A high-resolution age model for Early Triassic Carbon isotope fluctuations

The Early Triassic is a key interval to understand biotic and environmental recovery after the major disturbance of the planetary system at the Permo-Triassic Boundary (PTB). The collapse of biodiversity and the dramatic change of the global carbon cycle are thought to be the result of intrusions of mafic melts of the Siberian Large Igneous Province (S-LIP) into sediments rich in organic matter and evaporites. However, the Early Triassic is marked by a long record of global carbon cycle perturbations, suggesting that severe climatic instability and delayed biotic recovery may go beyond S-LIP volcanism.

To further explore this question, we compiled and constructed a continuous stratigraphic record in the platform-slope sequences of the Nanpanjiang Basin, South China, where bio- and chemostratigraphic control is supported by precise geochronology. We present here a U-Pb zircon-based Bayesian age model integrated with δ¹³C isotope data, providing precise temporal calibration of the carbon cycle fluctuations and of lithostratigraphic and biostratigraphic boundaries over 5.5 million years. This record is calibrated to absolute age via the Bayesian age model relying on 20 published and 6 new high-precision ID-TIMS U-Pb ages of volcanic zircon in interbedded ashes.

Our compilation covers six major δ¹³C excursions, three negative (termed N1 to N3) and three positive ones (termed P1 to P3): The initial sharp negative excursion N1 at the PTB (~252.02 Ma) reflects rapid input of volcanogenic CO₂ from the Siberian Large Igneous Province (S-LIP) activity. An early Griesbachian positive peak P1 (~251.88 Ma) is associated with transient recovery of organic carbon burial in microbial-rich environments. A negative δ¹³C shift (N2) in the Dienerian (~251.40 Ma) coincides with ecological stress and enhanced weathering. A sharp positive excursion P2 at the Dienerian-Smithian transition (~250.39 Ma) reflects carbon sequestration in a dynamic climate regime. The largest negative excursion N3 is found during the middle Smithian thermal maximum (~249.56 Ma) and correlates with the climax of global warming and marine anoxia. The Smithian-Spathian boundary positive excursion P3 (~249.33 Ma) marks a cooling phase and coincides with significant biotic turnover, particularly affecting nektonic faunas. The Spathian interval features a stepwise decrease in δ¹³C (N4; ~248.14 Ma), followed by a broad positive excursion near the Spathian-Anisian boundary (P4; ~246.88 Ma), reflecting stabilization of carbon cycling as ecosystems began recovering. The positive excursions P1 to P4 coincide with 100ky-long gaps in the sedimentary record, related to low sea-level and possibly to presence of continental ice.

Published and new U-Pb zircon and baddeleyite dates put the minimum estimate for intrusive as well as extrusive S-LIP activity into the Dienerian (ca. 251.0-250.0 Ma), pointing to a non-volcanic trigger of the P2, P3, P4, N3, N4 isotope excursions. In summary, the robust U-Pb calibrated early Triassic carbon isotope record is interpreted to reflect the interplay of volcanic influence, intrinsic carbon cycle feedback, and other extrinsic factors, which appear to have paced the alternation between extreme warming and transient cooling phases during the Early Triassic post-extinction recovery, and post-recovery period.