The early Triassic time scale: New constraints from the Nanpanjiang Basin in South China and a review of geochronological, biostratigraphical and carbon isotope data

Abstract

Following the Permian-Triassic Boundary mass extinction (PTBME), the Early Triassic biotic recovery witnessed several failed recoveries of the nekton and marked changes in the ecological associations of terrestrial plants during its 5 my time span. This period is characterized by a series of profound fluctuations of the global carbon cycle, associated with changes in global climate and closely related changes in sea level, weathering rates, redox conditions, and finally organic carbon burial. However, the pacing and the underlying cause(s) of these changes are not yet well constrained, nor understood. Episodic pulses of volcanism of the Siberian Large Igneous Province (S-LIP) are commonly proposed as a trigger for these Early Triassic climate disturbances. However, S-LIP magmatism does not extend over a sufficiently long period to explain the carbon cycle disturbances and associated biotic setbacks during the Smithian and the Spathian, that is during most of the Early Triassic.

This study develops a precise chronological framework for the Early Triassic substages by integrating new and published UPb zircon ages of volcanic ash beds with detailed biostratigraphic and carbon isotope data. The combination of these methods allows for the accurate delineation and bracketing of stage and substage boundaries and provides insights into the timing of biotic and environmental changes. This age-depth model spans from the latest Permian until the earliest Middle-Triassic and is accurately correlated to the carbonate C-isotope record, ammonoid and conodont biochronology of a composite section from the Nanpanjiang Basin (South China). In the studied outer shelf sections, unconformities terminate periods of condensed deposition that coincide with positive shifts in the δ¹³C_carb isotope record. Duration of main unconformities is assessed by the age-depth model and are mostly close to biostratigraphic boundaries such as the Permian-Triassic (PTB), Dienerian-Smithian (DSB), Smithian-Spathian (SSB) and Spathian-Anisian (SAB). This approach allows us to define the ages of Early Triassic substage boundaries and the pace of carbon cycle fluctuations more accurately at the millennial timescale. The age of Griesbachian-Dienerian boundary (GDB) is confined between 251.669 + 0.057/−0.057 Ma and 251.657 + 0.057/−0.059 Ma, the DSB between 251.005 + 0.084/−0.075 Ma and 250.772 + 0.067/−0.084 Ma, the SSB between 249.347 + 0.051/−0.053 Ma and 249.326 + 0.055/−0.056 Ma and the SAB between 247.183 + 0.040/−0.044 Ma and 246.883 + 0.082/−0.073 Ma. Furthermore, the durations of the δ¹³C_carb cycles and the ages of its excursion peaks (N1-P4) were determined and compared.