Carbon and nitrogen isotope disturbances and an end-Norian (Late Triassic) extinction event: Comment and Reply
REPLY
- 1School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
- 2Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2l 2A7, Canada
- 3Planetary and Space Sciences Research Institute, Open University, Milton Keynes MK7 6AA, UK
- *E-mail: wignall{at}earth.leeds.ac.uk.
We thank Hall and Pitaru for their comments and look forward to the publication of their work on the ammonites of the Black Bear Ridge section. This will undoubtedly provide much-needed details on this important Triassic-Jurassic boundary section. However, for now, the information they provide indicates that the Triassic-Jurassic boundary is lower than the level we chose, which was based on the published work of Tozer (1982). The first occurrence of Psiloceras is at 64.1 m and seems a reasonable choice for the placement of the boundary. This is 2.9 m above the top of the Norian stage, which is characterized by prolific Monotis. The affinity of the Primapsiloceras? at 63.4 m is debatable; it could be a true Psiloceras but it is also extremely similar to the Triassic taxon Rhacophyllites.
Despite this readjustment of the boundary level, we do not concur with Hall and Pitaru's proposal that the Rhaetian may be entirely absent at Black Bear Ridge. The strata between the last abundant Monotis and the first Psiloceras contain unequivocal Triassic taxa, as listed in Orchard et al. (2001) and noted by Hall and Pitaru. The suggestion that Monotis is reworked is not supported by any sedimentological data and is highly unlikely for such a thin-shelled taxon. Neither do we agree that the carbon and nitrogen isotope excursions we record are equivalent in time to end-Triassic carbon isotope excursions observed in other sections. Consider that Ward et al.'s (2001, their Fig. 1) data have a base Rhaetian δ13Corg positive excursion of similar magnitude and direction to ours (at a level that is also marked by the abrupt disappearance of Monotis) and, at a higher level, a negative excursion at the Triassic-Jurassic boundary. Hesselbo et al.'s (2002, their Fig. 3) data also have positive δ13C values within the lower Rhaetian followed by a complex negative anomaly beginning at a higher level taken to be the Triassic-Jurassic boundary. Thus, the Triassic-Jurassic boundary is marked by a negative, not a positive, excursion. The main contention of our paper, that a pre–end Triassic positive δ13Corg excursion is associated with an extinction of deep-water taxa is therefore unchanged.
Further clarification of the complex series of events that occurred during the Late Triassic will undoubtedly come from the study of other sections on Williston Lake. Our recent field investigations indicate that the top Monotis surface at Black Bear Ridge records an interval of early Rhaetian condensation, with much thicker Rhaetian successions present in sections to the west of this location.
- © 2003 Geological Society of America
