Temporal acuity and the rate and dynamics of mass extinctions.

In the excitement over the discovery of high levels of the element iridium at the Cretaceous-Paleogene (KPg) boundary and the high likelihood that the KPg mass extinction (66Ma) was caused by the collision of an extraterrestrial object with Earth (1), many envisioned a general theory of mass extinctions driven by impacts (2). Others favored an endogenous process, identifying correlations between the eruption of massive flood basalts and mass extinctions (3). A general model of mass extinctions and other biodiversity crises seemed to be supported by time-series analyses of compilations of the marine fossil record, which revealed a periodic pattern to extinctions over the past 250 million y (4). Over the past decade or so, evidence has grown for distinctive causes for the Permo-Triassic (PT) and the KPg mass extinctions, belying any general mechanism. However, in PNAS, the Burgess et al. (5) paper regarding very high-resolution radiometric dates for the PT event suggests that the similarities may lie not with the triggering mechanisms but in how the Earth’s biota responds to environmental insults. The PT boundary sections near the village of Meishan in Zhejiang Province, China, are the most intensively studied in the world. Easily accessible from Nanjing and Shanghai, they serve as the global reference sections for the boundary. More importantly, the mudstones and carbonates that outcrop along several kilometers of an old phosphate mine are interbedded with numerous layers of volcanic ash. For more than a decade geochronologic dating of the radiometric minerals in such ash beds have been producing dates of progressively higher resolution. In 1988, before the direct dating of these ash beds, the uncertainty on the age of the PT boundary was ±4 Myr (6) and there was no data on the duration of the extinction. By 1998 uncertainties on U-Pb dating of these ash beds at Meishan were ±300 ky and it was clear that the extinction was rapid (7). Now the uncertainties have improved to 31 ky (5), showing that the extinction was rapid indeed. [Astronomical (Milankovitch) cyclicity has also been used to evaluate the timing of this extinction (8), but has been difficult to test.] This improved temporal resolution has been driven by the need to distinguish the A bi ot ic P ha se