Constraining marine Si cycle and bedded chert formation in the Ediacaran-Cambrian transition (550-521 Ma)

The Ediacaran－Cambrian transition (550Ma-521Ma) is one of the most critical intervals in Earth's history. The widespread continuous Liuchaopo bedded-chert deposition in South China Block provides an opportunity to understand the marine Si cycle of the Ediacaran ocean. We analyzed major and trace element components of the Liuchapo cherts from six sections. Ge/Si ratios (0.3 0.5μmol/mol ) and the rare-earth elements distribution patterns of the Liuchapo cherts imply that the primary Si source of the Liuchapo cherts is seawater rather than hydrothermal fluid. To reconstruct the redox conditions of the late Ediacaran seawater, we adapted a mixing model to calculate the Ge/Si ratio of the clay component in cherts. The Ge/Si in clays ranges from 1 to 10μmol/mol, much lower than that of chert nodules from the early Ediacaran Doushantuo Formation, suggesting the total dissolved organic carbon in seawater was already low toward the Ediacaran－Cambrian transition.

In addition to the Liuchapo Formation in South China, the Ediacaran-Cambrian bedded cherts deposition have also been found all over the world, and the Ge/Si ratios are all significantly lower than that of the modern ocean (0.72μmol/mol). We simulated the global marine Ge-Si cycle with mass balance model. From this model, the global occurrence of low seawater Ge/Si ratio could be caused by a smaller marine Si budget, a higher fraction of riverine input (compared to hydrothermal fluids) and a larger Ge non-opal sink. The combination of the former two factors cannot justify the extremely low seawater Ge/Si ratio, and thus an expansion of non-opal Ge sink is requested. The larger Ge non-opal sink may be ascribed to the precipitation of iron oxides/oxyhydroxides. In the context of moderate oxygen level in atmosphere during the Ediacaran－Cambrian transition, the upper boundary of iron reduction zone was located near the water-sediment interface. The rapid transition between ferrous and ferric iron in sediments may scavenge more Ge from seawater, leading to a lower seawater Ge/Si ratio. Therefore the active Fe cycle between sediment and seawater might play an important role in the precipitation of bedded chert through the formation of Fe(III)-Si gels and polymerization of Si(OH)₄. Our study suggests seawater may not be supersaturated with respect to Si in Proterozoic oceans.