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Sea Level, Carbonate Mineralogy, and Early Diagenesis Controlled Δ13c Records in Upper Ordovician Carbonates David S

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Sea Level, Carbonate Mineralogy, and Early Diagenesis Controlled Δ13c Records in Upper Ordovician Carbonates David S https://doi.org/10.1130/G46861.1 Manuscript received 10 August 2019 Revised manuscript received 17 October 2019 Manuscript accepted 29 October 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 9 December 2019 Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates David S. Jones1, R. William Brothers1, Anne-Sofe Crüger Ahm2, Nicholas Slater2, John A. Higgins2 and David A. Fike3 1 Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA 2 Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA 3 Department of Earth & Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, USA ABSTRACT (Melchin and Holmden, 2006). However, these Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data models can require unrealistic changes to carbon for interpreting paleoenvironmental change throughout Earth history. However, the vast burial and/or weathering fuxes, or they predict majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms cross-platform δ13C gradients (δ13C increasing that may not refect global ocean chemistry. Here, we used calcium isotope ratios (δ44Ca) in with greater proximity to the coast) that con- conjunction with minor-element geochemistry (Sr/Ca) and feld observations to explore the tradict the variability observed in some basins. links among sea-level change, carbonate mineralogy, and marine diagenesis and the expres- Here, we developed a new hypothesis that sion of a globally documented interval of elevated carbon isotope ratios (δ13C; Hirnantian explains the HICE as a record of shallow-water isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow- aragonite δ13C that evolved during sea-level water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and change and was variably altered during early Utah, USA. The HICE on Anticosti is preserved in limestones with low δ44Ca and high Sr/Ca, marine diagenesis. We used measurements of consistent with aragonite as a major component of primary mineralogy. Great Basin strata δ44Ca, δ13C, and Sr/Ca to identify the primary are characterized by lateral gradients in δ44Ca and δ13C that refect variations in the extent mineralogy and early diagenetic history of car- of early marine diagenesis across the platform. In deep-ramp settings, deposition during bonates deposited across the Hirnantian interval. synglacial sea-level lowstand and subsequent postglacial fooding increased the preservation We propose a mechanism that links the observed of an aragonitic signature with elevated δ13C produced in shallow-water environments. In magnitude of the HICE to glacioeustasy, depo- contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater- sitional environment, and early diagenesis, and buffered conditions muted the magnitude of the shift in δ13C. The processes documented here we discuss the implications for understanding provide an alternative explanation for variability in a range of geochemical proxies preserved the links between elevated δ13C and global envi- in shallow-water carbonates at other times in Earth history, and challenge the notion that ronmental change in the geologic record. these proxies necessarily record changes in the global ocean. Diagenetic Framework for Shallow-Water INTRODUCTION Hirnantian δ13C and Sea Level Marine Carbonate Sediments Ancient shallow-water carbonate strata pro- Upper Ordovician (Hirnantian) strata around The pairing of calcium isotopes (δ44Ca) and vide important archives of the evolution of Earth’s the globe contain an interval of elevated δ13C, trace-element ratios (Sr/Ca and Mg/Ca) has climate and environments. In particular, the carbon referred to as the Hirnantian isotopic carbon ex- emerged as a promising tool to identify both isotopic composition (13C/12C) of ancient shallow- cursion (HICE). The HICE is associated with primary mineralogy and the style of early dia- water carbonate sediments has been widely ap- sea-level fall during the Hirnantian glaciation, genetic alteration of marine carbonate sediments plied as both a chronostratigraphic tool (Saltzman which lasted less than 1.3 m.y. and has a geo- (Fantle and DePaolo, 2007; Fantle and Higgins, and Thomas, 2012) and an indicator of the parti- graphically variable magnitude, ranging from 2014; Higgins et al., 2018). In particular, cross- tioning of global carbon fuxes between carbon- ∼+2‰ to ∼+7‰ (Melchin et al., 2013, and refer- plots of δ44Ca and Sr/Ca in carbonates of all ate and organic reservoirs, linking the global geo- ences therein). Geochemical anomalies in δ15N, geologic ages exhibit covariation that can be chemical cycles of carbon and oxygen (Kump and δ34S, 87Sr/86Sr, δ26Mg, and δ7Li (LaPorte et al., quantitatively related to the style of early marine Arthur, 1999; Saltzman et al., 2011). However, this 2009; Zhang et al., 2009; Hu et al., 2017; Kim- diagenesis. The covariation arises for three rea- interpretation of the carbon isotopic composition mig and Holmden, 2017; Pogge von Strand- sons. First, early diagenetic carbonate minerals of shallow-water carbonates has been questioned mann et al., 2017) are also found in Hirnantian are characterized by low Sr/Ca partition coeff- by studies of modern analogues (Swart and Eberli, strata. The origin of the HICE has been attrib- cients (Brand and Veizer, 1980) and small Ca iso- 2005; Swart, 2008; Higgins et al., 2018), where uted to increased global organic carbon burial tope fractionation factors (Fantle and DePaolo, local processes, early diagenetic alteration, and (Brenchley et al., 1994); synglacial changes in 2007; Jacobson and Holmden, 2008; Tang et al., changes in δ13C of carbonate minerals conspire to δ13C of the riverine weathering fux (Kump et al., 2008) compared to primary marine carbonate decouple the chemistry of shallow-water carbon- 1999); and/or local carbon cycling in shallow minerals (although Baker et al. [1982] showed ate sediments from the global ocean. basins partially isolated from the global ocean that deep-sea pelagic calcite can maintain high CITATION: Jones, D.S., et al., 2020, Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates: Geology, v. 48, p. 194–199, https://doi.org/10.1130/G46861.1 194 www.gsapubs.org | Volume 48 | Number 2 | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/2/194/4927147/194.pdf by Princeton University user on 22 July 2020 . 63° W Fm A B C EH D 25 m ecsci e Rhuddanian LFB . 49.5° Fm tian NACP N N Hirnan Ellis Bay LC 10km .B LEGEND Fm tian Ka NACP Silurian Vaureal Vaureal Laframboise Point Ellis vician Bay do Vauréal Or -1 031 2 4 5 -1.8 -1.6 -1.4 -1.2-1.0 -0.8 010.52.0 1.5 .0 2.5 3.0 δ C (‰-VPDB) δ44Ca (‰-SW) Sr/Ca (mmol/mol) Figure 1. Stratigraphic plots of geochemical data from Anticosti Island, Canada: New Associated Consolidated Paper (NACP) drill core and Laframboise Point outcrop limestones. (A) δ13C. (B) δ44Ca. (C) Sr/Ca. Laframboise Point carbon isotope data are from Jones et al. (2011). Hori- zontal gray bands indicate lower and upper Hirnantian isotopic carbon excursion (HICE) carbon isotope anomalies. (D) Simplifed geologic map of Anticosti Island, after Desrochers et al. (2010). EH—English Head, LFB—Laframboise Point, LC—Lousy Cove, Fm.—Formation, SW— seawater, VPDB—Vienna Peedee belemnite. Sr/Ca during marine diagenesis). Second, δ44Ca University (New Jersey, USA) following the HICE, were characterized by δ44Ca < −1.0‰ and Sr/Ca depend on primary mineralogy, with methods of Higgins et al. (2018). Ca isotope and elevated Sr/Ca. aragonite precipitation fractionating Ca isotopes measurements are reported as the relative abun- to a greater extent (Gussone et al., 2005) and in- dance of 44Ca to 40Ca using standard delta no- DISCUSSION corporating more Sr (Veizer, 1983) than calcite. tation, normalized to the isotopic composition Measured δ44Ca and Sr/Ca values of lime- Third, early marine diagenesis in shallow-water of modern seawater. The external reproducibil- stones and dolostones from Anticosti Island carbonate sediments can occur under both fuid- ity for SRM915b calcium carbonate standard and the Great Basin spanning the HICE carry and sediment-buffered conditions, depending on was −1.16‰ ± 0.19‰ (2σ, N = 38), and re- a geochemical fngerprint of mineralogy, early the extent to which pore-fuid exchange with producibility for an internal aragonite standard marine diagenesis, and dolomitization that is seawater occurs through advection or diffusion was −1.47‰ ± 0.15‰ (2σ, N = 12). New δ13C indistinguishable from Neogene platform top (Banner and Hanson, 1990; Fantle and DePaolo, data were generated for the NACP core follow- and margin strata from the Bahamas. 2007; Fantle and Higgins, 2014; Higgins et al., ing the methods of Jones et al. (2011); all other First, the range in δ44Ca and Sr/Ca values in 2018). Together, these properties of primary and δ13C data came from Jones et al. (2011, 2016). the Ordovician data set spans the range of val- diagenetic carbonate minerals and shallow-wa- ues observed for the Neogene Bahamas (Fig. 3; ter sedimentary systems lead to a characteristic RESULTS Fig. DR2), indicating that these sediments ex- relationship between bulk δ44Ca and Sr/Ca and The Anticosti Island and Great Basin data perienced a scope of early diagenetic conditions provide a means with which to characterize the varied stratigraphically and geographically from sediment-buffered (geochemical records effects of mineralogy and early marine diagen- in δ13C, δ44Ca, and Sr/Ca (Figs. 1 and 2; Fig. set by the chemistry of the primary sediment: esis on the geochemistry of shallow-water car- DR1 in the GSA Data Repository1). Ca isotope low δ44Ca; high Sr/Ca) to fuid-buffered (geo- bonate sediments (Ahm et al., 2018; Higgins values for limestones were between −0.9‰ chemical records modifed by the seawater-like et al., 2018).
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