Untangling the diagenetic history of uranium isotopes in marine carbonates: A case study tracing the δ238U composition of late Silurian oceans using calcitic brachiopod shells

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes

“…28 ). The simplest explanation for this uniformity is that these sites directly record contemporary δ 238 U sw , with little or no associated isotopic fractionation, as shown for some modern biogenic and abiotic calcites [41][42][43] . This implies that, under well-oxygenated conditions, U is incorporated into the carbonate lattice of syn-sedimentary cements as U(VI) whilst in communication with overlying seawater, and is not influenced by reducing conditions during early burial.…”

“…Previous work has suggested that some pelagic carbonates record lower δ 238 U CAU signatures, closer to seawater values compared to platform counterparts, and thus do not require correction 32,41 . Similarly, biogenic calcite from brachiopod shells is thought to better retain primary δ 238 U CAU signatures than metastable matrix components 43 . The better preservation of δ 238 U sw in pelagic carbonate samples is likely due to the stable mineralogy of these sample types (low-Mg calcite), but also the generally low organic carbon accumulation rates and low fluid flow rates of pelagic settings 43 .…”

“…Similarly, biogenic calcite from brachiopod shells is thought to better retain primary δ 238 U CAU signatures than metastable matrix components 43 . The better preservation of δ 238 U sw in pelagic carbonate samples is likely due to the stable mineralogy of these sample types (low-Mg calcite), but also the generally low organic carbon accumulation rates and low fluid flow rates of pelagic settings 43 . Here, increases in δ 238 U CAU and U/Ca at Sites 690 and 401, that likely reflect early diagenetic U(IV) uptake, are systematically tied to local environmental change related to the PETM hyperthermal (see below).…”

The uranium isotopic record of shales and carbonates through geologic time

“…In order to reconstruct the long-term redox evolution of the ocean, a large suite of marine carbonates ( Table 1) was assembled and the U concentrations and isotopic compositions of 95 carbonate samples spanning the Archean to Neoproterozoic were measured and combined with previously published data (Brennecka et al 2011;Dahl et al 2014Dahl et al , 2017Dahl et al , 2019Azmy et al 2015;Hood et al 2016;Lau et al 2016Lau et al , 2017Elrick et al 2017;Jost et al 2017;Song et al 2017;Bartlett et al 2018;Chen et al 2018aChen et al , 2018bClarkson et al 2018;Herrmann et al 2018;Phan et al 2018;White et al 2018;Zhang et al 2018aZhang et al , 2018bZhang et al , 2018cZhang et al , 2019aZhang et al , 2019bZhang et al , 2020aZhang et al , 2020cGilleaudeau et al 2019;Tostevin et al 2019;Brüske et al 2020a;Bura-Nakić et al 2020;Cheng et al 2020;Li et al 2020;del Rey et al 2020;Zhao et al 2020). The age and detailed description of the geological settings can be found in the references provided in Table 1 and in Supplementary Materials.…”

“…Indeed, modern primary carbonate precipitates are found to have a  238 U composition that is close to that of seawater (Stirling et al 2007;Weyer et al 2008;Romaniello et al 2013;Chen et al 2018aChen et al , 2018bTissot et al 2018), and the small detrital contribution can be minimized by partial dissolution of the carbonates in dilute acid. Based on the assumption that there is little fractionation between carbonates and seawater, several studies have used carbonates to track redox transitions during critical intervals such as at the Cambrian-Ordovician boundary, the end of the Sturtian Snowball Earth, and the Permian-Triassic boundary (Brennecka et al 2011;Dahl et al 2014Dahl et al , 2017Dahl et al , 2019Azmy et al 2015;Hood et al 2016;Lau et al 2016Elrick et al 2017;Jost et al 2017;Song et al 2017; Bartlett et al 2018;Clarkson et al 2018;Phan et al 2018;White et al 2018;Zhang et al 2018aZhang et al , 2018bZhang et al , 2018cZhang et al , 2019aZhang et al , 2019bZhang et al , 2020aZhang et al , 2020cGilleaudeau et al 2019;Tostevin et al 2019;Cao et al 2020;Cheng et al 2020;del Rey et al 2020;Li et al 2020;Zhao et al 2020). Most of these studies focused on variations in the U isotopic composition of carbonates during short time intervals to trace the expansion or contraction of anoxia in the oceans.…”