Zn-Sr isotope records of the Ediacaran Doushantuo Formation in South China: diagenesis assessment and implications

Abstract

Recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans. This study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks, a thorough evaluation of diagenetic effects, and systematic variations of Zn and Sr isotope ratios in lower part of the Ediacaran stratigraphic unit deposited in the aftermath of the Marinoan glaciation in South China. The influence of post-depositional diagenesis on Zn isotope compositions of the studied samples is assessed by comparing δ66Zn to other geochemical indexes (87Sr/86Sr, δ13C, δ18O and Mn/Sr ratios). In the five studied cap carbonate sections (Member I of the Doushantuo Formation), dolostones from four sections have δ66Zn values positively correlated with δ18O values and negatively correlated with 87Sr/86Sr ratios (0.7081–0.7204). These correlations suggest that these cap dolostone samples have been modified by post-depositional diagenesis. The light δ66Zn value (−0.02‰) suggests that initial Zn isotope ratios of cap dolostones could have been reset by hydrothermal fluids with relatively high Zn concentration and low δ66Zn values.By contrast, carbonates from Member II of the Doushantuo Formation above cap dolostones are relatively pristine based on their low 87Sr/86Sr ratios (0.7079–0.7086) being indistinct from the proposed early Ediacaran seawater 87Sr/86Sr values. Chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo-environmental changes of the early Ediacaran ocean. A rapid increase of δ66Zn from ∼0.3‰ to 1.1‰ occurs in the middle part of Member II, accompanying by relatively invariant 87Sr/86Sr ratios that imply insignificant changes in input from continental weathering. Considering the limited change in atmospheric oxygen during this period, the rapid δ66Zn raise indicates an increase in buried organic matters, which is consistent with the coupled positive shift of δ13Ccarb, as well as the fossil records found in the same strata. These results provide insights into Zn cycling in the post-Marinoan oceans and facilitate the application of Zn isotopes in carbonates as a proxy for the fate of marine organic matter.