Application of Cd as a paleo-environment indicator

Abstract

Cadmium is a micronutrient element and a tracer of biological productivity in the ocean. The contrasting Cd distributions in sediments from modern restricted basins, continental oxygen minimum zones (OMZs), and oxic settings make it possible to establish accurate thresholds to distinguish between various environments. By receiver operating characteristic analysis, we propose that Cd > 14.5 μg/g and Cd/Al > 4.33 × 10 −4 provide strong evidence for deposition within perennial OMZs, while Cd < 0.52 μg/g and Cd/Al < 0.08 × 10 −4 separate oxic sediments from anoxic sediments. With the modern Cd cycle as a guide, we present a case study of sedimentary Cd enrichment through a slope–basin transect in the early Cambrian Nanhua Basin, South China. With increasing water depth, the patterns of Cd concentrations and Cd/Al ratios are similar to those of the modern Peruvian margin, which are characterized by highest Cd enrichments in the OMZ core and muted Cd enrichments at the upper and lower edges of the OMZ. These data, combined with previous studies, indicate that the OMZ-like structure might be common in early Cambrian oceans. Furthermore, we analyzed Cd contents of shales from the Mesoproterozoic and assemble them with published data to explore the secular evolution of seawater Cd inventory. We find that a significant increase in Cd abundance and Cd/Al ratio occurs through the Ediacaran–Cambrian transition, reflecting enlarged Cd reservoir and widespread ocean oxygenation. Overall, Cd geochemistry can provide unique information concerning the redox state and hydrological condition of paleo-environment. • Contrasting Cd enrichments distinguish restricted basins, OMZs, and oxic settings. • Sedimentary Cd pattern reveals active upwelling in the early Cambrian Nanhua Basin. • Cd record reflects an increase in marine Cd reservoir through the E–C transition.