silurian of Jamtland, central Sweden: conodont biostratigraphy, correlation and biofacies: Transactions of the Royal Society of Edinburgh: Earth Sciences, v. 96, p. 1-19. 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Kaljo et al., 2000, Carbon istopic composition of Llandovery rocks (east Baltic Silurian) with environmental interpretation: Proceedings of the Estonian Academy of Sciences, v. 49, no. 4, p. 267-283. Lehnert et al., 1999, First record of Lower Silurian conodonts from South America: biostratigraphicand palaeobiogeographic implications of Llandovery conodonts in the Precordillera of Argentina: Geological Magazine, v. 136 (2), p. 119-131. Loydell et al., 2003, Integrated biostratigraphy of the lower Silurian of the Aizpute-41 core, Latvia, Geological Magazine, v. 140 (2), p. 205-229. Melchin M.J. and Holmden C., 2006, Carbon isotope chemostratigraphy of the Landovery in Arctic Canada: implications for global correlation and sea level change. GFF, 128, 173-180. Qing et al., 1998, The strontium isotopic composition of Ordovician and Silurian brachiopods and conodonts: Relationships to geological events and implications for coeval seawater. Geochim. Cosmochim Acta v.62, p. 1721–1733. Raymo et al., 1988, Influence of late Cenozoic mountain building on ocean geochemical cycles: Geology, v. 16, p. 649-653. Ruppel et al., 1996, High-resolution 87Sr/86Sr chemostratigraphy of the Silurian: Implications for event correlation and strontium flux: Geology, v. 24, p. 831-834. Veizer et al., 1999, 87Sr/86Sr, delta 13C, and delta 18O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59-88. Zhang et al., 2002, A new Llandovery (early Silurian) conodont biozonation and conodonts from the Becscie, Merrimack, and Gun River formations, Anticosti Island, Quebec: Supplement to Journal of Paleontology, v. 76, p. 1-46 Rhudd-anian Figure 3. Stratigraphic succession of Silurian Estonian K-Bentonites with graptolite zones (after Bergstrom et al., 1992) Discussion There are several possible explanations as to the rapid increase to more radiogenic Sr ratios during the late Llandovery. One explanation is that older, more radiogenic continental rocks became exposed and weathered during early Silurian continent-continent collisions, causing a flux of more radiogenic Sr into the oceans. Another possibility is that weathering of large scale felsic volcanics caused an increase in the radiogenic Sr flux to the oceans. This correlates well with abundant well-dated K-bentonite beds in the Estonian regions associated with the sedgwicki and convolutus graptolite zones, which is the same time when Sr in the Llandovery begins to rapidly become more radiogenic. As further evidence of this, a large influx of volcanic CO2, low in 13C, could cause a more negative shift in the 13C record, which is shown in the accompanying 13C record as occurring just after the rise in radiogenic Sr levels (Kaljo et al., 2000). A similar negative 13C excursion during this time can also be seen in the Canadian Arctic (Melchin et al., 2006), which could be evidence that this is a global excursion. Since these two events were probably acting in conjunction, it is possible that a combination of weathering due to tectonic activity and weathering of large scale volcanics, which both lead to a more radiogenic Sr flux to the oceans, is responsible for the rapid trend to more radiogenic Sr ratios in the carbonate rocks. Further research of the Sr and C record in the Llandovery, specifically work done in the Pancake range of Nevada, will help to assess if this shift to more radiogenic Sr levels, occurring with a negative 13C excursion, is a global occurrence. Also, better understanding of the composition of mid-Llandovery volcanics, primarily in Balto-Scandanavia, will help to link the volcanic weathering to radiogenic Sr ratios. Figure 1. 87/86Sr record of the late Ordovician and early Silurian based on studies by Ruppel et al., 1996, Shields, 2003, and Azmy et al., 1999 (after Azmy et al., 1999 and Shields, 2003).