High‐resolution δ13C and δ18O 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 δ13C 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 13C depletion contrasts with the sudden δ13C drop at the K/T boundary observed in many deep‐sea sections. The surface‐to‐bottom δ13C 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 δ13C values is a gradual decrease of about 2.5 permil in δ18C values which has not been observed at other localities. This 18O 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 18O and 13C 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.  and may be linked to sea level regression and environmental perturbations. The second extinction phase coincides with the minimum in δ13C and δ18O values in the Early Danian (Zone P0/Pla) 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 18O/16O and 13C/12C ratios in Tertiary sediments relative to specimens from Maastrichtian sediments, demonstrating the survival of this important Cretaceous taxon after the K/T boundary event.
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