Formation of the massive bedded chert and coupled Silicon and Iron cycles during the Ediacaran-Cambrian transition

Abstract

Direct inorganic silica precipitation was a major silicon sink in the Paleoproterozoic oceans, whereas the diagenetic silica replacement of peritidal carbonate rocks became an increasingly important sink for silica in Mesoproterozoic and Neoproterozoic. The unique widespread bedded chert documenting transition from platform margin to basinal environments in the Yangtze Block, South China, during the Ediacaran-Cambrian transition (ECT, 550-521 Ma), however, remains enigmatic. Here, we report consistently low Germanium/Silicon (Ge/Si) ratios (0.2-0.5 μmol/mol) of the ECT Liuchapo cherts. The low (Ge/Si) ratios, combined with rare earth elements (REE) analysis, point to normal seawater source for the Liuchapo cherts. The primary nature of the chert was verified by Oxygen isotopes (δ18O) of silica and thus the measured Ge/Si ratios of the cherts could represent the maximum estimate of seawater Ge/Si ratios during ECT. Seawater during ECT has a significantly lower Ge/Si ratio than that of the modern marine. Low Ge/Si requires a high fraction of seawater germanium to be buried by a non-SiO2 sink. We speculate that germanium removal may be associated with a very active iron cycle driven by oscillating redox during ECT. The iron redox cycle may also effectively shuttle seawater silicon to sediments, causing bedded cherts precipitation. This study suggests that silica precipitation may be directly linked to the redox cycling of iron, and that this linkage favors silica precipitation regardless of its saturation state in the ocean.