Fig. 7

BSE images and chemistry of calcites in the carbonate concretions of Xiamaling Formation. a and b BSE images, showing a calcite crystal with dark nucleus (low Mn–Fe contents) and light rim (high Mn–Fe contents) (Zhaojiashan section); c and d EDS spectrums showing that the Mn–Fe contents of the nucleus are lower than the detection limits of EDS, while the rim has higher Mn–Fe contents; e A cross plot showing that the nuclei of calcite crystals have low Mn and Fe concentrations, while the rims have high Mn and Fe concentrations; f Average Mn/Fe ratios under different redox conditions. Mn/Fe ratios are enriched in our samples compared with those of average global carbonate (0.29; Turekian and Wedepohl 1961) and of manganous samples (Tostevin et al. 2016). Bars represent median values; arrows indicate samples with exceptionally high Mn/Fe ratios that lie above the limit of the Y axis; g A cross plot of δ¹³C vs. δ¹⁸O; h REE patterns of the carbonate concretions, showing prominent positive Ce anomalies and slight enrichment of MREE