INTERSPECIES VARIATION IN STABLE ISOTOPIC SIGNALS OF MAASTRICHTIAN PLANKTONIC-FORAMINIFERA

We measured stable isotopic signals of the largest specimens of all ab undant species of planktonic foraminifera in late Maastrichtian sample s from three Atlantic mid-latitude and high-latitude sites. Each sampl e is marked by a relatively small interspecies range of delta(18)O val ues (1.2 to 0.8 parts per thousand). Each also exhibits an interspecie s range of delta(13)C values that resembles those of modern assemblage s at similar latitudes (2.8 to 0.9 parts per thousand, with greater ra nges at lower latitudes). These results indicate that Late Cretaceous planktonic foraminifera inhabited water masses characterized by a rela tively narrow range of temperatures but may have occupied an array of niches as ecologically diverse as that of modern taxa. Maastrichtian t axa exhibit more complex relationships between skeletal morphology and vertical and seasonal paleohabitat affinities than previous paleoecol ogic models have assumed. For example, keeled taxa were not limited to cold or deep habitats and serial taxa appear to have occupied both wa rm near-surface niches and the coolest or most saline habitats. The is otopic differences between species indicate that apparent variation in Maastrichtian stable isotopic signals strongly depends on the taxa an alyzed. Narrowly constrained size fractions of Rugoglobigerina species may provide a good estimate of relative variation in isotopic equilib ria of near-surface summer paleoconditions at low and middle latitudes . Gublerina species, Laeviheterohelix species, and Planoglobulina mult icamerata appear the likeliest candidates for consistently tracking re lative 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 be en only 3 to 5 degrees C in mid-latitude and high-latitude regions. Th ey also suggest that the western and central Atlantic mid-latitude sum mer seasurface may have been much cooler or saltier in the Maastrichti an than it is today. Finally, they corroborate previous estimates of s imilar to 10 degrees C surface water at southern high latitudes.