Passage A:

The abrupt disappearance of roughly three-quarters of species at the close of the Cretaceous, some 66 million years ago, has a leading suspect whose credentials are, on the face of it, overwhelming. A worldwide layer of clay marking the boundary is enriched in iridium, an element rare in the Earth's crust but abundant in asteroids, and it contains shocked quartz and glassy spherules - mineralogical signatures produced only by the extreme pressures of a large impact. The buried Chicxulub crater off the Yucatan Peninsula, dated to the boundary and some 180 kilometers across, supplies the requisite culprit. The attraction of the impact hypothesis lies in its capacity to explain suddenness. A body ten kilometers in diameter, striking in seconds, would have injected enough dust and sulfate aerosol into the stratosphere to darken the sky and suppress photosynthesis for months or years, collapsing food chains from the base upward. Such a mechanism accounts naturally for the geologically instantaneous character of the extinction and for its indifference to habitat: marine and terrestrial ecosystems fell together. Proponents stress that the boundary clay's iridium is a global, single-pulse signal, consistent with one catastrophic event rather than a protracted decline. Where the fossil record appears to show gradual losses in the strata below the boundary, they attribute the pattern to the incompleteness of sampling - the so-called Signor-Lipps effect, whereby the last known appearance of a species predates its true extinction simply because fossils are scarce. On this account, the impact was not merely a contributing factor but the proximate and sufficient trigger.

Passage B:

That an asteroid struck the Earth at the end of the Cretaceous is not in dispute; whether it did the killing is another matter. For hundreds of thousands of years spanning the boundary, the Deccan Traps of western India erupted in one of the largest volcanic episodes in the planet's history, releasing vast quantities of carbon dioxide and sulfur dioxide. These gases would have driven alternating pulses of warming and cooling and acidified the oceans, imposing a prolonged environmental stress quite unlike a single blow. Advocates of a volcanic cause point to evidence that many groups were already in decline before the boundary, their diversity thinning over the final stages of the Cretaceous in a manner difficult to reconcile with an instantaneous catastrophe. They note, too, that the impact and the most intense phase of eruption coincide closely in time, and argue that seismic energy from the impact may even have intensified the volcanism - making the asteroid, at most, an aggravating shock delivered to a biosphere already destabilized. The strongest form of the argument does not deny the impact's mineralogical fingerprint; it denies that fingerprint's sufficiency. A cause capable of explaining a sudden death may still be the wrong cause if the death was not, in fact, sudden. The dating of extinctions at the resolution required to settle the matter remains contested, and it is precisely this uncertainty about timing, rather than any dispute over the reality of either event, that keeps the question open.