Oceanic redox variations during the Cambrian Age 2–3 in the Yangtze Block of South China: Evidence from pyrite-based proxies

Abstract

The early Cambrian marks a pivotal period in metazoan evolution and ocean oxygenation through Earth's history, with sedimentary sequences in the Yangtze Block of South China preserving key records about biological and environmental evolution. Although previous studies have focused on redox conditions in shelf, slope, and basin environments, their controls on pyrite sulfur isotope (δ³⁴S_py) variations, and the influence of redox dynamics on biological activity within intraplatform basin remain unclear. This study presents results including diagnostic fossils, pyrite morphology, iron speciations, and δ³⁴S_py from the shales of the early Cambrian Qiongzhusi Formation in well W207, which is located in an intraplatform basin of the Yantze Block. These data are integrated with previous datasets from eight sections spanning shelf, slope, and basin environments. Our results indicate that during the transgression at the Cambrian Age 2–3 boundary (∼524–520 Ma), mid-depth euxinic waters on the slope expanded into the intraplatform basin, as supported by most samples with Fe_HR/Fe_T > 0.38, Fe_py/Fe_HR > 0.7, high Mo_EF/U_EF ratio (mean 5.2) and small sizes of framboidal pyrite (6.41 ± 0.70 μm). In contrast, in the early Cambrian Age 3 (∼520–517 Ma), these geochemical proxies collectively suggest widespread shelf oxygenation, coinciding with occurrence of abundant animal fossils in the strata. The seawater oxygenation probably facilitated metazoan diversification during the Cambrian explosion within the intraplatform basin. Deep-water oxygenation together with emerging bioturbations on the shelf shifted the oxic-anoxic redox interface from bottom water into sediments, promoting the formation of pyrite with heavy δ³⁴S_py values within seidments. Therefore, a spatial δ³⁴S_py gradient is observed during early Cambrian Age 3, with higher δ³⁴S_py values from the shallow shelf (more oxygen-rich) and deep basin (isotopically heavier sulfate in seawater) than those from middle-depth slope. The findings provide new insights into the redox landscape and its impact on biogeochemical cycling and metazoan evolution in the Yangtze Block during the early Cambrian.