Palaeogeography, Palaeoclimatology, Palaeoecology 399 (2014) 1–20 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo 13 Carbon isotope (δ Ccarb) stratigraphy of the Lower–Middle Ordovician (Tremadocian–Darriwilian) in the Great Basin, western United States: Implications for global correlation Cole T. Edwards ⁎, Matthew R. Saltzman School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, USA article info abstract 13 Article history: New stable carbon isotope data (δ Ccarb)fromLower–Middle Ordovician (Tremadocian to Darriwilian) carbon- Received 7 September 2013 ate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin Received in revised form 30 December 2013 region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a suc- Accepted 5 February 2014 cession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine Available online 13 February 2014 conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Keywords: Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North Carbon isotopes American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) 13 Ordovician to the Phragmodus polonicus Zone (Darriwilian). The δ C trend has four distinct characteristics recognized Chemostratigraphy in both Great Basin sections: 1) a drop in δ13Cfrom+1‰ at the base of the Ordovician (Tremadocian) Carbon cycling to −0.7‰,2)a1to2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Great Basin Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), fi Great Ordovician Biodiversi cation Event and 4) a steady δ13Cdecreasefrom0‰ to −4to−5‰ during Middle Ordovician (Dapingian–Darriwilian). (GOBE) In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic frame- work for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Cramer et al., 2011; Munnecke et al., 2011; Bergström et al., 2012; Calner et al., 2012). These δ13C excursions may allow for refinement of Stable carbon isotopes (δ13C) have been widely used to better un- biostratigraphic correlations between strata that were deposited in dif- derstand changes in global carbon cycling and organic carbon burial in ferent tectonic basins. However, the use of δ13C stratigraphy for global the geologic past (e.g. Kump and Arthur, 1999). In addition, numerous correlation can be problematic during time periods such as the Lower studies have focused on positive carbon isotope excursions present to Middle Ordovician in which the magnitude of the δ13C variability is throughout Lower Paleozoic strata and their utility in global correlations relatively small (Buggisch et al., 2003; Saltzman, 2005; Bergström (Brenchley et al., 1994; Saltzman et al., 1998; Finney et al., 1999; Kump et al., 2009; Munnecke et al., 2011). The problem with the use of these et al., 1999; Saltzman et al., 2000; Bergström et al., 2006; Kaljo et al., small magnitude or high frequency changes in δ13C for correlation is 2007; Ainsaar et al., 2010; Cramer et al., 2010; Young et al., 2010; that they cannot be assumed to be worldwide events because the global carbon cycle is only one of several variables that may affect δ13Cinma- rine carbonates. Specifically, recent studies have examined whether ⁎ Corresponding author. δ13 E-mail addresses: [email protected] (C.T. Edwards), [email protected] some C excursions are a result of differential exchange of isotopically (M.R. Saltzman). distinct local water bodies or diagenetic effects overprinting the original http://dx.doi.org/10.1016/j.palaeo.2014.02.005 0031-0182 © 2014 Elsevier B.V. All rights reserved. 2 C.T. Edwards, M.R. Saltzman / Palaeogeography, Palaeoclimatology, Palaeoecology 399 (2014) 1–20 signal imparted by global changes in the δ13C of the oceanic dissolved of well-preserved carbonate rocks that are more than 1 km thick and inorganic carbon (DIC) reservoir (Holmden et al., 1998; Immenhauser have been previously studied for conodont biostratigraphy (Ethington et al., 2002, 2003; Fanton and Holmden, 2007; Immenhauser et al., and Clark, 1981; Sweet and Tolbert, 1997). The Ibex area in particular 2008; Metzger and Fike, 2013). is one of the most intensively studied Lower Ordovician sections in In particular, the role of sea level change in producing local δ13C the world for its paleoecologic importance (e.g. Adrain et al., 1998; Li noise (i.e. diagenesis, mixing of water masses, or changes in carbon and Droser, 1999; Finnegan and Droser, 2005) and sequence stratigra- fluxes) that is superimposed on the global δ13C signal is widely phy (Ross et al., 1997; Miller et al., 2003, 2012) and provides a unique discussed. Immenhauser et al. (2002, 2003) interpreted a positive δ13C opportunity to directly compare δ13C and sea level. While local carbon excursion preserved in Carboniferous carbonates from northwest cycling and diagenesis significantly affected trends observed in the Spain to reflect a transgressive event that caused migration of isotopi- Middle Ordovician portion of these sections, the Lower Ordovician cally heavy open marine waters (with elevated δ13Candδ18O) into portion of the curve correlates well in timing and magnitude to other the more restricted platform environment. These authors further regions. Lower Ordovician δ13C correlations help integrate conodont argue that this relative sea level rise reduced the flux of 12C-enriched biostratigraphic data of different biogeographic realms including the carbon from oxidized organic matter and terrestrial inputs, thus Great Basin region, western Newfoundland (Azmy and Lavoie, 2009), creating a local positive δ13C excursion that can be traced in a coast- and the Argentine Precordillera (Buggisch et al., 2003; Bergström to-basin profile where the magnitude decreases from about 3‰ distally et al., 2009). In addition, Munnecke et al. (2011) report a limited to 1.5‰ in nearshore settings. In this example the change in δ13Cis Lower Ordovician data set from South China, and it is likely that more interpreted to be driven by changes in a relative sea level without any detailed correlations will be possible with the Great Basin in the future. significant changes in global carbon fluxes. A similar model of sea Ultimately, the observed global changes in δ13C observed here may have level-driven changes in δ13C has been interpreted to account for positive implications for the transitions in climate and life that took place during δ13C excursions in the epeiric sea over Laurentia during the Middle–Late the Early Ordovician (e.g. Trotter et al., 2008), but in order to address Ordovician (Holmden et al., 1998; Panchuk et al., 2005, 2006; Fanton this it will be necessary to couple these new data with other proxies and Holmden, 2007). Sea level rise introduces a cooler and nutrient- including global δ13C of organic matter, δ34S, and 87Sr/86Sr in future rich water mass onto the platform (referred to by Holmden et al. studies (e.g. Young et al., 2009; Gill et al., 2011; Saltzman et al., 2011). (1998) as an “aquafacies”) when, in conjunction with increased primary productivity and the burial of 12C-enriched organic matter, an increase 2. Geologic background in δ13C is preserved in authigenic carbonate sediments. In this model a δ13C excursion in a stratigraphic succession may simply record the 2.1. Depositional environments and sequence stratigraphy lateral movement of isotopically unique aquafacies during a sea level change. Therefore, sea level-driven δ13C excursions may offer little The depositional setting of the Great Basin region during the Early value for global correlation when considering the asynchronies of sea Ordovician is interpreted to have been a carbonate ramp with mixed level change from the combination of regional tectonic and eustatic siliciclastic and carbonate sedimentation that today comprises a effects (cf. Fanton and Holmden, 2007). 1–2 km thick succession known as the Pogonip Group (Ross et al., Metzger and Fike (2013) examined the Late