RAPID CHANGE IN STRONTIUM ISOTOPIC COMPOSITION OF SEA-WATER BEFORE THE CRETACEOUS TERTIARY BOUNDARY

A CRITICAL aspect of the debate over the origin of the chemical, biological and climatological perturbations that characterize the Cretaceous/Tertiary boundary (K/T boundary) is the timescale and detailed pattern of the extinctions and geochemical changes. Whether the development of the anomalies was catastrophic (over tens to hundreds of years) or more gradual (10(3) to 10(6) years), whether the anomalies occur exactly at the boundary or precede it, and whether the chemical and biological signatures represent one event or can be resolved into several discrete events, are issues fundamental to identifying the cause of the marked changes. To determine the timing and rate of change of strontium isotope evolution of sea water during the five million years preceding the K/T boundary, we analysed foraminifera from an exceptionally thick, palaeontologically well-characterized marine K/T section exposed at Bidart, southwest France. We found a rapid increase in Sr-87/Sr-86 of ocean water 1.5 to 2.3 Myr before the boundary. The increase may be explained by a 10% increase in continental strontium flux to the oceans over a period of about 1 Myr.