On stable isotopic variation and earliest Paleocene planktonic foraminifera

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Extant planktonic foraminifera display positive covariance between δ13C signals and test size. As documented by other studies, primary causes of increased δ13C 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 δ18O values also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between δ18O 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 δ13C and test size. The slope and magnitude of this trend varies between species, with Woodringina claytonensis displaying the largest shift (1.1‰ over a 130 µm range in mean sieve size) and Guembelitria cretacea displaying the smallest (0.2‰ over a 38 µm range). By analogy with modern planktonic foraminifera, this general relationship between δ13C 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 δ13C signals can lead to underestimation of early Paleocene surface ocean δ13C values by 1‰ or more. These size‐related δ13C effects provide an alternative explanation for decreases in whole‐rock δ13C values and some decreases in planktonic‐tobenthic foraminiferal δ13C gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of δ18O derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and δ18O signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene δ18O 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.