Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Québec, Canada) David S. Jones1,†, David A. Fike1, Seth Finnegan2, Woodward W. Fischer2, Daniel P. Schrag3, and Dwight McCay1 1Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, USA 2Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, MC 100-23, Pasadena, California 91125, USA 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA ABSTRACT approach provides evidence for the diachro- picture (Runkel et al., 2010). The biological crisis neity of the oncolite bed and Becscie lime- that accompanied the glaciation was the second Globally documented carbon isotope ex- stones; the former transgresses from west greatest mass extinction in the marine fossil cursions provide time-varying signals that to east, and the latter progrades from east rec ord (Sheehan, 2001; Bambach et al., 2004), can be used for high-resolution stratigraphic to west. The sea-level curve consistent with depleting the enormous diversity of marine or- correlation. We report detailed inorganic and our sequence-stratigraphic model indicates ganisms that had evolved during the Ordovician organic carbon isotope curves from carbonate that glacioeustatic sea-level changes and the Radiation (Webby et al., 2004). Marine carbon- rocks of the Ellis Bay and Becscie Formations positive carbon isotope excursion were not ate rocks deposited at the time preserve records spanning the Ordovician-Silurian boundary perfectly coupled. Although the start of the of a global disturbance to the carbon cycle rep- 13 on Anticosti Island, Québec, Canada. Strata of isotope excursion and the initial sea-level resented by positive excursions in I Ccarb and 13 the Anticosti Basin record the development of drawdown were coincident, the peak of the I Corg (Marshall and Middleton, 1990; Brench- a storm-dominated tropical carbonate ramp. isotope excursion did not occur until after sea ley et al., 1994; Underwood et al., 1997; Wang These strata host the well-known Hirnantian level had begun to rise. Carbon isotope values et al., 1997; Ripperdan et al., 1998; Kump et al., positive carbon isotope excursion, which at- did not return to baseline until well after the 1999). Some of the sedimentary records also host tains maximum values of ~4.5‰ in carbonate Anticosti ramp was reÁ ooded. The sea-level– geochemical evidence of global cooling and/or 13 carbon of the Laframboise Member or the I Ccarb relationship proposed here is consis- ice-sheet growth represented by parallel positive 18 Fox Point Member of the Becscie Formation. tent with the “weathering” hypothesis for the excursions in I Ocarb (Marshall and Middleton, 13 The excursion also occurs in organic carbon, origin of the Hirnantian I Ccarb excursion. 1990; Long, 1993; Brenchley et al., 2003). 13 13 and I Ccarb and I Corg values covary such Although much progress has been made in 13 13 13 that no reproducible ) C (= I Ccarb – I Corg) INTRODUCTION end-Ordovician correlation and carbon isotope excursion is observed. The most complete chemostratigraphy (Finney et al., 1999; Kump stratigraphic section, at Laframboise Point One of the largest mass extinctions of the et al., 1999; Brenchley et al., 2003; Melchin in the west, shows the characteristic shape of Phanerozoic took place at the close of the Ordo- and Holmden, 2006; Kaljo et al., 2008; LaPorte the Hirnantian Stage excursion at the global vi cian Period (Sepkoski, 1981; Brenchley et al., et al., 2009; Yan et al., 2009; Zhang et al., 2009; stratotype section and point (GSSP) for the 2001; Sheehan, 2001; Bambach et al., 2004), Ainsaar et al., 2010; Desrochers et al., 2010; Hirnantian Stage in China and the Silurian an interval also marked by the development of Young et al., 2010), uncertainty remains about System in Scotland. We therefore suggest extensive continental ice sheets on the south the causal relationship among glaciation, mass that the entire Hirnantian Stage on Anticosti polar super continent Gondwana (Hambrey extinction, and perturbation to the carbon cycle. Island is conÀ ned to the Laframboise and and Harland, 1981; Hambrey, 1985; Ghienne , Delabroye and Vecoli (2010) recently reviewed lower Fox Point Members. 2003; Le Heron et al., 2007; Le Heron and some of the outstanding issues in Hirnantian By documenting discontinuities in the Dowdeswell, 2009; Le Heron and Howard, 2010; event stratigraphy, emphasizing shortcomings archi tecture of the carbon isotope curve at Loi et al., 2010). This glaciation and the possible in the current biostratigraphic and chemostrati- multiple stratigraphic sections spanning a Early Silurian glaciation (Caputo and Crowell, graphic correlations. Although many pieces of proximal to distal transect across the sedi- 1985; Grahn and Caputo, 1992; Díaz-Martínez evidence suggest that the glaciation, extinction, mentary basin, we are able to reconstruct and Grahn, 2007) stand out as the only episodes and geochemical disturbance were synchronous glacioeustatic sea-level Á uctuations corre- in a 210 m.y. stretch (ca. 580–370 Ma) that other- (Marshall and Middleton, 1990; Long, 1993; sponding to maximum glacial conditions wise lacks direct geologic evidence for continen- Brenchley and Marshall, 1999; Brenchley et al., asso ciated with the end-Ordovician ice age. tal ice sheets (Hambrey and Harland, 1981). This 2003), no sedimentary succession contains The combined litho- and chemostratigraphic interval is thought to have occurred in a green- well-preserved records of all three events. house interval dominated by high atmo spheric Without a À rm chronological framework, it pCO (e.g., Berner, 2006), although sedimentary is difÀ cult to make strong statements regarding †Current address: Department of Geology, Am- 2 herst College, 11 Barrett Hill Road, Amherst, Massa- records of shoreline ice in tropical Laurentia in the causes, consequences, and interrelatedness chusetts 01002, USA; [email protected] Middle to Late Cambrian time complicate this of the events. GSA Bulletin; July/August 2011; v. 123; no. 7/8; p. 1645–1664; doi: 10.1130/B30323.1; 14 À gures; 2 tables; Data Repository item 2011133. For permission to copy, contact [email protected] 1645 © 2011 Geological Society of America Jones et al. In this paper, we present new high-resolution enced waning effects of the thermal subsidence Tower, Homard, and Joseph Point members, and stable isotope data coupled to lithostratigraphy (Waldron et al., 1998) that began in the Ediacaran the formal Mill Bay and Schmitt Creek Mem- from the Lousy Cove and Laframboise Mem- Period around 615 Ma (Kamo and Gower, 1994). bers (Long, 2007). The Vauréal is succeeded by bers of the upper Ellis Bay Formation and the In Middle Ordovician time, the advance of the the Ellis Bay Formation, which has been divided Fox Point Member of the lower Becscie For- Taconic arc from the south and east reinvigorated into the Grindstone, Velleda, Prinsta, Lousy mation on Anticosti Island in eastern Canada Á agging subsidence rates (Waldron et al., 1998; Cove, and Laframboise Members (Fig. 2A) (Fig. 1). Because the carbon isotopic compo- Long, 2007). Back-stripping analysis using ages (Long and Copper, 1987a). The Lower Silurian sition of the global ocean changes smoothly and water depths inferred from paleontological Becscie Formation overlies the Ellis Bay; it is (rather than discontinuously) in time, carbon data suggests an increase in sedimentation rate as composed of the Fox Point and Chabot Members isotope excursions can provide time-varying the foreland basin developed through the Sand- (Copper and Long, 1989; Sami and Desrochers, signals that allow for high-resolution strati- bian and Katian Stages of the Ordovician (Long, 1992; Long, 2007). The remainder of the Anti- graphic correlation. Brenchley et al. (2003) used 2007). Sediments continued to accumulate during costi succession includes the Merrimack, Gun the Hirnantian positive carbon isotope excursion the Early Silurian, but deposition waned once River, Jupiter, and Chicotte Formations (Fig. to achieve high-resolution intrabasinal correla- Taconic thrust loading ended in the Llandovery 2B). This paper focuses on strata of the Lousy tions within Baltica, and interbasinal correlation (Long, 2007), diminishing the production of Cove, Laframboise, and Fox Point Members. between Baltica and Laurentia, improving our addi tional accommodation space. Thermal matu- Most workers have interpreted the Anti- understanding of the relative timing of environ- ration data, basin modeling, and offshore seismic costi Basin sedimentary record to represent a mental changes, isotopic evolution, and biotic data demonstrate that Anticosti strata were subse- subtidal, storm-influenced, carbonate ramp- events associated with the end-Ordovician ex- quently buried no deeper than 2.5–3 km and are platform (Long and Copper, 1987a; Sami and tinction and glaciation. In this paper, we employ still in the oil window (Pinet and Lavoie, 2007). Desrochers, 1992; Farley and Desrochers, 2007; the Hirnantian positive carbon isotope excur- There has been little signiÀ cant structural defor- Long, 2007; Desrochers et al., 2010). The long sion for high-resolution intrabasinal correlation mation to the island, although structural analysis axis of the island is oblique to the paleoshore- within the Anticosti Basin.