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 δ 13 C carb records across a range of depositional environments on the Yangtze Platform and other basins. The δ 13 C carb curve of the Longwangmiao Formation fluctuates frequently approximately 0‰ with multiple excursions at its lower sequence, exhibits a conspicuous negative δ 13 C 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 δ 13 C curves, with a depleted trend upward and consistent timings of the largest-amplitude negative δ 13 C excursion, termed ROECE, while contemporaneous shallow-water platform sections have different δ 13 C profile shapes and timings of their largest-amplitude negative δ 13 C 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 δ 13 C 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 δ 13 C 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. [ABSTRACT FROM AUTHOR]