We measured stable isotopic signals of the largest specimens of all abundant species of planktonic foraminifera in late Maastrichtian samples from three Atlantic mid‐latitude and high‐latitude sites. Each sample is marked by a relatively small interspecies range of δ18O values (1.2 to 0.8‰). Each also exhibits an interspecies range of δ13C values that resembles those of modern assemblages at similar latitudes (2.8 to 0.9‰, with greater ranges at lower latitudes). These results indicate that Late Cretaceous planktonic foraminifera inhabited water masses characterized by a relatively narrow range of temperatures but may have occupied an array of niches as ecologically diverse as that of modern taxa. Maastrichtian taxa exhibit more complex relationships between skeletal morphology and vertical and seasonal paleohabitat affinities than previous paleoecologic models have assumed. For example, keeled taxa were not limited to cold or deep habitats and serial taxa appear to have occupied both warm near‐surface niches and the coolest or most saline habitats. The isotopic differences between species indicate that apparent variation in Maastrichtian stable isotopic signals strongly depends on the taxa analyzed. Narrowly constrained size fractions of Rugoglobigerina species may provide a good estimate of relative variation in isotopic equilibria of near‐surface summer paleoconditions at low and middle latitudes. Gublerina species, Laeviheterohelix species, and Planoglobulina multicamerata appear the likeliest candidates for consistently tracking relative variation in isotopic equilibria of the coldest or most saline paleohabitats of Maastrichtian planktonic foraminifera. These isotopic results suggest that upper water column thermal gradients may have been only 3 to 5°C in mid‐latitude and high‐latitude regions. They also suggest that the western and central Atlantic mid‐latitude summer seasurface may have been much cooler or saltier in the Maastrichtian than it is today. Finally, they corroborate previous estimates of ∼ 10°C surface water at southern high latitudes.
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