Cr isotopic composition of the Laobao cherts during the Ediacaran–Cambrian transition in South China

Abstract

In this study, we present the Cr isotopic compositions from the cherts in the Laobao Formation (~551–522 Ma) that were deposited in deep water in South China. The Cr concentrations and isotopic compositions in the Laobao cherts were dominated by the contemporaneous seawater and detrital phases. The lower part of the Laobao cherts show that the δ53Cr of contemporaneous deep seawater in the Huanan basin was approximately −0.1‰, and the δ53Cr of detrital materials was approximately ≥0.2‰. These findings imply that surface 53Cr-enriched Cr(VI) species did not affect the deep ocean until the late Ediacaran because of the strong redox stratification in the ocean, although the oxidative Cr cycle in terrestrial settings had emerged before that time. A slight rise in δ53Cr in the deep ocean (to approximately 0.08‰) was estimated for the upper part of the Laobao cherts. If the δ53Cr rise and its amplitude can be further determined, this will imply that shallow 53Cr-enriched Cr(VI) species had an impact on the deep ocean and will suggest an oxidation of the ocean in the Huanan basin around the Ediacaran–Cambrian boundary (~542 Ma).