IMPROVED RADIO-ISOTOPIC AND BIOCHRONOLOGICAL APPROACHES: APPLICATION TO THE EARLY-MIDDLE TRIASSIC BOUNDARY

The Early-Middle Triassic boundary (EMTB) is usually perceived as the end of a succession of upheavals and short term recoveries following the end-Permian mass extinction. Hence, accurate and robust biochronological definition and radio-isotopic calibration of the EMTB is of paramount importance. The recent confirmation that the first occurrence (FO) of the conodont Chiosella timorensis, previously used as an index for the base of the Anisian, ranges down into the Spathian led to a revision of the conodont faunas around the EMTB in former GSSP candidate sections and to a new proposal for the definition of the EMTB (Goudemand et al., 2012). Here we report on a new section at Monggan (NW Guangxi) where the same robust residual maximal associations of conodonts allow recognition of the EMTB uncertainty interval. Moreover, at Monggan this interval is bracketed by fourteen ash layers and volcanogenic greywackes. This well documented succession allows utilizing very selective criteria for the accuracy of individual U-Pb ages. Biases of magmatic (pre-eruptive zircons with longer residence in the magmatic system), sedimentary (reworking) and diagenetic (post-crystallization lead loss due to late hydrothermal circulations) origins are highlighted leading to a substantial reduction of the number of U-Pb ages that qualify for the calibration. Spline interpolation of the eight selected ages yields an EMTB age interval bounded by 247.19 ± 0.03 Ma and 247.03 ± 0.04 Ma dates. Although within the error margin of the previous 247.2 ± 0.4 Ma age of Lehrmann et al. (2006), which calibrated a FO of Chiosella timorensisand was obtained by linear interpolation between only two U-Pb ages, this new EMTB age interval is significantly more accurate and more precise and is devoid of any biostratigraphic distortion. In the Monggan section, the inception of a distinctive four per mil plateau in the carbonate carbon isotope record matches with the base of the Anisian, thus providing a potential second order age constraint for the correlation of the EMTB.

Goudemand et al. 2012 Geobios 45: 199-207

Lehrmann et al. 2006 Geology 34:1053-1056.