Response: Late Permian Extinctions

We agree with Caldeira that the mechanism we proposed (1) to account for large decreases in the C isotopic composition of surface sea water during Neoproterozoic and Permo-Triassic can work only if the negative excursions lasted for less than 10^6 years. It is likely that the Permo-Triassic boundary excursion persisted no longer than the 10^5-year time scale permitted in Kump's model (2). At the proposed Permo- Triassic boundary stratotype at Meishan, China, the stratigraphic interval containing the excursion is 1 meter thick (3); an earlier negative excursion described by Jin (4) from a Capitanian-Wujiapingian boundary section in southwestern China is confined to a comparably thin stratigraphic interval. For the one Neoproterozoic negative excursion closely constrained by radiometric data, uranium-lead zircon dates on volcanic rocks just below and just above the event are the same to within the measurement error of one million years (5)