Palaeogeography, Palaeoclimatology, Palaeoecology 222 (2005) 53–76 www.elsevier.com/locate/palaeo Upper Ordovician (Mohawkian) carbon isotope (y13C) stratigraphy in eastern and central North America: Regional expression of a perturbation of the global carbon cycle Seth A. YoungT, Matthew R. Saltzman, Stig M. Bergstro¨m Department of Geological Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, OH 43210, USA Received 2 September 2004; received in revised form 10 March 2005; accepted 10 March 2005 Abstract The Guttenberg carbon isotope excursion (GICE) documented from eastern North America demonstrates the effects that regional, geochemically distinct water masses, upwelling, and ocean circulation have on the carbon isotope record from carbonate platforms. Late Turinian–Chatfieldian carbonates from Oklahoma, Kentucky, Virginia, and West Virginia record a positive carbon isotope excursion (z+3.0x), the GICE excursion. The GICE excursion has relations to established biostratigraphy (beginning in the North American Midcontinent Phragmodus undatus Conodont Zone and continuing through the Plectodina tenuis Zone), sequence, and event stratigraphy. Previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the GICE excursion, where geochemically distinct water masses are defined for the Upper Ordovician. A major eustatic sea-level rise before the GICE promoted a greater exchange of open ocean waters onto the carbonate platform of Laurentia; however, restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent Iapetus Ocean were still apparent. Local variations documented in the GICE excursion are directly related to upwelling of nutrient rich isotopically light waters, increased primary productivity, and the subsequent organic carbon production and burial. D 2005 Published by Elsevier B.V. Keywords: Late Ordovician; Carbon isotope; Water masses; Upwelling; Carbonate platform 1. Introduction temperature–salinity-defined water masses based upon regional variations in faunal patterns and The Late Ordovician (Mohawkian) epeiric sea of carbon and neodymium isotopes in marine carbo- eastern North America has been divided into nates (Holmden et al., 1998). These proposed geochemically distinct water masses are referred to as baquafaciesQ because they are reflected in both T Corresponding author. faunas and lithologies of the underlying sedimentary E-mail address: [email protected] (S.A. Young). deposits. Recognition of these aquafacies across the 0031-0182/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.palaeo.2005.03.008 54 S.A. Young et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 222 (2005) 53–76 Mohawkian carbonate platform potentially indicates led to a fundamental shift in the style of platform sluggish or restricted circulation within epeiric seas sedimentation from tropical-type to temperate-type and between epeiric seas and adjacent oceans carbonates with associated chert and phosphate (Hol- (Holmden et al., 1998). However, this model has land and Patzkowsky, 1996; Kolata et al., 2001; Pope only been rigorously examined for a brief time slice and Steffen, 2003), the cause(s) of the inferred high of ~105 years defined by two prominent K-bentonite organic carbon burial remain uncertain. Patzkowsky et horizons (bDeicke–Millbrig time sliceQ). Much less al. (1997) suggested that a relative deepening in the is known about how the epeiric sea aquafacies Taconic foreland basin (Taconic baquafaciesQ)of patterns varied through time (N105 years), partic- eastern Laurentia, led to enhanced burial of isotopi- ularly in association with oceanographic events such cally light (13C-depleted) organic matter in poorly as a carbon isotope excursion (e.g., Patzkowsky et oxygenated deep water masses. Increased nutrient al., 1997). This is important because local water fluxes from the Taconic highlands may have con- mass differences in y13C can provide information tributed to locally high organic carbon burial by regarding nutrient cycling, which influences the stimulating surface ocean production (Holmden et al., strength of the biological pump, atmospheric CO2 1998). levels, and global climate (e.g., Sigman and Boyle, A role for upwelling as a source of increased 2000). nutrients to the Taconic aquafacies has also been The shallow epeiric seas that covered most of considered. However, because the observed offset of 13 eastern and central North America during the Late ~2.0x in y Ccarb values between the isotopically Ordovician record a significant positive carbon light Midcontinent water mass (affected by upwelling) isotope excursion of ~+3x (Hatch et al., 1987; and the heavier Taconic aquafacies remained constant Ludvigson et al., 1996; Patzkowsky et al., 1997; before and during the GICE, this suggests that the Simo et al., 2003). This excursion was first intensity of upwelling changed little during the documented in organic matter from the Guttenberg Middle to Late Ordovician (Patzkowsky et al., 1997; Limestone Member of the Decorah Formation in Holmden et al., 1998). The importance of the onset of the Upper Mississippi Valley (Hatch et al., 1987) vigorous upwelling in platform margin settings such and is here referred to as the Guttenberg carbon as Oklahoma (e.g. Pope and Steffen, 2003) has not isotope excursion (GICE). The GICE has since been examined as a source of nutrients and enhanced been documented at many sections in the Mohaw- productivity that influenced regional and global 13 kian (Chatfieldian Stage) Guttenberg Limestone y Ccarb values during the Chatfieldian. Examining Member of the Decorah Formation in the Upper the evidence for upwelling and y13Cvaluesin Mississippi Valley (Ludvigson et al., 1996, 2000, platform margin settings (e.g. Oklahoma) could 2004) and in the Salona Formation of Pennsylvania document any potential relation of upwelling to the (Patzkowsky et al., 1997) as well as time equiv- GICE. alent carbonates worldwide (Ainsaar et al., 1999). The purpose of this paper is to integrate high The correlation of the GICE worldwide is facili- resolution carbon isotope (y13C) data from several tated by its occurrence above several widespread K- Late Ordovician (Chatfieldian Stage) carbonate bentonite horizons deposited during extensive sections from widely separated basins across eastern explosive volcanism associated with the Taconic Laurentia as well as a platform margin section in Orogeny (Kolata et al., 1996; Simo et al., 2003). Oklahoma. These data are then used to establish Apart from the well known Hirnantian (latest relations of this positive y13C excursion (~+3.0x) Ordovician) positive excursion, which is associated to previously established aquafacies and changing with widespread glaciation and mass extinction paleoceanography during the Late Ordovician. (Brenchley et al., 1994, 2003), the GICE is the Carbon isotope data from these sections are most significant y13C excursion documented in the correlated across North America based upon rela- Ordovician. tions of the GICE to the established Ordovician While the GICE is known to coincide with biotic biostratigraphy, sequence and K-bentonite event and paleoceanographic changes that in North America stratigraphy. S.A. Young et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 222 (2005) 53–76 55 2. Geologic background 2.1. Conodont and graptolite biostratigraphy Most of eastern and central North America during Conodont biostratigraphy is well established the Late Ordovician was situated between 158 and (Sweet, 1984a; Hall, 1986; Haynes, 1994; Leslie, 308 S(Fig. 1). Four sections in this region are 1995; Richardson and Bergstro¨m, 2003) for the study particularly suitable for comparison with previously interval in the Upper Ordovician in North America. In published y13C data from the Midcontinent and the Mohawkian there are six conodont zones in the Taconic Foreland Basin of North America. The Midcontinent of North America, in ascending order, sections include (1) the Bromide and Viola Springs the Plectodina aculeata, Erismodus quadridactylus, Formations in the Arbuckle Mountains of south- Phragmodus undatus, Plectodina tenuis, and Belo- central Oklahoma (Alberstadt, 1973; Sweet, 1983); dina confluens Zones. Although first defined by (2) the Eggleston and Trenton Limestones in south- graphic correlation techniques (Sweet, 1984a), for western Virginia (Fetzer, 1973; Hall, 1986; Leslie, practical purposes these zones have been recognized 1995); (3) the Tyrone and Lexington Limestones of by the first appearance of the zonal index species. The north-central Kentucky (Sweet et al., 1974; Sweet, GICE begins near the top of the P. undatus Zone and 1979); and (4) the Nealmont and Dolly Ridge ends within the P. tenuis Zone. Formations in West Virginia (Perry, 1972; Keith Graptolite biostratigraphy is established for the and Hall, 1989; Keith, 1989). These sections were Middle and Upper Ordovician in North America. In chosen because they represent thick carbonate the North American Middle Ordovician, there are six sequences, where biostratigraphy (conodonts and/or graptolite zones, in stratigraphic order; the Nema- graptolites), sequence stratigraphy and/or K-benton- graptus gracilis, Climacograptus bicornis, Climacog- ite event stratigraphy have been previously inves- raptus americanus, Orthograptus ruedemanni, and tigated. Coupling biostratigraphic, sequence and Climacograptus spiniferus Zones. Previously,