Dwindling vanadium in seawater during the early Cambrian, South China.

Elemental vanadium (V), an essentially redox-sensitive metal in seawater, has had a significant impact on the understanding of the evolution of the atmosphere-ocean system throughout the history of the Earth. In fact, the geochemical cycle of V in early Cambrian seawater may have had an influence on the Chengjiang Biota in South China; however, it has not yet been well established. Given the authigenic vanadium accumulation is sensitive to the redox conditions of seawater, here, to constrain the geochemical cycle of V in seawater during the early Cambrian, the Mo, U and total organic carbon (TOC) distributions with high-resolution samples from both the outer shelf and slope facies (e.g., the Duoding and Longbizui sections), are applied to evaluate the redox conditions of ambient seawater. The Mo-U relationships indicate that the redox conditions of the mid-depth seawater evolved in a systematic way in South China, transitioning from an Fe-Mn reduction zone to anoxic/intermittently euxinic states and then to oxic conditions during the early Cambrian. As a consequence, the authigenic V enrichment, constrained by the marine redox conditions, was mainly controlled by the Fe-Mn particulate shuttle and the reduction and adsorption of organic matter in anoxic/euxinic conditions. However, the decoupling among V, Mo, U and TOC under anoxic/euxinic conditions suggests a dwindling vanadium concentration in the early Cambrian seawater of South China. The scavenging efficiency of V from seawater is much higher than those of Mo and U under anoxic/euxinic conditions. Ultimately, these trace elements (e.g., Mo, U, and especially V) in seawater could effectively be regulated and adjusted to a reasonable level under the widespread anoxic/euxinic conditions. The drawdown of trace elements in seawater might provide an early-stage preparation of the marine environment for the subsequent Chengjiang Biota. [ABSTRACT FROM AUTHOR]