Timing of the Early Triassic carbon cycle perturbations inferred from new U-Pb ages and ammonoid biochronozones

Based on analyses of single, thermally annealed and chemically abraded zircons, a new high-precision U-Pb age of 251.22 ± 0.20 Ma is established for a volcanic ash layer within the “Kashmirites densistriatus beds” of early Smithian age (Early Triassic) from the Luolou Formation (northwestern Guangxi, South China). This new date, together with recalculated uncertainties of previous U-Pb ages from the same section [M. Ovtcharova, H. Bucher, U. Schaltegger, T. Galfetti, A. Brayard, J. Guex. New Early to Middle Triassic U-Pb ages from South China: calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery. Earth Planet. Sci. Lett. 243 (2006) 463-475.] allows constraining the time framework of the Early Triassic and leads to an estimated duration of (i) ca. 0.7 ± 0.6 My for the Smithian and (ii) a maximal duration of ca. 1.4 ± 0.4 My for the Griesbachian-Dienerian time interval. The new U-Pb age considerably reduces the absolute age gap comprised between the Permian-Triassic boundary and the Spathian (late Early Triassic). The new age framework provides the basis for the calibration of a new carbonate carbon isotope and ammonoid records of the Early Triassic Luolou Fm., which in turn are of high significance for global correlations and for carbon cycle modeling. This calibration indicates that the most significant and fastest Early Triassic carbon isotope perturbations occur between the earliest Smithian and the early Spathian, thus spanning a time interval of about 1 My. Whatever caused these carbon cycle shifts of high intensity and short duration, there is evidence for connections between these fluctuations, the pulsate recovery of ammonoids and conodonts as well as climate changes