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Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275

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Palaeogeography, Palaeoclimatology, Palaeoecology

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

Jeremy C. Gouldey a,⁎, Matthew R. Saltzman b, Seth A. Young c, Dimitri Kaljo d a Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60202, United States b School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States c Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, United States d Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia article info abstract

Article history: A high-resolution 87Sr/86Sr curve and paired δ13C carbonate-organic data set is generated for the Llandovery Received 20 April 2009 Series from the Ikla drill core in Estonia. A δ13C carbonate curve is also presented from the Pancake Range in Received in revised form 4 May 2010 Nevada. Observed 87Sr/86Sr values in the Ikla drill core are at a minimum in the early Llandovery Accepted 26 May 2010 (∼0.7079 to 0.7080), and then trend to more radiogenic ratios in the basal part of the Stage. Available online 2 June 2010 An 87Sr/86Sr high near ∼0.7084 is observed in the Telychian at the top of the studied section. The range of values is in general agreement with the data from previous sample sets of and Keywords: recovered from localities in North America and Europe that record a rising trend in the 87Sr/86Sr ratio Silurian 87 86 Strontium isotopes throughout the Llandovery from approximately 0.7080 to 0.7084. The major increase in the Sr/ Sr ratio Carbon isotopes during the late Llandovery may be due to weathering of radiogenic source rocks that were uplifted during Weathering early Silurian continent–continent collisions. The Sr rise potentially coincides with the occurrence of an K-bentonites unusually thick sequence of K-bentonite beds representing large-magnitude ash falls in the early Telychian. A previously documented negative δ13C excursion in marine carbonates in the lower Telychian interval of the Ikla core is quasi-synchronous with the increase in 87Sr/86Sr. Our new organic matter δ13C data from the Ikla core confirm that this negative δ13C carbonate excursion is not a result of diagenesis. Furthermore, a negative δ13C excursion in carbonates from the early Telychian portion of the Pancake Range section in Nevada seems to confirm the global scope of this carbon cycle perturbation. Published by Elsevier B.V.

1. Introduction remain poorly understood, in part due to the lack of high-resolution, integrated geochemical investigations of changes in seawater chemistry. The Llandovery (∼443 to 428 Ma) was a time of biotic recovery Previous work on calcitic brachiopods and conodonts recovered from following the major episodes of Late () glaciation localities in North America and Europe record a rising trend in the 87Sr/ and mass extinction (Harris and Sheehan, 1996; Krug and Patzkowsky, 86Sr ratio throughout the Silurian, which started after the Late Ordovician 2004). It is generally described as a time of cyclic changes in sea level and glaciation (Ruppel et al., 1996; Azmy et al., 1999; Veizer et al., 1999). This climate (Melchin and Holmden, 2006) related to intermittent glacia- 87Sr/86Sr increase from ∼0.7079 to 0.7088 has been interpreted to reflect tions (Caputo, 1998). These Lower Silurian glaciations diminished in an overall warming of the Silurian climate that led to enhanced magnitude during the transition to a middle greenhouse weathering of relatively radiogenic continental silicate rocks (Azmy period that followed the major Late Ordovician glaciation (Harris and et al., 1999). The Llandovery portion of the 87Sr/86Sr seawater curve is Sheehan, 1996; Caputo, 1998; Kaljo and Martma, 2000; Brand et al., characterized by exceptionally high rates of increasing values (Azmy et

2006; Melchin and Holmden, 2006). Fluctuating atmospheric CO2 al., 1999), and significant inflection points have been linked to global sea- concentrations may have played a role in driving these changes in level changes (Ruppel et al., 1996). However, additional global studies climatic conditions (e.g., Azmy et al., 1999; Kaljo and Martma, 2000; are needed to more accurately establish correlations between Sr isotopes Kiipli et al., 2004). However, the timing and causes of paleoclimatic and eustatic sea-level changes (Johnson et al., 1991), tectonic events changes with possible links to carbon cycling during the Llandovery (e.g., Bergström et al., 1998), and climate changes (Caputo, 1998). Regions that contain both the evidence for these events and a detailed biostratigraphic framework within which to evaluate cause-and-effect relationships are required for these analyses. δ13 ⁎ Corresponding author. Fax: +1 847 491 8060. High-resolution records of changes in Ccarb have been previ- E-mail address: [email protected] (J.C. Gouldey). ously established for the Baltic region, and may also be used to infer

0031-0182/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.palaeo.2010.05.035 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 265 changes in global carbon cycling during the Llandovery (Heath et al., proximal lagoonal dolomites in the east to basinal graptolitic dark 1998; Kaljo and Martma, 2000; Kiipli et al., 2004). δ13C data from this shales and claystones in the west (Kaljo and Martma, 2000). In the Ikla interval is also available for Anticosti Island (Long, 1993; Azmy et al., core section, (Fig. 2) the studied interval is represented by deposits 1998; Munnecke and Männik, 2009). However, corresponding study formed in deep shelf to basinal environments on a carbonate ramp. It of Sr isotope stratigraphy has not been undertaken in these sections is one of the most complete Llandovery sections known in Estonia. Its 87 86 13 and therefore the relative timing of shifts in Sr/ Sr and δ Ccarb are distal location in the basin was the reason that even some of the not known in detail. In addition, because the reproducibility of these largest regressions did not reach the region to produce subaerial 13 δ Ccarb trends outside of the Baltic region has not been thoroughly unconformities (Kaljo and Martma, 2000). Lithologies are primarily tested, the relative roles of global versus local effects on the observed micritic limestones and marlstones, with interbeds of shales and of trends (e.g., Immenhauser et al., 2007) remain poorly understood. By carbonate nodules at the base and top of the Ikla core sequence. Near 13 13 comparing trends in δ Ccarb with that of coeval organic matter δ C the top of the studied interval, two submarine unconformities are 13 (δ Corg) in the same sections, it is also possible to address potential present above and below a thin sequence of argillaceous limestones diagenetic effects on the original global seawater values (e.g., Knoll et which correspond to the Rumba Formation (Kaljo and Martma, 2000). al., 1986). The Ikla core is biostratigraphically well-dated, mainly based on To better understand the timing and causes of rising seawater 87Sr/ graptolites (Kaljo and Martma, 2000; Fig. 4) and chitinozoans (Nestor, 86Sr and δ13C excursions during the Llandovery, we have constructed 1994). The Õhne Formation in this core is dated by the occurrence of new high-resolution Sr and C isotope datasets that can be tied to the globally widespread chitinozoan Conochitina electa and the established Llandovery biostratigraphic zones and previously gener- graptolite Dimorphograptus confertus. Conochitina electa is also well ated chemostratigraphic records. The Ikla core section from Estonia represented in two other nearby core sections, Kirikuküla and Ruhnu, (Figs. 1 and 2), which has previously been studied in great detail providing good criteria for correlations across the region. The 13 for δ Ccarb stratigraphy (Kaljo and Martma, 2000), graptolite and overlying Saarde Formation consists of five members (in ascending chitinozoan biostratigraphy (Kaljo and Martma, 2000), sequence order): Slitere, Kolka, Ikla, Lemme, and Staicele members. The stratigraphy (Johnson et al., 1991), and volcanic event (K-bentonite) boundary between the Kolka and Ikla members lies close to the stratigraphy (Kiipli et al., 2006) represent a relatively complete level of appearance of Demirastrites triangulatus (e.g., Kaljo and Llandovery sequence, and is studied here. A secondary, less biostrati- Martma 2000). In the Ruhnu core section from Ruhnu Island (located 13 graphically well-constrained δ Ccarb dataset comes from a section in about 55 km to the west from Ikla) the occurrence of graptolites the Pancake Range, Nevada, USA was also investigated (Harris and Coronograptus cyphus in the Kolka Member and Demirastrites Sheehan, 1998)(Figs. 1 and 3). The Pancake Range section, although triangulatus in the Ikla Member allow for precise graptolite zonations dolomitized and not an ideal target for geochemical investigation, still of these units, as corresponding to the C. cyphus and D. triangulatus represents one of the only opportunities to sample a thick, relatively graptolite zones, respectively (Kaljo and Martma, 2000). The first complete Llandovery sequence anywhere in North America (Sheehan, occurrence of chitinozoan Eisenackitina dolioliformis dates the Rumba 1980; Harris and Sheehan, 1998) and fill an important gap in the δ13C Formation as late to early Telychian (Kaljo and Martma, composite curve for the Great Basin region (Saltzman, 2005). 2000; Grahn, 2006). The occurrence of the Stricklandia laevis in the Rumba Formation in the Viki core section (western 2. Geological setting Saaremaa) further constrains this unit to the Telychian Stage (Johnson et al., 1991). In the Kirikuküla core (located about 100 km to the 2.1. Ikla drill core, Estonia (Baltica) northwest from Ikla) the cosmopolitan chitinozoan Angochitina longicollis and Baltic chitinozoan Conochitina proboscifera also appear, During the Llandovery, the Baltica Palaeocontinent moved toward in the Velise Formation, indicating Telychian age. a more equatorial setting from its temperate latitudinal setting in the Middle Ordovician. This resulted in significant climatic changes for the 2.2. Pancake Range, Nevada () region (Torsvik et al., 1996). In Estonia, the Llandovery sequence was formed along the western shoreline of the Baltica paleocontinent, and A Llandovery carbonate section representing a sequence deposited four distinct facies have been previously defined ranging from in a marginal marine to shelf environment during platform evolution

Fig. 1. Paleogeographic reconstruction of the Llandovery, with dots indicating the general areas of the Pancake Range (Nevada) and Ikla core (Estonia) localities (after Witzke, 1990). 266 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275

Fig. 2. Generalized map of the northern Baltic region, showing the location of the Ikla core. Key: 1, dolomites; 2, skeletal grainstones; 3, skeletal pack- and wackestones; 4, marlstones; 5, red and green marl- and mudstones; 6, eroded margin of the Rhuddanian; 7, facies boundaries; 8, borehole. Numbers on the map mark facies belt: 1, inshore lagoons and tidal flats; 2, nearshore high-energy shoals; 3, shallow mid-shelf; 4, deeper outer shelf. Shaded area indicates inferred land during the Llandovery (after Kaljo and Martma, 2000). from a carbonate ramp to rimmed shelf environment was sampled in Member tongue, and the Jack Valley and Decathon members (Harris Nevada (Fig. 3; Harris and Sheehan, 1998). The Pancake Range section and Sheehan, 1997). Additionally, six transgressive–regressive cycles lies somewhat east of the outer margin of the rimmed shelf which was separated by unconformities, identified as S1–S6, have been previ- located on the western margin of Laurentia, at about ∼10–15° S ously identified in the Laketown Dolostone Formation (Harris and latitude (Harris and Sheehan, 1998). Succession of eustatic changes in Sheehan, 1997). sea level has been reconstructed for this section using sequence The Pancake Range section is biostratigraphically dated based on stratigraphy based on methods in Johnson (1996) and on depositional macrofossils which are abundant throughout the section. The facies (Harris and Sheehan, 1998). The studied interval corresponds to Ordovician–Silurian boundary is marked by a rapid change in part of the Laketown Dolostone Formation, which here can be brachiopod fauna. At this level, in the early Silurian cosmopolitan subdivided into six separate members (in ascending order): the Virgiana community V. utahensis becomes the dominant brachiopod. Tony Grove, High Lake and Gettel members, the upper High Lake At the same time, diversity of brachiopods drops dramatically at the transition from Ordovician into Silurian. Characteristic of the Rhudda- nian and middle Aeronian stages is the occurrence of Virgiana. This brachiopod is abundant in some intervals in the Tony Grove and High Lake members. In the upper part of the High Lake Member Pentamerus is common, dating this interval as upper Aeronian to lower Telychian. Pentameroides in the uppermost High Lake and Gettel members assigns these strata to the middle and upper Telychian. In the strata transitional from Llandovery to Wenlock brachiopod diversity in- creases, and new communities such as Spirinella and Atrypina appear (Sheehan, 1980). In the High Lake Member, Verticillopopra dasyclada- cean algae are present, as well as Amplexoides radicosi and Tryplasma sp. corals. In the Gettel, Palaeocyclus sp. and Tryplasma sp. corals have also been identified (Harris and Sheehan, 1998).

3. Methods

3.1. Laboratory methods

A total of 135 carbonate samples from the Ikla core in Estonia and from field collecting in the Pancake Range of central Nevada, USA, were analyzed for 87Sr/86Sr and δ13C. Rock samples were first cut using a water-based diamond-bladed saw to produce thin-section billets, then cleaned using ultrapure water (deionized, 18 MΩ)inan ultrasonic bath to remove excess sediment. Fine-grained micritic components were preferentially microdrilled for analysis. Powders were analyzed for 87Sr/86Sr and Sr concentration ([Sr]) in the Radiogenic Isotope Laboratory at The Ohio State University using Sr purification and mass spectrometry procedures described in detail by Foland and Allen (1991). Sr was extracted from powders using ∼ Fig. 3. Generalized map of the Great Basin, showing the location of the Pancake Range ultrapure reagents; powder aliquots of 25 mg were pretreated with section and Llandovery depositional facies (after Harris and Sheehan, 1998). 1 M ammonium acetate (pH 8) and then leached in 4% acetic acid J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 267

Fig. 4. Stratigraphy of Llandovery rocks in southern Estonia (after Kaljo and Martma, 2000) correlated with generalized graptolite zones (after Koren et al., 1996) and zones (after Bergström et al., 1998; Männik, 2007). Black rectangle indicates approximate range of Early Silurian K-bentonites deposited in Estonia (Bergström et al., 1992).

(Montañez et al., 1996). The leachate solution was separated from 3.2. Primary versus secondary signals residue and then spiked with an 84Sr tracer. Samples were purified for Sr using a cation exchange resin and a 2 N HCl based ion-exchange. One of the most important issues in analyzing trends in 87Sr/86Sr is Purified Sr was then loaded with HCl on a Re double-filament config- the potential for secondary influences to alter the primary seawater uration. Isotopic compositions were measured using dynamic multi- values. In general, in samples that are diagenetically altered or in collection with a MAT-261A thermal ionization mass spectrometer. which non-marine strontium is present in Rb or Sr-rich siliciclastic The Laboratory value for the SRM 987 standard is (87Sr/86Sr)= phases (e.g., clays), the 87Sr/86Sr is shifted to more radiogenic values. 0.710242±0.000010 (one-sigma external reproducibility). For the We attempted to minimize leaching of Sr from non-carbonate phases 87Sr/86Sr values the associated uncertainties given are for two-sigma by pretreatment with 1 M ammonium acetate as described above mean internal reproducibilities, typically based upon 100 measured (after Montañez et al., 1996). In order to address diagenesis in this ratios. The 87Sr/86Sr reported ratios are normalized for instrumental study, the Sr contents of the analyzed rock were plotted against the fractionation using a normal Sr ratio of 86Sr/88Sr=0.119400. 87Sr/86Sr isotopic ratio (Table 1; Data Repository Fig. 1). When the For organic carbon isotope analysis, micritic (fine-grained) com- rock is diagenetically altered, Sr concentrations are in most cases ponents were microdrilled from the same cleaned thin-section billets reduced (Montañez et al., 1996; Azmy et al., 1999; Brand et al., 2006; used for the Sr analyses. Sample powders were accurately weighed Halverson et al., 2007). However, since initial ocean Sr contents can (∼1 g) and acidified using 6 N HCl to remove carbonate minerals. differ, as well as the original mineralogy (calcite versus aragonite), Insoluble fractions were then repeatedly rinsed and centrifuged in there is no set standard for rejecting 87Sr/86Sr values based on Sr ultrapure water, and then dried at 80 °C overnight. The remaining concentrations and evaluation must be made on a case-by-case basis. residues were homogenized using a mortar and pestle, and then Based upon the range of Sr contents in samples from the Ikla core, a accurately weighed into tin capsules. Samples were combusted with a threshold of 100 ppm was used to exclude data points from the 87 86 Costech Elemental Analyzer and the resulting CO2 gas analyzed for plotted Llandovery Sr/ Sr curve. Three samples with concentra- δ13C through a Finnigan Delta V plus stable isotope ratio mass tions of less than 100 ppm were considered to be diagenetically spectrometer under continuous flow using an open-split CONFLO III altered, and two of these were significantly more radiogenic than the interface in the Stable Isotope Biogeochemistry Laboratory at The surrounding data points (Table 1). However, we also note that some Ohio State University. Carbon isotope ratios presented here are 87Sr/86Sr outliers did not have Sr concentrations that were signifi- reported in per mil notation relative to the Vienna Peedee Belemnite cantly lowered relative to adjacent samples. 13 limestone standard (‰ V-PDB). Repeated measurements of the IAEA- Several earlier studies indicate that δ Ccarb values are largely rock- CH7 standards were ±0.15‰ for δ13C and ±1.0% for %C (1σ). Weight buffered (i.e. the carbon of the new mineral phase is derived from the percent of total organic carbon (TOC) in samples is determined by old mineral phase) during the diagenetic processes that typically comparison of voltages for the ion beam intensities of masses 44, 45, affect marine carbonates (Banner and Hanson, 1990). This appears to + and 46 CO2 between our samples and known wt.% carbon of the be the case even for dolostones, e.g. samples coming from the Pancake gravimetric standard Acetanilide. Range section analyzed in this study. For example, a global Upper 13 13 For δ Ccarb, samples of the Pancake Range section were drilled on δ C excursion (SPICE event) is recorded globally in clean carbonate surfaces for approximately 500 μg of powder. For limestones (Saltzman et al., 1998) as well as also in dolomitized 18 13 each sample, a 75–95 µg subsample was analyzed for δ O (All values sections (Glumac and Walker, 1998; Kouchinsky et al., 2008). δ Corg 13 are reported in permil relative to V-PDB) using an automated is likely to be inherently noisier than δ Ccarb, mainly due to the Carbonate Kiel device coupled to a Finnigan Delta V plus stable heterogeneity of the organic matter analyzed (e.g., Hayes et al., 1999). isotope ratio mass spectrometer in the Stable Isotope Biogeochemis- It is also possible that the differences in the carbon isotope curves try Laboratory at The Ohio State University. Samples were acidified from carbonates and from organic matter are related to changes in under vacuum with 100% ortho-phosphoric acid, the resulting CO2 atmospheric CO2 that can affect photosynthetic fractionation (Kump cryogenically purified, and delivered to the mass spectrometer. The and Arthur, 1999). One criterion for discerning primary versus 13 standard deviation of repeated measurements of an internal standard secondary signals in δ Corg includes consideration of the percentage was ±0.03‰ for δ13C and ±0.09‰ for δ18O(1σ). of organic matter in the samples. For example, in the Ikla core TOC 268 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275

Table 1 Table 1 (continued) Stable and radiogenic isotope data from Ikla Core, Estonia. Blank spaces indicate that 87 86 13 13 18 Meters Sr/ Sr Sr error Sr δ Ccarb δ Corg δ O Formation measurement was not taken. (ppm) 87 86 δ13 δ13 δ18 Meters Sr/ Sr Sr error Sr Ccarb Corg O Formation 470.0 0.708151 0.000009 260.3 2.117 −27.64 −3.905 Kolka (ppm) 471.2 1.580 −5.286 Kolka − 260.0 1.793 −3.726 Riga 473.0 2.297 3.937 Kolka − 265.0 3.659 −3.667 Riga 475.1 0.837 4.681 Kolka − 269.2 3.370 −3.311 Riga 477.3 2.412 3.702 Kolka − 275.0 3.413 −3.606 Riga 480.0 1.801 4.007 Kolka 481.0 2.014 −3.682 Slitere 280.4 3.588 −3.795 Riga 482.7 2.090 −4.060 Slitere 286.5 3.414 −26.69 −4.234 Riga − 288.5 3.114 −26.67 −4.104 Riga 484.9 1.982 4.077 Slitere − − 289.2 0.708441 0.000014 136.7 3.236 −26.40 −4.146 Velise 487.9 0.708105 0.000009 146.9 1.846 27.71 4.158 Slitere − 294.3 1.794 −5.212 Velise 489.7 1.799 4.645 Slitere − 296.0 0.708423 0.000009 138.3 2.474 −26.80 −4.746 Velise 494.0 1.388 4.866 Slitere − 300.8 0.708359 0.000008 148.9 2.241 −26.49 −5.437 Velise 496.0 0.347 4.187 Pusku beds − − 305.2 0.708334 0.000011 143.0 1.500 −27.73 −5.265 Rumba 497.5 0.708127 0.000007 392.2 0.112 27.40 4.767 Pusku beds 498.0 −0.890 −3.686 Pusku beds 308.1 0.708201 0.000008 227.2 0.386 −27.84 −4.988 Rumba 498.7 −0.953 −3.499 Pusku beds 310.1 0.169 −4.724 Rumba − 312.1 0.708217 0.000011 236.1 −0.028 −28.72 −5.038 Rumba 500.2 1.024 4.661 Ohne − − 313.2 −0.509 −5.621 Rumba 502.6 0.708171 0.000008 249.0 1.358 27.81 3.808 Ohne − 316.6 0.708184 0.000011 318.1 0.277 −30.02 −3.725 Rumba 505.0 1.392 3.913 Ohne − − 318.4 0.005 −4.563 Rumba 507.7 0.708135 0.000008 104.0 0.915 27.97 4.907 Ohne − 320.0 −0.946 −4.960 Rumba 510.5 0.759 3.823 Ohne − − 320.3 0.70814 0.00001 242.7 −29.50 Rumba 513.0 0.708341 0.000008 139.1 1.280 26.91 3.869 Ohne 515.0 1.465 −3.684 Ohne 322.0 0.335 −5.021 Rumba − − 323.1 0.70816 0.000008 247.1 −28.59 Staicele 516.8 0.708132 0.000012 154.7 1.368 27.21 3.478 Ohne − 323.5 0.960 −4.841 Staicele 518.4 1.970 4.034 Ohne − 324.5 1.506 −4.967 Staicele 520.0 0.956 4.727 Ohne − 327.5 2.119 −29.14 −5.065 Staicele 521.6 0.572 4.690 Ohne − 330.9 1.911 −5.061 Staicele 522.8 0.710 4.232 Ohne − 334.8 0.708067 0.000009 832.1 2.423 −28.79 −5.035 Staicele 524.5 0.773 4.904 Ohne 526.0 1.511 −3.503 Ohne 338.0 2.562 −5.084 Staicele 527.8 0.708464 0.000019 35.2 1.327 −26.54 −2.951 Ohne 340.1 0.708075 0.000009 1098.0 1.927 −27.56 −4.636 Staicele − 342.7 2.116 −5.226 Staicele 527.9 0.70802 0.000007 127.3 27.24 Ohne 344.9 0.70809 0.000011 887.1 2.075 −29.25 −4.989 Staicele 527.9 0.708034 0.000011 90.6 Ohne − 347.3 2.395 −4.976 Staicele 528.4 2.387 4.653 Saldus 350.0 2.416 −5.152 Staicele 529.1 0.707986 0.000011 174.3 Saldus − 352.5 2.428 −5.000 Staicele 530.0 2.520 4.615 Saldus − 353.0 0.70809 0.000008 917.8 −28.01 Staicele 531.1 0.708193 0.000016 120.6 26.76 Saldus 532.3 0.708111 0.000008 142.0 3.003 −26.60 −3.259 Saldus 354.5 0.708093 0.000008 341.9 2.376 −28.97 −5.050 Staicele 533.7 0.708154 0.000009 113.2 3.219 −26.39 −3.285 Saldus 357.0 2.586 −3.912 Staicele − 358.7 2.091 −4.958 Staicele 534.6 2.496 3.407 Saldus 361.5 2.338 −4.938 Staicele 363.8 0.708127 0.000008 423.1 1.562 −27.56 −4.025 Staicele 369.0 1.348 −4.246 Lemme − − 371.5 0.708068 0.000013 907.6 1.276 28.57 4.260 Lemme ranges from 0.06% to 0.34% and shows no discernable trend in relation 373.4 1.684 −5.703 Lemme δ13 376.1 1.969 −4.475 Lemme to Corg values (Data Repository Fig. 2). Parallel changes observed in 13 13 379.0 1.839 −5.175 Lemme both δ Ccarb and δ Corg in the Telychian and part of the Aeronian are 381.4 2.417 −4.335 Lemme likely a reliable indicator of preservation of primary seawater values 384.4 0.708069 0.000009 648.0 3.189 −27.45 −4.260 Lemme in the rock record (e.g., Joachimski et al., 2002), but different trends in 386.8 0.708108 0.000019 233.1 2.558 −28.82 −4.737 Lemme the Rhuddanian between δ13C and δ13C may indicate a potential 391.0 2.703 −4.494 Lemme carb org 393.5 2.828 −4.246 Lemme diagenetic overprint. 396.0 0.70811 0.000009 206.6 2.396 −28.27 −4.789 Lemme 398.0 3.211 −3.612 Ikla 4. Results 399.2 0.708056 0.000007 971.6 2.105 −27.38 −5.608 Ikla 402.0 0.708078 0.000009 1108.0 2.736 −28.73 −3.429 Ikla 403.1 2.881 −4.432 Ikla 4.1. Ikla drill core, Estonia 407.8 0.708027 0.000009 1083.0 2.959 −28.84 −3.613 Ikla 411.6 3.161 −4.337 Ikla Sr isotope data from Ikla core shows some variability in the − 415.3 3.119 4.535 Ikla Rhuddanian part (Õhne and lower Saarde formations) of the core, 419.7 0.708038 0.000008 1396.0 3.196 −28.22 −4.624 Ikla ranging from 0.708020 to 0.708341, averaging at 0.7081 (Table 1, 421.0 3.354 −4.397 Ikla 86 421.3 0.708044 0.000008 857.9 −27.94 Ikla Fig. 5). In the Aeronian middle and upper Saarde Formation the Sr/ 424.0 3.604 −4.448 Ikla 88Sr values are much less variable and stay between 0.708027 and 428.9 0.708072 0.000012 318.2 3.676 −28.46 −4.525 Ikla 0.708127. A rapid rise to more radiogenic values starts in the − − 434.0 0.708043 0.000013 967.9 3.258 29.05 4.647 Ikla Telychian part of the section, trending from 0.708075 in the upper 438.0 1.731 −4.587 Ikla 443.0 2.496 −29.41 −4.351 Ikla Staicele member of the Saarde Formation to 0.708441 in the upper 447.0 1.983 −5.077 Ikla Velise Formation. − − 13 451.3 0.708046 0.000006 510.3 2.209 28.16 5.105 Ikla The δ Corg curve from the Ikla core exhibits both similarities (in 455.0 2.356 −4.602 Ikla Telychian) and differences (Rhuddanian–Aeronian interval) when 458.3 0.708275 0.000011 69.3 1.960 −29.20 −4.996 Ikla compared with the δ13C curve generated by Kaljo and Martma 460.0 0.952 −4.473 Kolka carb 462.5 0.708239 0.000009 136.8 1.975 −28.82 −5.083 Kolka (2000) (Fig. 6, Table 1). While there is a general decrease in values in 466.0 2.332 −3.757 Kolka the Rhuddanian and a general increase in values in the lower Aeronian 469.1 1.196 −4.766 Kolka of both curves, here we wish to emphasize the significant parallel J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 269

Fig. 5. 87Sr/86Sr record from the Ikla core, stratigraphy after Kaljo and Martma, 2000. Thick lines in column represent submarine unconformities. Open circles represent samples which fall below the diagenetic threshold of 100 ppm of Sr. Generalized graptolite zones (after Koren et al., 1996): 1, acuminatus;2,vesiculosus;3,cyphus;4,pectinatus–triangulatus; 5, argenteus;6,convolutus;7,sedgwickii;8,guerichi–turriculatus–crispus;9,griestoniensis–crenulata–spiralis–insectus; 10, centrifugus–murchisoni. Abbreviations: W., Wenlock; S., ; Tely., Telychian; Rum., Rumba; Vel., Velise.

13 13 Fig. 6. δ Ccarb (Kaljo and Martma, 2000) and δ Corg data (this study) from the Ikla core, 3 point running average through the raw data, stratigraphy after Kaljo and Martma, 2000. Abbreviations and explanation of symbols as in Fig. 5. 270 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275

13 changes in the two curves in the Telychian. δ Corg values begin the Decathon Member lowering to approximately +0.5‰. Trends in 13 around −26.5‰, and trend toward lighter values through the the δ Ccarb data from the Pancake Range reflect similar trends in the 13 Rhuddanian. In the lower Aeronian, δ Corg values drop to their Ikla Core (Fig. 8), with a negative excursion present showing values of minimum of −29.5‰, and then increase up to −28‰. In the middle approximately 0‰ in the earliest Telychian. Similarly, the Pancake 13 and upper Aeronian, the δ Corg values tend to fluctuate (±1.5‰ Range data also trends to more positive values after this negative variations) around −28‰. A significant δ13C minimum occurs in the excursion, reaching +2.5‰ in the late Telychian, and even +3‰ in the 13 earliest Telychian Rumba Formation, with δ Corg values between early Wenlock, similar to the trends in the Ikla core (Table 2, Fig. 8). 13 −28.6 to −30.0‰ and δ Ccarb ∼0.0 to −1.0‰. Values in both curves begin to increase in the middle Telychian, through the lower Wenlock. 5. Discussion 13 13 δ Corg values rises with a magnitude of +3.5‰, and δ Ccarb increase by +4.5‰ (Table 1). Presumably, this increase marks the beginning of Our high-resolution Sr isotope curve from the Llandovery in the 13 13 87 86 the δ C excursion, a positive excursion in δ Ccarb values of Ikla core from Estonia demonstrates a similar rise of Sr/ Sr values as +3‰ to +4‰ in the early Wenlock (Saltzman, 2001; Cramer and previously recognized by Azmy et al. (1999), and also suggests that an Saltzman, 2005, 2007). increase in the rate of this rise may have occurred in the lower Telychian (Fig. 9). Furthermore, by analyzing 87Sr/86Sr as well as δ13C 13 13 4.2. Pancake Range section, Nevada, USA (δ Corg this study, and δ Ccarb from Kaljo and Martma, 2000), it is evident that both the strontium and carbon cycles underwent major The Pancake Range section is less biostratigraphically controlled changes during the Aeronian–Telychian study interval (Fig. 10). Our 13 than the Ikla Core and dolomitized, yet still records reliable δ Ccarb data trends from the earliest Llandovery Rhuddanian Stage are less values. In the basal Rhuddanian (present in the basal Tony Grove coherent than younger strata and will require additional study to 13 Member), δ Ccarb values are about +1‰, and increase up to +2.5‰ discern whether these trends indeed record primary, global changes through the member (Fig. 7; Table 2). Near the top of the Tony Grove in seawater chemistry or mainly secondary (local) influences. Thus, in 13 Member, δ Ccarb values drop to between −0.5 and 0‰, but then rise the discussion, which follows below, the focus is on the records again rapidly up to +2‰ in the lower High Lake Member. From the spanning the Aeronian and Telychian Stages only. lower High Lake Member just through the Aeronian/Telychian 13 boundary, δ Ccarb values decrease gradually reaching near 0‰. 5.1. Strontium isotopes and early Silurian tectonics 13 Above this level δ Ccarb values again increase and reach +2‰ at the base and +2.5‰ in the middle of the Gettel member. In the upper The observed increase in the rate of 87Sr/86Sr rise in early Telychian part of the Gettel Member, through the upper High Lake Member can potentially be linked to changes in fluxes of Sr into the oceans or, 13 tongue, δ Ccarb values decrease again to +0.75‰. The drop in the alternatively, it may be an artifact caused by a decrease in Gettel Member is followed by rapid increase of values in the Jack sedimentation rates (e.g., McArthur and Howarth, 2004). Based on Valley Member, up to +3‰. After this maximum, values decrease in the comparison with the Llandovery data by Azmy et al. (1999) from

13 Fig. 7. δ Ccarb data from the Pancake Range, NV with 3 point running average through the raw data. Stratigraphy after Harris and Sheehan (1997). Grayed boxes in column indicate dark colored carbonates, white boxes in column indicate light colored carbonates. Abundant burrows and laminae structures are present throughout the whole section. Member abbreviations: G, Gettel; HL, Upper High Lake Tongue; JV, Jack Valley; D, Decathon. J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 271

Table 2 Table 2 (continued) Stable isotope data from Pancake Range, Nevada. Blank spaces indicate that 13 18 Meters δ Ccarb δ O Formation Member measurement was not taken. 260.0 0.852 −4.62 Laketown Gettel δ13 δ18 Meters Ccarb O Formation Member 260.5 0.899 −5.26 Laketown High Lake − 2.0 −0.072 −2.78 Ely Springs Flouride 265.0 1.492 5.09 Laketown High Lake − 4.3 0.223 −3.22 Ely Springs Flouride 269.5 1.291 6.11 Laketown High Lake − 6.8 −0.098 −3.23 Ely Springs Flouride 274.0 0.700 7.33 Laketown High Lake − 8.3 0.589 −5.49 Ely Springs Flouride 278.5 1.135 6.73 Laketown High Lake − 8.4 0.808 −5.57 Ely Springs Flouride 282.0 1.183 6.24 Laketown High Lake 285.0 2.170 −3.98 Laketown Jack Valley 8.6 0.748 −4.61 Ely Springs Flouride 289.5 1.060 −5.84 Laketown Jack Valley 8.9 0.297 −3.97 Ely Springs Flouride − 9.1 0.938 −5.68 Ely Springs Flouride 294.0 3.006 4.19 Laketown Jack Valley − 9.2 0.483 −3.31 Ely Springs Flouride 298.5 1.647 3.77 Laketown Jack Valley − 9.3 0.300 −2.46 Ely Springs Flouride 303.0 1.280 6.03 Laketown Decathon − 9.6 0.415 −4.54 Ely Springs Flouride 307.5 0.753 6.33 Laketown Decathon − 9.8 −0.099 −4.45 Ely Springs Flouride 312.0 1.220 5.28 Laketown Decathon 10.1 0.626 −3.79 Ely Springs Flouride 10.3 0.597 −4.18 Ely Springs Flouride Anticosti Island, Canada, which also show an important increase in the 10.6 0.926 −5.31 Ely Springs Flouride rate of rise in the 87Sr/86Sr values at approximately at the same time in 10.8 0.463 −3.23 Ely Springs Flouride the Telychian (Fig. 9), we interpret this shift as a result of changes in 11.1 0.780 −2.90 Ely Springs Flouride 11.3 0.315 −3.60 Ely Springs Flouride the marine Sr cycle. Variations in marine Sr fluxes may be caused by 11.6 0.239 −2.45 Ely Springs Flouride changes in the rates of hydrothermal interaction with basaltic rocks, 13.1 1.758 −5.93 Ely Springs Flouride continental weathering, or changes in the 87Sr/86Sr of the continental − 15.1 0.881 1.61 Ely Springs Flouride source material being weathered (Hodell et al., 1990; Richter et al., 17.0 1.501 −5.66 Ely Springs Flouride 19.0 1.252 −6.42 Ely Springs Flouride 1992; Farrell et al., 1995). The weathering of continental, non-volcanic 87 86 23.0 2.378 −4.97 Ely Springs Flouride silicate rocks will introduce relatively radiogenic Sr (higher Sr/ Sr 25.0 2.742 −6.10 Ely Springs Flouride ratio) into rivers (Berner, 2006). The riverine input of Sr into the basin 29.0 1.529 −5.66 Ely Springs Flouride may increase during tectonic uplift caused by, e.g., continent– 32.5 1.588 −6.03 Ely Springs Flouride continent collision. Tectonic uplifts may also expose older, highly 35.0 2.272 −3.31 Ely Springs Flouride ∼ 37.0 0.573 −4.07 Ely Springs Flouride radiogenic ( 0.7116 or higher) silicates (Raymo et al., 1988; Richter 41.0 0.712 −6.69 Ely Springs Flouride et al., 1992). Hydrothermal interaction with fresh oceanic (basaltic) 41.0 1.086 −6.33 Laketown Tony Grove crust, or the weathering of continental mafic volcanics, both increase 43.0 1.038 −6.74 Laketown Tony Grove ∼ − the input of less radiogenic Sr ( 0.7035) to the oceans (Stern, 1982; 47.0 0.982 7.23 Laketown Tony Grove fi 51.0 0.845 −6.04 Laketown Tony Grove Palmer and Elder eld, 1985; Davis et al., 2003). 57.0 1.676 −3.50 Laketown Tony Grove The best evidence for a causal connection between tectonism and 63.0 1.018 −6.62 Laketown Tony Grove the early Telychian rise in seawater 87Sr/86Sr is based on the age of a 67.0 1.376 −6.26 Laketown Tony Grove major tectonic unconformity in the sedimentary succession of the 71.0 1.003 −5.69 Laketown Tony Grove Appalachian basin (Ettensohn and Brett, 1998). This unconformity, at 75.0 1.301 −8.03 Laketown Tony Grove 78.0 2.226 −10.57 Laketown Tony Grove the base of the Clinton Group in the eastern United States, is dated as 82.0 1.887 −9.57 Laketown Tony Grove early Telychian by Berry and Boucot (1970) and Rickard (1975).It 88.0 2.587 −10.82 Laketown Tony Grove likely resulted from the uplift and migration of the forebulge that − 97.0 2.191 9.84 Laketown Tony Grove formed during the early stages of the flexural subsidence creating the − 100.0 0.345 5.38 Laketown Tony Grove fl 106.0 −0.126 −2.84 Laketown Tony Grove Appalachian foreland basin (part of Salinic tectophase I, which re ects 115.0 2.123 −8.60 Laketown High Lake oblique subduction of Avalonian terranes under Laurentia) (Etten- 118.0 1.166 −8.79 Laketown High Lake sohn and Brett, 1998). A tectonic origin for this sequence boundary is 124.0 1.965 −8.90 Laketown High Lake also supported by an overlying condensed section, indicative of 133.0 1.732 −9.26 Laketown High Lake flexural subsidence and relative sea-level rise that cut off sediment 139.0 2.179 −8.80 Laketown High Lake 145.0 0.907 −6.28 Laketown High Lake supply to the shelf and basin (Ettensohn and Brett, 1998). 151.0 1.673 −7.57 Laketown High Lake While the final suturing of Baltica and Laurentia (Caledonian 157.0 1.603 −7.67 Laketown High Lake orogeny) occurred in the late Silurian–early , initial colli- − 163.0 1.672 8.61 Laketown High Lake sions of these continents and intervening terranes occurred in the 169.0 0.901 −5.81 Laketown High Lake 175.0 0.751 −7.84 Laketown High Lake Telychian and are collectively referred to as the Scandian orogeny 181.0 0.276 −6.31 Laketown High Lake (Gee, 1975). Evidence of Telychian tectonism is also supported by U– 184.0 0.277 −5.86 Laketown High Lake Pb dating of granites from the Canadian Appalachians in New 187.0 0.074 −6.25 Laketown High Lake Brunswick, Canada (Bevier and Whalen, 1990). − 190.0 1.057 6.57 Laketown High Lake The tectonic origin of Llandovery sequence boundaries in the 193.0 0.754 −6.33 Laketown High Lake 196.0 0.693 −7.30 Laketown High Lake Appalachian basin (Salinic tectophase I), together with the evidence 199.0 0.224 −4.72 Laketown High Lake for the Scandian orogeny in other regions, suggests that the early 202.0 0.137 −5.12 Laketown High Lake Silurian was an important period of global tectonic reorganization, − 208.0 0.132 2.96 Laketown High Lake particularly along the Caledonian suture (Ettensohn and Brett, 1998). 214.0 0.100 −3.15 Laketown High Lake 220.0 0.529 −5.30 Laketown High Lake Weathering of exhumed radiogenic silicate rocks may have continued 87 86 226.0 0.777 −6.66 Laketown High Lake to drive the Sr/ Sr in seawater towards more radiogenic values 232.0 1.108 −6.12 Laketown High Lake throughout the Telychian. 238.0 0.608 −5.91 Laketown High Lake 244.0 0.984 −7.69 Laketown High Lake 5.2. Early Silurian K-bentonites and the timing of tectonic events 247.0 1.863 −3.25 Laketown Gettel 251.5 2.062 −4.55 Laketown Gettel 255.5 2.534 −4.35 Laketown Gettel Stratigraphic evidence from K-bentonite studies of Silurian strata (Bergström et al., 1998) also reveals a potential link between the early 272 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275

13 13 Fig. 8. δ Ccarb data from the Ikla core (Kaljo and Martma, 2000) plotted against δ Ccarb data from the Pancake Range (this study). Gray box is emphasizing negative excursion present in both sections, which coincides with 87Sr/86Sr inflection point. Abbreviations: Tely., Telychian; S., Sheinwoodian; Rhuddan., Rhuddanian; Sheinwo., Sheinwoodian.

Telychian increase in seawater 87Sr/86Sr and convergent margin In some localities, the thicknesses of the Osmundsberg K-bentonite tectonics. Llandovery K-bentonites are known from sections in Europe bed reaches up to 115 cm, and the bed has been traced over a distance (Baltica and Avalonia) and eastern North America (Appalachians) about 2000 km across Estonia and the Baltic region (Bergström et al., sections (Bergström et al., 1992; Kiipli and Kallaste, 2002). The largest 1998). and most significant K-bentonite is the Osmundsberg, but several These Early Silurian K-bentonite beds have been interpreted to be additional beds have been documented from a number of localities in the result of a series of explosive ash falls following large-scale Europe, including Sweden, Estonia, Norway and the British Isles eruptions of felsic magma, with the event that caused the Osmunds- (Bergström et al., 1992, 1998; Lehnert et al., 1999; Kiipli et al., 2006). berg deposition likely having lasted a few weeks (Huff et al., 1998).

Fig. 9. Evolution of 87Sr/86Sr through the Llandovery and lower Wenlock based on this study, with comparison to data from Azmy et al. (1999) and Ruppel et al. (1996). Radiometric dates from Gradstein et al. (2004). Abbreviations and explanation of symbols as in Fig. 5. Additional abbreviations: Wenl., Wenlock; Shein., Sheinwoodian. J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 273

87 /86 13 Fig. 10. Sr Sr (this study) and δ Ccarb (Kaljo and Martma, 2000) record of the Ikla core, gray bar highlighting range of K-bentonite beds. Abbreviations and explanation of symbols as in Fig. 5.

13 These ash falls are well-dated by biostratigraphy as late Aeronian to Telychian (Fig. 6) may also be recorded in the δ Corg curve from the 13 early Telychian, correlating with the convolutus to turriculatus Canadian Arctic (Melchin and Holmden, 2006), the δ Ccarb curve graptolite zones range (Bergström et al., 1992, 1998). In Estonia, the from several other studied core sections in Estonia (Kaljo and Martma, 13 Osmundsberg K-bentonite occursinthesamestrata(Rumba 2000), and the δ Ccarb from the Pancake Range (Fig. 7). Carbon Formation) where rising of 87Sr/86Sr values are recorded, and several isotope excursions may have multiple origins related to changes in K-bentonites are recorded in the upper Aeronian, immediately nutrient cycling and organic carbon burial or preservation, or to preceding and coinciding with this rise, and in the lower Telychian weathering of carbonate versus organic matter on land (Kump and as well (Fig. 10). Arthur, 1999). Negative δ13C excursions may also potentially result

If a coincidence in timing between important K-bentonite beds from release of volcanically generated CO2 (e.g., Payne and Kump, and changes in 87Sr/86Sr is supported by future high-resolution 2007). biostratigraphic and chemostratigraphic studies, then the weathering A large influx of volcanic CO2 to the atmosphere would contain of old radiogenic crust with high 87Sr/86Sr ratios related to early isotopically light δ13C (about −5.0‰)(Kump and Arthur, 1999), and Silurian felsic volcanics may have been a contributing factor to the rise could have contributed in small part to the early Telychian negative in seawater 87Sr/86Sr. However, because these K-bentonite beds have δ13C shifts in the Rumba Formation observed in the Ikla core (Kaljo only been identified in nearby sections and were not preserved in the and Martma, 2000). However, only if this volcanism generated the same formations described from the Ikla core, it is difficult to release of CO2 from organic-rich sedimentary units (e.g., Svensen et determine how accurately deposition of these beds coincided with al., 2009) could the magnitude and timing of the negative excursion in the rise in 87Sr/86Sr values, and additional study is required to assess the Rumba Formation be reproduced by modeling (e.g., Kump and the validity of this connection. For example, recent work by Munnecke Arthur, 1999; Payne and Kump, 2007). Furthermore, Payne and Kump and Männik (2009) argues that the base of the Rumba Formation may (2007) modeled a negative δ13C shift related to the formation of a be as old as the upper Aeronian based on , large igneous province that represents one of the largest eruptions of and this also has implications for the age of the associated K- the past half billion , and thus a model would need to be bentonites. Furthermore, because it may not be possible to determine constructed that takes into account the aerial extent of explosive precisely where the rise in 87Sr/86Sr begins due to the fact that the volcanism observed for the early Telychian and its interaction with Rumba Formation is bracketed by two unconformities of uncertain associated sedimentary units (e.g., Ordovician black shales) to duration (Kaljo and Martma, 2000), additional Sr isotope work on estimate the effect on δ13C. Climate changes and associated episodes nearby cores may be needed. of glaciations in late Aeronian–early Telychian have been proposed by Caputo (1998), and may also have linkages to the global carbon cycle 5.3. Early Silurian carbon cycling and δ13C. Although the causes of the late Llandovery glaciation are not well understood, Early Silurian glaciations could be related to ongoing

Early Telychian volcanic ash falls (e.g., Osmundsberg K-bentonite) tectonic events if imbalances between volcanic outgassing of CO2 and and associated tectonic episodes (Salinic and Scandian Orogenies) consumption of atmospheric pCO2 via silicate weathering occurred could have influenced the Sr flux into the oceans, and may have also (e.g., Young et al., 2009). had a significant effect on carbon cycling in the Early Silurian. A Alternatively, the initial cooling and glaciation may have begun in negative δ13C shift similar to that which we observe in the Ikla core in response to enhanced bioproductivity during the preceding Aeronian both carbonate and organic matter during the late Aeronian–Early time (e.g. Kiipli et al., 2004) that lowered atmospheric pCO2. This 274 J.C. Gouldey et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 264–275 productivity event apparently caused an increase in organic carbon Appendix A. Supplementary data burial (e.g., basinal graptolitic black shales in the east Baltic) and may correlate with the broad positive mid-Aeronian δ13C excursion seen in Supplementary data associated with this article can be found, in the Ikla core and elsewhere (Kaljo and Martma, 2000). However, the the online version, at doi:10.1016/j.palaeo.2010.05.035. 13 mid-Aeronian positive δ Ccarb excursion seen in the Ikla core is not fi δ13 con rmed by the relatively low resolution Corg data presented here References (Fig. 6), and future high-resolution efforts are needed to confirm the global significance and correlation of δ13C trends in carbonate and Azmy, K., Bassett, M.G., Copper, P., Veizer, J., 1998. Oxygen and carbon isotopic organic matter (e.g., Young et al., 2008). Furthermore, even if this composition of Silurian brachiopods; implications for coeval seawater and glaciations. Geological Society of America Bulletin 110 (11), 1499–1512. δ13 fi broad positive trend in C in the mid-Aeronian is con rmed, Azmy, K., Veizer, J., Wenzel, B., Bassett, M.G., Copper, P., 1999. Silurian strontium alternative explanations for the excursion related to increased organic isotope stratigraphy. Geological Society of America Bulletin 111 (4), 475–483. matter preservation should be explored (Kump and Arthur, 1999). Banner, J.L., Hanson, G.N., 1990. Calculation of simultaneous isotopic and trace element variations during water–rock interaction with applications to carbonate diagenesis. Models for either enhanced productivity or preservation of organic Geochimica et Cosmochimica Acta 54, 3123–3137. matter may relate ultimately to sea-level change and the effects on Bergström, S.M., Huff, W.D., Kolata, D.R., Kaljo, D., 1992. Silurian K-bentonites in the sediment delivery and water column mixing (Sageman et al., 2003). Iapetus Region: a preliminary event-stratigraphic and tectonomagmatic assess- – – δ13 ment. GFF 114 (3), 327 334. The Late Aeronian Early Telychian negative C shift in the Bergström, S.M., Huff, W.D., Kolata, D.R., 1998. The Lower Silurian Osmundsberg K- Rumba Formation could in this context simply reflect a return to bentonite. Part I: stratigraphic position, distribution, and palaeogeographic lower productivity or decreased organic matter preservation (e.g., significance. Geological Magazine 135 (1), 1–13. Berner, R.A., 2006. Inclusion of the weathering of volcanic rocks in the GEOCARBSULF Cramer and Saltzman, 2005, 2007). This negative excursion in the Model. American Journal of Science 306, 295–302. 13 Rumba is followed by a prominent positive δ C excursion in the Berry, W.B.N., Boucot, A.J., 1970. Correlation of North American Silurian rocks. Velise and Riga formations of the Ikla core, which may correspond to Geological Society of America, Special Paper 102 289 pp. fi the beginning of the Sheinwoodian Ireviken δ13C excursion well Bevier, M.L., Whalen, J.B., 1990. Tectonic signi cance of Silurian magmatism in the Canadian Appalachians. Geology 18 (5), 411–414. documented in the Ruhnu and Viki core sections (Kaljo et al., 2003), Brand, U., Azmy, K., Veizer, J., 2006. Evaluation of the Salinic I tectonic, Cancaniri glacial and signal a return to anoxic deep oceans (Cramer and Saltzman, and Ireviken biotic events: biochemostratigraphy of the Lower Silurian succession 2005). However, because of the difficulties in precise correlations of in the Niagara Gorge area, Canada and U.S.A. Palaeogeography, Palaeoclimatology, Palaoecology 241, 192–213. beds with tillites found in regions of glaciations with the marine Caputo, M.V., 1998. Ordovician–Silurian glaciations and global sea-level changes. 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Halverson, G.P., Dudas, F.O., Maloof, A.C., Bowring, S.A., 2007. Evolution of the 87Sr/86Sr composition of Neoproterozoic seawater. Palaeogeography, Palaeoclimatology, The radiogenic isotope lab (Dr. Kenneth Foland and Jeff Linder) Palaeoecology 256, 103–129. and the stable isotope biogeochemistry lab (Dr. Andrea Grottoli, Yohei Harris, M.T., Sheehan, P.M., 1996. Upper Ordovician–Lower Silurian depositional Matsui and Abbey Chrystal) at The Ohio State University provided the sequences determined from middle shelf sections, Barn Hills and Lakeside Mountains, eastern Great Basin. In: Witzke, B.J., Ludvigson, G.A., Day, J. (Eds.), sample processing and technical support for this study. Assistance in Paleozoic Sequence Stratigraphy: Views from the North American Craton: Boulder, sample collection from the Pancake Range of Nevada from Brad Colorado: Geological Society of America Special Paper, 306, pp. 161–176. 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