STABLE ISOTOPE EVIDENCE FOR GRADUAL ENVIRONMENTAL CHANGES AND SPECIES SURVIVORSHIP ACROSS THE CRETACEOUS/TERTIARY BOUNDARY

High-resolution delta C-13 and delta C-18 records have been generated from analyses of the planktonic foraminiferal species Heterohelix globulosa and the benthonic foraminiferal taxon Lenticulina spp from 3 m of a cored section spanning the Cretaceous/Tertiary (K/T) boundary at Brazos River, Texas. These are the first stable isotope records across the K/T boundary based on monospecific and monogeneric foraminiferal samples. They show a gradual decrease in delta C-13 values of about 2.5 permil beginning at the K/T boundary, as defined by the first appearance of Tertiary planktonic foraminifera, and continuing 17-20 cm above the boundary, approximately 40,000 years later. Gradual C-13 depletion contrasts with the sudden delta C-13 drop at the K/T boundary observed in many deep-sea sections. The surface-to-bottom delta C-13 gradient decreased to less than zero approximately 25,000-30,000 years after the K/T boundary and remained negative for at least the next 140,000 years. Concomitant with change in delta C-13 values is a gradual decrease of about 2.5 permil in delta O-18 values which has not been observed at other localities. This O-18 depletion suggests changes in temperature and/or salinity in the earliest Paleocene Gulf of Mexico. No extinction of foraminiferal species is associated with the K/T boundary or the onset of O-18 and C-13 depletions. Instead, two phases of Cretaceous species extinctions occur. One extinction phase is below the K/T boundary and below the tsunami bed of Bourgeois et al. [1988] and may be linked to sea level regression and environmental perturbations. The second extinction phase coincides with the minimum in delta C-18 and delta O-18 values in the Early Danian (Zone P0/P1a) and appears directly related to environmental changes reflected in the isotopic record. H. globulosa, which is commonly present in Maastrichtian and Danian sediments, exhibits significantly lower O-18/O-16 and C-13/C-12 ratios in Tertiary sediments relative to specimens from Maastrichtian sediments, demonstrating the survival of this important Cretaceous taxon after the K/T boundary event.