Earth system instability explains redox paradox during late Cambrian SPICE event

The late Neoproterozoic-early Paleozoic (~600 to 520 million years ago) witnessed a fundamental shift under atmosphere-ocean redox conditions, where pulsed ocean oxygenation events (OOEs) were replaced by episodic ocean anoxic events (OAEs). The Cambrian Steptoean positive carbon isotope excursion (SPICE) event (494.5 to 492.5 million years ago) represents a major early Paleozoic OAE, yet its drivers and underlying dynamics remain controversial. Here, we present carbonate uranium isotope (δ²³⁸U_carb) data from North and South China, revealing a sharp decrease from -0.29 ± 0.12 per mil to -0.85 ± 0.03 per mil at the onset of the SPICE event, followed by a return to pre-excursion values. This indicates a rapid onset of global ocean anoxia before the rise of δ¹³C_carb values, followed by a transition to oxic conditions. Integrating δ²³⁸U_carb data with refined COPSE (Carbon, Oxygen, Phosphorus, Sulphur, and Evolution) biogeochemical modeling reconciles the pre-SPICE redox paradox of low atmospheric pO₂ coexisting with limited seafloor anoxia. We propose that the SPICE and its associated excursions reflect a non-steady-state Earth system behavior, involving oscillations in coupled phosphorus-oxygen-carbon-sulfur and uranium cycles, which influenced the evolution of early animals through redox-dependent biodiversity filters.