Bucher et al.'s Comment (2007) questioned the validity of our 247.2 Ma age estimate for the Early-Middle Triassic (Olenekian-Anisian, O-A) boundary based on a comparison of recent data from Lower Guandao section presented by us (Lehrmann et al., 2006) with preliminary data reported from the adjacent Upper Guandao section from a field-trip guide (Lehrmann et al., 2005). The ages presented in the field guide were provided without supporting data, and were not intended to be used for boundary age assignments or cited, as specified in the field guide (Lehrmann et al., 2005).
Bucher et al. suggest that the first appearance of the conodont Chiosella timorensis, which we used as a proxy for the O-A boundary, is diachronous based on comparison of the preliminary geochronology from Upper Guandao section (Lehrmann et al., 2005) and the high-precision dates presented in Lehrmann et al. (2006). Below we demonstrate that the conodont occurrences are isochronous within the constraints of paleomagnetic-reversal and carbon-isotope stratigraphy between the sections.
Figure 1 illustrates the correlation between Lower and Upper Guandao sections. The depiction of the Lower Guandao section is the same as our Figure 2 (Lehrmann et al., 2006) with the addition of high-resolution conodont data that resulted in a 2.6 m downward shift of the O-A boundary. The O-A boundary remains bracketed by volcanic-ash horizons PGD-2 and PGD-3. Adjustment in the boundary position yields a new interpolated boundary age of 247.24 Ma. The Upper Guandao section has been updated from the very preliminary form given in our field guide (Lehrmann et al., 2005) by integrating stratigraphic thicknesses of several measurements, adding high-resolution conodont data, paleomagnetic-reversals, and carbon-isotope data.
Lower and Upper Guandao sections occur in the deep-marine slope (Lower Guandao) to toe of slope (Upper Guandao) facies adjacent to a carbonate platform (Lehrmann et al., 2005). Rapid facies changes are the norm in such transitions. The thicker volcanic units in Upper Guandao section are tuffaceous siliciclastic mudstone. Thicker volcaniclastic units at Upper Guandao, and correspondingly thinner carbonate units, resulted from lower rates of carbonate accumulation and more rapid siliciclastic accumulation at the basin margin farther from the platform source of carbonate.
The argument of Bucher et al. that the first appearance of Cs. timorensis was diachronous in our sections is flawed because it rests on the preliminary age of sample GDGB-O from Upper Guandao section. Correlation between the two sections (Fig. 1) is corroborated by paleomagnetic reversals and a large positive isotope excursion, showing that the first occurrence of Cs. timorensis, and the occurrences of associated conodonts, are isochronous within the constraints of the data. In both sections, the O-A boundary is delineated by the first occurrence of Cs. timorensis (and faunal turnover of several associated species; Fig. 1) that occurs below the peak of the positive carbon isotope excursion near the base of the Aegean, and the shift from predominantly reversed to normal polarity near the base of the Bithynian. We agree with Bucher et al.'s suggestion that a boundary age should not be constrained solely by the first occurrence of one species, and we have used several conodont species in delineating the boundary.
We chose not to use the Upper Guandao section for delineation of the O-A boundary in Lehrmann et al. (2006) because the geochronological data for GDGB-O indicated a great deal of complexity. However, as discussed in Ramezani et al.'s Reply (2007) to Buchur et al.'s Comment, we can now confidently assign a depositional age to this ash.
Ovtcharova et al. (2006) interpreted the O-A boundary to lie between 248.1 Ma and 247.8 Ma on the basis of a single new age of 248.1 ± 0.4 Ma they obtained from the Upper Spathian, and the citation of a preliminary age date (GDGB-O) from our field trip guide (Lehrmann et al., 2005). The extensive and integrated chronostratigraphic and geochronological data presented by us (Lehrmann et al., 2006) provide a far more robust constraint on the boundary age at 247.2 (± 0.4).