Climatic-oceanic destabilization facilitated the euxinic expansion in the early Cambrian oceans

The early Cambrian (∼539–514 Ma) witnessed the dramatic evolution of atmospheric-oceanic oxygen level and biodiversification in Earth history. The turnover of the early life has been generally linked to climato-environmental factors, yet the detailed linkages between climate, oceanic circulation, and redox dynamics during this key interval are not fully understood. Here, we present the comprehensive records of Li isotope composition (δ⁷Li), Zn isotope composition (δ⁶⁶Zn) and other elemental geochemical data of the lower Cambrian Qiongzhusi Formation shales in the Yangtze Block, South China. These results combined with previous data in diverse depositional settings reveal the detailed record of severe expansion of bottom euxinic watermass followed by progressive shallow-water oxygenation in the marginal-marine Nanhua Basin from the middle Age 2 to Age 3 (∼526–514 Ma). Correlating with the climatic condition, we propose that the cool climate in sync with intense wind-flow may facilitate the invigorated seawater circulation and oceanic upwelling, which decoupled the redox evolution between surface water and bottom water. The high primary productivity and sulfate supply augmented by upwelling of open ocean caused the widespread euxinia in water column and photic zone. However, the establishment of euxinic seafloor and concomitant P regeneration further sustained the high bioproductivity and contributed to the burial of organic carbon, which in turn initiated the positive feedback in oxygenation of surface water. This study provides a window to reconcile the heterogeneous redox records between shallow water and deep water, and highlights the multiple interacting mechanisms for dynamic oceanic redox landscape during the early Cambrian, which significantly influenced the biological evolution.