Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 166–178 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo 13 13 Paired δ Ccarb and δ Corg records of Upper Ordovician (Sandbian–Katian) carbonates in North America and China: Implications for paleoceanographic change Seth A. Young a,⁎, Matthew R. Saltzman a, Stig M. Bergström a, Stephen A. Leslie b, Chen Xu c a Division of Geological Sciences, School of Earth Sciences, the Ohio State University, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, OH 43210, USA b Department of Geology and Environmental Science, James Madison University, MSC 6903, Harrisonburg, VA 22807, USA c State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, People's Republic of China article info abstract Article history: Late Sandbian–Early Katian marine carbonates from two sections in North America and one in south China Received 6 February 2008 13 13 13 13 were analyzed for paired δ Ccarb and δ Corg, and revealed similar δ Ccarb but varying trends in δ Corg Received in revised form 16 August 2008 13 13 stratigraphy while all sections recorded a well-known positive δ Ccarb shift. These δ Corg records are the Accepted 15 September 2008 13 first through the Guttenberg δ Ccarb excursion (GICE) in China, Oklahoma, and West Virginia. In south China a positive excursion in δ13C is associated with the latter part of the positive δ13C excursion and closely Keywords: org carb resembles trends from Iowa and Pennsylvania that have been interpreted to reflect a lowering of atmospheric Late Ordovician δ13 fi Katian pCO2 levels. The Corg trends from Oklahoma and West Virginia differ signi cantly from what is observed in Carbon isotopes China. This likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the Organic carbon dominant control on 12C fractionation in Oklahoma and West Virginia, masking a possible global signal of Water masses lowered atmospheric pCO2 levels observed in China. Carbon dioxide © 2008 Elsevier B.V. All rights reserved. 1. Introduction reflect a fall in pCO2 (and CO2(aq)) below a critical threshold for ice- sheet growth in the Late Ordovician (Patzkowsky et al., 1997; It is widely accepted that the Hirnantian Stage (445–443 Ma) of the Herrmann et al., 2003, 2004). Ordovician period was a time of major global environmental changes 13 Unlike the widely studied δ Ccarb excursion in the Katian, the that culminated in the re-emergence of large continental ice sheets 13 δ Corg trend has not been globally documented. Paired analysis of over polar landmasses (Brenchley et al., 1994). However, the 13 13 δ Ccarb and δ Corg in many different regions is critical because the circumstances surrounding the initiation of the Hirnantian climate 13 13 13 isotopic difference between δ Ccarb and δ Corg (Δ C) is known to transition remains controversial, and it has been suggested that the respond to variables other than pCO2, such as phytoplankton growth Late Ordovician ice age climate actually began ~10 m. y. prior in the rates, cell geometries (volume to surface area ratios), and the Katian Stage (e.g., Frakes et al., 1992; Pope and Steffen, 2003; Saltzman biological source of the organic matter preserved (Francois et al., and Young, 2005). In addition to lithological and biological indicators, 1993; Hinga et al., 1994; Bidigare et al., 1997; Popp et al., 1998). an important line of evidence that is consistent with global cooling Furthermore, in the case of the early Katian, the epeiric sea water 13 beginning as early as the Katian is a positive δ C excursion in marine masses that flooded landmasses have been shown to vary in critical δ13 carbonates ( Ccarb) (e.g., Ludvigson et al., 1996; Patzkowsky et al., parameters (i.e., nutrient concentrations) that affect growth rates and δ13 1997; Ainsaar et al., 1999; Young et al., 2005). This Ccarb excursion is algal ecology (e.g., Holmden et al., 1998; Young et al., 2005; Panchuk commonly interpreted to reflect enhanced burial of organic carbon on et al., 2006). a global scale. Furthermore, the positive δ13C excursion in the early 13 13 carb Presented here are paired analyses of δ Ccarb and δ Corg from Katian is also associated with an even larger positive shift in organic several Late Ordovician (late Sandbian to early Katian Stage) carbonate δ13 matter isotopes ( Corg)(Hatch et al., 1987; Ludvigson et al., 1996; sections in China and North America (Fig. 1) that can be correlated to Patzkowsky et al., 1997). This decreased isotopic difference between previously studied sections using biostratigraphy and/or K-bentonite δ13 δ13 Δ13 δ13 −δ13 13 Ccarb and Corg ( C= Ccarb Corg) has been interpreted to event stratigraphy. The documented Δ C trends are evaluated alongside with previously published data (Patzkowsky et al., 1997; Pancost et al., 1999) in the context of fluctuating local paleoceano- graphic conditions during an episode of global environmental change ⁎ Corresponding author. Present address: Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. in the Late Ordovician. Our results reveal evidence of local and also 13 E-mail addresses: [email protected], [email protected] (S.A. Young). what may be global carbon cycle effects on δ C in different sections, 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.09.006 S.A. Young et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 166–178 167 Fig. 1. Katian (Upper Ordovician) paleogeographic reconstruction (modified from Witzke, 1990; Scotese and McKerrow, 1991) showing locations of our study sections in Oklahoma 13 13 (OK), West Virginia (DR), and southeast China (PX) along with sections previously investigated for paired δ Ccarb and δ Corg analyses from Iowa (IA) and Pennsylvania (PA) (Ludvigson et al., 1996; Patzkowsky et al., 1997; Pancost et al., 1999). 13 13 and emphasize the importance of continued high-resolution study of Patzkowsky et al., 1997). Our new paired δ Ccarb and δ Corg curves 13 13 paired δ Ccarb and δ Corg trends in widely separated oceanic regions. include sections through (1) the Pagoda Formation, near Yichang City (Hubei Province) in south-central China; (2) the Bromide and Viola 2. Geologic background Springs formations near Fittstown (Pontotoc County), Oklahoma, USA; and (3) the Nealmont and Dolly Ridge formations, near Riverton Recent work by the International Subcommission on Ordovician (Pendleton County), West Virginia, USA. Previous work on conodonts Stratigraphy (ISOS) has led to the division of the Upper Ordovician and graptolite biostratigraphy, sequence and event stratigraphy, and 13 Series into three globally classified stages, in ascending order, δ Ccarb stratigraphy of these three sections allow us to correlate the Sandbian, Katian, and Hirnantian (Bergström et al., 2000). Katian three sections to other studied regions (Perry, 1972; Alberstadt, 1973; epeiric sea deposits of North America record regional extinctions of Sweet, 1983; Finney, 1986; An, 1987; Keith, 1989; Chen et al., 1995; marine benthos and a distinct change in the style of carbonate Young et al., 2005; Goldman et al., 2007; Bergström et al., in press). deposition (“tropical-type” to “temperate-type”) (e.g., Patzkowsky The recognition of upper P. undatus through lower B. confluens and Holland, 1993; Holland and Patzkowsky, 1996; Patzkowsky et al., Midcontinent Conodont Zones and upper C. bicornis through lower C. 1997; Pope and Read, 1998; Pope and Steffen, 2003). The associated spiniferus Graptolite Zones from the Bromide and Viola Springs 13 positive δ Ccarb shift, known as the Guttenberg carbon isotope formations at the Fittstown, Oklahoma section (Sweet, 1983; Finney, excursion (GICE), is the most widely documented excursion in the 1986; Goldman et al., 2007) allows for a unique and definitive Upper Ordovician outside of the Hirnantian. The GICE has been biostratigraphic framework for the δ13C stratigraphy presented here. documented in marine limestones from numerous localities in North Based upon the limited biostratigraphic studies conducted on the America, Europe, and recently in Asia (Fig. 1) (e.g., Ludvigson et al., Dolly Ridge Formation, West Virginia, the formation lies within the 1996; Patzkowsky et al., 1997; Ainsaar et al., 1999; Bergström et al., in upper P. undatus through basal B. confluens Conodont Zones (Keith, press). As a result of intensive study of the GICE over a wide area of 1989; Young et al., 2005). The presence of a prominent altered North America, it has been possible to observe local variations in the volcanic ash bed (Deicke K-bentonite) several meters below the magnitude of the excursion that are linked to differences in beginning of the GICE in both the Oklahoma and West Virginia paleoceanographic setting and nutrient cycling (e.g., Holmden et al., sections supports the conodont biostratigraphic correlations (Young 1998; Young et al., 2005; Panchuk et al., 2006). et al., 2005; Goldman et al., 2007). The GICE begins near the Phragmodus undatus/ Plectodina tenuis Additionally the GICE has been demonstrated to occur above the Midcontinent conodont zonal boundary, and ends near the P. tenuis/ M4–M5 sequence stratigraphic boundary in Eastern North America Belodina confluens zonal boundary (Young et al., 2005). The GICE lies (Young et al., 2005), and this