Oceanic and atmospheric oxygenation during the late Ediacaran based on multiple isotope records from South China

Abstract

The Neoproterozoic–Cambrian transition was a key period in Earth's history, particularly because changes in the oceanic redox environment were crucial to the Cambrian explosion of life. However, the redox state of the ocean during this transition is unclear. In this study, the carbonate strata of the Dengying Formation in Yunnan Province, South China, were subjected to δ98/95Mo, δ13C, 87Sr/86Sr, and trace element analyses. The low δ98/95Mo results (0‰ ∼ +2‰) of most samples indicate that a shallow-marine sedimentary environment dominated by low-O2 conditions during the late Ediacaran period (Dengying Formation), but a brief period of marine oxidation existed at this time, reflected by elevated δ98/95Mo values (+2.07‰ ∼ +2.43‰). At this time, δ13C values were increasing, which possibly indicate an increase in primary productivity and photosynthesis in shallow waters, causing oxidation of the shallow seawater. However, two anomalously high δ98/95Mo values (>2.65‰) could have resulted from additional isotope fractionation due to the upwelling of euxinic bottom waters abruptly. In addition, an increase in 87Sr/86Sr ratios lagged the δ13C and δ98/95Mo changes, suggesting that atmospheric changes appear to have lagged oceanic oxidation, but more evidence are needed to prove the relationship. The dynamic changes in the redox conditions of the shallow sea during deposition of the Dengying Formation provided favorable conditions for changes in the atmosphere and the Ediacaran biota.