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Toward a Neoproterozoic Composite Carbon-Isotope Record

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Toward a Neoproterozoic Composite Carbon-Isotope Record 25630 page 1 of 27 Toward a Neoproterozoic composite carbon-isotope record Galen P. Halverson† Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138-2902, USA and Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1126, Cam- bridge, Massachusetts 02139, USA Paul F. Hoffman Daniel P. Schrag Adam C. Maloof‡ Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138-2902, USA A. Hugh N. Rice Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria ABSTRACT framework for a new, high-resolution model Various workers have compiled composite carbon-isotope record for the Neoproterozoic δ13C records for the Neoproterozoic (e.g., Hayes Glacial deposits of Sturtian and Marinoan comprising new δ13C (carbonate) data from et al., 1999; Jacobsen and Kaufman, 1999; Wal- age occur in the well-studied Neoproterozoic Svalbard (Akademikerbreen Group) and ter et al., 2000), but like attempts to construct successions of northern Namibia, South Aus- Namibia (Otavi Group) and data in the lit- 87Sr/86Sr records for this time period (Melezhik tralia, and northwestern Canada. In all three erature from Svalbard, Namibia, and Oman. et al., 2001), these compilations have suffered regions, the Marinoan glaciation is presaged A new U-Pb zircon age of 760 ± 1 Ma from from low sample density, limited availability by a large negative δ13C anomaly, and the cap an ash bed in the Ombombo Subgroup in of chronostratigraphically well-constrained carbonates to both glacial units share a suite Namibia provides the oldest direct time-cali- data, and the consequent dependence on many of unique sedimentological, stratigraphic, bration point in the compilation, but the time tenuous correlations. In this paper we present and geochemical features. These global chro- scale of this preliminary δ13C record remains a new, high-resolution δ13C compilation that nostratigraphic markers are the bases of a poorly constrained. spans much of the Neoproterozoic and consists new correlation scheme for the Neoprotero- of both new and previously published data from zoic that corroborates radiometric data that Keywords: Neoproterozoic, chemostratigra- Svalbard, Namibia, and Oman. Like the others, indicate that there were three glacial epochs phy, carbon isotope, glaciation, geochronology. this record is based unavoidably on interpre- between ca. 750 and 580 Ma. Intraregional tive correlations (Knoll and Walter, 1992) and correlation of Neoproterozoic successions INTRODUCTION should be regarded as a preliminary model, but in the present-day North Atlantic region its strength is that it relies on only two linkages: suggests that glacial diamictite pairs in the Over two decades ago Knoll et al. (1986) Svalbard-Namibia and Namibia-Oman. Polarisbreen Group in northeastern Svalbard documented large and reproducible fl uctuations Any chronology for the Neoproterozoic must and the Tillite Group in eastern Greenland in the carbon-isotope composition of the Neo- reconcile the question of the number of glacia- were deposited during the Marinoan glacia- proterozoic oceans. Noting that these fl uctuations tions (e.g., Kennedy et al., 1998) that occurred tion, whereas the younger of a pair of glacials track Neoproterozoic ice ages in much the same during this era. We concur with various authors (Mortensnes Formation) in the Vestertana way that variations in marine δ18O composition who have recently proposed that there were Group of northern Norway was deposited defi ne Quaternary glacial cycles, Knoll and three glacial epochs (Knoll, 2000; Hoffman during the third (Gaskiers) Neoproterozoic Walter (1992) predicted that carbon-isotope and Schrag, 2002; Xiao et al., 2004) and use glaciation. Gaskiers-aged glacial deposits are chemostratigraphy would prove to be a particu- the now conventional names Sturtian and Mari- neither globally distributed nor overlain by a larly useful chronostratigraphic correlation tool noan (Dunn et al., 1971) to refer to the older widespread cap carbonate but are associated for late Precambrian rocks. The carbon-isotope two glaciations. Recently acquired radiometric with an extremely negative δ13C anomaly. database for the Neoproterozoic has proliferated age constraints (Bowring et al., 2003; Hoffmann The chronology developed here provides the (Shields and Veizer, 2002) over the past decade, et al., 2004) demonstrate unequivocally that the and a greatly improved picture of the relationship Gaskiers glacials in eastern Newfoundland post- 13 †E-mail: Present address: Laboratoire des Mé- between δ C patterns and glaciations is emerg- date the Marinoan glaciation (Knoll et al., 2004) canismes de Transferts en Géologie, Université Paul- ing. Nevertheless, the full potential of the carbon- (Fig. 1). Although glacial deposits of Gaskiers Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, isotope record has not been fully realized, owing age appear to be less widespread geographi- France; e-mail: [email protected]. in large part to scarce radiometric age constraints cally than their predecessors, intraregional ‡Present address: Department of Earth, Atmo- spheric, and Planetary Sciences, Massachusetts from key successions, limited biostratigraphic correlations proposed in this study suggest that Institute of Technology, Building 54, Cambridge, resolution, and the uncertainty in the number of the upper of a pair of well-studied glaciogenic Massachusetts 02139, USA. Neoproterozoic glaciations and δ13C anomalies. formations in the Vestertana Group in northern GSA Bulletin; September/October 2005; v. 117; no. 9/10; p. 000–000; doi: 10.1130/B25630.1; 15 fi gures; Data Repository item 2005xxx. For permission to copy, contact [email protected] © 2005 Geological Society of America 1 25630 page 2 of 27 HALVERSON et al. Norway is also post-Marinoan in age and likely 1 glacial A 2 3 2 3 sediments equivalent to the Gaskiers glacials (Fig. 1). 65 4 5 unconformity Stratigraphic, paleontological, and carbon- 76 10 Ediacaran isotope data also imply a Gaskiers age for the fossils Hankalchoug diamictite in northwestern China, 8 carbonate-rich where Marinoan-aged and putative Sturtian- 9 aged glacial deposits are also present (Xiao et al., 2004). This paper begins with a review of the Neo- B carbonate-poor proterozoic successions in northern Namibia, northwestern Canada, and South Australia, all of which contain Sturtian- and Marinoan-aged glacial deposits whose correlations are straight- Scotland (4) N Namibia (8) SW USA (7) SW USA Oman (10) Newfoundland (5) forward (Kennedy et al., 1998; Hoffman and Cambrian EGES (1-2) N Norway (3) NW Canada (6) Australia (9) S Schrag, 2002). Pertinent published stratigraphic, Precambrian isotopic, and radiometric data from Svalbard, the southwestern United States (Death Valley), Scotland, and Oman are reviewed to broaden Gaskiers / Wonoka ? ? the chronology. New carbon-isotope data from Namibia and Svalbard provide detailed pre-Stur- tian and post-Marinoan records, while new data from northern Norway support a Gaskiers age for the Mortensnes glacials. This glacial event Marinoan appears to correspond, at least temporally, to the largest negative δ13C anomaly in the Proterozoic and is the key to linking the Oman record to the composite δ13C curve. New Sr isotope data from Namibia and Svalbard and an U-Pb age on an Sturtian ? ? ? ash bed in the pre-Sturtian Ombombo Subgroup in Namibia reinforce the compilation. Ambigui- ties in the timing and duration of the glaciations and the scarcity of useful radiometric ages preclude a defi nitive time-calibration of the 13C record at this time. Two permissible but δ Figure 1. (A) Present-day locations of the Neoproterozoic successions discussed in this speculative time scales are suggested, but it is paper. (B) Schematic profi les of the successions from (A), showing their stratigraphic range, expected that the composite 13C record will be δ simplifi ed lithologies, distribution of glacial deposits, and occurrence of Ediacaran fossils. amended as new data emerge. See text for references. EGES—East Greenland-East Svalbard platform. ISOTOPIC DATA Nearly 2000 new carbon-isotope data from standard column chemistry techniques and ana- during recrystallization, dolomitization, and Svalbard, Namibia, and Norway are presented lyzed on a VG Sector 54 (TIMS) instrument with other diagenetic processes. The δ18O and 87Sr/ in this paper. All carbon-isotope data are pre- an average internal precision (2 σ) of 0.000012. 86Sr compositions and trace element concen- sented in per mil (‰) notation using the Vienna See Halverson et al. (2005) for a more detailed trations (e.g., Sr and Mn) of carbonates are Pee Dee Belemnite (VPDB) standard and, discussion of the analytical methods. especially prone to alteration during meteoric along with simultaneously acquired δ18O data, U-Pb geochronologic data for zircons from diagenesis and dolomitization, and these chemi- are available as supplementary online material an ash bed in Namibia were acquired by isotope cal tracers can be used as one tool to monitor (Tables DR1–DR5).1 Analytical methods are dilution–thermal ionization mass-spectrometry the extent of chemical alteration (Brand and described in detail in Halverson et al. (2004). (ID-TIMS) at the same laboratory as above. Veizer, 1980, 1981; Banner and Hanson, 1990). New 87Sr/86Sr data published here (Table DR- Analytical procedures
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