Germanium/silicon of the Ediacaran-Cambrian Laobao cherts: Implications for the bedded chert formation and paleoenvironment interpretations

Sedimentary strata of the terminal Ediacaran (635-542 Ma) to early Cambrian (542-488 Ma) Laobao-Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron-oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 mol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid-ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 mol/mol, which is comparable to the Ge/Si of typical marine clays, but 10-100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Laobao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the Ediacaran-Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si-accumulation bacteria, may have been responsible for the bedded Laobao-Liuchapo chert formation in South China Block.