THE FORMATION OF THE BEDDED CHERT IN THE LIUCHAPO FORMATION DURING THE EDIACARAN-CAMBRIAN TRANSITION IN SOUTH CHINA

It is widely accepted that, before the evolution of silica secreting organisms, inorganic silica precipitation might be the major Si sink of seawater. As a non-redox sensitive element, Si precipitation was thought to be linked to the concentration of silica in the ocean. The Liuchapo Formation, mainly composed of bedded chert with thickness of up to 100s of meters, was widespread in South China during the Ediacaran-Cambrian transition (ECT， 550-521 Ma). In order to understand the origin of the silica and the mechanism of the massive chert deposition, we measured the Germanium/Silicon (Ge/Si) ratios of chert samples from 6 sections in South China, which includes a variety of depositional environment, from shelf margin to basin. The Ge/Si ratio of seawater ((Ge/Si)_SW) is estimated to range from 0.3-0.5 μmol/mol in ECT, significantly lower than that of the modern seawater value. The modeling result indicates that low (Ge/Si)_SW requires a high fraction of seawater Ge being buried by non-opal sink, such as iron oxides/oxyhydroxides, which scavenge seawater Ge during Fe²⁺ oxidation. We speculate that a large non-SiO₂ sink might be attributed to active iron cycle in the redox oscillating ocean during ECT. On the other hand, active Fe cycle may also effectively shuttle seawater Si into sediments, resulting in the massive precipitation of bedded cherts. Our study suggests that the Fe cycle might be a more effective way to remove seawater Si before the evolution of silica-secreting organisms. Therefore, the Proterozoic ocean may not necessarily be supersaturated with respect to silica; instead, Si precipitation might be tightly coupled with the marine redox, which leads to the fluctuation of the remaining Si concentration in the ocean.