Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates

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.

The grandest of them all: the Lomagundi–Jatuli Event and Earth's oxygenation

“…In these aspects, the LJE is not a singularity in that the link between facies changes and positive δ 13 C carb excursions marked by sustained values of +5-10‰ and locally higher for stratigraphically significant intervals is known from other times in Earth history. Studies combining careful sedimentology and petrography with isotope geochemistry show that co-variation between facies and δ 13 C carb values is common and can modify global signals; examples include the Cryogenian Tayshir Member of the Tsagaan Oloom Formation in Mongolia (Macdonald et al 2009;Bold et al 2016) and Etina Formation in Australia (Rose et al 2012), Ediacaran Hüttenberg Formation in Namibia (Cui et al 2018), end-Ordovician Hirnantian strata (Jones et al 2020) and Silurian Ireviken and Lau Events in the Baltic Basin (Wigforss-Lange 1999; Rose et al 2019). Studies of C-isotope process-response in recent and modern depositional environments provide additional credence to our scenario that the LJE is a record of lateral isotopic gradients of contemporaneous DIC pools.…”

Calcium isotopes in deep time: Potential and limitations

“…Indeed, a significant positive correlation is observed for a carbonate-poor PETM deep-sea core (Griffith et al, 2015), which may indicate that a unique diagenetic regime characterizes predominantly siliciclastic d 44/40 Ca records. Data from the Late Silurian exhibit a linear inverse relationship between d 44/40 Ca and Sr/Ca, potentially reflecting variable precipitation rates and not diagenesis (Farkaš et al, 2016 (Lau et al, 2017), the Late Silurian (Farkaš et al, 2016), the end-Ordovician glaciation (Holmden et al, 1998;Kimmig and Holmden, 2017;Jones et al, 2020), the Ediacaran Shuram excursion (Husson et al, 2015), and the Marinoan cap carbonate sequence (Ahm et al, 2019 (Fantle, 2015;Fantle and DePaolo, 2007;Gussone and Heuser, 2016;Kısakürek et al, 2011), coccoliths (Gussone et al, 2007;Langer et al, 2007), and dinoflagellates , (5) Modern biogenic skeletal aragonite corals (Chen et al, 2016;Gothmann et al, 2016;Inoue et al, 2015), ( 6) authigenic aragonite clathrites (Teichert et al, 2005), ( 7) highmagnesium calcite from Site 1131 and high-magnesium foraminifers (Gussone et al, 2016), ( 8) inorganic calcite from laboratory experiments with varying precipitation rates (AlKhatib and Eisenhauer, 2017a;Tang et al, 2008b;Lemarchand et al, 2004), ( 9) authigenic carbonates from the Miocene Monterey Formation and the northern South China Sea (Blättler et al, 2015;Wang et al, 2012;, ( 10) Synthetic and natural ikarite (Gussone et al, 2011), (11) platform dolomites from the Great and Little Bahamas Bank Holocene, but timing and magnitude of the variation differ. Because of the larger isotope fractionation, the barite records…”

Geological evidences and mechanisms for oceanic anoxic events during the Early Paleozoic

“…当然 δ 13 C org 更容易受到局部碳储库循 环的影响，在不同深度的剖面幅度均有差别 [30,118] . 另外，浅水相碳酸盐台地易受到早期成岩 作用影响而造成 δ 13 C carb 在幅度和漂移起始点上失真 [119] . 另一方面，瑞典 Gotland 和美国 Nevada 地区兰多维列统-文洛克统界线附近产出的 δ 13 C carb 和 δ 34 S CAS 表现出同步正漂的特征， 但碳酸盐岩的 I/(Ca+Mg) 指示浅水相剖面底水氧化而深水相剖面水体是缺氧的 [120,121] .…”