Datensatz

Zusammenfassung

Cadmium concentrations and stable isotopic compositions in seawater are important tools for studying the biogeochemical cycling of Cd in the modern oceans and as a proxy for micronutrient utilisation by phytoplankton. It is now well established that Cd isotopes become “heavier” as the primary production in the surface ocean increases, even though the mechanism driving the isotopic fractionation is still debated. Here, we use this property of Cd isotopes to examine changes that took place in the oceans during the emergence of multicellular life in the Neoproterozoic. Isotopic compositions and concentrations of Cd, N and C are reported in shallow-water carbonates of Ediacaran age from the Xiaofenghe section on the Yangtze Platform, South China. The Cd isotope data - reported as ε112/110Cd - show positive excursions in the cap dolomites, while significantly lighter Cd is found in the overlying strata. After correction for salinity-controlled fractionation into inorganic calcite, calculated palaeo-seawater ε112/110Cdsw range from -2 to +1.5, overlapping values of modern surface seawater. Importantly, ε112/110Cdsw and δ13C show a general positive correlation, as would be expected in bio-productive environments. However, the trend to lighter ε112/110Cd up-section is not that explicitly expected for an “explosion of life” at the end of the Ediacaran. The upper Doushantuo also displays substantial fluctuations in REE abundances, δ15N and δ13C, which may be due to estuarine mixing. Our data suggest that the variations in ε112/110Cd are a result of biologically-induced fractionation in at least some of the Ediacaran carbonates at Xiaofenghe. Further Cd isotope fractionation processes are clearly playing a role as well, such as precipitation of sulphides under anoxic pore-water conditions and fractionation into inorganic carbonates under variable salinity conditions. These effects have to be evaluated carefully when using Cd isotope systematics in ancient marine carbonates to look for palaeo-productivity signals.