Isotopic and Elemental Evidence for Paleoenvironmental Evolution of Cambrian Stage 4 Longwangmiao Formation, East Chongqing, China

The Cambrian Stage 4 represents a key point in the history of life in Longwangmiao Formation, carbonate is main rocks and diamictite widely distributed at its lower-middle part, with the slight occurrence of bioclast. The major and trace element analyses show that Longwangmiao Formation carbonate has various contents of ∑(CaO+MgO+LOI) from 74.64% to 99.18%, high Na₂O, K₂O, Fe₂O₃ and MnO concentrations (with an average of 0.20%, 0.27%, 0.27% and 0.014%, respectively), low Sr contents (135×10^-6-407×10^-6), gradually decreased contents of ∑(SiO₂+Al₂O₃+Fe₂O₃+ P₂O₅+TiO₂) upward section, extremely positive Ce anomalies (δCe=5.6-6.2) and a distinctive LREE-enriched pattern all the section (∑LREE/∑HREE=2-4), coupled with relatively high contents of ∑REE+Y at its middle part. The whole carbonates possess δ¹³C values from -1.5‰ to 1.1‰ VPDB and δ¹⁸O values from -9.7‰ to -3.5‰ VPDB, while calcites have relatively high δ¹³C values (-3.4‰ to +0.5‰ VPDB) and approximate δ¹⁸O values (-9.8‰ to -5.0‰ VPDB), alongside a gradual increase in carbon isotopic composition of both whole carbonates and calcites from its lower part to the upper section. Geochemical indictors of different palaeoenvironmental factors, involved palaeoclimate (MgO/CaO), seawater temperature and salinity (δ¹³C, δ¹⁸O, 100×MgO/Al₂O₃ and B_{proof read}), relative seawater depth (Sr/Ba, 1 000×Sr/Ca) and redox condition (U/Th, V/(V+Ni)), etc., imply that Longwangmiao Formation deposited at salt-brackish water under 17-25℃ during the transition mainly from the relative oxidizing and paralic environment to the dysaerobic-anaerobic and shallow marine environment, with an abundant riverine influx at its early period and frequent fluctuations of paleoclimate from relatively humid to arid, which would build the environmental stress and then lead to the major biomass reduction.