Dynamic Changes of Redox and Hydrographic Conditions in the Early Cambrian Nanhua Basin 11

The Cambrian Explosion marks the emergence of ancestral lineages for most extant animal phyla and the establishment of a metazoan-dominated ecosystem. However, its driving mechanisms remain enigmatic. While improvements in marine environmental conditions may have played a pivotal role, existing reconstructions of early Cambrian oceanic redox conditions remain contentious. To address this issue, we present high-resolution geochemical datasets from three stratigraphic sections spanning shallow to deep-water settings within the South China Block across Cambrian Stage 2-3. Iron speciation and redox-sensitive trace element records reveal a progressive transition in basin water redox conditions from euxinic to weakly euxinic, coinciding with rising salinity trends inferred from B/Ga ratios. Furthermore, pronounced terrigenous input in shallow to mid-depth settings likely promoted substantial accumulation of detrital material on the basin floor, inducing basin-wide shallowing that enhanced vertical water column mixing. Our results demonstrate that seawater redox conditions in the South China Block were decoupled from the open ocean, with the observed covariation between redox state and salinity reflecting intensified internal vertical mixing driven by evolving basin configuration and connectivity. These hydrodynamic changes fostered redox conditions conducive to biological diversification. This study elucidates the intricate interplay among basin morphology, tectonic evolution, seawater exchange dynamics, and marine redox evolution, offering a novel framework for understanding how localized paleogeographic reorganization shaped Early Cambrian marine environments and potentially facilitated the Cambrian Explosion.