ON STABLE ISOTOPIC VARIATION AND EARLIEST PALEOCENE PLANKTONIC-FORAMINIFERA

Extant planktonic foraminifera display positive covariance between del taC-13 signals and test size. As documented by other studies, primary causes of increased deltaC-13 values with increased test size may incl ude increased reliance on ambient CO2 for calcification at larger test sizes, decreased kinetic fractionation during calcification at larger test sizes, and increased photosymbiotic activity in larger symbiont- bearing planktonic foraminifera. Planktonic foraminiferal deltaO-18 va lues also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between deltaO-18 signals and test size include changing habitat dept h over ontogeny, correlations between adult test size and environmenta l conditions, and changing isotopic disequilibrium with size, ontogene tic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene plan ktonic foraminifera, we measured the stable isotopic signals of multip le size fractions of 10 earliest Paleocene species. All of these taxa exhibit a strong positive correlation between delta-C13 and test size. The slope and magnitude of this trend varies between species, with Wo odringina claytonensis displaying the largest shift (1.1 part per thou sand over a 130 mum range in mean sieve size) and Guembelitria cretace a displaying the smallest (0.2 parts per thousand over a 38 mum range) . By analogy with modem planktonic foraminifera, this general relation ship between deltaC-13 and size probably resulted from increased relia nce on ambient CO2 for calcification at larger test sizes. The high ma gnitude of this shift in some taxa may reflect either photosymbiotic e nhancement of the general trend or relatively greater changes in the p roportions of metabolic and ambient CO2 used for calcification at diff erent test sizes. Failure to account for relationships between test si ze and deltaC-13 signals can lead to underestimation of early Paleocen e surface ocean deltaC-13 values by 1 parts per thousand or more. Thes e size-related deltaC-13 effects provide an alternative explanation fo r decreases in whole-rock deltaC-13 values and some decreases in plank tonic-to-benthic foraminiferal deltaC-13 gradients documented at marin e K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of deltaO-18 derived paleot emperatures. Despite this limited range, paleobiogeographic patterns a nd deltaO-18 signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic f oraminiferal species. Earliest Paleocene deltaO-18 and biogeographic d ata are consistent with a general trend of surface-to-deep diversifica tion of microperforate planktonic foraminifera following the K/T bound ary. Such a trend may simply result from exploitation of a near-surfac e open-ocean habitat by the epicontinental K/T survivor G. cretacea.