High-resolution carbon isotope records and correlations of the lower Cambrian Longwangmiao formation (stage 4, Toyonian) in Chongqing, South China

To gain a better understanding of the oceanic environment of the transitional Lower-Middle Cambrian boundary and provide additional data for the global ROECE correlation, we studied the carbon isotope records of shallow-water platform carbonates of the Cambrian Stage 4 Longwangmiao Formation in the Bandenggou section in Chongqing, South China, and correlated with previously reported contemporaneous δ13Ccarb records across a range of depositional environments on the Yangtze Platform and other basins. The δ13Ccarb curve of the Longwangmiao Formation fluctuates frequently approximately 0‰ with multiple excursions at its lower sequence, exhibits a conspicuous negative δ13C excursion with a minimum peak value of − 4.76‰ at the middle sequence, and then possesses a stable positive distribution upward. The correlation across the platform-to-basin transect on the Yangtze Platform shows that the Toyonian sections, from slope to basin, have similar shapes of δ13C curves, with a depleted trend upward and consistent timings of the largest-amplitude negative δ13C excursion, termed ROECE, while contemporaneous shallow-water platform sections have different δ13C profile shapes and timings of their largest-amplitude negative δ13C excursions, as do sections at a range of depositional environments from a multitude of continents. We infer that the carbon isotopic compositions from the Bandenggou section in the shallow-water environment layer may retain a regional signal. The negative δ13C distribution of the lower sequence is primarily due to a local, long-term riverine influx and low relative sea level at the early stage, which amplified organic matter decay and diminished primary productivity. Eventually, this elevated the oxidation and eutrophication of the shallow water, while the stable positive δ13C distribution upward is attributed to the integrated influence of a long-term scant riverine flux and high relative sea level. Additionally, the location of ROECE in the shallow-water sections still needs substantiation at the global scale.