ON STABLE ISOTOPIC VARIATION AND EARLIEST PALEOCENE PLANKTONIC-FORAMINIFERA

Extant planktonic foraminifera display positive covariance between deltaC-13 signals and test size. As documented by other studies, primary causes of increased deltaC-13 values with increased test size may include 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 values 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 depth over ontogeny, correlations between adult test size and environmental conditions, and changing isotopic disequilibrium with size, ontogenetic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene planktonic foraminifera, we measured the stable isotopic signals of multiple 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 Woodringina claytonensis displaying the largest shift (1.1 part per thousand over a 130 mum range in mean sieve size) and Guembelitria cretacea displaying the smallest (0.2 parts per thousand over a 38 mum range). By analogy with modem planktonic foraminifera, this general relationship between deltaC-13 and size probably resulted from increased reliance on ambient CO2 for calcification at larger test sizes. The high magnitude of this shift in some taxa may reflect either photosymbiotic enhancement of the general trend or relatively greater changes in the proportions of metabolic and ambient CO2 used for calcification at different test sizes. Failure to account for relationships between test size and deltaC-13 signals can lead to underestimation of early Paleocene surface ocean deltaC-13 values by 1 parts per thousand or more. These size-related deltaC-13 effects provide an alternative explanation for decreases in whole-rock deltaC-13 values and some decreases in planktonic-to-benthic foraminiferal deltaC-13 gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of deltaO-18 derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and deltaO-18 signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene deltaO-18 and biogeographic data are consistent with a general trend of surface-to-deep diversification of microperforate planktonic foraminifera following the K/T boundary. Such a trend may simply result from exploitation of a near-surface open-ocean habitat by the epicontinental K/T survivor G. cretacea.