Abstract

Abstract Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data for interpreting paleoenvironmental change throughout Earth history. However, the vast majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms that may not reflect global ocean chemistry. Here, we used calcium isotope ratios (δ44Ca) in conjunction with minor-element geochemistry (Sr/Ca) and field observations to explore the links among sea-level change, carbonate mineralogy, and marine diagenesis and the expression of a globally documented interval of elevated carbon isotope ratios (δ13C; Hirnantian isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow-water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and Utah, USA. The HICE on Anticosti is preserved in limestones with low δ44Ca and high Sr/Ca, consistent with aragonite as a major component of primary mineralogy. Great Basin strata are characterized by lateral gradients in δ44Ca and δ13C that reflect variations in the extent of early marine diagenesis across the platform. In deep-ramp settings, deposition during synglacial sea-level lowstand and subsequent postglacial flooding increased the preservation of an aragonitic signature with elevated δ13C produced in shallow-water environments. In contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater-buffered conditions muted the magnitude of the shift in δ13C. The processes documented here provide an alternative explanation for variability in a range of geochemical proxies preserved in shallow-water carbonates at other times in Earth history, and challenge the notion that these proxies necessarily record changes in the global ocean.

Highlights

  • Ancient shallow-water carbonate strata provide important archives of the evolution of Earth’s climate and environments

  • Measured δ44Ca and Sr/Ca values of limestones and dolostones from Anticosti Island and the Great Basin spanning the Hirnantian isotopic carbon excursion (HICE) carry a geochemical fingerprint of mineralogy, early marine diagenesis, and dolomitization that is indistinguishable from Neogene platform top and margin strata from the Bahamas

  • The range in δ44Ca and Sr/Ca values in the Ordovician data set spans the range of values observed for the Neogene Bahamas (Fig. 3; Fig. DR2), indicating that these sediments experienced a scope of early diagenetic conditions from sediment-buffered to fluid-buffered

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Summary

Introduction

Ancient shallow-water carbonate strata provide important archives of the evolution of Earth’s climate and environments. The carbon isotopic composition (13C/12C) of ancient shallowwater carbonate sediments has been widely applied as both a chronostratigraphic tool (Saltzman and Thomas, 2012) and an indicator of the partitioning of global carbon fluxes between carbonate and organic reservoirs, linking the global geochemical cycles of carbon and oxygen (Kump and Arthur, 1999; Saltzman et al, 2011). This interpretation of the carbon isotopic composition of shallow-water carbonates has been questioned by studies of modern analogues (Swart and Eberli, 2005; Swart, 2008; Higgins et al, 2018), where local processes, early diagenetic alteration, and changes in δ13C of carbonate minerals conspire to decouple the chemistry of shallow-water carbonate sediments from the global ocean. These models can require unrealistic changes to carbon burial and/or weathering fluxes, or they predict cross-platform δ13C gradients (δ13C increasing with greater proximity to the coast) that contradict the variability observed in some basins

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