Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

Abstract

The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O-and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated δ Sr signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few leastaltered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in postglacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration.

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil

“…Cr‐spinel) or silicates that show Cr isotope fractionation during sorption (e.g. kaolinite) are commonly used to interpret negatively fractionated values from cap dolomites following Neoproterozoic glaciations (Caxito et al., 2018; Rodler, Frei, et al., 2016; Rodler, Hohl, et al., 2016) and to evaluate paleoredox interpretations from δ 53 Cr of black shales (Frank et al., 2019). However, binary plots of selected geochemical data against δ 53 Cr values suggest that detrital contamination did not influence our measured chromium isotope values (Figure 10).…”

Chromium Isotope Systematics in Modern and Ancient Microbialites

“…The samples from Mono Lake and BSF showed low Cr/Al ratios in a range similar to or higher than the reference material and are thus unsuitable for detrital correction. In contrast, detrital Cr only contributed 5 to 24% to most samples from Marion Lake and ALG samples and was thus suitable for detrital correction (Figure 4; Table 3) [54,67]. However, the detritus-corrected δ 53 Cr values for Marion Lake and ALG samples were within the error of or only slightly higher than the uncorrected δ 53 Cr values (Table 3).…”

“…Powdered microbialite samples containing approximately 300 ng Cr were dissolved in 0.5 mol/L HCl, centrifuged, and the supernatant was collected and filtered through 0.45 µm nylon membrane filters (Advantec MFS) before ion chromatography. The microbialite samples were spiked with an adequate amount of 50 Cr− 54 Cr double spike and evaporated to dryness [3,52,53]. Samples were then re-dissolved in aqua regia to ensure equilibration between sample and spike and dried down again.…”

“…Samples were then re-dissolved in aqua regia to ensure equilibration between sample and spike and dried down again. To separate Cr from microbialite samples, chromatographic Cr separation modified after [54][55][56] was applied. In short, microbialite samples were re-dissolved with 20 mL of a 0.025 mol/L HCl solution containing 0.5 mL of 0.2 mol/L (NH 4 ) 2 S 2 O 8 , capped, and boiled for 1 h to oxidize Cr(III) to Cr(VI).…”

“…The δ 53 Cr values of sediment samples can be corrected for detrital contamination using concentrations of Al or Ti as immobile reference elements as outlined by [54,56,59]. Briefly, detrital Cr (Cr det ) is calculated as Cr det = Al sample /Al det × Cr det .…”