Redox condition of Cryogenian interglacial seawater in Nanhua Basin constrained by stable tungsten isotopes

The redox condition of atmosphere and ocean is essential to understand evolution of the Earth's system. Cryogenian is a critical period in the Earth's redox history with massive Fe and Mn depositions and possible emergency of the earliest animals that may be linked to the changes in redox condition of the ocean. The redox condition of the Cryogenian seawater, especially for the seawater in Nahua Basin on the margin of Yangtze Block, however, are still under debates, despite the extensive researches conducted due to its great significance for global synthesis. The debates are mainly related to the black shale layer deposited in the early Cryogenian interglacial that contains deposits of Mn carbonate. The high pyrite-associated Fe fraction in the total reactive Fe and the high reactive Fe fraction in the total Fe of the black shale layer suggest euxinic deep seawater. However, the very heavy Fe isotopic composition of pyrite suggests ferric deep seawater. This work investigates the redox condition of the Cryogenian interglacial seawater in Nanhua Basin using stable tungsten isotopes. The rational is that the formation of highly dissolvable thiotungstate under euxinic deep water would prevent deposition of W from seawater while the accumulation of tungsten can be widely observed under none-euxinic seawater column associated with highly partial reactivity of tungstate. However, determining the authigenic deposition of tungsten from seawater based on proxies such as W/Th ratio can be largely complicated by changes of lithological source and sediment sorting. The stable tungsten isotopic composition may help to resolve the existence of authigenic tungsten because a detrital origin would have a upper continental crust-like delta W-186/184 composition while the tungsten deposited from seawater would have a higher-than-UCC delta W-186/184 approaching that of the mean river water due to the preferential retention of light tungsten isotopes by Fe-Mn oxyhydroxides during weathering. The results show that Th/W ratio and delta W-186/184 of the Cryogenian interglacial sediments in Nanhua Basin can be explained by mixed contributions from the detrital input and the authigenic deposition. The layer of black shale shows the highest delta W-186/184 value, indicating accumulation of tungsten under none-euxinic bottom seawater. We propose that the deposition of authigenic W is linked to episodic oxygenation of bottom water in an analogue to the present-day Baltic Sea. Our investigation shows that the stable tungsten isotopic system has great potential to constrain the redox condition of seawater. Combination of multiple proxies including the stable tungsten isotope is essential to reconstruct the dynamic redox changes of the ocean in the geological past.