Size-related isotopic trends in some Maastrichtian planktic foraminifera: methodological comparisons, intraspecific variability, and evidence for photosymbiosis

The serial test dissection and sieve fraction methods for determining the p attern of size-related change in oxygen and carbon isotopic ratios are comp ared using four Late Cretaceous planktic foraminifer species (Racemiguembel ina fructicosa, Planoglobulina acervulinoides, Planoglobulina multicamerata , and Pseudoguembelina palpebra) from a subtropical site in the North Atlan tic (DSDP Hole 390A). Despite the extra labor required, we identify several clear advantages of the dissection method, including: (1) it provides a me ans of obtaining size-dependent changes in isotopic signatures that are une quivocally ontogenetic, whereas isotopic variation observed from sieve-sepa rated size fractions could be ontogenetic or ecotypic; (2) the taxonomic id entity of smaller sized specimens using the dissection method is unequivoca l, whereas species identification is increasingly ambiguous in smaller size fractions using the sieve method; (3) it reveals a greater total range and a greater complexity in the pattern of ontogenetic change in stable isotop ic values, whereas the sieve method averages the isotopic signal across the entire ontogenetic range preserved within the whole tests that are used. O ur results from serial dissections demonstrate that among the species analy zed, R. fructicosa and P, acervulinoides yield relatively negative adult de lta(18)O values, a large size-related change in delta(13)C values (1.32 and 2.05 parts per thousand, respectively), and virtually no correlation betwe en size-related delta(13)C and delta(18)O values. On this basis we suggest that these were photosymbiotic species that inhabited relatively shallow su rface waters. Evidence for photosymbiosis is not as compelling for P. palpe bra, as this species yields a 1.06 parts per thousand shift in delta(13)C a nd relatively negative delta(18)O values in adult chambers, but much strong er correlation between size-related delta(13)C and delta(18)O values (r(2) = 0.40) than in R. fructicosa and P. acervulinoides. Planoglobulina multica merata yields the most positive adult delta(18)O values of the species stud ied, a strong covariance between size-related delta(13)C and delta(18)O val ues (r(2) = 0.77), and a 0.97 parts per thousand shift in delta(13)C compos ition during ontogeny. We conclude that this species lacked photosymbionts and migrated to a deeper surface water paleohabitat as it increased in size . Single specimen analyses of tightly constrained size fractions reveal a h igh degree of intraspecific variation. delta(13)C and delta(18)O values var y by up to 0.70 and 0.28 parts per thousand in R. fructicosa, 1.41 and 0.80 parts per thousand in P. acervulinoides, 0.66 and 0.82 parts per thousand in P. palpebra, and 0.18 and 0.33 parts per thousand in P. multicamerata, r espectively. Such a range of isotopic variation has been observed in modern day planktic foraminifer assemblages, and likely results from growth of in dividuals during different phases of the seasonal cycle and/or the kinetic effect of intraspecific variation in shell calcification rates. As suggeste d by other investigators, large sample sizes should be analyzed to provide the most reliable correlation of stable isotopic stratigraphic records. (C) 1999 Elsevier Science B.V. All rights reserved.