Isotopic analyses of large (>250 µm) single specimens of a possible photosymbiotic (Racemeguembelina fructicosa) and an asymbiotic (Contusatruncana contusa) Cretaceous foraminifera exhibit surprisingly large within sample variability in both d ¹³C and d ¹⁸O. This variability complicates interpretation of their paleobiology but may provide clues to changing paleoceanography. We analyzed 10 well preserved specimens of each taxon from 25 samples spanning the last 3 million years of the Cretaceous, from the Ocean Drilling Program, hole 1050C (Western North Atlantic). Within sample variability for d ¹³C is up to 1.75‰ and 1.25‰ for R. fructicosa and C. contusa, respectively, and for d ¹⁸O is up to 1‰ for each taxon. In general R. fructicosa shows greaten but relatively constant variability; whereas, that variability for C. contusa decreases up section as average d ¹⁸O values decease (apparent warming). For both taxa there are no obvious size-related trends across the size range analyzed associated with the variability observed. The d ¹⁸O and d ¹³C are not strongly correlated.

Assuming the life cycle of Cretaceous foraminifera spanned weeks to months, the lack of correlation between size and isotopic composition suggests observed variability reflects interannual differences more than ontogenetic trends. If so, efforts to determine the paleobiology of these and other extinct foraminifera based on size related isotopic trends (particularly the possible presence of photosymbionts) might be affected by strong sampling biases. Analyses of large individuals typically use fewer specimens than analyses of small individuals. Because large individuals from the same sample can have very different isotopic signatures, which individuals are selected can have a large effect on the value measured. On the other hand, the decrease in within sample variability in C. contusa up section suggests that interannual variability decreased in C. contusa’s habitat. If C. contusa lived in relatively deep surface waters, decreasing variability through time suggests an increasingly stratified water column in the western Atlantic through the late Maastrichtian.