Oxygen levels regulate the co-evolution of non-terrigenous silica and organic matter in deep- to semi-deep waters during the Ediacaran–Cambrian transition

Abstract

During the Ediacaran–Cambrian (E–C) transition, marine black shales enriched in carbon (C) and silicon (Si) were widely deposited. However, the migration mechanism of Si in bathyal to abyssal zones remains unclear, and the key factors controlling the co-evolution of non-terrigenous silicon (NTS) and organic matter (OM) are still undefined. In this study, we focused on deep- to semi-deep water black shales in the upper Dengying Formation (Upper Sinian) and the lower Wangyinpu Formation (Lower Cambrian) of the Pukoucun section in northwestern Jiangxi Province, South China. Using an integrated approach that combines mineralogy, organic geochemistry, and element geochemistry, we investigated the factors controlling the co-evolution of the OM burial and NTS deposition in deep- to semi-deep water environments. The results suggest that the co-evolution of the OM and NTS in deep- to semi-deep water zones differs significantly from that in shallow-water environments. Hydrothermal fluids from the Jiangnan deep fault provided abundant hydrothermal Si, which diffused from hydrothermal vents into broader areas in the form of Fe(II)-silicate gels. In addition, the redox proxies (Mo_EF, U_EF, C_org/P, Ni/Co) show an increasing trend from the late Ediacaran to the early Cambrian, indicating that oxygen levels during this period exhibited an overall decreasing trend, accompanied by periodic fluctuations. Based on the co-variation relationships among Si_ex, TOC, Eu/Eu^⁎, and Fe/Al, it can be concluded that aqueous oxygen levels was the key controlling factor for the co-evolution of OM and NTS in deep- to semi-deep water settings along the southeastern margin of the Yangtze Platform during the E–C transition. We propose a new model of the co-evolution process, in which the extent of the marine dynamic anoxia zone influences the migration distance of Fe(II)-silicate gels, thereby regulating the sedimentation flux of NTS in distal vent areas. Based on these observations, NTS is predominantly removed from seawater in an inorganic form, and its enrichment may dilute the concentration of OM. These findings enhance our understanding of how marine redox evolution influenced the CSi cycle during the E–C transition and provide novel insights into shale OM enrichment mechanisms.