Calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late Ediacaran

Abstract

Highlights  δ 44 Ca from limestones aged ~550-539 Ma that host the earliest skeletal animal fossils  Negative shift in δ 44 Ca that lasted at least 11-14 Myrs  Unlikely to record a transition towards more sediment-buffered carbonate diagenesis  May record enhanced continental weathering or evaporite deposition  May record a change in timing of dolomitisation

Calcium Isotopic Constraints on the Transition From Aragonite Seas to Calcite Seas in the Cambrian

“…These observations from South China are also consistent with light Ca isotope signatures and petrographic data from uppermost Ediacaran limestones from the Nama Group of Namibia (−1.60‰ to −1.07‰, average −1.42‰; Pruss et al, 2018;Tostevin et al, 2019). Previous work in the Nama Group has led to suggestions that changes in continental weathering, evaporite deposition or style of dolomitization may have influenced stratigraphic δ 44 Ca variation in these carbonates (Tostevin et al, 2019). However, it is difficult to rule out short-lived changes in carbonate mineralogy (even if background seawater conditions, as reflected by sparser skeletal, cement, ooid and fluid-inclusion records, predominantly favored aragonite or dolomite precipitation through the broader interval of the late Ediacaran).…”

“…The recurrent prevalence of low δ 44 Ca values and high Sr contents in Ediacaran-Cambrian strata of the Gaojiaxi-Yanjiahe and Xiaotan sections therefore suggests that widespread aragonite precipitation persisted (and aragonite geochemical indicators were preserved) through the late Ediacaran to at least Cambrian Age 2. These observations from South China are also consistent with light Ca isotope signatures and petrographic data from uppermost Ediacaran limestones from the Nama Group of Namibia (−1.60‰ to −1.07‰, average −1.42‰; Pruss et al, 2018;Tostevin et al, 2019). Previous work in the Nama Group has led to suggestions that changes in continental weathering, evaporite deposition or style of dolomitization may have influenced stratigraphic δ 44 Ca variation in these carbonates (Tostevin et al, 2019).…”

“…Foremost, a pronounced increase in seafloor spreading, resulting in increased Mg removal from seawater, would also have decreased global ocean 87 Sr/ 86 Sr, which is opposite to the observed trend and the presence of anomalously high seawater 87 Sr/ 86 Sr values in Cambrian strata (McArthur et al., 2012). Enhanced continental weathering has also been proposed to have decreased global seawater Mg/Ca ratios in the late Neoproterozoic to early Cambrian, due to an inferred substantial input of Ca into seawater linked to Great Unconformity‐associated weathering (Peters & Gaines, 2012; Tostevin et al., 2019; Wood et al., 2017). Increases in marine oxygen and sulfate levels, as well as greenhouse conditions and high sea level have also been invoked as potential drivers for a hypothesized early Cambrian transition to a calcite sea (Kump, 2008; Ries, 2010; Sandberg, 1983; Stanley & Hardie, 1998).…”

“…This model can also be more broadly applied to other regions and datasets, such as to previously published δ 44 Ca data generated from correlative upper Ediacaran strata in Namibia (Tostevin et al., 2019). In both successions—the Shibantan and Dahai members of South China (this study) and the Nama Group of Namibia (Tostevin et al., 2019)—limestones are consistently characterized by low δ 44 Ca values and high Sr contents, providing robust evidence for deposition of marine aragonitic precursors through substantial portions of these successions. Changes in carbonate diagenetic regimes may have been related to different water depths of the depositional units.…”

“…Using this modeling approach, with parameters selected to represent a range of potential scenarios for Ediacaran-Cambrian shallow-marine carbonate successions (see Supporting Information S1 for details), we were able to reproduce a number of the salient features of the stratigraphic trends observed in the South China Ediacaran-Cambrian successions. Four main processes during early diagenesis of marine carbonates have been modeled, including dolomitization of primary aragonite (solid lines in Figure 5b) or primary calcite (solid lines and c) for the studied carbonates in South China as well as limestones from the Nama Group, Namibia (Tostevin et al, 2019), and results of application of a shallow-marine diagenetic model (Ahm et al, 2018). In (b), the solid lines represent pathways for marine dolomitization of primary aragonite, whereas the dashed lines represent neomorphism from primary aragonite to the present calcite.…”