A PRECISE AND ACCURATE SEAWATER SR CURVE FROM LATE CARBONIFEROUS – EARLY PERMIAN CONODONTS

Stratigraphic sections of the Southern Urals containing abundant and well-preserved fauna for precise biostratigraphic correlation and common instratified volcanic ash beds dated by U-Pb zircon geochronology offer a unique opportunity to constrain a temporally accurate Late Pennsylvanian-Early Permian seawater Sr curve. The ⁸⁷Sr/⁸⁶Sr compositions of conodonts (biogenic apatite) were measured by high-precision thermal ionization mass spectrometry following rigorous pretreatment protocols, and plotted within an age model calibrated by >10 high-precision U-Pb zircon ash bed ages. The resulting seawater Sr curve shows a significant reduction in data scatter by comparison to earlier curves (Denison et al., 1994; Veizer et al., 1999; Bruckschen et al., 1999), suggesting that our conodont pre-dissolution treatment was highly effective for retrieving the original seawater Sr signal. The relatively flat Late Moscovian through mid-Ghzelian seawater Sr curve of this study is generally consistent with that of Bruckschen et al. (1999). Beginning in the mid-Ghzelian, our data define a decreasing trend in ⁸⁷Sr/⁸⁶Sr through the mid-Sakmarian. This decrease contrasts with the whole rock carbonate data of Denison et al. (1994), but is similar to that of Veizer et al. (1999) although the slope of the latter curve is significantly steeper. The variation in slope may be an artifact of different biostratigraphic definitions of the basal Sakmarian used for the curves. In the mid-Sakmarian, our stratigraphically highest sample implies an abrupt increase in ⁸⁷Sr/⁸⁶Sr, although the fidelity of this sample as a record of seawater composition requires confirmation. Nonetheless, the highly resolved seawater ⁸⁷Sr/⁸⁶Sr record obtained for the Late Moscovian through mid-Sakmarian will aid in global carbonate chemostratigraphic correlation and contribute to our understanding of the timing of Late Paleozoic glacial and tectonic events.

Bruckschen, P., Oesmann, S., Veizer, J. (1999) Chem. Geol. 161:127-163. Denison, R.E., Koepnick, R.B., Burke, W.H., Hetherington, E.A., Fletcher, A. (1994) Chem. Geol. 112:145-167. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, J., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H. (1999) Chem. Geol. 161:59-88.