The Basque coastal sections of the K/T boundary – A key to understanding “mass extinction” in the fossil record

Resumen

 lnvestigations at the K/T boundary section al Zumaya, northern Spain, underline the importance of this section in (1) establishing a Maastrichtian ammonite zonation and (2) in understanding "mass extinctions" in the fossil record. "Mass extinction" at the K/T boundary can be restricted to two groups of oceanic surface plankton. Ammonites and inoceramids exhibit a gradual decline through the late Cretaceous, as probably also do the Temperate belemnites and the bulk of involved vertebrates. Most of these groups disappear a long time before the boundary.

Compared with the available environmental factors, the course of ammonite diversity through time perfectly parallels the course of global sea level changes. Times of high extinction rates (e.g. the era and system boundaries) coincide with times of maximum regressions, while the subsequent evolutionary radiations match with the new sea level rise. There is no need for any cosmic catastrophy to explain the decline of most invertebrate and vertebrate groups.

Coeval with the short-time extinction of oceanic surface plankton at the boundary itself, the following environmental and depositional changes were observed: minor (positive as well as negative) changes in temperature, boundary clay sedimentation, enrichment of iridium and noble metals, occurrence of sanidium spheres and shocked quartz, and changes in carbon and strontium isotopes. Ali these events were used to support the Alvarez hypothesis of a cosmic impact at the boundary. But they are much easier explained by increasing explosive volvanicity which, on the other hand, might correlate also with the time-equivalent magnetic reversals.