Spatiotemporal evolution and causes of marine euxinia in the early Cambrian Nanhua Basin (South China)

Marine redox conditions and their dynamic variations in the early Cambrian are thought to have strongly influenced the evolution of early animals. However, the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood. The Nanhua Basin (South China) was a failed rift basin located between the Yangtze Block (NW margin) and Cathaysia Block (SE margin) in the early Cambrian. In this study, we conducted a comprehensive study of marine redox conditions, sulfur cycling, and chemical weathering intensity of source areas for the lower Cambrian shales of the Qingxi Formation in the basinal Shimensuidao section, which was geographically close to the Cathaysia Block. We integrated these results with previously published data from the nearby Silikou section as well as the coeval inner-shelf Xiaotan, upper-slope Daotuo, lower-slope Longbizui, and basinal Zhalagou sections, where were closer to the Yangtze Block. At Shimensuidao, iron species and redox-sensitive trace element data suggest dominantly euxinic conditions for the organic-rich black shale interval (lower member; LM) and variable redox conditions (oxic-ferruginous-euxinic) for the overlying organic-lean shales (upper member; UM) of the Qingxi Formation. Integrated redox results show a similar lessening of euxinic conditions from LM to UM across the entire basin and that euxinia developed along both the Yangtze and Cathaysia basin margins, but it was mainly in upper-slope areas (e.g., Daotuo) on the Yangtze margin and reached basinal areas (e.g., Silikou and Shimensuidao) on the Cathaysia margin. A decline of delta S-34(py) from LM to UM was observed at Shimensuidao and other sections, which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections. Lower delta S-34(py) values in shallow-marine relative to deep-marine areas suggest increased terrestrial sulfate availability to the shallower sites. Chemical index of alteration (CIA) values at Shimensuidao are uniformly high for both the LM (CIA(corr1): 82.5 +/- 0.4, CIA(corr2): 88.6 +/- 0.5) and UM (CIA(corr1): 80.2 +/- 2.3, CIA(corr2): 86.2 +/- 2.5) compared with the low-to moderate values at Daotuo (CIA(cor)(r2): 61.9 +/- 3.9 for LM, 71.3 +/- 4.8 for UM). This pattern is consistent with intense chemical weathering of source areas on the Cathaysia Block and weaker chemical weathering of source areas on the Yangtze Block. Based on these observations, we propose that strong chemical weathering of the Cathaysia Block led to regionally high terrigenous sulfate fluxes, resulting in expansion of mid-depth euxinia into deep basinal areas on the Cathaysia margin of the Nanhua Basin. In contrast, weak chemical weathering of the Yangtze Block yielded lower terrigenous sulfate fluxes, limiting euxinia to upper slope areas on the Yangtze margin of the basin. Weaker euxinia during UM deposition may have been due to its decreasing organic availability and/or small seawater sulfate reservoir caused by persistent euxinic consumption during LM deposition. Our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early Cambrian marginal seas, providing new insights into the co-evolution of continents, oceans, and early animals.