Stable isotope evidence for gradual environmental changes and species survivorship across the Cretaceous/Tertiary Boundary

Abstract

High‐resolution δ 13 C and δ 18 O 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 δ 13 C 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 13 C depletion contrasts with the sudden δ 13 C drop at the K/T boundary observed in many deep‐sea sections. The surface‐to‐bottom δ 13 C 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 δ 13 C values is a gradual decrease of about 2.5 permil in δ 18 C values which has not been observed at other localities. This 18 O 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 18 O and 13 C 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 δ 13 C and δ 18 O 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 18 O/ 16 O and 13 C/ 12 C ratios in Tertiary sediments relative to specimens from Maastrichtian sediments, demonstrating the survival of this important Cretaceous taxon after the K/T boundary event.